Chapter 18 Neonatal Infection
Infection, both generalized and localized, is an important cause of morbidity and mortality in the large animal neonate; in both calves and foals, bacteria cause most infections. The prognosis of neonatal infection varies considerably depending on the type and severity of infection. The traditional view of sepsis includes a pivotal role for bacterial endotoxins with resultant overactivation of the host immune system and release of endogenous proinflammatory and antiinflammatory mediators. This response could then precipitate a cascade of metabolic and hemodynamic changes that may culminate in multiple organ system failure.1,2 This traditional model of sepsis has recently been challenged with a decreased emphasis on a primary role of bacterial endotoxins and a new focus on the regulation of Toll-like receptors (TLRs) in generation of the systemic inflammatory response syndrome (SIRS) and the sepsis syndrome.3 It has been suggested that TLR4, the putative endotoxin receptor, is under a constant state of inhibition. Endogenous factors generated in response to tissue inflammation lead to release of TLRs from inhibition as well as acting as TLR agonists. TLR activation results in a potent proinflammatory response. This model provides a common pathway of SIRS, irrespective of initiating causes (e.g., gram-negative bacteria, gram-positive bacteria, tissue trauma, neoplasia). Although further refinement is needed for acceptance of this model of sepsis, it may provide the window for novel therapies in the prevention and treatment of sepsis.
Much of the information related to sepsis and endotoxemia in horses has been derived from experimental endotoxin bolus or infusion in healthy animals. Endotoxin activates cytokine-mediated procoagulant effects on endothelial cells.4 The initial response to endotoxin is activation of coagulation.5 As the inflammatory response progresses, systemic hypercoagulability may progress to hypocoagulability, with fulminant signs of hemorrhage recognized as disseminated intravascular coagulation (DIC).4 Circulatory failure, perfusion deficits, and an inability of the body to use existing metabolic substrate effectively characterize septic shock, the end point of that continuum. If the neonate survives acute sepsis, localized areas of infection such as pneumonia, uveitis, synovitis, physitis, meningitis, hepatitis, and enteritis may then appear. In the past decade, survival rates of septicemic foals have improved considerably because of advances in early detection procedures and critical care management techniques,6-11 but it is far preferable to prevent12 septicemia by using good management techniques and by ensuring that the newborn acquires good colostrally derived immunity. The prognosis for septicemic neonates admitted for treatment in the late stages of the disease remains poor because bacteria are usually already well established in many organs, particularly the bones and joints. In these cases, even if the neonate survives on a short-term basis, a number of chronic complications result in an unfavorable long-term outcome. Foals with adequate transfer of maternal immunoglobulins can still succumb to generalized infection after birth, but animals with partial or total failure of passive transfer (FPT) have a greater risk of morbidity or mortality resulting from infection.13 See Chapter 53 for details on the detection, prevention, and treatment of FPT.
ETIOLOGY AND NEONATAL IMMUNITY
Most neonatal infections are caused by opportunistic bacteria that normally live in the genital tract, on the skin, or in the environment of normal horses, cows, sheep, and goats. Infection may be acquired prenatally, through the placenta, from the birth canal, or from the environment after birth. Portals of entry include the respiratory and gastrointestinal tracts, the placenta, and the umbilicus. A number of abnormalities in the late gestation mare make infection more likely in the neonate (see Chapter 15). Bacterial placentitis is a common cause of premature delivery and infection in the newborn foal; most cases result from ascending infection through the cervix. Perinatal stress, including chronic in utero hypoxia, prematurity, dystocia, and birth asphyxia, renders the neonate more susceptible to infection. Unsanitary environmental conditions, overcrowding, poor ventilation, contamination of the environment with pathogenic bacteria such as Salmonella species,12 and other poor management techniques also predispose to infection.9
The propensity for contagious and opportunistic infections in neonates reflects the immature status of their immune system. At birth, foals, calves, lambs, and kids are hypogammaglobulinemic or agammaglobulinemic and immunologically naive. Bovine colostrum contains approximately 45 mg/mL14 of immunoglobulin and 106 leukocytes/mL.15 Passively derived immunoglobulins enhance neonatal immunity by functioning as neutralizers and opsonins. The association between FPT of immunoglobulins and neonatal infection has been well established in the calf16,17 and suggested by several studies in the foal.13,18-20 Colostral leukocytes participate in regulation of the neonate’s immune response. Comparison of the immune response of calves fed leukocyte-replete or leukocyte-depleted colostrum indicates that colostral leukocytes enhance humoral immunity and phagocyte function.21-25 After experimental Escherichia coli infection, calves fed leukocyte-replete colostrum recovered more quickly and shed fewer bacteria than calves fed leukocyte-depleted colostrum.15 Transfer of cellular immunity via colostral leukocytes has also been demonstrated in sheep.26 Colostral leukocytes are destroyed when colostrum is frozen, pasteurized, or fermented.
The phagocytic and bacterial killing function of neutrophils (polymorphonuclear neutrophils [PMNs]) is a crucial component of the primary immune response against invading pathogens. Despite a larger number of neutrophils in the circulation of normal calves at birth, neutrophils from neonatal calves are functionally less effective than adult cells. Reduced Fc receptor expression in neonatal PMNs may contribute to impaired phagocytosis and antibody-dependent cellular cytotoxicity.27-29 Depressed PMN bacterial killing30 may be related to reduced superoxide anion31 and myeloperoxidase—hydrogen peroxide—halide antibacterial activity.29 Adult level superoxide activity in fetal PMNs suggests that some of the deficits in neonatal PMN function may be a manifestation of perinatal PMN suppression.32 Calves have elevated cortisol levels for the first 10 days of life, which may contribute to depression of neutrophil function.33 Dexamethasone depresses neutrophil phagocytosis, antibody-dependent cellular cytotoxicity, and bacterial killing.34,35 Undefined serum factors also appear to be important for PMN function, as phagocytosis and bacterial killing by neonatal PMNs is similar to that of adult PMNs when bacteria are opsonized with adult serum but is reduced when bacteria are opsonized with neonatal serum.30 T helper cells (CD4+) play a central role in humoral and cell-mediated immunologic memory. Lymphokines produced by CD4+ cells, interleukin (IL)-2, IL-4, and interferon gamma (IFN-γ) are essential components of antigen-specific immunity. Virtually all IL-4 and most IFN-γ produced by polyclonally activated adult human CD4+ cells are mediated by a subset of “functional memory cells.”36 Leukocyte production of IL-4 and IFN-γ is reduced in human neonates,36 and IFN-γ in bovine neonates,* possibly reflecting their antigenically naive status. Depressed lymphocyte proliferation and IL-2 activity in the perinatal period correlates with elevated cortisol levels.33
Acquisition of humoral immune competence is age and antigen dependent.37 Neonates are capable of producing humoral immune responses to good immunogens (protein antigens) but may fail to respond to lesser immunogens (sugars and lipids). Calves less than 3 months of age vaccinated with modified live or killed salmonella vaccines produce an adult-type humoral immune response to salmonella protein antigens but do not respond to salmonella lipopolysaccharide (LPS).38 Ingestion of colostrum suppresses the humoral response to some antigens but not to others.37
Adverse management and environmental conditions further compromise neonatal immunity. Cold stress depresses neutrophil chemotaxis, and vasoconstriction reduces delivery of leukocytes to peripheral tissues.39 Protein energy malnutrition in calves is associated with depressed lymphocyte IL-2 activity, lymphocyte proliferation, and humoral immune responses.33 Micronutrient deficiencies also depress immunity. Selenium, zinc, copper, and vitamin E deficiencies depress lymphocyte and phagocyte function.40,41
Inherited defects in immune function are sporadically observed in neonates and should be considered when recurrent or atypical infections are observed—for example, Pneumocystis carinii pneumonia in foals. Inherited immunodeficiencies are discussed in Chapter 53.
PATHOGENESIS OF SEPSIS
As discussed earlier, the pathogenesis of sepsis is yet to be fully elucidated.3 Systemic sepsis can be caused by a range of infectious agents including gram-positive bacteria, gram-negative bacteria, fungi, and viruses. Interaction between viruses, bacteria, or bacterial products and TLRs appears pivotal in the induction of the proinflammatory and antiinflammatory responses.42 Endotoxin is a mediator of gram-negative sepsis. Endotoxin, the LPS component in the outer cell membrane of gram-negative bacteria, is released whenever the bacterial cell membrane is disrupted, as occurs during rapid growth or cell death. Endotoxin is composed of hydrophilic heteropolysaccharide and hydrophobic lipid. The surface portion of LPS is the O-antigenic determinant consisting of sugar chains that are highly variable among species. Deeper within the LPS molecule is the lipid moiety, which is responsible for most of the endotoxin’s biologic activities. Binding of LPS to TLR4 receptors leads to expression of nuclear factor—kappa B (NF-κB)–controlled proinflammatory cytokines and IFN-regulatory factors and type I IFNs.42 The reticuloendothelial system is responsible for detoxification of endotoxin. Gram-positive bacteria do not produce endotoxin but elaborate lipoteichoic acid and peptidoglycan, which can effect a similar inflammatory response through activation of other TLR receptors, TLR2 and TLR6.42
The initiating event in the generalized bacterial sepsis syndrome is the presence of bacteria and/or bacterial products within the circulation. Subsequent binding with surface TLRs and other immune system components triggers intracellular signaling pathways that activate the genes and enzymes responsible for production and maturation of cytokines and IFNs. Proinflammatory products include the cytokines tumor necrosis factor alpha (TNF-α), IL-1, IL-6, transforming growth factor (TGF)–β, and IFN-γ. Antiinflammatory cytokines include IL-4, IL-10, and IL-13. The cascade yields other products including kinins, myocardial depressant factor, β-endorphins, free-radical oxygen species, lysosomal enzymes, and prostaglandins.1,2,43 Serum TNF concentration has been correlated with clinical criteria of sepsis in calves and foals.44,45 Many of the inflammatory mediators have direct effects on the vascular endothelium, resulting in increased endothelial permeability. The endothelium also releases two additional substances: endothelium-derived relaxing factor (EDRF), and endothelin-1. EDRF has been identified as nitric oxide (NO) and is responsible for relaxing smooth muscle, depressing myocardial function, decreasing vasopressor responsiveness, and inhibiting platelet aggregation.46,47 Endothelin-1 is a potent vasoconstrictor. After the increase in vascular permeability, interstitial and pulmonary edema, hypovolemia, and decreased cardiac output develop. Pulmonary and systemic hypotension develops. Splanchnic perfusion decreases, and coagulation pathways are activated, resulting in varying degrees of DIC. The majority of patients that die of septic shock suffer multiple organ failure.2
Respiratory failure is a frequent complication of septic shock. As pulmonary capillary permeability increases, leukocytes accumulate and degranulate in the pulmonary microvasculature, resulting in endothelial damage, increased capillary leakiness, and alveolar flooding. Lung collapse (atelectasis), intrapulmonary shunting, and mismatching of ventilation and perfusion develop. Terminally, hypoxemia, pulmonary hypertension, progressive lung collapse, pulmonary edema, and respiratory failure develop.2
Myocardial depression occurs during sepsis. Endotoxin and NO exert a direct inotropic depressant effect on the heart. During early sepsis stimulation of the sympathetic nervous system results in tachycardia, increased cardiac output, improved myocardial contractility, and increased oxygen consumption. As sepsis progresses, there is a decrease in vascular tone and oxygen extraction by peripheral tissues, accompanied by development of metabolic acidosis and anaerobic metabolism. During late sepsis, myocardial failure develops, accompanied by decreased cardiac output and severe hypotension. A recent study reported elevations in the cardiac biomarkers cardiac troponin I and creatine kinase MB in septic foals compared with healthy neonatal foals.48 The magnitude of either of these markers did not predict outcome.
Sepsis is best characterized nutritionally by hypermetabolism, catabolism, and protein wasting.49 During sepsis, intermediary metabolism is disrupted and the foal loses sequentially the ability to use glucose, then fat, and finally protein as energy sources. Elevated concentrations of catecholamines and glucagon contribute to insulin resistance and increased lipolysis. These changes explain the hyperglycemia and lipemic serum occasionally observed in foals with sepsis.
INFECTIOUS AGENTS ASSOCIATED WITH NEONATAL DISEASE
In all studies conducted in the United States, Europe, and Australia in the past 20 years, gram-negative bacteria have been the predominant cause of infection in large animal neonates, and E. coli has been by far the most common bacterial species isolated.6,7,9,10,50-55 Other common bacterial organisms include Actinobacillus species (foals), Pasteurella species (calves and foals56 [rare]), Klebsiella species, Salmonella species (calves and foals), and less commonly Pseudomonas species, Listeria monocytogenes,57 Clostridium perfringens and Clostridium septicum,58 Staphylococcus aureus, and Streptococcus species. Although Streptococcus species are most commonly isolated in mixed infections with gram-negative bacteria,6,7 both alpha- and beta-hemolytic Streptococcus species have been isolated in pure culture in foals with large subcutaneous abscesses and physitis and septic arthritis. Streptococcus pneumoniae type 3, typically a human pathogen, was identified as the cause of severe respiratory distress in a neonatal foal.59 Polymicrobial infections are common in calves with septicemia (28%).52 Surveys examining blood culture results of foals admitted to neonatal intensive care units reflect a trend in the type of bacterial organisms contributing to septicemia. There has been an increase in the number of gram-positive bacterial isolates including Streptococcus (alpha and beta) species, Staphylococcus species, Enterococcus species, and Clostridium species.60 These statistics serve to remind clinicians not to ignore the pathologic significance of gram-positive organisms when selecting an antibiotic regimen for the septic neonate. Although newer antibiotics including some of the cephalosporins, β-lactam antibiotics, and fluoroquinolones have an extended gram-negative spectrum, their gram-positive spectrum may be inadequate for pathogens such as Streptococcus species.
Most organisms known to cause placental and fetal disease may cause disease of the newborn. Infectious agents associated with abortion, stillbirths, and birth of weak ruminant neonates are listed in Box 15-3. Infections of neonates contracted in utero are uncommon compared with postnatally acquired infections. Clinical signs of in utero infections are dependent on the age of the fetus at the time of infection and the tissue tropism and virulence of the infecting organism. Abortion storms and outbreaks of perinatal weakness, congenital defects, and mortality are often manifestations of widespread herd or flock infections. Subclinically infected neonates may remain chronically infected, providing a continuing reservoir of infection in the population. Transplacental infection is important in the epidemiology of a number of diseases including Johne’s disease (Mycobacterium paratuberculosis), bovine virus diarrhea, and bovine leukosis.61-63
Although viral agents are most commonly associated with abortion in the mare, viral infections may also occasionally cause disease in the newborn foal. Equine herpes viral infection may result in a weak newborn foal that responds very poorly to conventional supportive care; antemortem diagnosis is very difficult.64,65 Equine viral arteritis has been reported in neonatal foals.66,67 Equine influenza may be associated with interstitial pneumonia in foals.68
Fungal infections have been observed in neonatal foals. The development of severe, generalized candidiasis has been observed in debilitated or immunocompromised foals undergoing intensive care. A history of prolonged antimicrobial use is common. Diagnosis has been made using blood cultures or cultures of joint aspirates.
CLINICAL SIGNS
The spectrum of clinical signs associated with septicemia depends on the integrity of the host immune system, the duration of illness, the severity, the route of infection, and the target organs. Early in the clinical course, the clinical signs are usually nonexistent or vague, nonspecific, and easily attributed to other diseases. During the early hyperdynamic phase of sepsis, clinical signs include lethargy, hypotonia, more time spent sleeping, decreased nursing frequency followed by complete loss of suckle reflex, hyperemic mucous membranes with rapid capillary refill time associated with peripheral vasodilation and increased cardiac output, tachycardia, bounding peripheral pulses, extremities that are still warm, tachypnea, and variable body temperature. Capillary leakiness contributes to the early appearance of petechiae on the gums, sclera, inside of the ears, and coronary bands. As soon as the foal’s nursing vigor decreases, the mare’s udder becomes warm and distended and may stream milk spontaneously. When the weak foal does nurse from the overdistended udder, it often comes away with a milk-stained face owing to the spontaneous milk letdown that it is too weak too swallow. Dehydration develops rapidly, resulting in decreased urine output and constipation. Obviously, the earlier the infection is diagnosed, the better chance the treatment has of being successful. Localizing signs may or may not be present. Prompt and aggressive intervention at this stage of the disease process frequently results in a successful outcome.
During late sepsis, when infection overwhelms the host’s immune system, septic shock develops. Affected foals are usually recumbent, dehydrated, and almost moribund. Clinical signs include severe hypotension associated with hypovolemia and decreased cardiac output, tachycardia, altered mentation, cold extremities, weak peripheral pulses, and dry, injected mucous membranes with a toxic ring and prolonged capillary refill time. Hypothermia is common. Gut motility is usually decreased and is accompanied by gastric reflux, abdominal distention, and constipation or diarrhea. Colic may be present if the ileus and abdominal distention are severe. Liver dysfunction is associated with cholestasis and increasing clinical jaundice. Decreased pulmonary perfusion and increased vascular permeability contribute to progressive respiratory compromise. Signs of respiratory distress include tachypnea, dyspnea with nostril flare, rib retractions, and expiratory grunting. Salvage of the patient in late septic shock is usually not successful. Although an encouraging response to intensive therapy may be noted initially, most neonates presented in late septic shock do not survive.
Fever is inconsistently present in infected foals; the possibility of infection should never be ruled out because of the absence of fever.6 Foals in septic shock often have a subnormal rectal temperature. In one study diarrhea was the most common early localizing sign in a group of foals with septicemia.6 The pathophysiologic basis of the diarrhea is not known but may relate to altered intestinal perfusion. E. coli is the common cause of systemic sepsis in foals, but it is a rare cause of primary enteric disease. Other signs include seizures (with or without the presence of meningitis or encephalitis), colic, respiratory distress, uveitis, subcutaneous abscesses, joint distention and/or periarticular edema, and umbilical abscessation. The signs of osteomyelitis or physeal infection may be extremely subtle, with no obvious areas of inflammation detectable on the limbs. The only clues may be a reluctance to move, a choppy, stilted gait, and/or an inflammatory hemogram (increased fibrinogen, in particular).
The time of onset of clinical signs of infection in the neonate depends on whether the infection was acquired in utero or postnatally. Animals infected in utero generally begin to show signs sometime during the first 24 hours of life, whereas postnatally infected animals often appear relatively normal for the first 2 days of life or longer. Actinobacillus infections in foals may become apparent somewhat earlier (24 to 48 hours of age) and are commonly characterized by an acute onset of depression, diarrhea, or rapidly distending, painful joint(s). Bone and joint infections in neonates may not be obvious for several days to weeks, and their appearance may either follow an improvement in systemic signs of illness or not be accompanied by any signs of systemic illness.
As with foals, septicemia in ruminant neonates commonly involves multiple organs with the respiratory and gastrointestinal systems most commonly affected.52 Clinical signs are often nonspecific and may include lethargy, poor suckle reflex, weakness, dehydration, tachycardia, tachypnea, and recumbency. Findings suggestive of involvement of a particular organ system include diarrhea, lameness, omphalophlebitis, neurologic and ocular signs, and cardiac murmurs. Depressed mentation, diarrhea, and dehydration are the most common clinical signs of sepsis in neonatal ruminants, but clinical presentation is variable.54 Rectal temperature, heart rate, and respiratory rate are poor predictors of sepsis in calves.54,55 Fecteau and co-workers have developed a clinical score model for predicting sepsis in calves to promote rational antimicrobial use by producers.55 Criteria included in the model include fecal consistency, hydration status, attitude (mental awareness), and umbilical and scleral vessel assessment. Sensitivity and specificity of the model were 76% and 75%, respectively. In the calves examined by Fecteau and colleagues the presence of severe diarrhea or a localized infection (e.g., infected navel) was associated with an increased probability of bacteremia as determined by blood culture.
DIAGNOSIS OF BACTERIAL INFECTION IN NEONATES
It is not difficult to diagnose an infected neonate when overwhelming sepsis is present. Bacteria may be observed on a peripheral blood smear in cases of advanced sepsis. Unfortunately, if treatment is instituted at this time, it is unlikely to be successful. Because of the necessity of early diagnosis and treatment of infection for a favorable outcome, a reliable, rapidly available field test to establish the presence of infection would be highly desirable. Such a test is not currently available. Positive blood cultures are the only definitive antemortem proof of bacteremia, but a minimum of 24 hours is usually required for preliminary results.
Because of the difficulty in identifying early sepsis at a treatable stage, a scoring system has been developed for the neonatal foal that incorporates a group of historic parameters, physical examination findings, and laboratory values, which together have been found to be considerably more accurate than any one single parameter in establishing the likelihood of infection.69 The sepsis score is intended for use as a diagnostic aid only and is not 100% accurate. False-negative results occur in older foals, and false-positive results are common in foals born prematurely. If the score is low for an individual foal but clinical suspicion of infection is high, antibiotic therapy should be instituted, and further assessment should then be performed.
The white blood cell (WBC) count and differential are important parts of the sepsis score. A number of foals in the early stages of septicemia have a normal total WBC count, but most have either an increased number of immature (band) neutrophils (>50 cells/μL) or toxic changes (Döhle bodies, toxic granulation, vacuolization) in the neutrophils. Foals that die of septicemia generally have very low WBC counts with considerable toxicity, but a low WBC count in a patient with sepsis does not necessarily predict death. The WBC count may undergo dramatic changes in a short period of time, often preceding changes in clinical condition. Foals with infection acquired in utero as a consequence of bacterial placentitis typically are born with elevated WBC counts. There is some evidence that very high WBC counts are positively correlated with successful outcomes. The fibrinogen concentration is also useful in detecting newborn foals that have been infected or exposed to inflammatory placental disease in utero. Fibrinogen values in these cases may be 1000 mg/dL or greater at birth (normal is 300 mg/dL or less), and, again, high values are often positively related to outcome. In the early stages of postnatally acquired infections, fibrinogen values may be only mildly increased (400 to 500 mg/dL), but with increasing chronicity resulting from pneumonia or bone and joint infections, plasma fibrinogen levels may increase dramatically.
Total plasma protein concentration is highly variable and may be influenced by dehydration, catabolism, and ingestion of colostral immunoglobulins. The range of presuckle protein concentration varies so much between foals that it is not a reliable indicator of FPT. IgG determination has been shown to be an important component of the sepsis evaluation, in that low IgG levels have strongly correlated with the presence of sepsis. Severe, overwhelming infections are seen far less commonly in foals with normal immunoglobulin G (IgG) levels (>800 mg/dL) but can occur, particularly in individuals with in utero acquired infections and severe enteric infections caused by pathogenic bacteria such as Salmonella species and clostridial intestinal infections. Because serum IgG levels can change dramatically as a result of protein catabolism associated with sepsis, it is often difficult to determine whether hypogammaglobulinemia in a sick foal is the cause or result of sepsis.
Hypoglycemia (glucose <60 mg/dL; <3.3 mmol/L) commonly accompanies generalized infection and is associated with bacterial consumption and reduced glycogen reserves. Serum glucose values can be very low, with the animal showing few signs other than depression and weakness.
The potential application of hematologic evaluation of neonates for early detection of sepsis is illustrated in a study reported by Adams and colleagues.70 Hematologic values in 35 newborn beef calves were evaluated; five calves subsequently developed clinical signs of sepsis at 3 weeks of age. Comparison of hematologic values from the five diseased calves with values for healthy calves revealed significant differences at each sample collection time (birth, 24 hours, 48 hours, and 3 weeks), although disease was not clinically evident at the three early sample times. Compared with the clinically normal calves the five septic calves had more band neutrophils and a higher neutrophil-to-lymphocyte ratio at birth. At 24 hours the monocyte count was higher, and at 48 hours total leukocyte, mature neutrophil, and monocyte counts and neutrophil-to-lymphocyte ratio were higher in the five calves. At 3 weeks, when clinical signs of disease were detectable in the five calves, the total leukocyte, band neutrophil, and mature neutrophil counts, neutrophil-to-lymphocyte ratio, and plasma total protein and fibrinogen concentrations were higher.70
Hematologic abnormalities observed in septic calves with clinical signs of disease are not consistent. In a retrospective study of 25 septic calves a noticeable feature of the pattern of laboratory abnormalities was the contrast of severe clinical signs with minor complete blood count (CBC) and serum biochemical alterations in numerous calves.52 Abnormal laboratory findings included neutrophilia or neutropenia, immature neutrophils, toxic neutrophils, and hyperfibrinogenemia.52 Low serum immunoglobulin concentrations are also commonly observed in calves with sepsis.52,55 The most common abnormalities observed in calves with DIC include activated partial thromboplastin time (aPTT) and prothrombin time (PT), observation of schistocytes, and elevated fibrin degradation products. Thrombocytopenia is less frequently observed.71 Abnormalities of the coagulation and the fibrinolytic systems are also common in neonatal foals with sepsis.72 Prolongation of PT and aPTT are common, as are increased concentrations of fibrin degradation products.73 Foals with advanced sepsis may show signs of spontaneous hemorrhage or vascular thrombosis.
Lactate measurement is an important indicator of foal sepsis and if taken at admission may provide important prognostic information.74 Lactate is normally <2.5 mmol/L; levels between 2.5 and 5 mmol/L are typically not associated with acidosis, whereas values<5 mmol/L are often associated with acidosis. Substantial reductions in blood lactate in response to 24 hours of therapy often translate into a favorable outcome.75 Other abnormal serum chemistries associated with sepsis include metabolic acidosis (bicarbonate <19 mEq/L) resulting from increased anaerobic metabolism and azotemia (creatinine <2 mg/dL) secondary to dehydration as well as primary renal damage. During the terminal stages of septicemia, foals frequently display a mixed respiratory and metabolic acidosis accompanied by hypoxemia. Lipemia and hyperbilirubinemia reflect altered endocrine and hepatic function.
Positive blood cultures are essential to make the diagnosis of septicemia. However, it is clear that treatment cannot be delayed until results of blood cultures are obtained. Although it does not help to make the initial decision regarding therapy, a positive blood culture allows a more accurate prognosis to be given to the owner, provides information about the type of bacteria and its susceptibility pattern, and helps guide the decision as to length of antibiotic treatment.
Blood cultures are easy to perform but must be done carefully for accurate results. The hair should be shaved, and the site of venipuncture surgically scrubbed. Depending on the type of culture bottle or tube, a set amount of blood is withdrawn from the vein aseptically and deposited into both anaerobic and aerobic blood culture bottles. A liquid or solid blood culture medium can be used. One popular culture medium is Columbia broth medium with sodium polyanetholsulfonate as anticoagulant for aerobic cultures (Septi-Chek, Roche Laboratories, Nutley, NJ) and a brain-heart infusion medium for anaerobic cultures. If the medium is not readily available, the sample can be transferred in a yellow-top tube containing anticoagulant citrate dextrose (ACD). Serial cultures are preferred. A clean needle is used for transferring the blood into each bottle. The bottles are then incubated. A positive culture is characterized by marked turbidity, usually within 12 to 48 hours of incubation. The medium with bacterial growth is then Gram stained and plated out for identification and susceptibility testing. Working with a local human hospital may provide the ideal resource when trying to identify bacteria and establish antibiotic susceptibility patterns.
Other samples that can be used for culture in addition to urine and blood are synovial fluid, cerebrospinal fluid (CSF), peritoneal fluid, feces, and transtracheal aspirate. In cases of physeal osteomyelitis a physeal aspirate may be beneficial.
Many referral hospitals perform blood culture on all abnormal neonatal foals on admission, whether or not they received antibiotics previously, and the foals should then be placed on antibiotics if infection is suspected. Bacterial cultures are also taken from specific areas if local infections are suspected (CSF, joint, feces, trachea). With only one blood culture routinely taken per foal, some false-negative results have been obtained, but the number of positive results has been surprising.6,51 However, foals with in utero—acquired pneumonia have rarely had positive results on blood culture, and additional culture specimens (e.g., tracheal aspirate) should also be taken. If a fever spike occurs during the hospital stay, if the WBC count changes dramatically, or if the clinical condition of an infected foal deteriorates, the blood is recultured. The development of resistant infections has been observed in both community- and hospital-acquired infections, and their prompt detection is very important.
THERAPY FOR BACTERIAL INFECTION
Antibiotic therapy is currently the cornerstone of treatment of neonatal infection. Because sepsis can progress with devastating speed in the neonate, antibiotic therapy should be started as soon as sepsis is suspected. Broad-spectrum coverage should be initiated pending culture results, anticipating the preponderance of gram-negative bacteremia, the reemergence of gram-positive pathogens, and the possibility of polymicrobial infections. Bactericidal drugs are favored for treatment of sepsis in neonates because of the immaturity of the neonatal immune system. Immaturity of mechanisms involved in drug absorption, distribution, biotransformation, and excretion contribute to altered pharmacokinetics of antimicrobials in neonates. Implications of altered pharmacokinetics in neonates include the potential for suboptimal therapeutic concentrations, toxic effects, and, of importance in food animals, violative residues if adult dosing regimens are employed. In general terms, antimicrobials have longer elimination times in neonates (less than 2 weeks of age) than in adults and therefore larger doses are administered with a longer dosage interval to achieve similar peak and trough antimicrobial concentrations. More complete discussions of antibiotic therapy in neonatal foals10,76,77 and food animals can be found in other texts.78-81 The use of immediate antimicrobial therapy in advanced cases of sepsis is controversial. It has been suggested that lysis of circulating bacteria could further contribute to endotoxin load, hastening circulatory collapse and death. In vitro data indicate that β-lactam antibiotics, such as ceftiofur or ampicillin, but not aminoglycosides, may lead to an increase in endotoxin concentration.82
The duration of antibiotic therapy in infected neonates depends on the clinical status of the patient and what type of infection has been documented. One week to 10 days of therapy may be adequate for suspected but undocumented sepsis if the CBC, fibrinogen, and patient are normal at the end of therapy. A minimum of 2 weeks of treatment is suggested in blood culture—positive patients with no evidence of localized infections. Three to 4 weeks (or more) of antibiotic treatment are often required when the infection has localized, particularly in the joints or lungs. Therapy is usually discontinued when the WBC count, fibrinogen, and radiographs have returned to normal. Although there has been concern about the use of aminoglycoside antibiotics in the neonatal foal, the well-hydrated neonatal foal tolerates these drugs very well, even after extended periods of treatment (2 to 4 weeks).
Established sepsis in the neonate carries a poor prognosis, with less than 12% survival of calves with sepsis in a referral hospital.52 Client education to facilitate early recognition and treatment of neonates with sepsis improves outcome and reduces cost.
Antimicrobial Therapy in Foals
The combination of a penicillin and an aminoglycoside, such as gentamicin (6.6 mg/kg given intravenously [IV] or intramuscularly [IM] once per day [sid]) or amikacin (21 to 25 mg/kg IV or IM, sid), provides good antimicrobial coverage. Some clinicians have used gentamicin at 6.6 mg/kg twice per day (bid) as long as in-hospital monitoring is available. Amikacin is often preferred to gentamicin because it seems to be less nephrotoxic and less likely to be associated with the development of resistant bacterial infections.10,77,83 Ideally, peak and trough serum aminoglycoside concentrations are monitored to ensure that the proper dose and dosing interval are used, but this is not often possible in a field setting. Unfortunately, no studies have been conducted in the foal to determine the specific peak and trough serum concentrations resulting in optimal survival rates in individuals with sepsis.84 Based on work in other species a target peak concentration of 15 to 30 μg/mL and trough concentration of 1 to 3 μg/mL have been suggested.85 During long-term aminoglycoside therapy, efforts are made to prevent dehydration, and the patient’s urinalysis and serum creatinine are monitored at least weekly.
Other antibiotics that may be useful in the empirical treatment of the infected neonate include certain third-generation cephalosporins, such as ceftiofur (2.2 to 4.4 mg/kg IV or IM bid), cefotaxime (20 to 30 mg/kg IV or IM three times per day [tid]), ceftriaxone, and ceftazidime. In studies of susceptibility patterns of bacteria cultured from foals undergoing treatment in intensive care units, antibiotics such as ampicillin, kanamycin, and tetracycline were of very little value for treatment of gram-negative infections. Twenty percent to 40% of isolates were resistant to trimethoprim-sulfonamide (TMS) combinations (15 mg/kg bid IV, orally [PO]), ceftiofur (2.2 to 6.6 mg/kg IM bid), chloramphenicol (25 to 50 mg/kg IV or PO four times per day [qid]), and ticarcillin-clavulanate (50 mg/kg IV, tid or qid).7,9,51,86 Some, however, have reported success with tetracyclines in the management of osseous infection. A susceptibility pattern should document that the organism is indeed susceptible before these antibiotics are selected for use. Fluconazole may be effective in treatment of localized and generalized candidiasis. One recommended dosage for foals is a loading dose of 400 mg followed by 200 mg at 24-hour dosing intervals.10 See Chapter 45 for further discussion of antibiotics.
Antimicrobial Therapy in Neonatal Ruminants
Common medical conditions in neonatal calves that require antimicrobial therapy include diarrhea, pneumonia, bacteremia, omphalophlebitis, osteomyelitis, meningitis, and septic arthritis.
Bacteremia is a common problem in debilitated neonatal ruminants. Rapid recognition and treatment of sepsis improves the likelihood of a successful outcome. Blood culture studies of debilitated calves indicate that gram-negative bacteria account for approximately 80% of bacterial isolates; E. coli is the most common species of bacteria isolated.52,54,87 In a study of 190 recumbent calves on a large calf-raising facility, 31% were determined to have bacteremia. E. coli accounted for 51% of the isolates; other gram-negatives, 25%; gram-negative anaerobes, 5.9%; gram-positive cocci, 11.8%; and gram-positive rods, 5.9%.87 Empiric antimicrobial therapy should include a gram-negative and gram-positive spectrum. Other considerations pertinent to antimicrobial selection include the pharmacokinetics and pharmacodynamics of the drug in neonates, likelihood of antimicrobial resistance, and potential for violative antimicrobial tissue residues. Determination of antimicrobial susceptibility (minimal inhibitory concentration [MIC]) before therapy is desirable but often not possible. Alternatively, a drug may be selected and given at a dosage that has been shown to be effective for ≥90% of similar isolates tested (MIC 90%). The objective of measuring antimicrobial MIC’ is to facilitate antimicrobial selection that is likely to achieve a therapeutic concentration of drug for the target pathogen. The MIC data are of limited value without information on serum and tissue concentrations attainable using the intended drug dose. Data regarding microbial susceptibility to antimicrobial drugs are provided in Table 18-1, and pharmacologic data regarding the volume of distribution, half life, and breakpoint MICs of common antimicrobial drugs are presented in Table 18-2. The data presented in the tables are intended as a guide. Being from different studies, they reflect the magnitude of differences that may be observed over time and among sources. The data reflect intravenous administration. The half-life of drugs is often longer after intramuscular injection, reflecting absorption rate—dependent elimination.114
Antimicrobial drugs with a gram-negative spectrum of activity include third-generation cephalosporins (ceftiofur), TMS, fluoroquinolones (enrofloxacin), aminoglycosides, sulfonamides, and tetracyclines. Although florfenicol has a gram-negative spectrum, the MIC90 for E. coli is very high at 25 mg/mL.88 Intramuscular injection of florfenicol (20 mg/kg IM) fails to reach the MIC90 value in plasma, and intravenous injection (11 to 20 mg/kg IV) exceeds the MIC90 value for only 60 minutes.89-91 The National Cattlemen’s Association (NCA) recommends that “until further scientific information becomes available alleviating safety and efficacy concerns, aminoglycoside antibiotics should not be used in cattle except as specifically approved by the FDA” (Herd Health Memo, No. 9, p. 82, 1993–1994). The bacteriostatic action and frequency of antimicrobial resistance to tetracyclines and nonpotentiated sulfonamides limits their effectiveness in neonates with sepsis. TMS may be used to treat sepsis in neonatal calves, but its half-life rapidly declines as ruminal function develops. In ruminating (6- to 8-week-old) calves, subcutaneous or oral administration of TMS leads to high serum levels of sulfadiazine but little or no serum trimethoprim.92 Bacterial resistance to TMS is less common than resistance to sulfa drugs alone but still may be high.54,93 Fluoroquinolones such as enrofloxacin are bacteriocidal and have an appropriate gram-negative spectrum of activity suitable for treatment of gram-negative sepsis. However, in the United States, enrofloxacin is conditionally licensed for treatment of respiratory disease in beef cattle. In countries where it is legal to use enrofloxacin for treatment of neonatal sepsis, a dosage rate of 2.5 to 5 mg/kg every 24 hours has been suggested as appropriate for calves.94 Enrofloxacin has demonstrated good efficacy in the treatment of E. coli septicemia, Salmonella enteritis, and Mycoplasma and Pasteurella pneumonia in calves.95-97 Prolonged administration of enrofloxacin (weeks) is not recommended, as it produces articular erosions in immature animals of other species. Ceftiofur has an appropriate antimicrobial spectrum, is bacteriocidal, and has been used to treat calves with sepsis with good clinical results using a dose of 5 mg/kg twice a day.* The label dose of 1 mg/kg once a day may not achieve minimal inhibitory tissue drug concentrations for some bacteria commonly isolated from neonates. Deviation from the labeled dose requires implementation of a withholding period. Information regarding drug withholding times for extra-label use of antimicrobials is available from the Food Animal Residue Avoidance Databank (FARAD), and a database of U.S. Food and Drug Administration (FDA)–approved drugs is accessible via the World Wide Web (http://www.farad.org/index/html).
Circulatory Support
Maintenance or restoration of effective circulating volume is a top priority in cases of sepsis. Aggressive intravenous fluid therapy is the mainstay of cardiovascular support and should be administered at the maximal rate tolerated by the foal. Severe septic shock may require fluid rates of 40 to 80 mL/kg/hr. Volume expansion should be achieved using a balanced electrolyte solution (crystalloid) or plasma (colloid). Colloid solutions are preferred and may reduce the incidence of pulmonary and systemic edema during fluid resuscitation. Infusion of crystalloid solutions equivalent to 0.5 to 1.5 times the estimated blood volume of the patient has been used, but hemodilution is a common consequence.122 If hemodilution is severe, or if hypotension or vasoconstriction continue or recur, additional fluid administration in the form of colloid, such as plasma, or hypertonic crystalloid fluid should be considered. Additional discussion of hypertonic saline administration may be found in Chapter 44 and in other references.123,124
When fluid resuscitation alone is inadequate to improve cardiovascular function and restore acceptable blood pressure, pharmacologic intervention using sympathomimetic agents is necessary. Peripheral and cardiac adrenergic receptor downregulation necessitates the use of larger doses of pressor agents than usual. In patients that are hypotensive, dopamine with its combined α- and β-adrenergic and dopaminergic activity is preferred. Higher doses are required for patients in severe septic shock. If the foal fails to respond to high doses (>10 to 15 μg/kg/min) then norepinephrine, a more potent α-adrenergic agent, can be tried. Recently, NO has been shown to play a role in sepsis-induced hypotension. IV administered new methylene blue, an NO antagonist, has been used to try and reverse severe life-threatening hypotension. If oliguria continues in spite of restoration of circulating volume, diuretics such as furosemide or mannitol are used to promote renal vasodilation and urine flow.122
Because most foals with sepsis are hypoglycemic, a slower continuous infusion of dextrose-containing solutions should be run simultaneously with the rehydration fluids. Avoid too rapid an infusion of dextrose to avoid hyperglycemia, which can induce an osmotic diuresis that further exacerbates the dehydration.
Treatment with antiprostaglandin drugs has been found to counteract a number of the clinical and hemodynamic changes associated with endotoxemia and septic shock, including the decrease in cardiac output and systemic hypotension. They have little effect, however, on the leukopenia, thrombocytopenia, or coagulopathies that develop in septic shock.122 Based on the effect of these drugs in models of endotoxemia in the adult horse125,126 and neonatal calf127 and of septic shock in other species,122 they would be expected to be of some benefit in treatment of the septic large animal neonate. Pharmacokinetic studies of flunixin meglumine in neonatal foals suggest that, in spite of prolonged elimination of flunixin in healthy newborn foals, the physiologic activity appears similar to that in the adult, and the adult dose of 1.1 mg/kg of body weight would be appropriate in at least some patients.128 Lower doses of flunixin meglumine, 0.25 mg/kg IV tid may be effective in ameliorating some of the signs of endotoxemia. Other treatments include plasma administration from hyperimmunized donors to treat not only FPT but to provide opsonins and improve foal neutrophil function. Plasma also represents an ideal colloid for rapid volume expansion.
Other therapies include naloxone, an opiate antagonist, which has been used experimentally to counteract the detrimental vasodilatory effects of endorphins released during sepsis. An NO inhibitor, new methylene blue, has been used to treat refractory hypotension in septic patients.46,47 Pentoxifylline, a methylxanthine derivative, has been used for treatment of conditions characterized by inadequate regional blood flow.129,130 The drug increases red cell deformability, reduces blood viscosity, decreases platelet aggregation, and decreases thrombus formation. Pentoxifylline administration also results in decreased plasma fibrinogen, increased action of plasminogen activators, and antithrombin III, decreased platelet thromboxane synthesis, and increased prostaglandin 1-2 synthesis. The net effect of the drug is to increase regional blood flow and inhibit coagulation. When the drug was used in animal models of endotoxic shock, it increased overall survival rates and prevented endotoxin-induced renal failure, synthesis of TNF, and coagulopathies. Horses that received the drug showed a decrease in packed cell volume (PCV) and red blood cell (RBC) sedimentation rate and beneficial effects of RBC deformability. IV doses used experimentally in horses include a single bolus of 7.5 mg/kg of body weight followed by a continuous infusion of 1.5 mg/kg/hr.129
Polymyxin B administered at low, nontoxic doses is a new investigational treatment modality being used to neutralize systemic endotoxin. Low doses of the drug result in decreased concentrations of circulating endotoxin, improved immune function, and decreased mortality rates among shock patients. Polymyxin B binds and neutralizes endotoxin in vitro and has been shown to removed endotoxin from the circulation in vivo. Use of this drug in the horse is investigational. A suggested dose is 6000 IU/kg diluted in 300 to 500 mL of 5% dextrose and given as a slow intravenous infusion.
Immunologic Support
When FPT of antibodies accompanies neonatal infection, plasma is routinely used to increase immunoglobulin levels. From 1 to 4 L of plasma have been infused IV to raise IgG levels, but the optimum amount is not known. Important factors influencing the amount of plasma indicated include the total IgG content of the plasma and the specific antibody concentration against foal pathogens, as well as the degree of circulatory impairment of the neonate. The efficacy of plasma in the prevention or treatment of septicemia in foals has not been established as of this writing. See Chapter 53 for more information on treatment of FPT.
Several immunologic products are currently under investigation as potential treatments of foal septicemia; their efficacy has not yet been proven. These include serum or plasma containing high levels of antibodies to the common core structures of LPS,126,131 and granulocyte colony-stimulating factor, which markedly increases WBC counts in foals.132 Monoclonal antibodies to TNF and other inflammatory mediators may also be of use in the future.133
Bovine colostrum supplements are commercially available. Immunoglobulin content varies among products and should be considered when comparing prices. Colostrum supplements generally fail as colostrum substitutes because of the relatively low mass of immunoglobulin delivered per dose.134 In two separate clinical trials, peak serum immunoglobulin concentrations were not significantly different in calves fed 3 or 4 L of maternal colostrum with or without colostrum supplement.134,135 Administration of a colostrum supplement before 18 hours of age is indicated when an adequate supply of colostrum is not available. If a colostrum supplement is to be used as a colostral substitute, multiple doses are required to deliver a minimum of 100 g of immunoglobulin.
Supportive Therapy
Nutritional support of neonates with sepsis is critical. Gram-negative bacterial sepsis disrupts intermediary metabolism, increases metabolic rate, and sequentially hinders use of carbohydrates, lipids, and finally protein for energy. Endotoxin release precipitates a neurohormonal cascade of events mediated by TNF and increased levels of catecholamines, glucocorticoids, and glucagon. Elevated concentrations of antidiuretic hormone, aldosterone, and thyroxin and low levels of insulin accompany low perfusion states associated with septic shock. Sepsis results in glycolysis, lipolysis, and proteolysis, increased urinary excretion of potassium and nitrogen, and water and sodium retention. Suppressed insulin production and peripheral insulin resistance result in glucose intolerance and hyperglycemia. During sepsis the transport, oxidation, and clearance of free fatty acids (FFAs) is impaired owing to a deficiency of the carrier peptide carnation and decreased lipoprotein lipase activity. Lipemia develops. The final fuel source becomes protein degradation. Uremia, production of false neurotransmitters, hepatoencephalopathy, and neurologic signs occur when excessive amino acid degradation overwhelms hepatic metabolic capacity.
Provision of adequate nutrition is vital for a successful outcome in the treatment of infected neonates. Poor nutritional support leads to debilitation, a poorly functioning immune system and poor healing, persistent infection, and other complications, such as decubital ulcers. The healthy newborn foal requires 15% to 25% of body weight in milk per day. Foals that are too weak to nurse from the mare or a bottle should be tube-fed a minimum of 10% of their body weight per day in milk administered in small feeds every 2 to 3 hours. Because many sick foals have poor gut function, enteral nutrition is not a viable option initially. If a foal is not consuming at least 10% of body weight in milk within the first 36 to 48 hours, it should be started on parenteral nutrition using a formula containing dextrose, lipids, amino acids, vitamins, and trace minerals. Solutions of 5% to 10% glucose can be administered to help maintain a normal blood glucose level. These solutions provide temporary nutritional support but do not contain nearly enough calories for long-term nutritional support. It would require 35 L of a 5% dextrose solution per day to provide a 50-kg foal with adequate calories (120 kcal/kg/day). The use of a combination of oral and parenteral nutritional support has considerable merit in treating the infected neonate.
Foals with sepsis are susceptible to pulmonary dysfunction because of a variety of factors including dependent lung atelectasis, pneumonia, pulmonary edema, and surfactant dysfunction. The focus of respiratory support is to minimize ventilation and perfusion mismatching. Fluid therapy helps increase left ventricular, left atrial, and diastolic pressures to create more uniform lung perfusion. Recumbent foals should be turned and repositioned frequently to minimize dependent lung atelectasis and pulmonary edema formation. Mild to moderate hypoxemia can be treated with humidified intranasal oxygen (2 to 10 L/min). Severe hypoxemia (PO2 < 50 mm Hg), despite oxygen supplementation, and persistent hypercapnia (PCO2 <65 to 70 mm Hg) require positive pressure ventilation with positive end expiratory pressure (PEEP) to prevent further lung collapse, reduce interstitial edema, and prevent respiratory muscle fatigue. Debilitated foals with sepsis that require mechanical ventilation and nasotracheal intubation are at increased risk for nosocomial infections. Nebulization using bronchodilators, wetting agents, and mucolytic agents, accompanied by coupage therapy, also helps relieve foals with respiratory distress and facilitates removal of tracheal secretions.
The nursing care of the neonate with sepsis is very important. Maintenance of body temperature; fluid, blood gas, and acid-base balance; and a clean environment is critical for a successful outcome. Every neonate undergoing intensive care, regardless of its primary problem, should be monitored closely for fever spikes, neutropenia, increasing lethargy, and localizing signs of infection that could indicate early sepsis or a different, bacterial infection resistant to the antibiotics being used.
PROGNOSIS AND COMPLICATIONS OF SEPTICEMIA AND RELATED INFECTIONS
If a large animal neonate has FPT and is septicemic with several organ systems involved, its long-term outcome must be considered guarded, even with intensive nursing care.8 A recent study reported a short-term survival rate of 81% for neonatal intensive care unit survivors.11 Secondary complications that often accompany multifocal bone and joint infections may exert an adverse effect on the final outcome. A retrospective study that examined the factors associated with prognosis for survival and athletic use in foals with septic arthritis showed that with treatment the prognosis for survival was favorable, whereas the prognosis for ability to race was unfavorable. Approximately 78% of treated foals survived, and a third of those foals raced. Multisystem disease, isolation of Salmonella species from synovial fluid, involvement of multiple joints, and synovial fluid neutrophil count <95% were associated with a poor prognosis.136 If blood cultures are negative and localized infection (enteritis, pneumonia) is present, the outcome can be much more positive with aggressive therapy. If in utero acquired infections in the foal are treated appropriately early in the clinical course and good IgG levels are attained by the newborn, the outcome can be quite favorable (>75% survival).
1 Bone RC. The pathogenesis of sepsis. Ann Intern Med. 1991;115:457.
2 Rackow EC, Astiz ME. Pathophysiology and treatment of septic shock. JAMA. 1991;266:548.
3 Brunn GJ, Platt JL. The etiology of sepsis: turned inside out. Trends Mol Med. 2006;12:10.
4 Weiss DJ, Rashid J. The sepsis-coagulant axis: a review. J Vet Intern Med. 1998;12:317.
5 Gando S, Nauzaki S, Sasaki S. Activation of the extrinsic coagulation pathway in patients with severe sepsis, and septic shock. Crit Care Med. 1998;26:2005.
6 Koterba AM, Brewer BD, Tarplee FA. Clinical and clinicopathological characteristics of the septicaemic neonatal foal: review of 38 cases. Equine Vet J. 1984;16:376.
7 Paradis MR. Update on neonatal septicemia. Vet Clin North Am Equine Pract. 1994;10:109.
8 Koterba AM. Equine neonatal intensive care at the University of Florida 1982-1987: an update. Proceedings of the 33rd Annual Meeting of the American Association of Equine Practitioners. 1987:805.
9 Brewer BD. Neonatal infection. In: Koterba AM, Drummond WH, Kosch PC, editors. Equine clinical neonatology. Philadelphia: Lea & Febiger; 1990:296.
10 Baggot JD. Drug therapy in the neonatal foal. Vet Clin North Am Equine Pract. 1994;10:87.
11 Axon J. Short and long term athletic outcome of neonatal intensive care unit survivors. Proceedings of the 45th Annual Convention of the American Association of Equine Practitioners. 1999:224.
12 Clinical pathological conference. Cornell Vet. 1969;59:648.
13 Robinson JA, Allen GK, Green EM, et al. A prospective study of septicemia in colostrum-deprived foals. Equine Vet J. 1993;25:214.
14 Besser TE, Gay CC, Pritchett L. Comparison of three methods of feeding colostrum to dairy calves. J Am Vet Med Assoc. 1991;198:419.
15 Riedel-Caspari G. The influence of colostral leukocytes on the course of an experimental Escherichia coli infection and serum antibodies in neonatal calves. Vet Immunol Immunopathol. 1993;35:275.
16 Tyler JW, Hancock DD, Thorne JG, et al. Partitioning the mortality risk associated with inadequate passive transfer of colostral immunoglobulins in dairy calves. J Vet Intern Med. 1999;13:335.
17 Selim SA, Smith BP, Cullor JS, et al. Serum immunoglobulins in calves: their effects and two easy reliable means of measurement. Vet Med. 1995;90:387.
18 McGuire TC, Crawford TB, Hallowell AL, et al. Failure of colostral immunoglobulin transfer as an explanation for most infections and deaths of neonatal foals. J Am Vet Med Assoc. 1977;170:1302.
19 Stoneham SJ, Digby NJ, Ricketts SW. Failure of passive transfer of colostral immunity in the foal: incidence, and the effect of stud management and plasma transfusions. Vet Rec. 1991;128:416.
20 Clabough DL, Levine JF, Grant GL, et al. Factors associated with failure of passive transfer of colostral antibodies in standardbred foals [see comments]. J Vet Intern Med. 1991;5:335.
21 Riedel-Caspari G, Schmidt FW. [Review article: colostral leukocytes and their significance for the immune system of newborns]. Dtw Dtsch Tierarztl Wochenschr. 1990;97:180.
22 Riedel-Caspari G, Schmidt FW, Marquardt J. The influence of colostral leukocytes on the immune system of the neonatal calf. IV. Effects on bactericidity, complement and interferon; synopsis. Dtw Dtsch Tierarztl Wochenschr. 1991;98:395.
23 Riedel-Caspari G, Schmidt FW. The influence of colostral leukocytes on the immune system of the neonatal calf. III. Effects on phagocytosis. Dtw Dtsch Tierarztl Wochenschr. 1991;98:330.
24 Riedel-Caspari G, Schmidt FW. The influence of colostral leukocytes on the immune system of the neonatal calf. II. Effects on passive and active immunization. Dtw Dtsch Tierarztl Wochenschr. 1991;98:190.
25 Riedel-Caspari G, Schmidt FW. The influence of colostral leukocytes on the immune system of the neonatal calf. I. Effects on lymphocyte responses. Dtw Dtsch Tierarztl Wochenschr. 1991;98:102.
26 Schnorr KL. Cell tracing with fluorescein isothiocyanate and maternal transfer of cellular immunity in sheep. Fort Collins: Colorado Colorado State University, 1983. [PhD Thesis]
27 Zwahlen RD, Wyder-Walther M, Roth DR. Fc receptor expression, concanavalin A capping, and enzyme content of bovine neonatal neutrophils: a comparative study with adult cattle. J Leukoc Biol. 1992;51:264.
28 Toman M, Psikal I, Mensik J. Phagocytic activity of blood leukocytes in calves from birth to 3 months of age. Vet Med. 1985;30:401.
29 Hauser MA, Koob MD, Roth JA. Variation of neutrophil function with age in calves. Am J Vet Res. 1986;47:152.
30 Moiola F, Spycher M, Wyder-Walther M, et al. Comparative in vitro phagocytosis and F-actin polymerization of bovine neonatal neutrophils. Zentralbl Veterinarmed A. 1994;41:202.
31 Dore M, Slauson DO, Neilsen NR. Decreased respiratory burst activity in neonatal bovine neutrophils stimulated by protein kinase C agonists. Am J Vet Res. 1991;52:375.
32 Clifford CB, Slauson DO, Neilsen NR, et al. Ontogeny of inflammatory cell responsiveness: superoxide anion generation by phorbol ester-stimulated fetal, neonatal, and adult bovine neutrophils. Inflammation. 1989;13:221.
33 Griebel PJ, Schoonderwoerd M, Babiuk LA. Ontogeny of the immune response: effect of protein energy malnutrition in neonatal calves. Can J Vet Res. 1987;51:428.
34 Reddy PG, McVey DS, Chengappa MM, et al. Bovine recombinant granulocyte-macrophage colony-stimulating factor enhancement of bovine neutrophil functions in vitro. Am J Vet Res. 1990;51:1395.
35 Blecha F, Baker PE. Effect of cortisol in vitro and in vivo on production of bovine interleukin 2. Am J Vet Res. 1986;47:841.
36 Lewis DB, Yu CC, Meyer J, et al. Cellular and molecular mechanisms for reduced interleukin 4 and interferon-gamma production by neonatal T cells. J Clin Invest. 1991;87:194.
37 Smith AN, Ingram DG. Immunological responses of young animals II. Antibody production in calves. Can Vet J. 1965;6:226.
38 Roden LD, Smith BP, Spier SJ, et al. Effect of calf age and Salmonella bacterin type on ability to produce immunoglobulins directed against Salmonella whole cells or lipopolysaccharide. Am J Vet Res. 1992;53:1895.
39 Olson DP. In vitro migration responses of neutrophils from cows and calves. Am J Vet Res. 1990;51:973.
40 Pollock JM, McNair J, Kennedy S, et al. Effects of dietary vitamin E and selenium on in vitro cellular immune responses in cattle. Res Vet Sci. 1994;56:100.
41 Graham TW. Trace element deficiencies in cattle. Vet Clin North Am Food Anim Pract. 1991;7:153.
42 Takeuchi O, Akira S. Signaling pathways activated by microorganisms. Curr Opin Cell Biol. 2007;19:185.
43 Polin RA, St Geme JW. Neonatal sepsis. Adv Pediatr Infect Dis. 1992;7:25.
44 Basoglu A, Sen I, Sevinc M, et al. Serum concentrations of tumor necrosis factor-alpha in neonatal calves with presumed septicemia. J Vet Intern Med. 2004;18:238.
45 Morris DD, Moore JN. Tumor necrosis factor activity in serum from neonatal foals with presumed septicemia. J Am Vet Med Assoc. 1991;199:1584.
46 Symeonides S, Balk RA. Nitric oxide in the pathogenesis of sepsis. Infect Dis Clin North Am. 1999;13:449.
47 Lowenstein CJ, Dinerman JL, Snyder SH. Nitric oxide: a physiologic messenger. Ann Intern Med. 1994;120:227.
48 Slack JA, McGuirk SM, Erb HN, et al. Biochemical markers of cardiac injury in normal, surviving septic, or nonsurviving septic neonatal foals. J Vet Intern Med. 2005;19:577.
49 Cerra FB, Siegel JH, Coleman B, et al. Septic autocannibalism. A failure of exogenous nutritional support. Ann Surg. 1980;192:570.
50 Platt H. Septicaemia in the foal. A review of 61 cases. Br Vet J. 1973;129:221.
51 Wilson WD, Madigan JE. Comparison of bacteriologic culture of blood and necropsy specimens for determining the cause of foal septicemia: 47 cases (1978-1987). J Am Vet Med Assoc. 1989;195:1759.
52 Aldridge BM, Garry FB, Adams R. Neonatal septicemia in calves: 25 cases (1985-1990). J Am Vet Med Assoc. 1993;203:1324.
53 Joshi DV, Kaul PL, Shan NM. Bacterial agents associated with lamb mortality. Indian J Anim Sci. 1992;62:120.
54 Hariharan H, Bryenton J, St Onge J, et al. Blood cultures from calves and foals. Can Vet J. 1992;33:56.
55 Fecteau G, Van Metre DC, Pare J, et al. Predicting bacteremia in the bovine neonate. Proceedings of the 12th American College of Veterinary Internal Medicine, San Francisco. 1994:626.
56 Bourgault A, Bada R, Messier S. Isolation of Pasteurella canis from a foal with polyarthritis. Can Vet J. 1994;35:244.
57 Wallace SS, Hathcock TL. Listeria monocytogenes septicemia in a foal. J Am Vet Med Assoc. 1995;207:1325.
58 Jones SL, Wilson WD. Clostridium septicum septicemia in a neonatal foal with hemorrhagic enteritis. Cornell Vet. 1993;83:143.
59 Meyer JC, Koterba AM, Lester G, et al. Bacteraemia and pneumonia in a neonatal foal caused by Streptococcus pneumoniae type 3. Equine Vet J. 1992;24:407.
60 Marsh PS, Palmer JE, Fitzsimmons M. A survey of results of bacteriologic culture of blood from critically ill neonatal foals: 56 cases (1996-1998). Proceedings of the 2nd Dorothy Havemeyer Foundation Neonatal Septicemia Workshop, Boston. 1998:33.
61 Agresti A, Ponti W, Rocchi M, et al. Use of polymerase chain reaction to diagnose bovine leukemia virus infection in calves at birth. Am J Vet Res. 1993;54:373.
62 Moerman A, Straver PJ, de Jong MC, et al. A long term epidemiological study of bovine viral diarrhoea infections in a large herd of dairy cattle. Vet Rec. 1993;132:622.
63 Sweeney RW, Whitlock RH, Rosenberger AE. Mycobacterium paratuberculosis isolated from fetuses of infected cows not manifesting signs of the disease. Am J Vet Res. 1992;53:477.
64 Bryan LA, Fenton RA, Misra V, et al. Fatal, generalized bovine herpesvirus type-1 infection associated with a modified-live infectious bovine rhinotracheitis and parainfluenza-3 vaccine administered to neonatal calves. Can Vet J. 1994;35:223.
65 Whitwell KE. Investigations into fetal and neonatal losses in the horse. Vet Clin North Am Large Anim Pract. 1980;2:313.
66 Szeredi L, Hornyak A, Denes B, et al. Equine viral arteritis in a newborn foal: parallel detection of the virus by immunohistochemistry, polymerase chain reaction and virus isolation. J Vet Med B Infect Dis Vet Public Health. 2003;50:270.
67 Vaala WE, Hamir AN, Dubor EJ, et al. Fatal, congenitally acquired infection with equine arteritis virus in a neonatal thoroughbred. Equine Vet J. 1992;24:155.
68 Buergelt CD, Hines SA, Cantor G, et al. A retrospective study of proliferative interstitial lung disease of horses in Florida. Vet Pathol. 1986;23:750.
69 Brewer BD, Koterba AM. The development of a scoring system for the early diagnosis of equine neonatal sepsis. Equine Vet J. 1988;20:18.
70 Adams R, Garry FB, Aldridge BM, et al. Hematologic values in newborn beef calves. Am J Vet Res. 1992;53:944.
71 Irmak K, Sen I, Col R, et al. The evaluation of coagulation profiles in calves with suspected septic shock. Vet Res Commun. 2006;30:497.
72 Sanchez LC. Equine neonatal sepsis. Vet Clin North Am Equine Pract. 2005;21:273.
73 Barton MH, Morris DD, Norton N, et al. Hemostatic and fibrinolytic indices in neonatal foals with presumed septicemia. J Vet Intern Med. 1998;12:26.
74 Corley KT, Donaldson LL, Furr MO. Arterial lactate concentration, hospital survival, sepsis and SIRS in critically ill neonatal foals. Equine Vet J. 2005;37:53.
75 Franklin RP, Peloso JG. Review of the clinical use of lactate. Proceedings of the 52nd Annual Convention of the American Association of Equine Practitioners, San Antonio. 2006:305.
76 Brumbaugh GW. Clinical pharmacology and the pediatric patient. Proceedings of the 45th Annual Convention of the American Association of Equine Practitioners. 1999:226.
77 Koterba AM. Antibiotic therapy. In: Koterba AM, Drummond WH, Kosch PC, editors. Equine clinical neonatology. Philadelphia: Lea & Febiger; 1990:712.
78 Prescott JF, Baggot JD, editors. Antimicrobial therapy in veterinary medicine, ed 2, Ames, Iowa: Iowa State University Press, 1993.
79 Wilcke JR. Clinical pharmacology of antimicrobial drugs for the treatment of septic neonatal calves. Vet Clin North Am Food Anim Pract. 1991;7:695.
80 Schwark WS. Factors that affect drug disposition in food-producing animals during maturation. J Anim Sci. 1992;70:3635.
81 Prescott JF, Baggot JD. Principles of antimicrobial drug disposition. In Antimicrobial therapy in veterinary medicine, ed 2, Ames, Iowa: Iowa State University Press; 1993:37.
82 Bentley AP, Barton MH, Lee MD, et al. Antimicrobial-induced endotoxin and cytokine activity in an in vitro model of septicemia in foals. Am J Vet Res. 2002;63:660.
83 Orsini JA, Benson CE, Spencer PA, et al. Resistance to gentamicin and amikacin of gram-negative organisms isolated from horses. Am J Vet Res. 1989;50:923.
84 Golenz MR, Wilson WD, Carlson GP, et al. Effect of route of administration and age on the pharmacokinetics of amikacin administered by the intravenous and intraosseous routes to 3 and 5 day old foals. Equine Vet J. 1994;26:367.
85 Green SL, Conlon PD, Mama K, et al. Effects of hypoxia and azotemia on the pharmacokinetics of amikacin in neonatal foals. Equine Vet J. 1992;24:475.
86 Brewer BD, Koterba AM. Bacterial isolates and susceptibility patterns in foals in a neonatal intensive care unit. Compend Cont Educ (Pract Vet). 1990;12:1773.
87 Fecteau G, Van Metre DC, Pare J, et al. Bacteriological culture of blood from critically ill neonatal calves. Can Vet J. 1997;38:95.
88 Neu HC, Fu KP. In vitro activity of chloramphenicol and thiamphenicol analogs. Antimicrob Agents Chemother. 1980;18:311.
89 Adams PE, Varma KJ, Powers TE, et al. Tissue concentrations and pharmacokinetics of florfenicol in male veal calves given repeated doses. Am J Vet Res. 1987;48:1725.
90 de Craene BA, Deprez P, D’Haese E, et al. Pharmacokinetics of florfenicol in cerebrospinal fluid and plasma of calves. Antimicrob Agents Chemother. 1997;41:1991.
91 Lobell RD, Varma KJ, Johnson JC, et al. Pharmacokinetics of florfenicol following intravenous and intramuscular doses to cattle. J Vet Pharmacol Ther. 1994;17:253.
92 Guard CL, Schwark WS, Friedman DS, et al. Age-related alterations in trimethoprim-sulfadiazine disposition following oral or parenteral administration in calves. Can J Vet Res. 1986;50:342.
93 Hariharan H, Bryenton J, St Onge J, et al. Resistance to trimethoprim-sulfamethoxazole of Escherichia coli isolated from pigs and calves with diarrhea. Can Vet J. 1989;30:348.
94 Berg JN. Clinical indications for enrofloxacin in domestic animals and poultry. In: Quinolones: a symposium: a new class of antimicrobial agents for use in veterinary medicine. Lawrenceville, NJ: Veterinary Learning Systems; 1988:25.
95 Bauditz R. Results of clinical studies with Baytril in calves and pigs. Vet Med Rev. 1987;2:122.
96 Giles CJ, Grimshaw WT, Shanks DJ, et al. Efficacy of danofloxacin in the therapy of acute bacterial pneumonia in housed beef cattle. Vet Rec. 1991;128:296.
97 Lekeux P, Art T. Effect of enrofloxacin therapy on shipping fever pneumonia in feedlot cattle. Vet Rec. 1988;123:205.
98 Watts JL, Yancey RJJr., Salmon SA, et al. A 4-year survey of antimicrobial susceptibility trends for isolates from cattle with bovine respiratory disease in North America. J Clin Microbiol. 1994;32:725.
99 Burrows GE, Morton RJ, Fales WH. Microdilution antimicrobial susceptibilities of selected gram-negative veterinary bacterial isolates. J Vet Diagn Invest. 1993;5:541.
100 Post KW, Cole NA, Raleigh RH. In vitro antimicrobial susceptibility of Pasteurella haemolytica and Pasteurella multocida recovered from cattle with bovine respiratory disease complex. J Vet Diagn Invest. 1991;3:124.
101 Salmon SA, Watts JL, Yancey RJJr. In vitro activity of ceftiofur and its primary metabolite, desfuroylceftiofur, against organisms of veterinary importance. J Vet Diagn Invest. 1996;8:332.
102 Ewert KM. Food animal pharmacology III: Baytril. Proceedings of the 17th American College of Veterinary Internal Medicine Annual Convention, Chicago. 1999:259.
103 Simmons RD, Varma KJ, Johnson JC. Food Animal Pharmacology II: Nuflor. Proceedings of the 17th American College of Veterinary Internal Medicine Annual Convention, Chicago. 1999:256.
104 Hannan PC, Windsor GD, de Jong A, et al. Comparative susceptibilities of various animal-pathogenic mycoplasmas to fluoroquinolones. Antimicrob Agents Chemother. 1997;41:2037.
105 Ayling RD, Baker SE, Nicholas RA, et al. Comparison of in vitro activity of danofloxacin, florfenicol, oxytetracycline, spectinomycin and tilmicosin against Mycoplasma mycoides subspecies mycoides small colony type. Vet Rec. 2000;146:243.
106 Lechtenberg KF, Nagaraja TG, Chengappa MM. Antimicrobial susceptibility of Fusobacterium necrophorum isolated from bovine hepatic abscesses. Am J Vet Res. 1998;59:44.
107 Hirsh DC, Indiveri MC, Jang SS, et al. Changes in prevalence and susceptibility of obligate anaerobes in clinical veterinary practice. J Am Vet Med Assoc. 1985;186:1086.
108 Samitz EM, Jang SS, Hirsh DC. In vitro susceptibilities of selected obligate anaerobic bacteria obtained from bovine and equine sources to ceftiofur. J Vet Diagn Invest. 1996;8:121.
109 Jang SS, Hirsh DC. Broth-disk elution determination of antimicrobial susceptibility of selected anaerobes isolated from animals. J Vet Diagn Invest. 1991;3:82.
110 Yoshimura H, Kojima A, Ishimaru M. Antimicrobial susceptibility of Arcanobacterium pyogenes isolated from cattle and pigs. Zentralbl Veterinarmed B. 2000;47:139.
111 Nouws JF, van Ginneken CA, Hekman P, et al. Comparative plasma ampicillin levels and bioavailability of five parenteral ampicillin formulations in ruminant calves. Vet Q. 1982;4:62.
112 Brown SA, Chester ST, Robb EJ. Effects of age on the pharmacokinetics of single dose ceftiofur sodium administered intramuscularly or intravenously to cattle. J Vet Pharmacol Ther. 1996;19:32.
113 Burrows GE, Griffin DD, Pippin A, et al. A comparison of the various routes of administration of erythromycin in cattle. J Vet Pharmacol Ther. 1989;12:289.
114 Soback S, Paape MJ, Filep R, et al. Florfenicol pharmacokinetics in lactating cows after intravenous, intramuscular and intramammary administration. J Vet Pharmacol Ther. 1995;18:413.
115 Burrows GE, Barto PB, Martin B. Comparative pharmacokinetics of gentamicin, neomycin and oxytetracycline in newborn calves. J Vet Pharmacol Ther. 1987;10:54.
116 Nouws JF, van Ginneken CA, Ziv G. Age-dependent pharmacokinetics of oxytetracycline in ruminants. J Vet Pharmacol Ther. 1983;6:59.
117 Guard CL, Byman KW, Schwark WS. Effect of experimental synovitis on disposition of penicillin and oxytetracycline in neonatal calves. Cornell Vet. 1989;79:161.
118 Boxenbaum HG, Fellig J, Hanson LJ, et al. Pharmacokinetics of sulphadimethoxine in cattle. Res Vet Sci. 1977;23:24.
119 Shoaf SE, Schwark WS, Guard CL. Pharmacokinetics of sulfadiazine/trimethoprim in neonatal male calves: effect of age and penetration into cerebrospinal fluid. Am J Vet Res. 1989;50:396.
120 Nielsen P, Rasmussen F. Half-life, apparent volume of distribution and protein-binding for some sulphonamides in cows. Res Vet Sci. 1977;22:205.
121 Burrows GE, Barto PB, Martin B, et al. Comparative pharmacokinetics of antibiotics in newborn calves: chloramphenicol, lincomycin, and tylosin. Am J Vet Res. 1983;44:1053.
122 Haskins SC. Management of septic shock. J Am Vet Med Assoc. 1992;200:1915.
123 Constable PD, Schmall LM, Muir Wd, et al. Respiratory, renal, hematologic, and serum biochemical effects of hypertonic saline solution in endotoxemic calves. Am J Vet Res. 1991;52:990.
124 Bertone JJ, Shoemaker KE. Effect of hypertonic and isotonic saline solutions on plasma constituents of conscious horses. Am J Vet Res. 1992;53:1844.
125 Bottoms GD, Fessler JF, Roesel OF, et al. Endotoxin-induced hemodynamic changes in ponies: effects of flunixin meglumine. Am J Vet Res. 1981;42:1514.
126 Moore JN, Garner HE, Shapland JE, et al. Prevention of endotoxin-induced arterial hypoxemia and lactic acidosis with flunixin meglumine. Equine Vet J. 1981;13:95.
127 Semrad SD. Comparative efficacy of flunixin, ketoprofen, and ketorolac for treating endotoxemic neonatal calves. Am J Vet Res. 1993;54:1511.
128 Semrad SD, Sams RA, Ashcraft SM. Pharmacokinetics of and serum thromboxane suppression by flunixin meglumine in healthy foals during the first month of life. Am J Vet Res. 1993;54:2083.
129 Barton MH. Use of pentoxifylline for the treatment of equine endotoxemia. Proceedings of the 12th American College of Veterinary Internal Medicine Forum, San Francisco. 1994:740.
130 Geor RJ. Effects of pentoxifylline on blood flow properties in the horse. Proceedings of the 10th American College of Veterinary Internal Medicine Forum, San Diego. 1992:818.
131 Morris DD, Whitlock RH. Therapy of suspected septicemia in neonatal foals using plasma-containing antibodies to core lipopolysaccharide (LPS). J Vet Intern Med. 1987;1:175.
132 Zinkl JG, Madigan JE, Fridmann DM, et al. Haematological, bone marrow and clinical chemical changes in neonatal foals given canine recombinant granulocyte-colony stimulating factor. Equine Vet J. 1994;26:313.
133 Moore JN, Morris DD. Endotoxemia and septicemia in horses: experimental and clinical correlates. J Am Vet Med Assoc. 1992;200:1903.
134 Zaremba W, Guterbock WM, Holmberg CA. Efficacy of a dried colostrum powder in the prevention of disease in neonatal Holstein calves. J Dairy Sci. 1993;76:831.
135 Abel Francisco SF, Quigley JD. Serum immunoglobulin concentrations after feeding maternal colostrum or maternal colostrum plus colostral supplement to dairy calves. Am J Vet Res. 1993;54:1051.
136 Steel CM, Hunt AR, Adams PL, et al. Factors associated with prognosis for survival and athletic use in foals with septic arthritis: 93 cases (1987-1994). J Am Vet Med Assoc. 1999;215:973.