Teat Canal

The teat canal and surrounding musculoelastic tissue provide the primary physical barrier to microbial invasion and also prevent leakage of milk between milkings.^6,7 The teat canal is a narrow, longitudinally folded cylinder lined with stratified squamous epithelium. The tortuous shape of the canal provides physical protection against infection, as does protein-rich keratin, which is continually produced by the epithelial cells and lines the canal. Keratin physically plugs the teat canal and traps invading microbes, which are then expelled with sloughed keratin at milking time. Keratin also contains fatty acids and proteins that have bacteriostatic or bacteriocidal effects in vitro,^8,9 although their role in mammary gland defense is uncertain. Loss of keratin greatly increases the risk of intramammary infection.¹⁰

Management practices and the cow’s physiologic state influence teat function and mastitis resistance For example, the integrity of the teat canal is compromised for up to 2 hours after milking while the elastic fibers surrounding the canal recoil and keratin begins to be renewed.^6,11 Therefore, it is recommended that cows be fed after milking, to keep them standing and to minimize exposure to pathogens while the teat canal recovers. When a cow is dried off at the end of lactation, the teat canal does not fully fill with keratin for days to months. A delay in keratin plug formation is associated with an increased risk of intramammary infection.^12,13 When antibiotics are infused into the teat, full insertion of the cannula disrupts the keratin and can transport microorganisms from the teat canal or skin into the teat cistern. Therefore, the “partial-insertion” method of administering antibiotics, whereby the cannula is inserted only 2 to 3 mm into the canal, is recommended.¹⁴ Using the partial-insertion method to infuse antibiotics at dry-off can reduce the incidence of intramammary infection or clinical mastitis during the dry period.^14,15

Callosity (hyperkeratosis) of the teat end and abrasion of the teat orifice epithelium increase the risk of intramammary infection.^16,17 Trauma to the teat end compromises teat canal function and predisposes to infection. Trauma can occur when a cow steps on her teat or as a consequence of the milking process. Ineffective pulsation, excessive milking vacuum, or poorly fitting teat cup liners can damage teat tissue during milking.¹⁸ Air admission into the milking cluster, as occurs with liner slips, can cause reverse-pressure gradients that propel bacteria-laden milk droplets up through the teat canal into the teat cistern.¹⁹ Even the shape of the teat end and length and diameter of the teat canal influence infection risk; short, wide teat canals and flat or inverted teat ends predispose to mastitis, as does teat canal protrusion.^7,20,21

Because of the importance of the teat in mammary gland defense, dairy producers should routinely monitor teat condition and use a combination of genetic selection and management practices to promote teat health and function. Teat Club International, a group of researchers, veterinarians, and udder health advisors, has developed a standard method for scoring teat condition; the method and images are published on an educational CD available through the National Mastitis Council (NMC; www.nmconline.org).

Cellular Defense Mechanisms

Once microbes have breeched the teat canal, leukocytes and mammary epithelial cells constitute the next line of defense. Milk from healthy mammary glands contains a low concentration (<100,000/mL) of cells, the majority of which are macrophages. However, when microbes are detected, macrophages and epithelial cells release cytokines and chemokines that trigger an influx of neutrophils.^22,23 The rapidity and extent of neutrophil influx are critical determinants of infection outcome.^24,25 Neutrophils are the predominant cells in mastitic milk, with concentrations often exceeding 1 million/mL during acute infection. Macrophages, lymphocytes, and epithelial cells are present in much lower concentrations, with the total cell concentration referred to as the somatic cell count (SCC).

Noncellular Defense Mechanisms

Defense Mechanisms in the Periparturient Period

Mammary gland defense mechanisms are greatly reduced in the periparturient period, making cows most vulnerable to mastitis at that time. Indeed, both the incidence and the severity of mastitis are higher in the periparturient period than at other times.^29,55-57 Neutrophil functions, particularly chemotaxis and intracellular killing, are impaired in periparturient cows, as is phagocytosis by macrophages. The immunosuppression accompanying parturition is partly caused by a high circulating cortisol concentration, which impairs neutrophil margination and recruitment. Neutrophils of periparturient cows also undergo apoptosis more rapidly than do neutrophils of cows in later lactation, and early onset of apoptosis reduces phagocytic and intracellular killing activity.²⁶ Low circulating concentrations of insulin-like growth factor 1 (IGF-1) in periparturient cows may reduce neutrophil viability and impair cytokine secretion.⁵⁶ Because a cow’s ability to resist microbial challenge is reduced during the periparturient period, producers must pay special attention to the housing and management of periparturient cows.

Detection of Subclinical Mastitis

Detection of Clinical Mastitis

Clinical mastitis results in obvious abnormalities of the milk, mammary gland, or cow and is easier to recognize than subclinical mastitis. Most episodes of clinical mastitis are mild to moderate in severity. With mild clinical mastitis, milk from the affected quarter is abnormal in color, viscosity (watery, thick), or consistency (contains flakes or clots of inflammatory material). In moderate cases the affected mammary gland is noticeably inflamed (swollen, firm, warm, red, painful), and milk production may be somewhat decreased. With severe clinical mastitis, milk and mammary gland changes are accompanied by systemic illness. Systemic signs may include decreased feed intake or milk production, lethargy, depression, tachypnea, enophthalmos, weakness, recumbency, or low-volume diarrhea that may contain mucus or blood. Physical examination of the cow often reveals fever, tachycardia, decreased rate or strength of rumen contractions, prolonged skin tent or capillary refill time, tacky mucous membranes, or cold extremities.^76-79 Hypothermia may precede death in fatal cases. Occasionally, gangrenous mastitis occurs, in which case the skin of the mammary gland or teat may be cold, black, or crepitant, and gas may be expelled when milking the teat.

The severity of clinical mastitis influences the choice of treatment, as well as the prognosis, so efficient detection of clinical mastitis and determination of severity are critical. Detection of clinical mastitis is accomplished by stripping foremilk onto the floor of the milking parlor at each milking. Milk that is grossly abnormal must be discarded or fed to calves and must not enter the bulk tank. Grossly abnormal milk generally has a very high SCC and altered composition, so immediate exclusion from the bulk tank helps to protect milk quality. Currently, no automated methods reliably detect clinical mastitis in the milking parlor, although such tests are being sought for robotic milking systems.

Milking personnel should routinely check mammary glands for signs of inflammation while cows are in the milking parlor. When cows with abnormal milk or an inflamed mammary gland have reduced milk yield or appear to be systemically ill, they should be examined promptly and systematically to determine mastitis severity and appropriate actions. Relevant farm staff must therefore be trained in assessment of attitude, rectal temperature, hydration status, and ideally, rumen motility.

Resolution of clinical signs is not synonymous with resolution of intramammary infection. Clinical signs may persist for a time after the pathogen has been eliminated, as often occurs with E. coli mastitis, or infection may persist after clinical signs have resolved, as with S. aureus mastitis. Regardless, clinical mastitis is always followed by a period of subclinical inflammation. If the infection has resolved, SCC usually returns to normal within 2 weeks.⁸⁰ Persistent elevation in SCC suggests chronic intramammary infection, which can be accompanied by recurrent bouts of clinical mastitis. Therefore, monthly CMSCC and periodic CMT are useful tools for assessing the likelihood of cure after clinical mastitis.

Contagious Mastitis

Infected mammary glands are the predominant reservoirs of contagious mastitis pathogens. Transmission occurs when milk from an infected gland contacts the teat of an uninfected gland during the milking process. The teat cup liners of the milking cluster are important fomites if infected cows are milked before uninfected cows. Other fomites include the hands of the milking personnel and towels used to wash or dry the teats of multiple cows. Once pathogens have been deposited on the teat skin, the gland is at risk of infection, especially when teat defense mechanisms are compromised. In some cases, abrupt vacuum fluctuations within the milking cluster (reverse-pressure gradients) can propel contagious pathogens from the milk of an infected gland into the teat canal of an uninfected gland.¹⁹

Streptococcus agalactiae and Staphylococcus aureus are major contagious mastitis pathogens that cause persistent elevation in SCC and reduction in milk yield. Corynebacterium bovis is a minor contagious mastitis pathogen with a lesser impact on SCC and milk yield. Infection with these three pathogens can be controlled by implementing a five-point mastitis control program consisting of (1) postmilking germicidal teat disinfection (postdipping), (2) antibiotic treatment of all quarters of all cows at dry-off (“blanket” dry-cow antibiotic therapy), (3) culling of chronically infected cows, (4) prompt recognition and treatment of clinical mastitis cases, and (5) proper use and maintenance of the milking machine.⁸¹ Together, these measures reduce both the incidence and the prevalence of infection. Mycoplasma bovis and other Mycoplasma species are usually categorized as contagious mastitis pathogens even though they colonize body sites other than the mammary gland (see section on Mycoplasma mastitis). This is because once intramammary infection is established, Mycoplasma spreads among cows and glands in a contagious manner.

Environmental Mastitis

With environmental mastitis, the predominant reservoir is the environment. Transmission occurs when teats become contaminated with environmental pathogens between milkings or at milking time. Common sources of infection are fecal material, bedding, soil, and contaminated water. Once environmental pathogens have been deposited on the teat skin, the gland is predisposed to infection, particularly when defense mechanisms are compromised. Environmental pathogens can also be transported into the teat canal or cistern if the teat end is not sufficiently disinfected before infusing antibiotics, or when a contaminated infusion preparation or cannula is used. The predominant environmental mastitis pathogens are coliform bacteria and streptococci other than S. agalactiae (environmental streptococci). However, many other environmental organisms are capable of causing mastitis.

Sample Selection

Sample Collection, Storage, and Handling

Selection of Media

The media used for quarter or composite milk samples depends on the pathogen(s) of concern. Blood agar (5%), the most common medium, allows growth of most aerobic mastitis-causing pathogens. Addition of esculin or staphylococcal beta-toxin facilitates identification of streptococci. Clinical mastitis samples are often plated on MacConkey medium, which selects for gram-negative bacteria and facilitates diagnosis of coliform mastitis. Biplates are available that contain both blood agar with esculin and MacConkey agar. A variety of selective media for streptococci and staphylococci are also available. Triplates and quadplates incorporate streptococcal and staphylococcal media along with blood and MacConkey agar. The use of biplates, triplates, or quadplates facilitates pathogen identification by farm personnel.¹⁰⁵Mycoplasma spp will not grow on blood agar and requires special media and enhanced—carbon dioxide (CO₂) incubation conditions. Therefore, if milk samples are to be tested for Mycoplasma spp, it is important to notify the laboratory.

Bulk tank milk samples should be plated on a variety of media to facilitate detection and quantification of streptococci, staphylococci, coliform bacteria, and (when of concern) Mycoplasma species.

Inoculum Size and Laboratory Methods

The amount of milk plated determines the lower limit of pathogen detection. For example, a 0.01-mL (10-μL) inoculum allows detection of approximately 100 CFU/mL of milk, whereas a 0.1-mL (100-μL) inoculum detects concentrations as low as 10 CFU/mL. With the traditional method of milk culturing, a swab or 0.01-mL loop is used to plate the sample onto one quadrant of a plate; this allows four samples to be cultured per plate. Larger (0.05- or 0.10-mL) inocula increase recovery rates for pathogens shed in low concentrations, such as E. coli or S. aureus. Other ways to increase recovery are preculture centrifugation,¹⁰⁶ incubation,^94,107,108 or freezing¹⁰⁸ of the milk, or a combination of these methods. The optimal method is pathogen dependent. Augmented methods are most useful for detecting subclinical S. aureus infections and reducing false-negative results for clinical mastitis samples. Some reports suggest that freezing negatively impacts the isolation of E. coli from milk,^109,110 but this is not the case in all studies.¹¹¹

A selective culture system marketed for isolation and enumeration of specific pathogens from food products (Petrifilm plates, 3M, Minneapolis) was more sensitive (88%) than traditional milk culture (66%) for detecting S. aureus in milk and provided results within 24 hours. However, this test requires subjective interpretation of color changes, which leads to false-positive results if the person reading the films is inexperienced.⁹⁴

The inoculum volume used for bulk tank milk varies among laboratories. Larger inocula (e.g., 0.1 or 0.2 mL) enhance detection of pathogens that are present in low concentrations. Preculture incubation (95° F for 18 hours) of bulk tank milk dramatically increased detection of S. aureus in one study.¹¹²

Interpretation of Culture Results

Pathogen identification procedures are described in detail in the Laboratory Handbook on Bovine Mastitis published by the NMC.¹¹³ In most cases, it is not essential to identify mastitis pathogens to the species or strain level. For routine diagnosis and treatment purposes, it is sufficient to categorize isolates as S. agalactiae, other Streptococcus or Enterococcus species, coagulase-positive Staphylococcus species, coagulase-negative Staphylococcus species, coliform bacteria, Corynebacterium species, Arcanobacterium (Actinomyces) pyogenes, Bacillus species, yeast, or “other.” Most isolates can be tentatively categorized on the basis of colony morphology, hemolysis pattern, Gram stain results, and catalase testing. Use of blood agar that contains esculin and staphylococcal beta-toxin allows differentiation of S. agalactiae from other streptococci and enterococci; alternatively, separate CAMP and esculin tests can be performed. Coagulase testing distinguishes coagulase-positive staphylococci (assumed to be S. aureus) from less pathogenic coagulase-negative species. As previously mentioned, use of biplates, triplates, or quadplates can facilitate pathogen categorization.¹⁰⁵ When speciation or further confirmation is desired, a variety of biochemical and nonbiochemical testing methods can be used. Mycoplasma isolates must be speciated to distinguish potential mastitis pathogens from incidental, nonpathogenic species.

Deoxyribonucleic acid (DNA) fingerprinting, phage typing, ribotyping, pulse-field gel electrophoresis, multilocus gene sequencing, and other molecular methods of pathogen typing can provide valuable information about the source and spread of infection in herds with mastitis problems. For example, ribotyping identified a cat with chronic sinusitis as the probable source of a herd outbreak of Streptococcus canis mastitis.¹¹⁴ DNA fingerprinting demonstrated that many cows with recurrent episodes of clinical E. coli mastitis had chronic infections,¹¹⁵ challenging the assumption that E. coli infections resolve spontaneously shortly after the onset of disease. Typing techniques allow bacterial isolates from potential environmental reservoirs to be compared with those causing high SCC or clinical mastitis¹¹⁶ and can help identify the source of bacteria in bulk tank milk.¹¹⁷ As molecular technologies become more affordable, their use in mastitis investigations is likely to increase.

Definition of Infection

No universally accepted criteria exist for defining intramammary infection. Researchers require growth of the same pathogen in at least two of three consecutive milk samples, growth of the same pathogen in duplicate milk samples collected simultaneously, or growth of a pathogen in conjunction with indirect evidence of infection (high SCC, positive CMT, or clinical signs of mastitis). In practice, it is too costly to collect consecutive or duplicate milk samples. Therefore, diagnosis is usually made on the basis of a single milk culture result; fortunately, most cultures are from cows with clinical mastitis or high SCC, providing indirect evidence of infection.

Confidence in culture results is greatest when a single pathogen is isolated in pure culture. However, co-infections do occur. When three or more colony types are isolated from a given sample, the sample is considered contaminated. However, even in a contaminated sample, growth of a single colony of S. agalactiae or S. aureus is considered significant.

Alternative Methods of Pathogen Detection

Clinical mastitis episodes caused by gram-positive bacteria are usually treated with antibiotics, whereas antibiotics are not required for many coliform mastitis episodes (see section on clinical mastitis treatment). A variety of nonculture schemes using historical data (e.g., parity, stage of lactation, season of year) and clinical signs (e.g., rectal temperature, rumen contraction rate, appearance of milk) have been developed to distinguish coliform mastitis from mastitis caused by gram-positive bacteria.^118-121 Unfortunately, none of these schemes is sufficiently accurate to be used as a basis for antibiotic treatment decisions. Although veterinary practitioners often believe they can recognize coliform mastitis, one study showed that practitioners correctly identified only 23 of 36 (63%) coliform mastitis episodes and misidentified 32 of 82 (39%) noncoliform episodes.¹²² An artificial neural network and a model using inductive inference were only able to accurately classify bacterial pathogens about 60% of the time.¹²³

Cow-side tests that detect LPS in milk have been used to identify gram-negative mastitis episodes.¹²⁴ However, these tests are not available in the United States. A method that distinguishes gram-positive and gram-negative bacteria in milk by adding a reagent, incubating the mixture in hot tap water for 2 minutes, filtering the mixture through a membrane, and staining the membrane has been described.¹²⁵ Unfortunately, the limit of detection of that method was 10⁶ CFU/mL or greater for E. coli and S. aureus, which is too high for many mastitis episodes.

Cows with gram-negative clinical mastitis are more likely to be neutropenic and monocytopenic and to have higher blood hemoglobin concentration than cows with gram-positive clinical mastitis. The sensitivity of the combination of these hematologic parameters for detecting gram-negative mastitis was 93%, and specificity 89%, in one study.¹²⁶ If a laboratory is not available or a complete blood count (CBC) is considered too costly, the percentage of segmented neutrophils and monocytes on a blood smear can be used; sensitivity and specificity of differential cell counts were 87% and 71%, respectively.

Polymerase chain reaction (PCR) technology has allowed development of a number of PCR tests for mastitis pathogens. With PCR, pathogens are detected and definitively identified more rapidly than by milk culture. Unfortunately, because PCR primers are specific for particular pathogens, PCR cannot be used to test milk for a wide range of mastitis-causing pathogens. Multiplex PCR overcomes this limitation to some extent. For example, a recently developed multiplex real-time PCR method for simultaneous detection of S. aureus, S. agalactiae, and Streptococcus uberis correctly identified 96% of quarter milk samples.¹²⁷ PCR is also used for detecting Mycoplasma mastitis because culture results can take more than a week.¹²⁸ Now that techniques have been developed to overcome the inhibiting effects of milk on PCR, PCR testing will likely become more widespread.

Streptococcus agalactiae Mastitis

Herds that conscientiously use the control measures described earlier in the five-point plan can eliminate or greatly reduce S. agalactiae mastitis within a few years. However, S. agalactiae spreads rapidly when effective control measures are not in place and can cause substantial economic loss. Most episodes of S. agalactiae mastitis are subclinical and last for months or years if not treated. Subclinical infection may be interspersed with bouts of mild clinical mastitis, but cows do not become systemically ill.

Streptococcus agalactiae adheres to epithelial cells in the mammary gland and localizes primarily in the ducts. Milk production declines when ducts become blocked with cells and debris and the associated alveoli involute. Early treatment can restore milk production because permanent damage to the gland is minimal. However, tissue damage, fibrosis, and a permanent decrease in milk production can occur if infection becomes chronic. S. agalactiae–infected glands tend to shed high concentrations of bacteria and SCC in milk. Even a small number of infected cows can increase the SCC or SPC of bulk tank milk and result in milk quality penalties.^129-131

Streptococcus agalactiae mastitis should be suspected in herds that have a high or increasing BTSCC or a large or increasing proportion of cows with persistently elevated SCC. Growth of esculin-negative, CAMP-positive streptococci in milk from the bulk tank or from individual cows confirms S. agalactiae infection. Composite milk from all cows in the herd must be cultured to determine the prevalence of infection and identify positive cows. PCR is a promising alternative to milk culture.^127,131-133

Staphylococcus aureus Mastitis

Most dairy herds have cows with S. aureus mastitis, but the prevalence and economic impact vary. As for S. agalactiae, most S. aureus intramammary infections are chronic and subclinical, with periodic bouts of clinical mastitis. However, clinical S. aureus mastitis ranges in severity from mild to gangrenous. Herds with a high prevalence of S. aureus mastitis cannot be distinguished from those with a high prevalence of S. agalactiae mastitis without culturing milk; in some cases, both pathogens are prevalent. Herds with S. aureus or S. agalactiae mastitis tend to have a high or increasing BTSCC and a large or increasing proportion of cows with persistently elevated CMSCC; however, S. agalactiae mastitis tends to cause higher SCC and a higher bulk tank bacteria count.

Staphylococcus aureus mastitis is presumptively diagnosed by isolating coagulase-positive staphylococci from bulk tank milk or infected glands, particularly if colonies are surrounded by a double zone of hemolysis (complete hemolysis surrounded by incomplete hemolysis). Because certain S. aureus strains are coagulase negative¹⁴³ and other Staphylococcus species (e.g., S. hyicus) can be catalase positive,¹⁴⁴ definitive identification requires additional diagnostic testing. Herds typically have a predominant strain of S. aureus, with additional strains appearing sporadically.¹⁴⁵ Strains of S. aureus differ in contagiousness, virulence, and antibiotic susceptibility, leading to variable responses to treatment and control measures among herds and among cows.^146-148 In some cases, multiple strains of S. aureus co-inhabit a gland or cow.¹⁴⁹ Resolution of infection with one strain can be followed by infection with another strain.¹⁵⁰

Staphylococcus aureus adheres to and invades mammary epithelial cells and interstitial tissue.^151-153 It also resists phagocytosis by neutrophils. Antiphagocytic mechanisms include a capsule that inhibits opsonization and a surface protein (protein A) that binds the Fc portion of host Ig. Leukotoxin production enables some S. aureus strains to kill phagocytes.¹⁵⁴ Other mechanisms allow S. aureus to survive within leukocytes and epithelial cells and induce apoptosis.^153,155,156S. aureus strains produce a variety of enzymes and exotoxins that damage mammary cells and result in fibrosis and abscess formation.¹⁵⁷ Some strains produce β-lactamase, conveying resistance to antibiotics such as penicillin and amoxicillin. Others convert to L-form or small colony variants, which can persist with antibiotic therapy.^158-160 Certain S. aureus strains form adherent colonies surrounded by a biofilm, making them inaccessible to phagocytes and antibiotics^161,162; biofilm-associated proteins are associated with persistent S. aureus mastitis.¹⁶¹ For these reasons, S. aureus intramammary infections seldom resolve spontaneously and are difficult to treat with antibiotics.

Mycoplasma Mastitis

Mycoplasma species are found in a variety of extramammary body sites of cattle, such as the respiratory and urogenital tracts. Colonized cattle may be asymptomatic or may develop bronchopneumonia, otitis, polyarthritis, or mastitis. In dairy herds with Mycoplasma mastitis, otitis media-interna, respiratory disease, or swollen joints may be seen in calves, especially if unpasteurized milk is fed.¹⁸⁵ Cows may or may not exhibit clinical signs other than mastitis. Most Mycoplasma intramammary infections are subclinical and chronic, as with S. agalactiae and S. aureus. However, when clinical mastitis occurs, it often affects multiple glands simultaneously or in succession and fails to respond to conventional antibiotic therapy. Although the milk from cows with clinical Mycoplasma mastitis is visibly abnormal, the cows are usually systemically healthy.

The pathogenesis of Mycoplasma mastitis is not fully understood. Asymptomatic carrier animals may infect herdmates by shedding the organisms in nasal or vaginal secretions, feces, or milk. Purchased animals are often blamed for introducing Mycoplasma mastitis, but outbreaks can occur even in closed herds. Cow-to-cow transmission of intramammary infection is believed to occur through milk-contaminated fomites at milking time, as with other contagious mastitis pathogens. However, it appears that Mycoplasma species can be transmitted from other body sites to the mammary gland, or vice versa, via the bloodstream or lymphatics. For example, experimental intramammary inoculation of Mycoplasma species into a single mammary gland was followed by detection of phenotypically identical Mycoplasma species in extramammary body sites, blood, and noninoculated mammary glands,^186,187 even when gland-to-gland transmission was prevented by a special milking device.¹⁸⁷ Recently, Biddle et al.¹⁸⁸ found that isolates of Mycoplasma species from the milk, mammary gland parenchyma, and supramammary lymph nodes of infected cows all had the same pulse-field gel electrophoresis pattern, and that all cows had at least one extramammary isolate with the same pattern. These findings suggest that cows with Mycoplasma mastitis frequently have disseminated infections, and that internal transmission may occur.

Results of bulk tank surveys suggest that less than 10% of U.S. dairy herds have cows with Mycoplasma mastitis; however, the economic impact in infected herds can be substantial. Many species of Mycoplasma can infect the mammary gland and cause mastitis, but M. bovis is isolated most frequently. Mycoplasma infection will be missed when milk is cultured unless appropriate media and incubation conditions are used. Freezing of samples reduces the concentration of viable organisms, whereas broth enrichment before culturing can enhance detection of Mycoplasma species.¹⁸⁹ Colonies usually take 7 to 10 days to appear, and isolates must be speciated to differentiate pathogenic from nonpathogenic Mycoplasma strains. Because of the time required for culture and secondary speciation, PCR-based diagnostic methods have been developed. Recent PCR-based methods have acceptable limits of detection (<100 CFU/mL) and use primers that differentiate among several Mycoplasma species.¹⁹⁰ PCR-based methods likely will increasingly replace culture for detection of Mycoplasma mastitis.

When pathogenic Mycoplasma organisms are isolated from the bulk tank or milk from an individual cow, the herd probably contains more than one infected animal. Pooled milk can be cultured after milking each string of cows to help identify infected groups for further testing. Cows with clinical mastitis and those with high SCC are good candidates for testing but may not include all infected individuals.

Corynebacterium bovis Mastitis

Although highly contagious, C. bovis is considered a minor mastitis pathogen because it typically inhabits the teat canal, induces a lesser increase in SCC than other contagious mastitis pathogens, causes subclinical infection, and has little impact on milk yield.¹⁹⁴C. bovis can cause clinical mastitis,^195,196 but the episodes are usually milder than those caused by other gram-positive bacteria or coliform bacteria.¹⁹⁶ As with S. aureus, there is often a predominant strain of C. bovis in a herd.¹⁹⁷

Corynebacterium bovis mastitis can be controlled by measures that control other contagious mastitis pathogens.¹⁹⁸ A high prevalence of C. bovis mastitis is often attributable to poor teat-dipping practices.^195,199

There is some evidence that C. bovis infection may protect against mastitis caused by major pathogens. For example, in a large case-control study, the odds of infection with a major pathogen were significantly reduced when a gland was infected with C. bovis than when a gland was uninfected (odds ratio [OR] = 0.52).²⁰⁰ However, failure to control C. bovis means that contagious mastitis control measures are insufficient and the herd is at risk for an outbreak of S. aureus, S. agalactiae, or Mycoplasma mastitis should those pathogens be introduced.

Environmental Streptococcal Mastitis

Mastitis-causing Streptococcus species other than S. agalactiae are referred to as “environmental streptococci.” These include S. uberis, S. parauberis, S. dysgalactiae, S. equinus, S. saccharolyticus. S. salivarius, S. canis, and others; enterococci, such as Enterococcus faecalis and E. faecium, and Aerococcus viridans are often grouped with the environmental streptococci. These catalase-negative, gram-positive cocci have reservoirs in the environment and act as opportunistic pathogens, whereas the main reservoir for S. agalactiae is infected mammary glands. This does not mean that environmental streptococcal mastitis cannot spread contagiously. Indeed, S. dysgalactiae is sometimes categorized as a contagious pathogen. Also, S. canis and some strains of S. uberis have been implicated in mastitis outbreaks that appear to involve cow-to-cow transmission.^114,201,202 Still, environmental streptococcal mastitis is relatively more prevalent and costly in herds that have achieved control of contagious mastitis. Most research on environmental streptococcal mastitis has focused on S. uberis, which is considered the most important of these pathogens.²⁰³

Environmental streptococci are shed in the feces of cattle and are ubiquitous in the environment on dairy farms. Fecal shedding probably accounts for isolation of environmental streptococci from water, soil, plant material, and flies.¹¹⁶ Infected dogs and cats can carry S. canis in their respiratory and urogenital tracts and serve as potential sources of infection for cows.^114,201S. uberis is found in the vagina of cows,¹¹⁶ but genital tract secretions probably play little role in the transmission of infection compared with feces.

Bedding is considered to be a major source of environmental streptococcal exposure for cows housed in confinement. Environmental streptococci multiply in a wide range of bedding materials, particularly when the bedding is contaminated with feces and urine.^204,205 Straw, in particular, supports the growth of S. uberis. However, S. uberis mastitis also affects pastured cows and cows in lots that are not bedded. High-traffic races and areas of pastures or lots where cows congregate and defecate harbor S. uberis and serve as reservoirs.¹¹⁶

Approximately 50% of environmental streptococcal infections are initiated during the dry period.²⁰⁶ The mammary gland is highly susceptible to environmental streptococcal infection during the early and late stages of the dry period, when mammary secretions accumulate and host defense mechanisms are compromised.^13,207-209 However, infection may develop at any stage of lactation. Infection is accompanied by a rapid influx of neutrophils into the mammary gland and a resultant increase in SCC. Based on experimental inoculation studies with S. uberis, the recruited neutrophils are unable to fully prevent bacterial multiplication.²¹⁰ Ineffective host response probably explains the prolonged infections and persistently high SCC seen in some cases.

Environmental streptococcal infections often remain subclinical, with the main impact being the increase in SCC. When the prevalence of environmental streptococcal mastitis is high, the increase in BTSCC may result in penalties or loss of quality premium payments.^201,211 In some cases, environmental streptococcal counts in bulk tank milk may exceed the regulatory limit of 100,000 CFU/mL.¹¹⁷ Although a portion of the environmental streptococci in bulk tank milk come from contaminated teat skin, strain typing has demonstrated identical strains of S. uberis in bulk tank milk and infected mammary glands. This suggests that infected cows may contribute to excessively high bulk milk bacteria counts.¹¹⁷

Although most environmental streptococcal infections resolve within 30 days, about one third of infections persist for long periods, sometimes more than a lactation. Also, up to 50% of environmental streptococcal infections result in clinical mastitis.²⁰⁶ Clinical mastitis causes economic loss as a result of discarded milk and treatment costs. Fortunately, most clinical mastitis episodes are of mild to moderate severity and do not result in marked milk production loss or death of the cow.

Coliform Mastitis

Coliform mastitis is a major cause of disease in many well-managed dairy herds, accounting for approximately 30% to 50% of clinical mastitis episodes.²⁴⁰ Although E. coli causes most coliform mastitis episodes, Klebsiella species are important contributors on some farms, with Enterobacter species isolated less frequently. Other gram-negative mastitis-causing bacteria, such as Serratia, Pseudomonas, Salmonella, Proteus, and Pasteurella, are discussed later. Coliform mastitis can be distinguished from noncoliform mastitis by culturing milk on MacConkey agar and observing the characteristic pink colonies indicative of lactose fermentation.

As with environmental streptococci, coliform bacteria are shed in the feces and are ubiquitous on dairy farms. A wide variety of strains are capable of causing mastitis.²⁴¹ Mastitis develops after teats are exposed to feces or to contaminated bedding, water, or soil. Organic bedding materials, such as straw, wood chips, sawdust, recycled manure, pelleted corncobs, and newspaper, support the growth of coliform bacteria, particularly under warm, wet conditions.^242,243 Sawdust bedding has been implicated in outbreaks of Klebsiella mastitis,²⁴⁴ but Klebsiella species can proliferate rapidly in other bedding materials as well. Intramammary infections and clinical coliform mastitis tend to increase during summer.²⁴⁵ Rainfall, stocking density, frequency of manure removal, and frequency of pasture rotation can also influence the extent of exposure and risk of intramammary infection.

The majority of coliform intramammary infections develop during the early and late phases of the dry period.^208,246 These infections usually remain subclinical until after parturition. Most clinical coliform mastitis episodes that develop during early lactation are the result of infections acquired during the dry period.^208,246 Also, quarters that were infected during the dry period are at higher risk of clinical mastitis in early lactation than are quarters that were not infected.²⁴⁶ In one study, E. coli infection during the late dry period was associated with an increased risk of culling in the next lactation.²⁰⁹ Although the dry period and early lactation are times of high risk for coliform mastitis, it can develop at any time during lactation, particularly if exposure is high and cows are stressed.

The majority of clinical coliform mastitis episodes are mild, with less than 10% causing systemic illness or a marked drop in milk production.^120,247 However, coliform mastitis accounts for the majority of severe clinical mastitis episodes on most farms.²⁴⁸ The outcome of clinical coliform mastitis depends on the severity, which reflects the cow’s immune response (see later). Systemic signs are better than local signs or a combination of systemic and local signs for predicting coliform mastitis outcome.²⁴⁹ High or subnormal rectal temperature, reduced rumen contraction rate and amplitude, marked dehydration, and marked depression are associated with increased risk of death, culling, or poor return to milk production.^76,79 Severe neutropenia⁷⁹ and high bacterial concentration in the milk^79,250 are also indicative of a poor prognosis. Cows in early lactation cows do not develop neutropenia to the extent observed in later lactation; therefore the leukon is not a good indicator of mastitis severity in early lactation cows.^126,251

Once in the mammary gland, coliform bacteria multiply rapidly but do not adhere to or invade the epithelial cells.²⁵² Therefore, if the cow’s immune response is rapid and efficient, infection will be eliminated quickly, with little long-term impact on cow health or productivity. This is what usually happens when healthy cows are experimentally inoculated with E. coli and when cows develop coliform mastitis in mid- to late lactation. In such cases, cows are referred to as “mild responders” or “moderate responders.” However, when influx of neutrophils is delayed or phagocytosis or intracellular killing mechanisms of neutrophils impaired, bacterial multiplication continues, resulting in high bacterial concentrations in the milk and severe clinical disease. This happens most frequently in the periparturient period and early lactation. In such cases, cows are referred to as “severe responders,” and prognosis for recovery is worse.^{25,55,253,254} Responsiveness does not depend solely on stage of lactation and varies among cows. Indeed, it is believed that cow factors, rather than bacterial factors, are the predominant determinants of coliform mastitis outcome.²⁵²

As coliform bacteria multiply and die in the mammary gland, LPS (endotoxin) is released from the cell wall. Binding of LPS to host cells results in release of TNF-α, which is largely responsible for initiating the inflammatory cascade that causes the local and systemic signs of coliform mastitis.^255,256 Important contributors to the inflammatory response include prostaglandins,²⁵⁷ IL-1, IL-6, and IL-8,²⁵ C5a,²⁵⁸ nitric oxide,^256,259 and acute-phase reactants.²⁵⁴ Although LPS concentration in the milk may be high, LPS is usually undetectable or in low concentration in the plasma.²⁶⁰ This suggests that the fever, tachycardia, reduced rumen motility, and signs of shock that accompany severe coliform mastitis are not a consequence of endotoxemia but of production and absorption of other inflammatory mediators, such as TNF-α. The concentration of TNF-α in milk is positively correlated with coliform mastitis severity, and a high concentration of TNF-α in blood is seen only in severe responders.^25,254-256 Hematologic and plasma biochemical changes that often accompany clinical coliform mastitis include leukopenia (neutropenia, lymphopenia, monocytopenia), hypocalcemia, and reductions in plasma concentrations of zinc, copper, and iron.^126,261

Bacteremia does not occur during experimental coliform mastitis but develops in 30% to 40% of severely ill cows with naturally occurring clinical coliform mastitis.^77,262 The risk of coliform bacteremia increases with the severity of clinical signs.²⁶² Also, cows that remain neutropenic for 4 or more days, or that have high metamyelocyte and myelocyte concentrations in their blood, are more likely to be bacteremic than cows with a normalizing leukon.²⁵¹ Cows with coliform bacteremia are at greater risk of death or culling compared with nonbacteremic cows.²⁶²

Most coliform intramammary infections are of short duration, which means that less than 5% of quarters in a herd are infected at a given time.^245,247 However, up to 10% of E. coli infections can become chronic. Chronic infections put cows at risk for recurrent clinical mastitis in the affected quarter and for coliform infections in other quarters.^115,263 Clinical mastitis accompanying persistent infections tends to be milder than clinical mastitis accompanying newly acquired infections.²⁶³

Coagulase-Negative Staphylococcal Mastitis

Coagulase-negative staphylococci (CNS) are the most prevalent bacteria isolated from mammary secretions of lactating cows, dry cows, and prepartum heifers (see section on heifer mastitis). Coagulase-negative staphylococci can also be isolated from teat skin and teat canals and found in the environment. Staphylococcus hyicus, S. chromogenes, S. haemolyticus, S. epidermidis, S. simulans, and S. sciuri are among the most frequently isolated CNS species, but many others have been cultured from bovine milk; S. hyicus is usually grouped with the CNS, even though some strains are coagulase positive.

Although the CNS are usually considered as a group, different species and strains may differ in epidemiology, virulence, and response to treatment. For example, novobiocin-sensitive CNS predominate in intramammary and teat canal infections, whereas novobiocin-resistant CNS are more likely to be found in the environment and on teat skin.²⁸⁸

Most CNS infections are subclinical, manifested only by an increase in SCC. However, infections tend to be persistent.²⁸⁹ Compared with most other pathogens, the increase in SCC with CNS mastitis is much less.²⁰⁰ In a meta-analysis the geometric mean SCC for CNS-infected quarters was 138,000/mL, compared with 357,000/mL for S. aureus and greater than 1 million/mL for S. uberis and E. coli.²⁹⁰ For this reason, the CNS (as with C. bovis) are referred to as “minor” mastitis pathogens. Several studies suggest that CNS infection protects the mammary gland against infection with major pathogens^291,292 or shortens the duration of infection.²⁰⁰ However, the protective effect differs for the various major pathogens,^292,293 and some studies have shown no protective effect or even a detrimental effect of CNS infection.^200,294 Therefore, relying on a high prevalence of CNS intramammary infections to protect a herd against major mastitis pathogens is unrealistic.

Coagulase-negative staphylococci can be isolated from the milk of cows with clinical mastitis in both poorly managed and well-managed herds.²⁹⁵ However, this typically occurs in less than 10% of clinical mastitis episodes, and signs are usually mild. For these reasons, the role of CNS in clinical mastitis is usually downplayed. Care must be taken when collecting milk samples from cows with clinical mastitis to avoid culturing CNS from the skin or teat canal, which could lead to misdiagnosis.

The CNS are often susceptible in vitro to antibiotics in intramammary infusion products in the United States. Antibiotic treatment of clinical mastitis is logical, especially given the persistent nature of many CNS infections. However, controlled trials specifically investigating the efficacy and economics of antibiotic treatment for clinical CNS mastitis are lacking. Infusing intramammary antibiotics at dry-off successfully treats subclinical CNS mastitis. Treatment of subclinical CNS mastitis during lactation is unlikely to be profitable because the potential increase in milk yield after treatment is small.

There are no specific control measures for CNS mastitis. Effective germicidal teat dipping, antibiotic treatment of cows at dry-off, and prompt treatment of clinical mastitis are recommended. Although such practices can reduce the prevalence of CNS mastitis, CNS are still the most prevalent isolates in herds that have adopted these practices. Because of the wide variety of CNS and the limited economic loss associated with CNS mastitis, vaccines are not available.

Arcanobacterium pyogenes Mastitis

Arcanobacterium pyogenes (formerly Actinomyces pyogenes and Corynebacterium pyogenes) is the predominant pathogen involved in “summer mastitis,” a condition that affects prepartum heifers and nonlactating cows, mainly during summer.²⁹⁶ In most cases, anaerobic bacteria (e.g., Peptococcus indolicus, Bacteroides species, Fusobacterium necrophorum) and facultative anaerobes (e.g., Streptococcus dysgalactiae) are also involved.^296,297 Summer mastitis occurs mainly in northern Europe and Japan, but cases have been reported throughout the world. It is characterized by a swollen, hard, painful mammary gland containing purulent, foul-smelling secretion. Acutely affected animals may show systemic signs ranging from lethargy and reduced feed intake to high fever, severe depression, abortion, and death. Chronically affected animals may have palpable or draining mammary abscesses. Systemic administration of procaine penicillin G often resolves systemic illness. However, neither systemic nor intramammary antibiotics are effective at eliminating intramammary infection.²⁹⁸ The outcome of most episodes of summer mastitis is chronic clinical mastitis or a nonfunctional quarter. Therefore, affected cows are often culled.

Horn flies are believed to be responsible for transmitting summer mastitis. Flies that were allowed to feed on summer mastitis—causing bacteria transmitted the bacteria to teats, with subsequent development of intramammary infection²⁹⁹; damaged teat skin facilitated infection. However, the occasional occurrence of summer mastitis in winter suggests that other methods of transmission are possible. Because A. pyogenes is often found in the environment on farms and on body sites of cattle, teat skin contamination may predispose to infection, as occurs with coliform or environmental streptococcal mastitis, particularly if teat defenses are compromised.

Control of summer mastitis is accomplished by fly control, infusion of mammary glands with antibiotics at dry-off, appropriate environmental hygiene practices and stocking density, prevention and treatment of teat lesions, and prompt recognition and segregation of infected cows. Amputation of the teat, which facilitates drainage of secretions, increases environmental contamination and should not be done unless the cow can be segregated.

A condition similar to summer mastitis occurs sporadically in lactating cows.²⁹⁷A. pyogenes is usually involved, often in conjunction with other bacteria. Mastitis almost always follows teat injury or teat skin damage. The result is clinical mastitis and a very high SCC. Milk yield loss persists for at least 70 days.³⁰⁰ The majority of episodes result in a blind quarter or loss (culling, death) of the cow.²⁹⁷

Prototheca Mastitis

Prototheca species are unicellular algae. These organisms have been isolated from bovine feces, as well as from soil, mud, vegetation, standing water, pond water, drinking water, feed troughs, and barn floors on dairy farms.³⁰¹Prototheca zopfii is the main species associated with mastitis. Mastitis can be subclinical or clinical and can occur sporadically, endemically, or as an acute outbreak. Once infected, cows carry P. zopfii chronically in milk, mammary tissue, and supramammary lymph nodes, which results in pyogranulomatous inflammation.³⁰² Infected cows usually have persistently high SCC and reduced milk production. An elevation in BTSCC sufficient to threaten marketability of grade A milk has occurred in some herds.³⁰³ Shedding of P. zopfii in milk can be persistent or intermittent.³⁰⁴ Diagnosis is facilitated by culturing milk on selective media, such as Prototheca Isolation Medium; however, P. zopfii does grow on blood agar and can be detected by Gram staining of suspect colonies. Fine-needle aspiration of mammary tissue³⁰⁵ and antibody testing of whey³⁰⁶ are alternatives to milk culture but are not typically performed. Most Prototheca intramammary infections are probably acquired from the environment, but spread from shedding to uninfected cows at milking time may play a role in transmission.

Prototheca zopfii is resistant to antimicrobial therapy, and infection can persist through the dry period.³⁰⁴ Therefore, identification, segregation, and progressive culling of infected animals are required to control Prototheca mastitis. Culling must be coupled with identification and avoidance of contaminated environmental sites, enhanced cleaning of feed bunks and watering devices, and use of milking procedures that ensure clean teats and minimize contagious spread of infection.

Yeast Mastitis

Yeast can be isolated from feed, bovine skin and feces, and a variety of environmental sites on dairy farms.³⁰⁷Candida species, Trichosporon beigelii, and other species can cause clinical mastitis in individual cows or herds. Yeast infection usually follows intramammary antibiotic treatment or teat skin injury. Outbreaks occur when homemade, multidose infusion products or associated equipment (e.g., needles, syringes, bottles, cannulas) are contaminated or when the teat is not adequately disinfected before infusion.^140,141 In one study, six cows treated with a homemade intramammary antibiotic solution contaminated with T. beigelii all developed fever, hypogalactia, and swollen mammary glands within 2 weeks, and two of the six died. When the producer continued to use homemade solutions, an additional 23 cows (over half the herd) became infected, and the herd was dispersed.¹⁴⁰ Yeast can be cultured from towels or teat cup liners used on infected cows, suggesting that contagious transmission may occur.^140,308

On blood agar, yeast colonies appear similar to CNS colonies, so it is important to examine the colonies microscopically. Sabouraud dextrose agar with antibiotics is a common alterative medium that selects for yeasts. Yeast infections are not susceptible to antibiotic treatment, and infusion of antibiotics may potentiate infection. Antifungal agents have not been demonstrated to be effective and should not be used. Infection (especially with Candida species) often resolves spontaneously within 2 to 4 weeks,^141,308,309 but cows with severe or recurrent clinical mastitis, reduced milk production, or persistently high SCC should be culled.¹⁴⁰ Control of yeast mastitis includes aseptic intramammary infusion procedures; use of single-dose, commercially prepared antibiotic infusion products; segregation of infected cows at milking time; and use of individual rather than shared towels. Feeding contaminated silage promotes fecal excretion of yeast and has been blamed for outbreaks of yeast mastitis in cows that have not received intramammary infusions.³⁰⁸ Bulk tank SCC can increase greatly during yeast mastitis outbreaks.

Pseudomonas Mastitis

Intramammary infection with Pseudomonas species can result in severe clinical mastitis, similar to that caused by E. coli.³¹⁰ Gangrene develops in some cases.³¹¹ However, chronic subclinical mastitis with periodic bouts of mild clinical mastitis is more common. Pseudomonas aeruginosa is the most common Pseudomonas species associated with mastitis. Intramammary infection with P. aeruginosa can occur sporadically or can affect more than one third of lactating cows in a herd.³¹² A high incidence or prevalence of P. aeruginosa intramammary infections should trigger an investigation of the water system in the milking parlor. Hoses used to wash teats before milking have been implicated in numerous outbreaks of Pseudomonas mastitis.³¹³ Water from the hoses or the hoses themselves have cultured positive, particularly after water has been sitting in the hoses for several hours. When wash water is contaminated with Pseudomonas species, the hoses and nozzles must be replaced, but not without first culturing potential sources of bacteria. For example, water tanks, water heaters, or water lines may need to be replaced or decontaminated to prevent colonization of new hoses.³¹³ Other steps that can be taken are to flush stagnant water from the hoses before each milking and ensure that iodine concentration in the wash water is 25 ppm.³¹³ Alternatively, the teats can be predipped rather than washed before milking.

Pseudomonas mastitis has also been attributed to contaminated intramammary infusion products³¹⁴ or teat cannulas, inadequate teat end sanitation before drug infusion, contaminated teat wipes,³¹⁵ and exposure of cows to stagnant water. Therefore, investigation of a herd problem should include intramammary infusion practices and housing conditions. Once P. aeruginosa mastitis is established, it usually persists, and antibiotic therapy is unsuccessful. This is caused in part by production of factors that inhibit host defenses and reduce antibiotic efficacy, such as biofilm.³¹ Cows with persistently high SCC or repeated bouts of clinical mastitis are usually culled, but infusing glands with germicide to induce permanent cessation of lactation is another option for cows with a single infected gland.³¹⁶

Serratia Mastitis

Serratia mastitis is caused mainly by S. marcescens and S. liquefaciens. Mastitis can be clinical or subclinical, with the subclinical form predominating. More than 15% of lactating cows can be infected in problem herds, which can cause substantial elevation in BTSCC.^317,318 Although Serratia species produce bright-red colonies on blood agar, they are easily outcompeted by other pathogens. Therefore, infection prevalence may be underestimated unless special media and cultural conditions (e.g., incubation at 20° C) are used.^317,318

Serratia intramammary infections can originate during lactation or the dry period. In one longitudinal study, 48% of Serratia infections were acquired during the first half of the dry period and 31% during the second half, with only 21% initiated during lactation.³¹⁹ Almost half the infections resulted in clinical mastitis. Cows with clinical Serratia mastitis do not exhibit severe clinical signs, such as those accompanying acute E. coli mastitis.^319,320 However, Serratia infection persists longer than E. coli. A mean duration of 55 days has been reported,³¹⁹ but many infections persist for 6 to 10 months.^317,321

Serratia mastitis has been associated with contaminated teat dip or teat dip cups,^322,323 frostbitten teats,³²⁰ contaminated organic bedding material,³¹⁷ and dirt packs in cow lots.³¹⁸ In some cases the reservoir of infection is never identified.³²¹ Antibiotic therapy is usually ineffective,^319,320 but some infected quarters eventually self-cure.³¹⁹ Cows with repeated bouts of clinical mastitis or persistently high SCC should be culled.

Nocardia and Mycobacterium Mastitis

Nocardia species occasionally cause sporadic cases or herd outbreaks of mastitis. Infections can be subclinical or clinical, with some clinical episodes being severe or fatal. Fibrosis and draining abscesses may accompany chronic infections. Clinical or subclinical mastitis can also be caused by Mycobacterium fortuitum. This organism causes granulomatous inflammation and encapsulated granulomas in the mammary gland.³²⁴ Other rapidly growing Mycobacterium species, such as M. chelonei, M. smegmatis, M. phlei, M. vaccae, and M. flavescens, have been isolated from cows with acute or chronic mastitis.^325-327 M. avium subspecies paratuberculosis, the causative agent of Johne’s disease, does not readily cause mastitis, despite being shed in the milk of infected cows.

Milk culture is required to diagnose and differentiate Nocardia and Mycobacterium species. Nocardia species produce characteristic cottony white, adherent colonies on blood agar, but the colonies may not appear for 72 to 96 hours; the color changes from white to yellowish orange as the colonies age. On the other hand, M. fortuitum produces nonadherent, smooth colonies.³²⁴ The other mycobacteria may or may not grow on blood agar.^325,327 With all these organisms, a tentative diagnosis can be made by observing acid-fast rods or gram-positive rods with branching filaments in a milk smear.^324,325

Nocardia species and mastitis-causing Mycobacterium species are saprophytes and therefore may cause intramammary infection when teats are exposed to unhygienic environmental conditions. However, most outbreaks are associated with contaminated intramammary drugs or infusion equipment or poor teat hygiene before infusion.^324,326 Results can be devastating. In one Nocardia outbreak, 450 of 3300 cows died, and 500 more were culled.³²⁸ Use of oil-based dry-cow infusion products appears to facilitate growth of Nocardia and Mycobacterium species. Both types of pathogens cause intramammary infections that persist for months, even through the dry period. Antibiotic therapy is futile. Cows with chronic Nocardia mastitis should be culled or the quarter dried off to prevent continued exposure of humans to this potentially zoonotic pathogen.³²⁹ Cows with Mycobacterium mastitis are often culled because of chronic mastitis and low productivity. Control of Nocardia and Mycobacterium mastitis involves proper intramammary infusion practices and general environmental hygiene.

Pasteurella multocida Mastitis

Pasteurella multocida, a normal upper respiratory tract inhabitant of cattle and contributor to the bovine respiratory disease complex, occasionally causes mastitis. Most episodes are sporadic, but herd outbreaks can occur.^330,331 In a recent report a high incidence of clinical mastitis caused by P. multocida was accompanied by an increase in BTSCC from approximately 200,000/mL to over 900,000/mL in about 5 months.³³⁰ BTSCC increased partly because milk from cows with clinical mastitis was not diverted from the bulk tank. Hematogenous or lymphatic spread of P. multocida from the respiratory tract to the mammary gland has been proposed, and P. multocida has been isolated from the blood of cows with severe coliform mastitis.²⁶² However, it is unlikely that hematogenous or lymphatic spread is responsible for herd outbreaks. Contamination of teats by nasal secretions (e.g., from nursing calves or intersucking cows), cow-to-cow spread at milking time, or inappropriate infusion practices may contribute to outbreaks.³³⁰ Antibiotic treatment has historically been unrewarding, and infected cows are often culled. However, spontaneous resolution appears to occur in some cases.³³⁰

Food-Borne Disease Agents

Salmonella species and Listeria monocytogenes can cause mastitis. However, mastitis is an uncommon manifestation of these infections. With the exception of Salmonella dublin, a cattle-adapted species that is shed in the milk of infected cows, Salmonella species or L. monocytogenes in bulk tank milk are most likely to originate from unclean teats or fecal contamination of milking units, rather than from the milk itself. Good milking hygiene practices reduce the risk of milk contamination.

Antibiotic Treatment

Antibiotics can be avoided without adversely impacting the outcome of many clinical mastitis episodes. However, treatment protocols that exclude antibiotic use altogether are likely to have a detrimental effect at both the cow and the herd level. Judicious use of antibiotics should be the goal, rather than complete avoidance. This section discusses some principles and factors to consider when making antibiotic treatment decisions. More specific recommendations can be found in the sections pertaining to each pathogen.

Supportive Treatment

A variety of supportive measures are used in cows with clinical mastitis, often in conjunction with antibiotic therapy. Fluids and electrolytes are administered to combat circulatory and electrolyte disturbances. Steroidal and nonsteroidal antiinflammatory agents are used to reduce pain, inflammation, and fever. Oxytocin and frequent milk-out are used to promote milk ejection and removal of secretions. Dairy producers and veterinarians use many other systemic and local treatments, most of which have not been scientifically evaluated or shown to be effective. This section discusses the most frequently used supportive treatments.

Subclinical Mastitis

In most prevalence surveys, approximately one third to one half of cows have intramammary infections, with the vast majority of infections being subclinical. Subclinical infection is accompanied by an increase in SCC and reduction in milk yield, the extent and duration of which depend on the causative pathogen and effectiveness of host defense mechanisms. The reduction in milk yield associated with subclinical mastitis is usually estimated by extrapolating from crude SCC or a logarithmic transformation of SCC. The relationship between crude SCC and milk yield loss is nonlinear, whereas logarithmic transformation results in a linear relationship. For example, each twofold increase in crude SCC above 50,000/mL on monthly test day reports was associated with an average reduction in milk yield of 0.6 kg/day for multiparous cows and 0.3 kg/day for heifers.³⁹² Each unit increase in log₁₀SCC on weekly test day reports was associated with an average reduction in milk yield of 2 kg/day for multiparous cows and 1.3 kg/day for heifers.³⁹³ In the United States, linear score (LS), a log₂-based transformation of SCC that results in scores of 0 to 9, is the usual parameter used. Each unit increase in lactational average LS above 2 results in an average milk yield reduction of 0.7 kg/day or 180 kg/lactation for multiparous cows, with losses for heifers being approximately half.³⁹⁴ Such associations provide a rough estimate of the magnitude of milk yield loss associated with subclinical mastitis in a herd, but do not accurately estimate milk yield loss in individual cows.

Sometimes, herd milk yield loss is estimated from the SCC of the bulk tank (BTSCC). For example, a decrease in milk yield equating to a loss of at least $100/cow/year was demonstrated in herds with BTSCC greater than 200,000/mL.³⁹⁵ One problem with using BTSCC to estimate milk yield loss is the dilutional effect of milk from high-yielding cows on BTSCC.³⁹⁶

Milk from cows with subclinical mastitis is not usually withheld from the bulk tank. This causes an increase in BTSCC and reduction in milk quality. The milk quality changes associated with subclinical mastitis adversely affect the processing properties, organoleptic qualities, and shelf life of milk and milk products.³⁹⁷ In the United States, milk processors routinely monitor BTSCC, and dairy producers may be paid premiums for low BTSCC or penalized for high BTSCC. Premium and penalty programs vary widely by location; however, in some cases, penalty payments or unearned premiums account for the greatest loss associated with subclinical mastitis.³⁹⁸ A substantial economic loss is incurred when BTSCC exceeds 750,000/mL because the milk cannot be marketed as grade A; this BTSCC limit is much higher in the United States than in Canada and other developed countries.³⁹⁹ High-BTSCC milk also has a greater risk of condemnation because of violative antimicrobial residues.^400-402

Cows that maintain persistently high SCC, have untreatable intramammary infections, or have low milk production often are culled prematurely. The cost of premature culling depends on the perceived value of the cow, the market prices for culled cows and replacement heifers, and the potential value of the replacement heifer.³⁹⁸ A recent retrospective study revealed that cows with lactation average SCC greater than 700,000/mL had more than twice the risk of being culled than cows with SCC of 200,000 to 250,000/mL; the risk tended to be greater in herds with low SCC than those with high SCC.⁴⁰³ Cows that are not culled sometimes require segregation or special milking procedures that increase labor costs; for example, cows with chronic S. aureus or Mycoplasma mastitis may need to be milked last or in separate strings.

Clinical Mastitis

Approximately 15% to 20% of cows experience clinical mastitis during the course of a lactation.^404,405 However, the incidence of clinical mastitis varies among herds, from less than 10% of lactations to greater than 50% of lactations. An acute drop in milk production often accompanies the onset of clinical mastitis. Some cows rapidly cease lactating or produce substantially less milk than predicted for the rest of the lactation. Others experience little drop or rapidly return to normal or near-normal production.⁴⁰⁶ Average lactational milk yield losses reported for clinical mastitis range from less than 50 kg to 749 kg per lactation, but a loss of 5% of lactational milk production is usually assumed.⁴⁰⁶ Milk yield loss is generally higher when clinical mastitis occurs in early to peak lactation than in late lactation, and for multiparous cows than for heifers.⁴⁰⁶

Milk that is visibly abnormal or contains drug residues must be diverted from the bulk tank and withheld from sale. The cost associated with diverted milk is a major component of the cost of clinical mastitis. Actual cost depends on whether the milk is discarded or fed to calves in place of milk replacer, as well as the price of milk and milk replacer and difference in labor associated with the two feeding methods. Failure to detect clinical mastitis and divert milk from the bulk tank adds to the reduction in milk quality caused by subclinical mastitis. Cows with repeated episodes of clinical mastitis produce the majority of diverted milk in some herds.⁴⁰⁷

A small proportion (<5%) of cows with clinical mastitis die or are euthanized because of the severity of their condition. More cows fail to respond adequately to treatment or experience recurrent episodes of clinical mastitis, resulting in premature culling. Mastitis is consistently ranked as one of the top reasons for culling dairy cows. In a national survey, U.S. dairy producers reported that mastitis and other udder problems were responsible for an average 27% of culls⁴⁰⁸; mastitis may also have contributed indirectly to the 19% of cows culled because of low milk production. Neerhof et al.⁴⁰⁹ reported that the risk of culling Danish Black and White cows was 1.7 times higher for cows that had experienced clinical mastitis than for those without mastitis. Rajala-Schultz and Gröhn⁴¹⁰ reported similar findings for Finnish Ayrshire cows. In a study examining the effect of mastitis on herd life in two New York dairy herds, the first episode of clinical mastitis caused by Streptococcus species, S. aureus, Staphylococcus species, E. coli, or Klebsiella species reduced herd life, with pathogen-associated hazard ratios ranging from 1.19 to 3.18.⁴¹¹ The hazard of culling for clinical mastitis differs with stage of lactation and increases with age of the cow.^411,412

The cost of diagnosing and treating an episode of clinical mastitis depends on its severity, the causative pathogen, the treatments administered, the labor required, whether the milk is cultured, and whether a veterinarian is involved. Antibiotic treatment carries a risk of inadvertently introducing antibiotics into the bulk tank and causing violative drug residues, which is costly. However, if antibiotic treatment improves the cure rate and reduces lost milk, SCC, and risk of mastitis transmission, economic losses are reduced.⁴¹³ The appropriate selection of cases for antibiotic treatment is critical for optimizing returns and avoiding unnecessary losses, as discussed in other sections.

Effects of Mastitis on Reproduction

One increasingly recognized cost associated with mastitis is reduced reproductive performance. Development of clinical mastitis between days 15 and 28 postpartum delayed the onset of ovarian cyclicity and estrus by approximately 6 days, regardless of the causative pathogen (gram positive or gram negative).⁴¹⁴ Clinical mastitis episodes caused by gram-negative pathogens induced a substantially higher rate of premature luteolysis (47%) than those caused by gram-positive pathogens (8%) and prolonged the follicular phase of the estrus cycle when mastitis occurred at that time.⁴¹⁴ Development of clinical or subclinical mastitis before first service caused increases in days to first service, days open, and services per conception, compared with nonmastitic cows.⁴¹⁵ Induction of S. uberis mastitis before ovulation decreased luteinizing hormone (LH) pulses, impaired 17β-estradiol production, and prevented ovulation.⁴¹⁶ Maintenance of pregnancy can be impaired when mastitis occurs shortly after a cow is bred, presumably as a result of embryonic death.^417,418 Cows with LS greater than 4.5 before breeding were 2.4 times more likely to experience early embryonic death than cows with LS less than 4.5.⁴¹⁹ Both experimental and natural clinical mastitis episodes have been reported to induce abortion.^420,421 Together, these findings imply that both clinical and subclinical mastitis caused by a variety of pathogens can be detrimental to reproductive performance and can contribute to economic loss.

Total Cost of Mastitis

The total cost of mastitis is impossible to quantify accurately, varies over time, and is herd dependent. No studies have included all the major categories of economic loss in their calculations.⁴⁰⁶ However, imperfect estimates of average costs have been generated and are useful for appreciating the impact of the disease. The cost of clinical mastitis has been estimated at $107 per case⁴²² and $30 to $50 per cow in the herd.³⁹⁸ In one herd the cost associated with loss of saleable milk and treatment alone averaged $201 or $295 per lactation, depending on the treatment protocol.³⁴⁰ The total cost of mastitis has been estimated at $200/cow/year or $1.5 to $2 billion per year nationally.^398,423

The economic loss resulting from mastitis is not equivalent to the economic returns available from mastitis control, because it is impossible to eliminate mastitis completely. Also, the costs associated with mastitis control can be substantial. In one survey the cost of mastitis prevention accounted for 50% of the cost for preventing all diseases.⁴²² Return on investment in mastitis control measures is more important than the absolute cost. Return on investment ranged from −$20 to $275/cow/year in nine studies,⁴²⁴ depending on the prevalence and type of mastitis in the herd, control measures already in place, efficacy of the control measures to be implemented, and producer compliance.^425,426

Stochastic modeling has been used to predict return on investment under a variety of scenarios.^426,427 Using one model, mastitis control programs that included antibiotic treatment of all cows at dry-off and preventive measures in the milking parlor (forestripping, cleaning, or predipping of teats and postdipping of teats), with or without antibiotic treatment of lactating cows, yielded positive annual net benefits, regardless of whether the primary pathogens in the herd were contagious or environmental. On the other hand, lactating-cow antibiotic therapy alone was not profitable in any circumstance.⁴²⁶ Such models can assist in making rational decisions about mastitis control, but results depend on the assumptions used, which are seldom universally applicable.

Teat Condition

Teat injuries can be avoided through appropriate housing and stall design. Milking machine—associated teat damage can be minimized by providing appropriate pulsation and teat end vacuum and by using properly sized teat cup liners. Premilking teat preparation methods can promote oxytocin release and allow teat cups to be attached after milk ejection, thus minimizing milking time and associated risks to the teats. Teat chapping, which facilitates bacterial colonization, can be avoided by providing protection against adverse weather and using teat dips that condition the skin. Teat cannulas and dilators compromise the teat canal and should be avoided. Intramammary antibiotics should be administered using the partial-insertion method of infusion.¹⁴ Internal teat sealant can enhance physical defense against pathogen invasion during the dry period.

Nutrition

Negative energy balance, or more specifically ketogenesis, is a risk factor for mastitis.^428,429 Ketone bodies adversely affect neutrophil function in vitro.⁴³⁰ Experimental E. coli mastitis is more severe in ketonemic cows than nonketonemic cows.^429,431 In one field study, 29% of cows with circulating β-hydroxybutyrate (BHB) concentration of 1400 μmol/L or greater in the week before calving developed clinical mastitis after calving, compared with 9% of cows with BHB concentration less than 1400 μmol/L.⁴³² In another study, cows with serum BHB concentration of 1000 μmol/L or greater 1 to 3 days after calving were at increased risk of developing clinical mastitis caused by environmental pathogens, compared with cows with BHB concentration less than 1000 μmol/L.⁴³³ Heifers with BHB concentration of 100 μmol/L or greater in milk during the first or second week after calving were more likely to have subclinical mastitis before calving and develop new intramammary infections after calving, compared with heifers with less than 100 μmol/L BHB in milk.⁴³⁴ These findings suggest that dietary management to minimize ketosis in the periparturient period should reduce the incidence and possibly the severity of mastitis. Ionophore antibiotics also may help to reduce energy-associated mastitis risk. Administering a controlled-release monensin-containing capsule before calving reduced serum BHB concentration in periparturient cows,⁴³⁵ and feeding monensin after calving reduced clinical mastitis incidence and rate of intramammary infection.⁴³⁶

Dietary vitamin and mineral concentrations influence mastitis susceptibility, particularly in the periparturient period when feed intake declines. Vitamin E and selenium have received the most attention because of their synergistic antioxidant actions and beneficial effects on the immune system, especially neutrophil function.^437,438 Heifers fed a selenium-deficient diet before and during their first lactation experienced more severe and persistent E. coli mastitis than heifers fed a selenium-supplemented diet.⁴³⁹ The prevalence of intramammary infection, rate of clinical mastitis, and BTSCC all decline as blood selenium concentration increases.^437,440 Supplementing 1000 or 4000 IU vitamin E/day during the last 2 weeks of the dry period reduced clinical mastitis in the first week after calving by 30% and 88%, respectively, compared with 100 IU/day.⁴⁴¹ Current recommendations are to feed 1000 IU vitamin E/day during the late dry period and early lactation and 500 to 1000 IU/day at other times.⁴⁴² The traditional practice of feeding 0.1-ppm dietary selenium may result in blood selenium concentrations that are too low to protect against mastitis; 0.3 ppm is considered advantageous.⁴⁴¹ Rations for U.S. dairy cows often contain suboptimal concentrations of both vitamin E and selenium, which creates an opportunity to improve mastitis resistance through dietary supplementation. In contrast, parenteral injection of vitamin E or selenium provides less consistent beneficial effects.^437,443

Additional dietary ingredients considered important in immune system function and mastitis resistance are vitamin A, β-carotene,⁴⁴⁴ and antioxidant trace minerals and vitamins such as zinc, copper, and ascorbic acid.⁴⁴⁵ Ensuring adequate vitamin and micronutrient supplementation should assist in mastitis control. Protein balance, particularly a balance of amino acids such as glutamine, may also be important.⁴⁴⁶

Genetics

Although mastitis occurrence is largely determined by physiologic and environmental factors, susceptibility is partly attributable to genetic variability. Direct selection against clinical mastitis is difficult because heritability is low (<0.05) and producers do not consistently record clinical mastitis events. Somatic cell count (SCC) is recorded more consistently and has higher heritability (0.10 to 0.30), making it a better selection parameter.⁴⁴⁷ Genetic correlation between SCC and intramammary infection is almost 1.0, and correlation between SCC and clinical mastitis is fairly high (0.5 to 0.8), making SCC a reasonable proxy.⁴⁴⁸ For instance, daughters of sires that transmit low somatic cell score (SCS) have a lower prevalence of intramammary infection at first parturition and shorter and less severe episodes of clinical mastitis during first lactation than daughters of sires that transmit high SCS.^449,450 Lactation average SCS, clinical mastitis incidence, and duration of clinical mastitis in first lactation are all influenced by predicted transmitting ability (PTA) for SCS, as are total number of lactations, days of productive life, and total days in milk, which means that daughters from sires with high PTA SCS are culled sooner.⁴⁵¹ The concern that breeding for low SCS will increase clinical mastitis susceptibility does not appear to be founded.⁴⁵²

Both clinical mastitis and SCC are moderately correlated (0.2 to 0.7) with udder depth and udder attachment, meaning that selecting for high, tightly attached udders should reduce mastitis susceptibility.⁴⁴⁸ Clinical mastitis is negatively correlated with body condition score (−0.16) and positively correlated with dairy character (0.27), which suggests that placing more emphasis on body condition and less on dairy character might promote positive energy balance and reduce the risk of ketosis-associated mastitis.⁴⁵³

Unfortunately, clinical mastitis is positively correlated (∼0.40) with milk production and milking speed.⁴⁴⁸ Therefore, selecting solely for increased milk production is detrimental to udder health.⁴⁵⁴ This may be the result of shorter, wider teat canals and increased pressure in the teats of high-yielding cows, both of which compromise the barrier function of the teats. Use of bovine somatotropin (rBST), which enhances milk yield, increases the risk of clinical mastitis by approximately 25%.⁴⁵⁵ High-yielding cows also have more severe negative energy balance in the periparturient period, when immune function is compromised and mastitis risk is high.⁴²⁹ Therefore, the economic benefits of higher production and faster milking must be weighed against the economic and welfare costs associated with clinical mastitis.

It may be possible to use immune function measures to select for mastitis resistance. Neutrophil function in vitro and antibody production in response to vaccination in vivo are more heritable than SCS and influence mastitis susceptibility or severity.^448,456 Therefore, immune function testing of sires might be helpful in selecting for cows that can resist mastitis. Differences in alleles of class I and class II major histocompatibility complex (MHC) genes also are associated with clinical mastitis susceptibility or SCC. For example, MHC class I alleles A26 and A7(w50) are associated with mastitis resistance and the class II DRB3.2*24 allele with susceptibility.⁴⁴⁸ Similarly, quantitative trait loci for SCS and clinical mastitis are found on almost all chromosomes.⁴⁴⁸ Therefore, in the future, producers may be able to use haplotype or quantitative trait loci data to supplement other selection criteria. The challenge will be to develop the most appropriate combinations of criteria to protect udder health without serious detriment to other health or production parameters.

Stress

Mastitis susceptibility is increased in the periparturient period, when stress hormones are high. The role of stress at other times on mastitis susceptibility is not clear. However, because a variety of stressors affect immune function, it is logical that stress avoidance should promote mastitis resistance. Such measures as sprinklers, shades, and tunnel ventilation can help prevent heat stress. Good housing and stall design, appropriate stocking density, and sufficient feedbunk space can reduce housing and social stress. Handling cows calmly and humanely, taking advantage of natural behaviors, can minimize handling stress.

Other Practices That Influence Host Defense

Vaccination of cows against core LPS antigens boosts immunity against gram-negative pathogens and reduces the severity of coliform mastitis. Unfortunately, effective vaccines are not available for most mastitis pathogens. Manipulation of photoperiod (short daylength exposure) during the dry period enhances cellular immune responses in vitro at calving,⁴⁵⁷ but the effect on mastitis incidence and severity needs further investigation.

Antibiotic therapy can assist host defenses in resolving mastitis in some cases (e.g., with streptococcal infection). However, several classes of antibiotics often used to treat mastitis, such as β-lactams and tetracyclines, have detrimental effects on neutrophil phagocytosis or killing ability in vitro.^26,458 It is not known if this translates to a negative impact on mastitis outcome in vivo.

Clinical Mastitis

The incidence of clinical mastitis in small ruminants is low (<5% of lactations).⁴⁵⁹ In most cases, clinical mastitis is sporadic, but outbreaks occasionally occur. Ewes or does with clinical mastitis can experience severe clinical illness or develop chronic mastitis with abscessation. The reduction in milk yield that accompanies clinical mastitis reduces income on dairy operations and can adversely impact growth or viability of nursing neonates. Dairy producers must divert clinically abnormal or residue-containing milk from sale. Ewes or does that develop clinical mastitis are often culled at weaning or at the end of lactation, if not sooner. Therefore, despite its low incidence, clinical mastitis can still have an adverse economic impact on sheep and goat farms.

Staphylococcus aureus is the most common clinical mastitis pathogen in meat and dairy sheep, as well as in goats.^460,461S. aureus is more likely to cause severe clinical signs or gangrenous mastitis in small ruminants than in cattle, and small ruminants are more likely to develop abscesses in the mammary gland if infection persists. Another important clinical mastitis pathogen of sheep, particularly meat sheep, is Mannheimia haemolytica (formerly Pasteurella haemolytica).⁴⁶⁰ S. aureus, M. haemolytica, and coagulase-negative staphylococci are responsible for most clinical mastitis episodes in sheep, with streptocooci, corynebacteria, coliform bacteria, and Arcanobacterium pyogenes isolated less frequently.⁴⁶⁰ These pathogens also cause clinical mastitis in goats, but M. haemolytica is isolated less frequently than in sheep.⁴⁵⁹ Outbreaks of clinical mastitis in small ruminants are usually caused by S. aureus, streptococci, or opportunistic pathogens, such as Pseudomonas or fungi.⁴⁵⁹

When clinical mastitis cannot be explained by bacterial or fungal infection, Mycoplasma infection should be suspected. A variety of Mycoplasma species can cause clinical mastitis outbreaks in small ruminants. Mycoplasma mycoides subspecies mycoides (large-colony type) is an important mastitis pathogen in goats in the United States, especially in California.^462,463Mycoplasma putrifaciens is involved in some outbreaks,⁴⁶⁴ and M. agalactiae is isolated on rare occasions.⁴⁶⁵ Does with Mycoplasma mastitis often exhibit severe clinical illness and reduced milk yield, in addition to an inflamed udder and visibly abnormal milk; however, subclinical intramammary infections can occur. Adult goats and kids in herds with Mycoplasma mastitis may experience polyarthritis and pneumonia, and pregnant does may abort. Morbidity and mortality can be very high, with more than 90% of goats dying or euthanized in some herds.⁴⁶⁴

Clinical mastitis is detected at milking time in dairy sheep and dairy goats but may not be readily apparent in animals that nurse their young. Lethargy, depression, or abnormal gait may be the most obvious sign if the mammary gland is not greatly enlarged. Abnormal gait, which is often mistaken for lameness, occurs when the animal attempts to keep its hindlimb away from the painful gland. Lambs or kids of dams with clinical mastitis may make frequent attempts to nurse but have poor abdominal fill because of the reduced milk production.

Little research has been done on the treatment of clinical mastitis in small ruminants. Ewes or does with severe or recurrent clinical mastitis are often sacrificed rather than treated, and the goal of treatment is often to maintain survival until the animal can be slaughtered or sold. When treatment is performed, practices for bovine mastitis are usually followed. These include intramammary and systemic antibiotics, antiinflammatory agents, and fluid therapy (see section on clinical mastitis treatment). No intramammary or systemic antibiotics are labeled for treatment of mastitis in sheep or goats in the United States, so extralabel drug use is necessary. Beta-lactam and macrolide antibiotics are often used because of the predominance of staphylococci and (in sheep) M. haemolytica.⁴⁶⁶ Without efficacy data, it is impossible to make evidence-based recommendations.

Because of the small volume of the ovine and caprine mammary glands, producers often administer one half of the bovine dose when infusing intramammary antibiotics. This is likely to be adequate, provided the pathogen is susceptible, the antibiotic can reach the site of infection, and treatment is continued for a sufficient duration. Depletion of antibiotics from the mammary gland of goats after intramammary infusion is similar to that for cows.⁴⁶⁷ However, when using antibiotics labeled for cows, the milk-withholding and slaughter-withholding times should be extended to avoid violative residues.

Economics often drive treatment decisions in small ruminants, but the welfare of the animal must be addressed. Because clinical mastitis in small ruminants is usually caused by gram-positive bacteria, in contrast to the high rate of coliform mastitis in cattle, antibiotics should be administered. Also, analgesics (NSAIDs) are indicated to control pain. Flunixin meglumine administration hastens clinical recovery in both sheep⁴⁶⁸ and goats.⁴⁶⁹ Neither intramammary nor systemic antibiotics are likely to resolve Mycoplasma infection, making euthanasia the most humane option in animals with clinical mycoplasmosis. Euthanasia is also the best option in most cases of gangrenous mastitis; if euthanasia is not performed, it may be necessary to amputate the teat or surgically remove necrotic tissue. Animals with gangrenous mastitis should be segregated to avoid contaminating the environment and transmitting the pathogen (usually S. aureus) to herdmates.⁴⁷⁰

Subclinical Mastitis

Subclinical mastitis is more common than clinical mastitis in both sheep and goats. Intramammary infection is identified in 20% to 40% of ewes and does in most surveys.^461,471,472 The prevalence of intramammary infection ranges from less than 10% to more than 90% among sheep flocks^460,473 and less than 10% to more than 60% among goat herds.^461,474 Prevalence increases with parity in goats.^471,475

Subclinical mastitis is accompanied by reduced milk production and altered milk composition. On dairy farms, this reduces income and adversely impacts the quality of milk and milk products. On farms raising sheep or goats for meat, the weaning weights of nursing animals may be reduced.^476-478

Coagulase-negative staphylococci (CNS) cause the majority of subclinical mastitis episodes in sheep and goats.⁴⁵⁹ Although CNS are considered minor pathogens in dairy cows, this is not the case in small ruminants. Mammary glands infected with CNS, particularly novobiocin-sensitive species, have substantially higher SCC than do uninfected glands.^{461,472,474,477} Also, CNS infections in small ruminants usually persist for months. Other subclinical mastitis pathogens of sheep and goats include S. aureus, streptococci, enterococci, corynebacteria, and Mycoplasma species.⁴⁵⁹ Within-herd/flock prevalence of S. aureus is typically lower for small ruminants than for dairy cattle.

Subclinical mastitis is most often detected by palpating mammary glands for firmness and abscesses. In meat animals this is done at weaning, breeding, or lambing/kidding. In dairy animals it is done more routinely. Ultrasonograpy can be helpful in detecting abscesses or areas of fibrosis.⁴⁷⁹ Although milk culture is the “gold standard” method for diagnosing subclinical intramammary infection, most sheep and goat producers do not routinely culture milk. Indirect diagnostic tests for subclinical mastitis include SCC, CMT, electrical conductivity/impedance, and NAGase activity (see Intramammary Infection and Mastitis). SCC is a more reliable indicator of intramammary infection than CMT,⁴⁸⁰ but CMT is more practical. In one trial, electrical impedance was of no predictive value in sheep and little value in goats.⁴⁸⁰ NAGase activity increases with intramammary infection in sheep and goats,^474,481 but more studies are needed to determine the ability of NAGase activity to predict infection accurately.

Somatic cell counts are more likely to be monitored in dairy sheep/goats than meat animals. An SCC threshold of 250,000/mL was suggested for sheep in one report, because 82% of milk samples from uninfected halves had SCC less than 250,000/mL and 91% of samples from infected halves had SCC greater than 250,000/mL.⁴⁸² In contrast, the mean SCC for uninfected halves of sheep was 500,000/mL in another study.⁴⁸⁰ Thresholds ranging from 500,000/mL to more than 1 million/mL have been recommended for both sheep and goats. The choice of SCC threshold and its value for predicting infection status depend on the prevalence of infection in the flock/herd, as is the case for cattle (see section on mastitis detection). In general, a low SCC is more predictive of a healthy gland than a high SCC of an infected gland.^480,483 SCCs from sequential tests are more valuable than a single test result.

An interesting difference between goats and sheep (or cattle) is that milk secretion in goats is an apocrine process. Anucleated cytoplasmic particles are expelled from the secretory cells into the milk during secretion of milk components. These particles are approximately the same size as somatic cells, average 150,000/mL in goat milk, and they can be mistaken for somatic cells by some counting methods. Therefore, only direct microscopic observation or methods that identify DNA should be used for quantifying somatic cells in goat milk.⁴⁸⁴ Another difference between goats and sheep is a higher proportion of neutrophils in normal goat milk. Lastly, the SCC of goat milk increases substantially in late lactation, even in does free of intramammary infection. Counts can exceed 1 million/mL in healthy glands, making SCC an unreliable predictor of intramammary infection in late lactation.^484,485 To explore mastitis prevalence in a goat herd using SCC, goats should be sampled after the postparturient period but before 130 days in milk, to avoid the influence of stage of lactation on SCC.⁴⁷⁴

Geometric mean SCC values for CMT scores of negative, trace, 1, 2, and 3 are 170,000, 309,000, 760,000, 1,911,000, and 8,819,000/mL, respectively, for dairy sheep and 135,000, 148,000, 359,000, 1,110,000, and 7,505,000/mL, respectively, for dairy goats, showing a clear positive relationship between CMT score and SCC.⁴⁸⁰ A CMT threshold of 1 is usually recommended for differentiating infected from uninfected halves. However, the CMT has the same drawbacks as SCC, with low scores (negative or trace) being highly predictive of uninfected glands but high scores being poorly predictive (<45%) of infected glands, unless infection prevalence is very high.⁴⁸⁰ Bulk tank SCC is useful for estimating the intramammary infection prevalence in dairy sheep and goats,⁴⁵⁹ but the proportion of goats in late lactation must be considered when interpreting BTSCC.

Subclinical mastitis in small ruminants is most effectively treated by infusing antibiotics into the mammary glands at dry-off/weaning. Antibiotic products labeled for dairy cows are used. Antibiotic infusion at dry-off/weaning significantly increases cure rate compared with spontaneous cure. Antibiotic infusion at dry-off/weaning also reduces the incidence of new intramammary infections at parturition and the prevalence of intramammary infection throughout lactation, and results in lower SCC and higher milk yield.^481,486,487 There appears to be little difference in efficacy between a full dose of antibiotic and a half dose, so producers typically split one antibiotic tube between two udder halves. When this is done, the cannula should be swabbed with alcohol before inserting it into the second gland.⁴⁸⁶ As in cattle, inadequate disinfection of the teat or poor intramammary infusion technique in sheep and goats can lead to opportunistic intramammary infections. Withholding times for milk and meat must be extended and milk tested for antibiotic residues after parturition if it is to be sold for human consumption. The risk of antibiotic residues by 5 to 7 days after lambing appears to be negligible.⁴⁸¹

Other mastitis control measures for small ruminants include culling of chronically infected animals, hygienic housing conditions, and good premilking udder hygiene practices. Contagious ecthyma control will reduce the risk of teat lesions and secondary mastitis. Identification and removal of milk-robbing lambs may reduce spread of pathogens from teat to teat. Control measures for contagious mastitis in dairy cows are recommended for dairy sheep and goats; these include postmilking germicidal teat dipping and use of gloves and individual towels when preparing teats for milking. Apparently healthy goats can carry pathogenic Mycoplasma species in their external ear canals, often in conjunction with ear mites,⁴⁸⁸ but the importance of the ear as reservoir for Mycoplasma mastitis is uncertain.⁴⁸⁹

Heritability of SCS in sheep is .10 to .15, with a negative genetic correlation (−0.35 to −0.11) between SCS and milk yield. Genetic selection for mastitis resistance is practiced in certain breeds in some countries.⁴⁵⁹ Identification of regions of the ovine genome involved in mastitis resistance has led to use of quantitative trait loci programs for genetic selection in some countries.⁴⁵⁹

Retroviral Mastitis

Retroviral (lentiviral) infections in sheep (ovine progressive pneumonia, maedi-visna)⁴⁹⁰ and goats (caprine arthritis-encephalitis)^491,492 can cause interstitial mastitis. Mammary glands become diffusely and homogenously firm, and milk production declines. Viral infection results in an interstitial accumulation of lymphocytes in the mammary gland. The SCC of seropositive (presumptively infected) goats is higher than the SCC of seronegative goats, but not as high as with bacterial intramammary infection.⁴⁹² The increase in SCC in seropositive goats may be caused by an increase in mononuclear cells in the milk or a reduction in milk yield. In contrast, retroviral infection in sheep appears to have little effect on SCC.

Retroviral mastitis should be suspected when the udder is firm but not inflamed and the milk appears normal. A positive serologic test result and other signs of retroviral infection (chronic weight loss, increased respiratory effort, arthritis) support the diagnosis. There is no effective treatment for retroviral mastitis, so affected animals must be culled. Feeding colostrum or milk from affected animals to their offspring can transmit the infection.