Anaerobic Infections

Anaerobes differ from other bacteria in that they do not require the presence of molecular oxygen for metabolic activity and growth but instead depend on fermentative processes. Strict anaerobes cannot tolerate the presence of oxygen, and few are clinically significant. Growth of obligate anaerobes depends on an environment characterized by reduced oxygen tension and low oxidation-reduction (redox) potential. Most clinically significant anaerobic organisms are obligate and include the genera Fusobacterium, Bacteroides, Clostridium, (selected species), Peptostreptococcus (enteric Streptococcus), and Peptococcus spp. Although oxygen-tolerant anaerobes cannot utilize oxygen, they can grow in its presence. Oxygen-tolerant organisms include Clostridium perfringens and Propionibacterium spp. Finally, facultative anaerobes are characterized by flexible oxygen requirements and can grow in the presence or absence of oxygen. Facultative anaerobes, which are clinically important in small animals, include Staphylococcus spp. and the enteric gram-negative bacilli such as E. coli and Klebsiella and Pasteurella spp.^211,²¹² Like aerobic organisms, anaerobic organisms are also classified by their gram-staining characteristics. Gram-positive cocci include Peptococcus and Peptostreptococcus. Gram-positive rods include those that form spores (Clostridium spp.) and those that do not (Actinomyces and Propionibacterium spp.). Gram–negative anaerobic rods include Bacteroides and Fusobacterium spp.

Results of studies investigating the incidence of anaerobic organisms as causative agents of infections in veterinary medicine have been relatively consistent. The most frequently isolated organisms in one study²¹³ were Bacteroides, Peptostreptococcus, Fusobacterium, and Porphyromonas spp. These organisms represented 70% of the isolates. Other previously reported clinically significant isolates include Clostridium, Propionibacterium, Actinomyces, and Peptococcus spp.²¹² Some older studies suggest that Clostridium spp. are more commonly isolated, but this finding may reflect inappropriate culturing techniques or regional differences in prevalence of selected genera.

Pathogenesis

Anaerobic infections are endogenous in origin because the causative organisms are most commonly members of the normal bacterial flora that occur in surrounding uninfected tissues. The normal bacterial flora of the body are predominantly anaerobic. Anaerobic organisms are particularly prevalent on mucous membrane surfaces, as exemplified by an anaerobic to aerobic ratio of 1000:1 in the large intestine (colon) and 10:1 in the oral cavity.¹⁹³ Anaerobic organisms, and particularly Bacteroides spp., are also prevalent in the female reproductive tract (vagina). Infections by anaerobic organisms usually require a break in the normal skin or mucosal defense barriers, thus allowing penetration and contamination, or a break in the host’s immune defenses. Thus certain areas of the body are more predisposed to the development of anaerobic infections. The most common sites of anaerobic infections reported in small animals are the oropharynx, skin (including bite wounds), respiratory tract, abdomen, reproductive tract, musculoskeletal system, and CNS. Anaerobic organisms are also causes of bacteremia.

Anaerobic bacteria, particularly gram-negative rods, are often considered serious pathogens because they are capable of producing toxins and enzymes, which enhance their pathogenicity. Both Bacteroides (especially B. fragilis) and Fusobacterium spp. produce potent toxins that not only cause tissue necrosis but also enhance the spread of infection. Bacteroides spp. may also produce collagenase. Clostridial organisms also produce a variety of toxic compounds that, in addition to tissue necrosis, may cause hemolysis, disseminated intravascular coagulation, and renal failure.²¹¹ Several organisms produce compounds that destroy leukocytes, thus debilitating a component of the host’s immune system.²¹¹

Diagnosis of anaerobic infections is often difficult. Bacteroides and Fusobacterium spp. are often difficult to visualize in Gram stains when in the presence of exudates and tissue debris. Gram-positive organisms may appear as gram-negative, or they may assume usual morphology in older exudates or after antibacterial therapy.²¹¹ Improper culturing techniques are probably the most common cause for failure to isolate all infective organisms.^211,²¹⁴

Anaerobic infections should be suspected when an infection is characterized by one or more of the following: close proximity to a mucous membrane; a putrid, foul-smelling exudate; necrotic or gangrenous tissue; gas; a blackish discoloration of tissues (which may fluoresce under ultraviolet light if caused by Bacteroides melaninogenicus); sulfur granules (indicating infection caused by a variety of different organisms, including Actinomyces spp.)185 a subacute onset of inflammation; and leukocytosis associated with a high fever.^211,²¹⁵ Anaerobic infections also should be suspected when cultures are negative despite observation of organisms with a Gram stain and in cases of endocarditis associated with negative blood cultures. Closed-space infections such as pyothorax; pyometra; and brain, lung, or intraabdominal abscesses are frequently caused by anaerobic infections. Other infections commonly caused by anaerobes include aspiration pneumonia, peritonitis associated with bowel contamination, chronic osteomyelitis associated with open fractures, bite wounds, penetrating foreign bodies, and solid tumors with a necrotic center.

Factors Affecting Selection of Antimicrobials

Factors that contribute to therapeutic (antimicrobial) failure for anaerobic infections include improper culturing techniques, mixed infections, and inactivation of the antimicrobial. Culture information important for the proper management of anaerobic infections. Failure to isolate anaerobic organisms caused by improper culturing techniques should be suspected whenever routine aerobic cultures of purulent exudate yield no growth.

Most clinically significant anaerobes are obligate and thus cannot survive exposure to oxygen for more than a few seconds. In addition, facultative bacteria generally grow faster than anaerobic organisms and thus may overgrow and mask anaerobic organisms, particularly if specimen culturing is delayed.²¹¹ The best specimens for anaerobic culture are aspirates or tissue biopsy specimens (1 cm²) rather than swabs.^216,²¹⁷ Suitable aspirates include those of infected body fluids (i.e., peritoneal, pericardial, pleural), aspirates of pus (from abscesses, deep wounds, or pyometra), tracheal or percutaneous lung aspirates, surgical tissue specimens (including samples from deep infections, bone biopsy material, and sequestra), and blood.^216,²¹⁷ All air or gas bubbles should be removed from both the needle and syringe immediately after sample collection.²¹⁵ Urine; swabs for the oropharynx, upper airway, external airway, and external reproductive tract; sputum or nasal exudates; and bronchoscopy brushings or aspirates (unless obtained with a double-lumen sleeve) are generally considered inappropriate. Culture samples should be placed in culture vials.²¹⁵ Specialized transport receptacles should be devoid of oxygen and contain anaerobic media that will also allow isolation of aerobic organisms. Specimens collected in plastic syringes should also be placed in transport tubes because oxygen can slowly permeate through the syringe. Specimens generally should be kept at room temperature until transported.

Many anaerobic infections are caused by more than one organism. Both anaerobic–anaerobic and anaerobic–aerobic mixed infections have been reported in small animals. A mean of 1.7 to 1.9 anaerobic organisms were isolated per specimen in several studies. Samples usually also contained facultative anaerobes.^214,²¹⁸ According to one study, 80% of infections with obligate anaerobes simultaneously contained facultative anaerobic or aerobic organisms, with members of the family Enterobacteriaceae being the most common organisms isolated.²¹³ These include E. coli, followed by Pasteurella spp. and S. intermedius. Mixed bacterial infections are often more virulent than infections caused by single organisms.²¹¹ Because synergistic mechanisms develop between facultative and anaerobic organisms, infection by multiple organisms may promote the overgrowth of commensal anaerobes such as Actinomyces spp.^208,^212,²¹⁹ Bacteroides spp. can inhibit phagocytic activity of surrounding white blood cells when present in mixed infections. Commensal bacteria may act as symbiotes by producing growth factors required by pathogenic anaerobic organisms. Facultative organisms may also provide a more favorable environment for anaerobes by removing oxygen and adding reducing substances.²¹² The number of microorganisms located within an abscess may also have an effect on drug efficacy because antimicrobial inactivation is more likely.

The environmental conditions surrounding an anaerobic infection can be detrimental to the activity of antimicrobial drugs. These effects, discussed in Chapter 6, are very likely to be marked in some anaerobic infections. The inflammatory exudate that usually accompanies anaerobic infections can have profound effects on drug efficacy. Cellular membranes, breakdown products of phagocytic cells, and inflamed tissues are all capable of binding to and reducing the effective concentration of pharmacologically active antimicrobial drugs.¹⁵⁵ Host tissues may also produce local enzymes that destroy antimicrobials.¹⁵⁵

The anaerobic environment that characterizes anaerobic infections can also profoundly alter the efficacy of antimicrobials. The oxidation-reduction potential (referred to as EH), which measures the anaerobiosis associated with an abscess, is estimated to be approximately −400 mV in human abscesses, which is indicative of an environment free of oxygen. The lack of oxygen has profound effects on two activities important to the success of antimicrobial therapy. The first is its effect on white blood cells. To kill selective organisms, white blood cells must be able to initiate oxidative bursts, an activity that is very difficult to accomplish in the anaerobic environment.^220,²²¹ In addition, white blood cell chemotaxis in response to bacterial factors is reduced in anaerobic environments.²¹⁵ The second detrimental effect of an anaerobic environment relates to the mechanism of transport or action of the drugs. The efficacy of aminoglycosides and the combination of trimethoprim–sulfamethoxazole are particularly affected. Aminoglycosides require active transport into cells by mechanisms dependent on oxygen. In addition, the mechanism of action of these antimicrobials depends, in part, on cellular respiration processes that utilize oxygen. The lack of oxygen renders these antimicrobials ineffective; all anaerobic organisms are thus resistant to aminoglycosides. The aerobic component of a mixed infection may also be resistant to aminoglycoside therapy because the oxidative transport systems of such organisms (e.g., E. coli) may shut down in an anaerobic environment.

The development of resistance by anaerobic organisms to selected antimicrobials is an important cause of therapeutic failure. As with other bacteria, plasmid-mediated, transferable drug resistance and the inability of drugs to penetrate bacterial cells are important mechanisms by which anaerobic organisms develop resistance to antimicrobials.²¹⁵ Several studies have concentrated on the development of resistance to beta-lactam antibiotics. Anaerobic organisms, and particularly B. fragilis, develop resistance to these drugs primarily by blocking all penetration by the drug or by producing beta-lactamase enzymes that inactivate the drug. B. fragilis is particularly adept at developing resistance because of the production of beta-lactamases (penicillinases or cephalosporinases), enzymes that cleave the beta-lactam ring, thus effectively destroying antimicrobial activity. Most strains of B. fragilis produce a chromosomally mediated beta-lactamase, which inactivates many cephalosporins, particularly first-generation drugs. In addition to this cephalosporinase, many strains of B. fragilis can acquire novel beta-lactamases that are characterized by greater impact on the penicillins than the cephalosporins. One study found that some strains of B. fragilis could produce beta-lactamases that are capable of inactivating cefoxitin and imipenem, two drugs that historically have been effective for the treatment of anaerobic infections.²²¹ Several of these strains are, however, capable of producing massive quantities of the cephalosporinase. Although this enzyme does not have much specific activity against cefoxitin, the drug is inactivated because of the vast quantities produced. Cefoxitin resistance of this nature can be transferred to other strains of Bacteroides spp. A study that investigated the emerging resistance patterns of B. fragilis to various antimicrobial agents over a 3-year period found that resistance to cefoxitin, originally the most active drug against this species, doubled in 2 years.221

Bacteroides spp. also possess a sophisticated system of resistance. In the aforementioned 3-year study, resistance to clindamycin did not increase, and no organisms were resistant to metronidazole or chloramphenicol. Although tetracyclines were the drug of choice for infections caused by B. fragilis in the early 1950s, almost 66% of the organisms were resistant to the drug. Variation in anaerobic (e.g., B. fragilis) resistance patterns have been described for different geographic regions. In addition, various patterns have been described according to the site of tissue or sample collection. One study found that organisms isolated from the blood were more resistant to piperacillin, cefoxitin, and clindamycin than were the same isolates obtained from the abdominal cavity. Experimental studies have shown that resistance genes can be naturally transferred from B. fragilis to E. coli. This finding may have profound implications for the treatment of mixed anaerobic–aerobic infections, particularly those originating from colonic bacteria. One study²¹³ has documented a 29% incidence of resistance of Bacteroides spp. to ampicillin; in contrast, 100% were susceptible to amoxicillin, clavulanic acid, and chloramphenicol, and most were susceptible to metronidazole. Only 83% were susceptible to clindamycin.

Clostridium difficile has been associated with hospital outbreaks of illness in hospital environments (based on the author’s personal experience).²²² Its ability to form spores to minimize exposure to oxygen renders it resistant to most environmental disinfectants, with the exception of chlorine bleach. Animals receiving anticancer or antibacterial chemotherapeutic agents are at a risk of developing infection when exposed to environmental clostridial spores. Treatment in such situations generally is accomplished with metronidazole until clinical signs resolved; feces should be negative for enzyme-linked immunosorbent assay (ELISA) toxin (1 to 15 days).

Antimicrobial Drugs

The successful treatment of an anaerobic infection depends on altering the local environment in a manner designed to reduce bacterial proliferation and checking the spread of infection into adjacent tissues. The first goal is achieved by surgical débridement of dead tissue. At the time of surgery, pockets of pus should be drained, trapped gas released, and any obstructions to drainage eliminated. Surgery should also improve circulation to the site of infection, which will improve oxygenation of tissues. Local spread of infection is managed by administration of an antimicrobial. Although the number of antimicrobials that can be used to treat anaerobic infections is limited, there are several that are effective and safe. Resistance does not yet seem to have impaired empirical selection of anaerobes (Table 8-12), although caution is still indicated in patients that previously received antimicrobials or those at risk for therapeutic failure for other reasons.

Table 8-12 Susceptibility Data for Selected Anaerobic Pathogens Collected from Antimicrobial-Free Dogs and Cats⁷⁶

Beta-Lactam Antibiotics

Penicillin G, a natural antibiotic, is the prototype penicillin. The anaerobic susceptibility of the semisynthetic aminopenicillins ampicillin and amoxicillin is similar to that of penicillin G. In contrast to penicillin G, they are effective after oral administration. Penicillins are the drug of choice for clostridial infections and for Peptostreptococcus spp. They are generally effective against Fusobacterium and Actinomyces spp. Although the penicillins are effective against many Bacteroides spp., several species, including B. melaninogenicus and B. fragilis, both of which are clinically important, are uniformly resistant to most penicillins,²¹¹ with the exception of piperacillin.²²⁰ A 33% rate of resistance by Bacteroides spp. to penicillin G has been reported in veterinary patients. A large percentage of these strains was also resistant to ampicillin and cephalothin.²¹⁴ Jang and colleagues²¹³reported that 29% of Bacteroides isolates in veterinary medicine are resistant to ampicillin. The efficacy of penicillin (and cephalosporin) antibiotics against B. fragilis can be improved by combining the antibiotic with clavulanic acid or sulbactam. Indeed, Jang and colleagues²¹³ reported all Bacteroides isolates to be susceptible to amoxicillin–clavulanic acid. Both of these drugs have a greater affinity for beta-lactamases than do the beta-lactam antibiotics and preferentially bind to and inactivate the bacterial enzyme. The efficacy of amoxicillin against B. fragilis is greatly enhanced when combined with clavulanic acid.²²³ The addition of clavulanic acid also enhances the efficacy of amoxicillin against several gram-negative enteric organisms, which may be important factors in the pathogenesis of mixed infections. Jang and coworkers²¹³ found 100% of clostridial organisms to be susceptible to ampicillin, compared with 80% of isolates that were susceptible to clindamycin.

The cephalosporins are very similar to the penicillins in pharmacokinetic characteristics but as a class are less efficacious against anaerobes. First-generation cephalosporins (e.g., cephalothin, cefazolin, and cefadroxil) may be effective against many anaerobic organisms, with the exception of B. fragilis; use should be based on susceptibility testing. The second-generation cephalosporin cefoxitin is, however, one of the most effective antibiotics against anaerobic infections, including B. fragilis, although resistance is increasing, as was previously noted. As a second-generation cephalosporin, cefoxitin is also effective against many gram-negative enteric organisms, which enhances its utility for the treatment of mixed infections. Many Clostridium spp. are resistant to most cephalosporins; thus alternative antimicrobials should be considered for infections caused by these organisms. Currently, piperacillin and cefoxitin remain the most active beta-lactam antibiotics for the treatment of human anaerobic infections. Because of the variable nature of susceptibility patterns and the unique mechanisms and characteristics of beta-lactams exhibited by anaerobic bacteria, however, culture and susceptibility monitoring of these pathogens in crucial. The carbapenems are the newest class of beta-lactam antibiotics and are among the the most effective beta-lactam antibiotics against anaerobic infections.

Other Antimicrobials

Chloramphenicol is one of the most effective antimicrobial drugs against all strains of anaerobic bacteria, including penicillin-resistant B. fragilis.²¹³ Its utility, however, is decreased by its bacteriostatic nature as well as by its tendency to cause adverse reactions, particularly in the cat, and concerns regarding human exposure. Clindamycin is a lincosamide antimicrobial whose mechanism of action is similar to that of chloramphenicol. Like chloramphenicol, it is bacteriostatic against anaerobic organisms. Clindamycin is more efficacious for the treatment of anaerobic infections than its parent drug, lincomycin. Clindamycin is also concentrated in white blood cells at the site of infection, which is considered to be an important factor in the efficacy of this drug. This is an active energy process, however, which may not occur in the oxygen-deficient environments that characterize anaerobic infections. The spectrum of activity of clindamycin includes anaerobic organisms. It is generally very effective against most strains (83% cited by Jang et al.²¹³) of B. fragilis, including those resistant to penicillin, and is very effective against veterinary isolates of Clostridium. Clindamycin is not, however, effective against C. difficile, and its use may be associated with pseudomembranitis or other gastrointestinal disorders related to microbial overgrowth. Although fluoroquinolones are generally not considered sufficiently broad in spectrum to include anaerobes, newer-third generation drugs may be very effective. An example is pradofloxacin, whose efficacy includes anaerobes associated with gingival disease in dogs and cats.²⁶⁷

Selected Anaerobic Infections

Clostridum difficile

C. difficile is reaching epidemic proportions in human patients. Antimicrobial exposure results in suppression of normal flora and is a major risk factor with clindamycin, penicillins, and cephalosporins. A more recent study in humans found fluoroquinolones to be the primary risk factors for the emergence of a clonal-based outbreak in a hospital.²²⁴ Transmission is fecal to oral through contaminated environment and hands of health care personnel in human patients (as with MRSA), and a new strain with increased virulence has emerged. Outbreaks have increased in the hospital setting, although community-acquired infections are increasing in Europe.²²⁵ Outbreaks have been reported in veterinary hospitals as well.²²² Clinical signs range from mild diarrhea to pseudomembranous colitis and toxic (and potentially fatal) megacolon. Toxin binds to intestinal cells and disrupts tight junctions, leading to inflammation and watery diarrhea, the hallmark of infection. Toxins are downregulated by the tcdC gene; deletions of this gene leads to clinical signs. A binary toxin also has been described, although its role in human disease is not well defined. A seasonal pattern has been described for selected human hospitals.²²⁶ Epidemiologic description in humans is complicated by diagnosis based on the presence of toxin rather than organism culture. Resistance to newer 8-methoxy fluoroquinolones used to treat the organism (e.g., gatifloxacin and moxifloxicin) has already emerged.²²⁷

C. difficile may massively increase its exotoxin production in the gastrointestinal tract in the presence of subinhibitory plasma clindamycin concentrations because of increased bacterial synthesis of exotoxin.²²⁸ The use of metronidazole, a popular antigiardial drug, for the treatment of anaerobic infections in dogs and cats has markedly increased. The drug is consistently effective against most anaerobes, including B. fragilis as well as most other strains of Bacteroides. Metronidazole is also effective for treatment of human cases of C. difficile. It is generally not effective against Actinomyces or Propionibacterium. Its mode of action probably results from disruption of bacterial DNA after bacterial metabolism to toxic metabolites. The low oxidation-reduction potential of the anaerobic environment is conducive to bacterial formation of toxic metabolites.

Vancomycin is the drug of choice for humans for the treatment of antibiotic-associated colitis caused by C. difficile. Its mechanism of action results from inhibition of bacterial cell wall synthesis. Thus its actions are primarily bactericidal. Its spectrum includes Clostridium, gram-positive bacilli (Bacillus, Actinomyces), and Propionibacterium.

The sulfonamides can be effective against many anaerobic organisms. Caution should be taken, however, because of the patterns of resistance that have been noted for veterinary anaerobic isolates. For serious or difficult anaerobic infections, sulfonamide use should be based on susceptibility data.

Clostridum tetani

Tetanus is caused by the neurotoxin tetanospasmin produced by vegetative Clostridum tetani, a motile, gram-positive spore-forming anaerobe.²²⁹ Spores contaminating an anaerobic site (e.g., in the presence of a foreign body or tissue necrosis) vegetate. The toxin, tetanospasmin, enters axons of local motor neurons, and by retrograde movement reaches the axonal body in the spinal cord. Eventually the toxin reaches the brain, where it acts to cleave the protein responsible for fusion of the gamma-aminobutyric acid–containing synaptic vesicle with the neuronal cell membrane.²³⁰ As such, it inhibits the release of inhibitory neurotransmitters gamma-aminobutyric acid as well as glycine, resulting in presynaptic blockade for selected synapses of the spinal cord (predominant site) and brain. Dogs and cats are considered fairly resistant to tetanus and therefore are not generally vaccinated; indeed, vaccination is not protective in humans, necessitating revaccination with exposure.²³¹ Clinical signs generally occur within 5 to 10 days of wound acquisition, although delays of up to 3 weeks have been reported.²³¹ Wounds closer to the head are associated with more rapid onset of generalized tetanus. Relative resistance of carnivores to the toxin may result in signs localized to the wound area, manifested as stiffness that usually spreads to the opposite limb or beyond. Generalized tetanus is associated with a guarded prognosis and is characterized as extreme muscle rigidity, manifested as a stiff gait, difficulty in standing or lying down, and hyperthermia. Intracranial signs accompany localized tetanus, resulting in hypertonic facial muscles (e.g., trismus or lockjaw) and reflex muscle spasms. Animals have difficulty with prehension or swallowing solid food. Dysuria, urine retention, constipation, and gas distention are not unusual. Autonomic clinical signs may not appear until several days after rigidity has occurred. Signs include ptyalism, tachycardia, tachypnea, and hypertension.²³¹ Death, when it occurs, usually results from respiratory compromise.²²⁹ Untreated cases can prove fatal, although natural resistance often limits the disease to localized or mild cases.

Treatment focuses on antimicrobial therapy in an attempt to decrease formation of neurotoxin and neutralization with antitoxin of any toxin not yet bound to nerve tissue. The intravenous route is preferred to the intramuscular or subcutaneous routes (the latter may require 48 to 72 hours for therapeutic concentrations to be reached). Antitoxin (100 to 1000 U/kg, <20,000 U total) should be administered every 5 to 10 minutes; the dose should be decreased for larger dogs. Pretreatment of anaphylaxis is appropriate; administration of a test dose (0.1 to 0.2 mL) subcutaneously or intradermally may help identify those at risk (formation of a wheal at the site). Localized injection at the wound site (1000 U) may be helpful; likewise, intrathecal (I–10 U) injection has been shown experimentally to reduce morbidity and mortality rates in dogs, probably because of better access to neurotoxin bound to nervous tissue.In humans 500 U is injected proximal to the wound. Recovery is slow and progressive, even when antitoxin is administered. Recovery at sites were tetanospasmin is bound requires formation of new receptors, which takes approximately 3 weeks.²³¹

Local and systemic antibiotic therapy should be used to eradicate vegetative clostridial organisms, thus reducing the amount of toxin released. Metronidazole is superior to penicillin G and tetracyclines. Penicillins inhibit type A gamma-aminobutyric receptors, which is the target of tetanus toxin. Studies comparing the time to recovery and mortality of penicillin with those of metronidazole for treatment of tetanus in humans are controversial, with studies ranging from improved outcome for metronidazole to no differences between treatments.²³⁰ The studies are complicated by differences in the form of penicillin studied, limiting conclusions. However, because of the possible impact of penicillin, metronidazole might be preferred; at the very least, procaine penicillin probably should be avoided because it may exacerbate CNS symptoms.²³⁰

Animals should be removed from stimulating environments. Supportive therapy for tetanus includes control of autonomic signs, reflex spasms, or convulsions. Anticholinergics may be indicated to treat bradyarrhythmias. Phenothiazines (chlorpromazine preferred) combined with barbiturates (pentobarbital or phenobarbital) have been recommended as the preferred treatment.²²⁹ Caution with the use of phenothiazines is recommended for patients with seizures; phenothiazines lower seizure threshold and can worsen seizures associated with other disorders. However, use for treatment of muscle rigidity may preclude concerns regarding seizures. Further, a retrospective study of acepromazine use in dogs experiencing seizures found no increase in seizure activity associated with acepromazine. However, acepromazine may be sufficiently different from other phenothiazines to warrant caution in extrapolating information to phenothiazines as a class.²³² Phenobarbital should be administered as a loading dose (12 mg/kg to achieve the lower end of the recommended range for epilepsy). Glycopyrrolate should be administered to control bradycardia that may occur with the combined use of these drugs. Benzodiazepines (e.g., diazepam) may be useful for controlling spasms and hyperexcitability; methocarbamol is a less effective centrally acting skeletal muscle relaxant. Methocarbamol, dantrolene (a peripherally acting skeletal muscle relaxant), or gamma-aminobutyric acid antagonist may be useful, including in combination, for the control of spasticity. Tetanus-induced respiratory compromise requires intubation and respiratory support, which may require sedation, paralytic agents, or both. Linnenbrink²³¹ discussed the limited use of supraphysiologic concentrations (2 to 4 mmol/L) of magnesium for control of autonomic dysfunction in human patients. Magnesium decreased rigidity and imparted a dose-sparing effect on sedatives. Hypocalcemia occured but generally did not need correcting. The loss of tendon reflexes was monitored as an indication of toxicity. Successful management of tetanus in two dogs has been reported.²³³ Therapy included a combination of general anesthestics (pentobarbital, propofol), anticonvulsants (diazepam, including by constant-rate infusion, phenobarbital), and muscle relaxants (diazepam, methocarbamol), including phenothiazine sedatives (for muscle rigidity). Supportive therapy was intensive and crucial to therapeutic success, with full recovery requiring several weeks.

Clostridium botulinum

Botulism in dogs is almost exclusively caused by type C Clostridium botulinum organisms that release the neurotoxin in a vegetative state.¹¹⁰ Although released in the inert form, cleavage by tissue or bacterial proteinases generates the dichain metalloproteinase neurotoxin, which is similar to tetanus toxin. Generally, clinical signs result from ingestion of the preformed toxin, in part because adult animals generally are resistant to colonization by the organism, although antimicrobial therapy can facilitate colonization. The toxin is absorbed by intestinal lymphatics as with nutrient proteins; toxin complexed with other proteins appears to protect the toxin from intestinal degradation. At the nerve terminal, the toxin prevents the presynaptic release of acetylcholine by irreversibly binding. The toxin is characterized by a very high affinity for the presynaptic membrane receptors, making it one of the most potent toxins. At this point the toxin is susceptible to inactivation by antitoxin. However, once receptor-mediated endocytosis has moved the toxin into the cell, antitoxin activation can no longer occur. Of the two chains, the L chain is more likely to inhibit acetylcholine release. Clinical sequelae, evident within hours but delayed for up to 6 days, include paralysis and degeneration of the affected synapse with subsequent lower motor neuron and parasympathetic dysfunction. Death generally does not occur unless paralysis extends to the respiratory tract. Recovery follows formation of new terminal axons and neuromuscular junctions with cranial nerve, neck, and forelimb functions generally returning to normal first.

Therapy focuses on supportive care; spontaneous recovery generally will occur in all animals if respiratory function is not affected. Antitoxin is ineffective if nerve endings have been penetrated, which generally occurs rapidly; however, treatment may be beneficial if oral absorption of the toxin is ongoing. Type C antitoxin (10,000 to 15,000 U, intravenously or intramuscularly) should be administered twice, with each dose 4 hours apart. The antitoxin remains in the system for 40 days, mitigating the need for follow-up therapy. Penicillin or metronidazole can be administered to reduce the intestinal growth of C. botulinim, although the efficacy of this practice is questionable and may be contraindicated if organism death is thought to increase toxin release. Neuromuscular potentiators (aminopyridine, diaminopyridine, guanidine hydrochloride) have not proved effective.

Clostridium perfringens

C. perfringens capable of producing an enterotoxin can cause severe diarrhea.²³⁴ Binding of the toxin to intestinal epithelial cells increases permeability, causing fluid and ion secretion, cell death, and sloughing of the intestinal mucosa. Sporulation leading to enterointoxication can occur after antimicrobial therapy, increased intestinal pH, dietary alteration, and immunosuppression; it also is associated with viral enteritis. Enterotoxemia can cause rapid death. Very occasionally, clinical signs may occur after ingestion of contaminated meat or as a result of nosocomial infections. Therapy includes antimicrobials and intensive fluid therapy. Antimicrobials that are variably effective include metronidazole, ampicillin, amoxicillin–clavulanic acid, tylosin, clindamycin, and tetracyclines.

Miscellaneous Infections

L-Form Bacterial Infection

L-form bacteria either have a deficient cell wall or lack one altogether. Their role in causing infections in animals is not well understood. A number of organisms are capable of becoming cell wall deficient, including Staphylococcus and Streptococcus species. Information regarding the causes of an organism assuming this structural state are not clear, nor is the role of L-forms in the cause of disease. Presumably, the infection starts in the skin and progresses as multiple abscesses form and then dehisce. Bacteremia can lead to polyarthritis. Because of their cell wall–deficient state, microorganisms are difficult to detect by light microscopy but can be visualized inside phagocytes by electron microscopy. Drugs whose antibacterial effects do not target the cell wall are indicated. Tetracyclines have been the drug of choice.²³⁵

Hemobartonellosis (Hemotropic Mycoplasmosis)

The causative agent of feline infectious anemia was recently reclassified as Mycoplasma on the basis of RNA sequence analysis, leading to a change in terminology to hemotrophic mycoplasmosis. Mycoplasma are gram-negative organisms that lack a cell wall. At least two variants have been identified in the cat, with the large form, Ohio strain (Mycoplasma haemofelis) being more pathogenic compared with the small form, California strain (Candidatus Mycoplasma haemominutum); the latter apparently has not been associated with disease.²³⁶ Mycoplasma haemocanis has been associated with anemia in splenectomized dogs, although the distinction between M. haemofelis and M. haemocanis is not clear.

Experimentally, inoculation is followed by a period of 2 to 34 days before clinical signs and anemia associated with parasitemia appear; this second period persists from 18 to 30 days. Mortality rates are highest during this phase, with hematocrit levels returning to near normal in surviving cats. Recovered cats remain carriers for years and may experience recrudescence of clinical signs. Anemia reflects predominantly extravasular rather than intravascular hemolysis. Inclusions and other changes in the surface of the red blood cell result in osmotic fragility, reduced deformability, and subsequent removal from the bloodstream by the spleen. Afflicted animals may have lowered resistance to concurrent infections; an association with feline leukemia virus (but apparently not feline immunodeficiency virus) may worsen the pathopyhysiology of either syndrome.²³⁶

Doxycycline is the treatment of choice (5 mg/kg orally every 24 hours for 21 days) but will not necessarily eradicate the infection. Care should be taken to either administer a liquid preparation or follow capsules with a liquid wash to prevent esophageal damage. Experimentally, azithromycin was ineffective at 15 mg/kg orally twice daily for 7 days for treating large-form mycoplasma in cats.²³⁷ Cats remained PCR positive and anemic after treatment. In contrast to azithromycin, fluoroquinolones may be effective for treatment. Dowers²³⁸ prospectively compared 2 weeks of treatment with enrofloxacin at a low (5 mg/kg po qd) and high dose (10 mg/kg po qd) to doxycycline (5 mg/kg po bid) in cats (n = 16, 4 per group, including a no treatment group) experimentally infected with large form H. felis. All treatment cats responded. The high dose enrofloxacin group had fewer days of anemia compared to the other groups; 1 doxycycline and 2 high-dose enrofloxacin cats cleared of organisms based on PCR and remained disease free for at least 6 months. Ishak et al.²⁶⁸ studied the efficacy of marbofloxacin (2.75 mg/kg daily for 2 weeks beginning 16 days after infection) for treatment of experimentally induced M. haemofelis infection in cats (n = 12; including 6 untreated control). Marbofloxacin was associated with more rapid hematologic response. However, PCR-based detection of organisms (viral numbers not quantified) did not differ from that of the nontreated group, indicating that infection was not consistently eradicated. Nonetheless, the authors concluded that marbofloxacin was a reasonable treatment option. Tasker et al.²⁶⁹ also studied the efficacy of marbofloxacin (2 mg/kg orally daily for 4 weeks) for treatment of M. haemofelis in experimentally infected cats with or without chronic feline immunodeficiency virus (n = 6/group, 3 cats from each group treated). Organism counts were reduced and animals improved, but animals were not cleared.

Dowers et al.²³⁹ also compared the efficacy of pradofloxacin at a low (5 mg/kg orally qd) and high dose (10 mg/kg orally qd) to doxycycline (5 mg/kg orally bid) in cats (n = 23) experimentally infected with M. haemofelis. Therapy was initiated when cats became positive on PCR and continued for 6 weeks. However, cats that became PCR negative by day 42 (the last day of treatment) were also treated with methylprednisolone sodium acetate (20 mg/kg intramuscularly) in an attempt to cause recrudescence after immunosuppresion. The number of days of anemia did not differ among treatment groups; all were less than the control group. Organism copy numbers were significantly lower in the low-dose group compared with the doxycycline group for 4 of the 5 posttreatment weeks. Four of 6 cats in the high-dose and 2 of 6 cats in the low-dose pradofloxacin group tested negative (cPCR) on day 42 of treatment and were immunosuppressed. None of the doxycycline cats yielded negative PCR results. Immunosuppression yielded transiently positive results in 5 of the cats; 1 cat became persistently negative. By day 28 of glucocorticoid therapy, however, 4 of the 6 transiently positive cats (3 low doses and 1 high dose) became negative. These studies consistently indicated that, under experimental conditions, fluoroquinolones may play a role in the treatment and possible clearing of M. haemofelis and are reasonable alternatives when doxycycline fails or cannot be used. Lappin²⁴⁰ prospectively evaluated the efficacy of imidocarb (5 mg/kg intramuscularly; repeated in 2 weeks) for treatment of experimentally induced (11 cats) chronic haemobartonellosis. Both the large form (M. haemofelis; n = 4; 3 treated, 1 control), and small form (M. hemominutum; n = 3, 2 treated, 1 control) were studied. Three cats were infected with both organisms. Side effects were limited to irritation at the injection site. Both control cats remained PCR positive throughout the study. One cat inoculated with both forms and one cat inoculated with M. haemofelis became persistently negative at weeks 4 and 6 after treatment (the remaining cats became negative at week 2 but positive at weeks 4 and 6) and were subsequently treated with methylprednioslone. One cat died as a result of undetermined causes, and the second became PCR positive. Imidocarb does not appear to be a reasonable alternative for clearing chronically infected cats.

KEY POINT 8-37

Doxycycline remains the drug of choice for treatment and eradication of feline mycoplasma, with high-dose fluoroquinolones a potential treatment alternative.

The concurrent use of immunosuppressive doses of glucocorticoids is controversial. Glucocorticoids may increase the number of microogranisms in the blood and may exacerbate accompanying concurrent infections.²³⁶

Bartonellosis and Coxiella

Bartonella is an aerobic, gram-negative, intracellular organism, probably transmitted by ticks. It targets cells of the immune system, living in endothelial and red cells. The organism is associated with a reduction in CD8+ cells and the expression of adhesion molecules. The association with Bartonella and IE has been described.²⁴¹ All seropositive dogs were also seropositive for Anaplasma phagocytophilum. Its association with valvular disease in people results in a poor prognosis. Treatment should be at least 2 weeks in duration. Potentially effective drugs include doxycycline.

Brucellosis

Pathogenesis

An infectious disease, brucellosis draws attention because of its insidious nature, difficulty to treat, and zoonotic potential. Brucella canis, the causative agent of brucellosis in dogs, is a small gram-negative coccobacillus. Among Brucella, B. canis stands out in morphology, biochemistry, and immunology. Among its differences compared with other organisms is its zoonotic potential: The disease can be transmitted only between members of Canidae. Cats are only transiently infected after experimental infection. Dogs are susceptible to infection with Brucella abortus but do not appear to be important to the spread of infection. Infection by Brucella involves penetration of mucous membranes after contact with contaminated fluids from an infected urogenital tract or an aborted fetus. Phagocytized organisms are transported to lymphatic and genital tissues, where multiplication occurs. Leukocyte-associated bacteremia can persist for years. Organisms can be intermittently shed from infected animals (during estrus, breeding, or abortion).²⁴²

Although antibodies are generated against Brucella species, as with most intracellular organisms, cell-mediated immunity is the primary mechanism of host defense. Cell-mediated immunity can provide protection against reinfection, although persistent infection may be necessary to provide persistent protection. Immune response to inflamed infected tissues (e.g., spermatozoa) contributes to infertility. Immunosuppressive drugs appear to increase the risk of infection but may not alter the course of the disease in dogs already infected. Spontaneous recovery can occur but may take up to 5 years. During this period infection might be associated with bacteremia. As with other blood-borne organisms, Brucella may localize in tissues other than the urogenital tract. Discospondylitis; anterior uveitis; and, less commonly, glomerulopathy or meningoencephalitis may result. Diagnosis is based on agglutination tests, agar-gel immunodiffusion, or ELISA.²⁴² Therapy with antibiotics does alter the progression of the disease.

Antimicrobial and Adjuvant Therapy

A number of antimicrobial drugs are effective in vitro against Brucella but may not be able to eradicate infection. Antimicrobial therapy can relieve symptoms, shorten the duration of illness, and reduce the likelihood of complications yet not eradicate the organism.²⁴³ Success will be enhanced by a combination of effective drugs at high doses for at least 4 weeks. Drugs should be able to penetrate cell membranes; accumulation in phagocytic cells should further facilitate success. Among effective antimicrobials, lipid-soluble tetracyclines (minocycline and doxycycline) are particularly efficacious when used at a high dose and in combination with an aminoglycoside. The World Health Organization has also recommended doxycycline in combination with rifampin, although doxycycline with streptomycin was found to be more effective in the treatment of discospondylitis.²⁴³ Fluoroquinolones should also be effective when combined with aminoglycosides,²⁴³ although clinical trials documenting efficacy in dogs have not been reported. An added advantage of these drugs is intracellular accumulation. Fluoroquinolones may not, however, be effective as sole agents against brucellosis. Human patients intolerant of tetracyclines also responded well to combinations of trimethoprim–sulfonamides and aminoglycosides. Antimicrobial therapy should not be discontinued too early; sequential titers can be used to assess response to therapy.

Infected dogs should be neutered as soon as possible. Glucocorticoids or other immunosuppressive therapy may be indicated for treatment of infections associated with life-threatening or organ-threatening inflammation (e.g., CNS). Currently, prevention is best implemented through good hygiene practices, neutering of infected animals, and vaccination of cattle.

Leptospirosis

The genus Leptospira belongs to the family Treponemataceae, which contains all spirochetes, including Borrelia species. Leptospirosis is primarily a disease of wild and domestic animals; humans are infected only occasionally.²⁴⁴ However, it is a reemerging disease for reasons that are not yet known.²⁴⁵

Pathogenesis

Leptospirosis generally causes an acute generalized infection or interstitial nephritis. Leptospires are motile spirochetes; Leptospira interrogans tends to be the pathogenic organism in animals. At least three antigenically distinct serovars cause disease in dogs; infection in cats is rare.²³⁴ Infection occurs through the mucosa or abraded skin and can occur between animals or in animals that come in contact with the contaminated environment. Once the mucosa is penetrated, the organism multiplies rapidly in the bloodstream. Inflammation causes parenchymal damage, particularly in the liver and kidney. Persistence in renal tubular cells leads to colonization of the kidney.²⁴⁴ Chronic active hepatitis can be a sequela of infection in dogs.

Leptospirosis can present as a peracute infection manifested by shock and death. Less acute presentation involves fever, anorexia, vomiting, and dehydration. Clinical laboratory tests may reveal thrombocytopenia, electrolyte alterations, and liver damage. Diagnosis is based on serologic testing (paired serum titers). Leptospires are very difficult to recover, with urine analysis by dark field microscopy providing the best chance of discovery.

Antimicrobial Therapy

Treatment focuses on eradication of the causative organism and supportive therapy. Acute renal failure may require intensive management. Antibiotics should inhibit the multiplication of the organism as well as eradicate it. Response manifested as a decrease in fever should be apparent within several hours of administration.²³⁴ Drugs shown to be effective in vitro include most penicillins, third-generation cephalosporins, and tetracyclines but not first- and second-generation cephalosporins. Not all penicillins appear to be able to eradicate the organisms, but high doses of penicillin G, ampicillin, and amoxicillin are among the more effective choices in clearing the urine.^244,²⁴⁵ Procaine penicillin G (40,000 units/kg intramuscularly or subcutaneously every 24 hours or a divided dose given every 12 hours) is the antibiotic of choice for leptospiremia. It may be necessary to decrease the dose if the animal is in renal failure (e.g., dividing the dose by the serum creatinine), although the real risk of side effects in the presence of renal disease is not clear. Penicillin clears infection with leptospirosis if continued for a sufficient period of time. An injectable form of penicillin should be used for 14 days or until azotemia resolves; an oral form (amoxicillin [22 mg/kg every 8 hours]) can then be initiated. Intravenous ampicillin or amoxicillin (22 mg/kg every 6 to 8 hours) have also been used successfully during initial therapy. Doxycycline can also be used for initial therapy and to clear leptospiremia.^234,²⁴⁵ Initial penicillin therapy of 2 weeks’ duration can also be followed by doxycycline (5 mg/kg twice daily) for 6 to 8 weeks to achieve elimination of the carrier state.

KEY POINT 8-38

Procaine penicillin is the antibiotic of choice for leptospiremia and may clear the infection if administered for a sufficient length of time.

In human patients doxycycline appears to be beneficial for the prevention of infection even after exposure, whereas the penicillins do not appear to prevent infection after exposure (despite their ability to eradicate infection).²⁴⁴ Clearing tissues may be more difficult than clearing urine; doxycycline for 2 weeks appears to be the most frequently recommended for clearing the carrier state in dogs.²⁴⁵ Fluoroquinolones (ciprofloxacin and enrofloxacin) also appear to be effective against leptospires, although clinical studies supporting their use are lacking. Hamsters experimentally infected with L. interrogans responded to ciprofloxacin.²⁴⁶ In vitro studies of five serovars of Leptospira indicated efficacy on the basis of MICs (0.05 to 0.20 μg/mL), although the effect was bacteriostatic; bactericidal concentrations were tenfold to a hundredfold higher.²⁴⁷ Orbifloxacin has proved ineffective.²³⁴ Experimentally, the macrolides also appear effective.²³⁴ Protection is best implemented by prevention of exposure and elimination of reservoirs.

Most dogs presenting with leptospirosis have developed renal disease.²⁴⁵ Thus supportive therapy should focus on treatment of acute renal failure and its associated signs (i.e., uremic gastritis, hypertension). Leptospirosis may be the most widespread zoonotic disease in the world, and vaccination is important to prevention. Gloves should be worn when handling carrier animals.

Lyme Borreliosis

Like Leptospira species, Borrelia species belong to the family Treponemataceae. Borrelia are motile spirochetes that have an outer slimelike layer. They are rapidly killed by desiccation and ultraviolet lights. Borrelia infect both humans and animals; up to 15 Borrelia species cause disease. The Centers for Disease Control and Prevention has designated Lyme disease as the most common vector-borne infection in the United States. However, the disease may be overdiagnosed in humans.²⁴⁸

Ixodes species are the primary insect vectors of the spirochete Borrelia burgdorferi. Lyme borreliosis afflicts primarily humans and dogs, although cats can be infected experimentally. Organisms are able to adhere to many different types of mammalian cells as well as avoid elimination by phagocytic cells.²⁴⁸ The immune response appears to be initially suppressed during infection, apparently allowing the organisms to spread. The disease is multisystemic, with nonspecific clinical signs such as relapsing fever (during which borreliae are present in blood), anorexia, lethargy, and lymphadenopathy. Polyarthritis might result in episodic lameness; clinical signs often do not develop until several months after infection.^248,²⁴⁹ Heart block, which occurs in humans,²⁴⁸ has been reported in one infected dog. Clinical laboratory changes are also variable, depending on the site of localization of the infection. Diagnosis is facilitated by serum titers, although interpretation is complicated by interlaboratory variability and overlap between titers indicative of clinical versus subclinical infection. Resolution of natural disease in humans appears to be dependent on class II major histocompatibility complex genes.

According to the the 16th Consensus Conference on Anti-infective Therapy,²⁵⁰ the goal for treatment of borreliosis is eradication of infection (not negative serology) such that progression to chronic disease might be prevented. Treatment should be implemented for 14 to 21 days. Based on in vitro testing, antimicrobials effective against B. burgdorferi include, in order of most to least effective, ceftriaxone, erythromycin, amoxicillin, cefuroxime, doxycycline, tetracycline, and penicillin G.²⁴⁹ In human medicine the first line of oral therapy for the primary phase of illness is amoxicillin or doxycycline, with cefuroxime–axetil implemented as second line. Azithromycin is indicated if first or second line cannot be administered. Treatment for the subsequent phases include ceftriaxone (intravenous or intramuscular) or penicillin (intravenous), doxycycline, and amoxicillin (orally) for 21 to 28 days. Antimicrobial therapy based on clinical response includes, in order of preference, tetracyclines (doxycycline is the drug of choice), high doses of ampicillin or amoxicillin, and erythromycin and its derivatives.²⁴⁸ Third-generation cephalosporins (e.g., ceftriaxone) are also generally effective. One study in humans found similar clinical cure rates for ceftriaxone (85%) compared with doxycycline (88%).²⁵¹ No information appears to be available regarding the efficacy of cefpodoxime; however, initial studies by the manufacturer may have indicated a lack of in vitro susceptibility.^251a Accordingly, the use of either cefpodoxime or cefovecin should be based on susceptibility data. The organisms appear to be resistant to ciprofloxacin (and presumably enrofloxacin and other veterinary fluoroquinolones) and to the aminoglycosides. Oral therapy is sufficient except in cases of neurologic signs; intravenous therapy should be instituted in such cases. B. burgdorferi is a potent inducer (in vitro) of TNF-alpha and interleukin-1β. Antimicrobial therapy in humans is occasionally characterized by a Jarisch–Herxheimer reaction, which may reflect release of spirochetal endotoxin and subsequent septic shock syndrome.²⁴⁸ Supportive therapy includes nonsteroidal antiinflammatories as needed for joint lameness. Disease-modifying chondroprotective agents should be strongly considered. Care should be taken when combining nonsteroidals with doxycycline or minocycline because of the competition for protein-binding sites and the potential for adverse reactions to the nonsteroidal drug. Glucocorticoids should be limited to treatment of acute spirochetemia; recrudescence of spirochetemia will be facilitated with their use. Prevention focuses on eradication of the vector.

Higher Bacteria: Nocardiosis and Actinomycosis

Pathogenesis

Nocardiosis is generally presented as a localized infection. The organism is an acid-fast, aerobic, soil-borne actinomycete, usually introduced through the respiratory tract. Traumatic penetration may result in a localized skin infection.^252,²⁵³ Actinomyces are commensal anaerobic organisms found in the oral cavities of animals. Infection commonly follows penetrating wounds, such as inhalation of grass awns. Actinomyces are distinctive in their configuration, presenting as filamentous growth with true branching.

The taxonomy of Nocardia is currently evolving, but Nocardia asteroides is among the more commonly identified pathogens. Culture is made difficult sometimes by the slow growth that characterizes this organism when present in mixed cultures from clinical material. Rapidly growing bacteria often obscure the smaller Nocardia colonies. Colony characteristics may take up to 2 to 4 weeks to be noticed. Gram staining may help identify the organisms earlier, although smears may also be negative. Nocardia, but not Actinomyces, stains acid fast, although acid-fast staining in Nocardia is also variable. Nocardia appears as beaded, branching filaments when Gram stained but will not stain with hematoxylin and eosin preparations or in periodic acid-Schiff stains for fungi (see Figure 6-14)).²⁵² In human patients, positive cultures and smears occur in only one third of the cases. Pus from a fistula or abscess will facilitate identification. Nocardia and Actinomyces also stain gram positive, but again staining is irregular.²⁵⁴

Nocardiosis is an opportunistic infection. In humans a number of underlying diseases, most of which are accompanied by an altered immune system, predispose the patient to nocardiosis. Infections occur because the organism is able to evade bacterial protective mechanisms of the host. Immune T cells and neutrophils are important to eradication of Nocardia. Nocardia may be resistant to oxidative bursts of neutrophils. Filamentous log-phase cells of Nocardia are more virulent and toxic to macrophages than are the coccoid stationary-phase organisms, which can be easily phagocytized.

Nocardia produces suppurative necrosis and abscess formation. In humans Nocardia in skin seldom causes a marked fibrotic response; rather, granulation tissue will be loose, with bands of fibrous tissue surrounding the lesions. Confluent abscesses form with little to no encapsulation. Extension to the pleura or chest wall may result in empyema, subcutaneous abscesses, or sinus tracts. Occasionally, bony involvement may occur. Calcium-containing “sulfur granules” present a barrier to bacterial penetration and may indicate reversion of the organism (Actinomyces) to an L-form.

Antimicrobial and Adjuvant Therapy

A number of antimicrobials are effective against both Actinomyces and Nocardia. An anaerobic environment and hence infection with Actinomyces should be assumed unless proved otherwise, thus leading to more conservative antimicrobial selection. Penicillins (penicillin G, amoxicillin) are generally preferred, although resistance and L–form organisms can preclude their efficacy.²⁵⁴ Clavulanic acid can reduce the risk of resistance. The formation of protective calcium granules by certain strains of Actinomyces limits antibiotic penetration into the organisms. The formation of these granules is stimulated by the presence of penicillin antibiotics. Trimethoprim–sulfonamide combinations are also very effective, although high doses are recommended. Combinations of penicillins with trimethoprim–sulfonamide result in synergistic actions against Nocardia and Actinomyces¹⁸⁶ (high doses and frequent intervals). Clindamycin and erythromycin are also effective, particularly against L-forms.²⁵⁴ The aminoglycosides are highly effective against Nocardia and Actinomyces, and synergistic activity has been documented against this organism with combinations. Minocycline (and presumably doxycycline) also is effective, although use of this bacteriostatic drug may preclude combinations with other antimicrobials.

Treatment for both Nocardia and Actinomyces should occur for at least 6 weeks, with high doses at frequent intervals (see the discussion of respiratory tract infections in this chapter). Treatment should continue beyond resolution of clinical signs. Adjuvant therapy should include drainage and lavage of empyema and, when indicated, chest tube drainage or surgical débridement. Lavage should continue for several days until cytologic examination of aspirated fluid indicates resolution of infection and fluid accumulation decreases (5 to 10 days; see previous discussion of empyema). The author has often recommended initial hospitalization and intravenous therapy with amikacin and amoxicillin–clavulanic acid or, for serious, life-threatening infections, a carbapenem followed 10 to 14 days later with very high (45 to 60 mg/kg twice daily; note the risk of thyroid gland suppression at this dose) oral sulfadiazine, with trimethoprim replacing the aminoglycoside and high, frequent doses of amoxicillin-clavulanic acid replacing the carbapenem. The amoxicillin–clavulanic acid combination should be continued with the sulfonamide for 4 to 6 weeks or more. The beta-lactam should be administered at 6- to 8-hour intervals.

Mycobacteria

Mycobacteria are composed of a number of aerobic, acid-fast bacteria. They vary markedly in host affinity and ability to cause disease. Disease is frequently accompanied by granulomatous inflammation because of their ability to survive phagocytosis. The acid-fast nature of these microorganisms reflects the large amount of lipid material in the cell wall. Constituents of the cell wall stimulate the granulomatous response. The organisms are more resistant than most organisms to environmental changes (e.g., pH, heat) and are more resistant to disinfection. Some organisms (most notably Mycobacterium avium) can survive in the environment for several years. They are, however, very susceptible to 5% phenol or 5% household bleach. Generally, organisms causing disease are characterized by one of three forms.²³⁵ Tuberculosis generally is internal in location. Infecting organisms include Mycobacterium tuberculosis (more common in dogs) and Mycobacterium bovis (more common in cats). Leprosy is characterized by localized cutaneous nodules; Mycobacterium lepraemurium is probably the most common infecting organism (in cats). Atypical mycobacteria generally presents as a spreading subcutaneous inflammatory disease (Figure 8-11); among the several organisms causing this complex is M. avium. Dogs and cats are most commonly infected by owners with disease or exposure to infected farm animals.

Figure 8-11 Atypical mycobacterium in a cat before (A) and 3 months after (B) treatment with a combination of enrofloxacin and sulfadiazine–tribrissen. Note the granulomatous tissue and multiple fistulous tracts.

(Photographs courtesy Katrina Mealey, DVM, PhD, DACVIM, DACVCP, Washington State University, Pullman, WA.)

Tuberculosis

Dogs and cats are more susceptible to infection by tuberculous mycobacteria than by atypical mycobacteria. Infection generally occurs through the respiratory or alimentary tract. Local multiplication at the site of infection results in a granulomatous response at the primary complex (site of deposition) and local lymph nodes. Particularly in cats, however, a granulomatous response may develop only in surrounding lymph nodes. For tuberculosis respiratory infections are more common in dogs; intestinal infections are more common in cats. Infection can be followed by elimination of the organisms in animals with a sufficient immune response. The more common sequelae are location within phagocytic cells, intracellular multiplication, and granuloma formation as the body attempts to eradicate the organism. Organisms that outpace the host immune system can cause disseminated disease. Immunity is incurred by the cell-mediated response, but factors that facilitate an adequate response in the host are not known. Diagnosis can be facilitated with intradermal skin testing in dogs with the highest concentration of antigen used in humans. Cats do not react strongly to intradermal testing. Diagnosis can be facilitated by the presence of acid-fast organisms in tissue biopsy material.

Antimicrobial therapy is complicated by the fastidious nature of the organism. Drug penetration into the organism is likely to be more difficult than for other organisms; intracellular survival further complicates efficacy of drugs reaching the site of infection. Treatment should generally include combination therapy for at least 6 to 9 months. A combination of isoniazid (10 to 20 mg/kg orally once daily) plus rifampin (10 to 20 mg/kg orally every 12 to 24 hours) plus ethambutol (15 mg/kg orally every 24 hours) is the most effective therapy (in humans), although isoniazid-resistant organisms have become increasingly difficult to treat. More rapid remission is likely with intravenous administration. The isoniazid can also be administered prophylactically (6 to 12 months) in cases of exposure. The fluoroquinolones are also effective against selected species of mycobacteria. M. avium (see later discussion of atypical mycobacteria) is an exception, although it does also respond to other drugs used to treat atypical mycobacteria. Infected animals remain a health hazard because they serve as temporary sources of dissemination in the environment.

Leprosy

Feline leprosy is caused by M. lepraemurium, also the causative organism of rat leprosy. The infection causes rapidly growing, soft, fleshy nodules in the skin and subcutaneous tissues, usually on the head or extremities. Infected cats are generally healthy, and the lesions are not painful. Feline leprosy may comprise two different clinical syndromes, one tending to occur in young cats and caused typically by M. lepraemurium and another in old cats caused by a single novel mycobacterial species.

Diagnosis is generally based on the presence of granulomatous inflammation and acid-fast organisms in biopsy specimens or impression smears; PCR may also be available. Treatment includes surgical removal and antimicrobial therapy. Drugs include dapsone (1 mg/kg [dogs] orally every 8 hours or 50 mg [cats] orally every 12 hours) for 2 weeks or clofazimine (8 mg/kg orally every 24 hours for 6 weeks and then twice weekly thereafter). Rifampin may also be useful. Cats with feline leprosy caused by a slow-growing mycobacterium responded well to a combination of two or three antibiotics:²⁵⁵ rifampicin (10 to 15 mg/kg once a day), clofazimine (25 to 50 mg once daily or 50 mg every other day) or clarithromycin (62.5 mg per cat every 12 hours).

Atypical Mycobacteria

Both slow-growing organisms (M. avium) and rapid-growing organisms (e.g., Mycobacterium fortuitum, Mycobacterium chelonei) cause disease in dogs and cats, although the rapid-growing organisms are more common. These organisms are ubiquitous in natural environments (especially wet soils) and generally are not pathogenic. Infection is generally acquired after trauma to the skin; the location of entrance into the body determines the presentation of the disease. Penetration into subcutaneous tissue appears to promote pathogenicity. Generally, infection presents as a localized but spreading infection characterized by granulomatous inflammation and acid-fast organisms. Multiple fistulous draining tracts are evident, usually in the caudal abdominal, inguinal, or lumbar subcutaneous tissues (see Figure 8-11). Cats are usually clinically healthy even if cutaneous involvement is extensive. Less commonly, fever, anorexia, and weight loss occur. Hypercalcemia as a result of the release of parathormone-like hormone from macrophages associated with the inflammatory response may occur,²⁷⁵ and has, in the author’s experience, occurred in a cat infected with atypical mycobacteria. Diagnosis of atypical mycobacterium is difficult in part because organisms are not abundant. Tissue biopsy specimens should be taken from the subcutaneous tissues because organisms are more likely to be located in the panniculus. Nocardiosis should be considered as a differential. Bacterial culture provides the definitive diagnosis.

Therapy focuses on antimicrobial drugs. Antitubercular drugs are generally ineffective against atypical mycobacterial species. Quinolones, aminoglycosides (particularly amikacin), and doxycycline (or minocycline) are effective and can be used in combination if more aggressive therapy is desired. Other combinations include clofazimine, rifampin and the azolides, azithromycin, or clarithromycin. Other drugs that might be beneficial include trimethoprim–sulfonamide combinations and clofazimine. High doses are recommended to maximize drug delivery into the granulomatous tissue. Surgical debulking may be indicated for large granulomatous masses. Care should be taken not to discontinue drugs too early. At least 4 to 6 weeks of therapy should be anticipated.

Horne and Kunkel²⁵⁶ retrospectively reported on 10 cases of rapidly growing mycobacterium in cats (n = 10); 6 of 10 yielded M. fortuitum on culture, 2 of 10 Mycobacterium abcessus, and 1 of 10 Mycobacterium goodii. Five of the cases resolved. Susceptibility patterns for the drugs were as follows (“I” considered resistant): amiikacin (9 of 10), kanamycin (3 of 10), gentamicin (7 of 10), cefoxitin (6 of 10), imipenem (8 of 10), doxycycline (4 of 10), minocycline (7 of 10), ciprofloxacin (7 of 10), moxifloxacin (7 of 8), clarithromycin (5 of 10), azithromycin (5 of 10), trimethoprim sulfamethoxazole (6 of 8), linezolid (2 of 8), and amoxicillin–clavulanic acid (3 of 10). Organisms were resistant to ceftriaxone, cefepime, cefotaxin, and erythromycin. The drugs used to treat all cases varied but included marbofloxacin, clarithromycin, minocycline or doxycycline, cefodroxil, and a potentiated sulfonamide. The drug most commonly associated with resolution appeared to be clarithromycin. The duration of treatment ranged from 3 to 21 months (median 7 months).

Melioidosis and Tularemia

Melioidosis is caused by Burkholderia (Pseudomonas) pseudomallei, a bipolar, aerobic, gram-negative motile bacillus occurring predominantly in Southeast Asia, northern Australia, and the South Pacific. It is characterized by fever, myalgia, dermal abscesses, and epididymitis.²⁵⁷ Effective antimicrobials include tetracyclines, chloramphenicol, trimethoprim–sulfonamide combinations, amoxicillin–clavulanic acid, and novobiocin–tetracycline. High doses of parenteral ceftazidime for 2 weeks has proved most effective; imipenem–cilastin may also be effective.

Tularemia, caused by Francisella tularensis, is a tick-transmitted disease of both dogs and cats.²⁵⁸ Infection begins with localized lymphadenopathy followed by bacteremia and multiple organ involvement. Clinical signs vary and include fever, mucopurulent nasal or ocular discharge, abscess at the site of inoculation (or associated with lymphadenopathy), myalgia, shivering, and signs indicative of septicemia. Preferred antimicrobial therapy in human patients includes the aminoglycosides. Chloramphenicol or tetracyclines may be associated with relapses. The fluoroquinolones also may be effective.

Rickettsial and Anaplasmid Diseases

Rickettsial and anaplasmid organisms are fastidious, obligate intracellular parasites that appear as pleomorphic coccobacilli. They multiply by binary fission, contain both DNA and RNA, and are capable of synthetic and energy-producing reactions. Their life cycle involves insect reservoirs (primarily ticks) and mammals.²⁵⁹ The rickettsial organisms have undergone reclassification such that two families exist: Anaplasmataceae and Rickettsiaceae Ehrlichia has recently been reclassified from the family Rickettsiaceae to the family Anaplasmataceae; other animal pathogens in this family include Anaplasma and Neorickettsia. Reclassification has resulted in renaming Ehrlichia platys as Anaplasma platys, Ehrlichia risticii as Neorickettsia risticii, and Cowdria sp. as Ehrlichia sp. The family Rickettsiaceae is limited to Rickettsia. Coxiella burnetii, the causative agent of Q fever, is not included in either group and stands out additionally because of its robust nature outside of host cells and presentation of disease.²⁵⁹

Although advances in recent years have increased our knowledge regarding the physiology of these organisms and the pathophysiology of the disease, much information is still missing. The pathogenesis of the other rickettsial organisms reflects vasculitis caused by proliferation of the organisms in endothelial cells. Diagnosis is generally based on serologic testing. Serologic evidence of disease generally does not, however, occur until several weeks after clinical signs have developed.

Ehrlichiosis

Pathophysiology

Diseases caused by Ehrlichia can be varied in presentation and often mimic other diseases, making diagnosis difficult.²³⁶ Disease can present in an acute, clinical, or subclinical phase. The acute phase lasts 2 to 4 weeks, during which the organisms replicate in the mononuclear phagocytic cells of the liver, spleen, and lymph nodes. Infected cells travel to the lung, kidney, and meninges, where endothelial inflammation can occur. Nonspecific clinical signs during this phase include fever, anorexia, weight loss, ocular and nasal discharge, and edema. Additional clinical signs depend on the severity of infection in each organ and include dyspnea, neurologic abnormalities, and lymphadenopathy. Platelet consumption, sequestration, and destruction contribute to thrombocytopenia, which often characterizes this phase. Leukopenia and anemia become more likely as the disease progresses. The subclinical phase occurs at 6 to 9 weeks and is characterized by pancytopenia. An adequate immune response should eradicate the disease, but immunoincompetence leads to chronic infection. Clinical signs in chronic disease vary with the severity of infection and can range from asymptomatic to severe. Bleeding tendencies, anemia, chronic weight loss, and debilitation are nonspecific clinical signs. Abdominal tenderness, ophthalmic complications (anterior uveitis, retinal detachment), and neurologic abnormalities may be present. Secondary infections (bacterial and, less commonly, fungal) may reflect immune suppression. Diagnosis is based on clinical laboratory changes and serologic diagnosis using indirect fluorescent antibody. Treatment is oriented toward eradication of the infecting organism and supportive therapy based on clinical signs. Therapy in chronic stages may also require targeting opportunistic infections in the immunosuppressed animal.

Causative Organisms

Disease caused by canine ehrlichiosis generally targets blood-forming units. Ehrlichiosis is caused by E. canis, the brown dog tick, and Rhipicephalus sanguineus serves as the primary insect vector. Other infections carried by this vector (Babesia canis and Hepatozoon canis) can simultaneously infect the host. E. canis is a pleomorphic organism that circulates in peripheral monocytes. Infection is transmitted through the saliva of the tick into the bloodstream of the host. Transmission has also occurred between hosts (patients) through blood transfusions. Although the dog is the primary target of E. canis, infections in cats is suspected but has not yet been documented.²⁶⁰ Other Ehrlichia organisms infecting animals include Ehrlichia chaffeensis in dogs (mononuclear cells), Ehrlichia ewingii (granulocytes, dogs), Anaplasma phagocyophila (formerly Ehrlichia equi, probably responsible for a significant number of granulocytic ehrlichiosis in dogs in northeastern and upper Midwestern states and California²⁶⁰), Anaplasma platys (previously Ehrlichia platys; platelets in dogs) and N. risticcia (formerly Ehrlichia risticcia), which can cause infection in monocytic and granulocytic cells of dogs or cats (or horses).

Antimicrobial and Supportive Therapy

Doxycycline or minocycline are the treatment of choice (10 mg/kg orally every 24 hours for 4 weeks) for ehrlichiosis.²⁶⁰ Although fluoroquinolones (e.g., enrofloxacin) appear to be effective for the treatment of Rickettsia rickettsii at a dose of 3 mg/kg orally twice daily, based on experimental infection, they do not appear to be effective at 10 mg/kg orally twice daily for the treatment of ehrlichiosis.²⁶¹ The effects of combination therapy with doxycycline and enrofloxacin on antimicrobial efficacy against ehrlichiosis have not been established, but the combination might be considered for patients that do not respond to doxycycline. Chloramphenicol is also effective against ehrlichiosis but is less ideal clinically than tetracyclines and enrofloxacin; its use might be limited to puppies if tetracyclines are not acceptable because of brown discoloration of teeth. However, doxycycline is less likely to cause discoloration (see Chapter 7). Imidocarb diproprionate is as effective as doxycycline (5 mg/kg intramuscularly followed by a second injection 2 weeks later), but care must be taken to minimize the side effects of this drug. It recently has been approved in the United States. It should be used to treat patients that have not responded to doxycycline or enrofloxacin. Because chloramphenicol targets a different ribosomal target site than doxycycline, combination therapy with the two drugs might be considered but should be based on clinical trials,

Imidocarb (6.6 mg/kg intramuscularly, two injections, 2 weeks apart) was ineffective in clearing Ehrlichia in dogs (n = 10 treated and n = 5 untreated controls) experimentally infected.²⁶² However, twice-daily administration of doxycycline at 5 mg/kg orally bid for 4 weeks cleared five of the imidocarb-treated dogs; two control dogs spontaneously cleared as well.²⁶³

Supportive therapy for ehrlichiosis includes fluid and electrolyte therapy; blood or blood component transfusions; hematinics (vitamins, iron if bleeding has been extensive); and, less commonly, drugs that stimulate erythropoiesis. Anabolic steroids should be used cautiously in the presence of liver involvement. Short-term gluocorticoids may be indicated in severe cases to minimize immune-mediated destruction of platelets or immune-mediated arthropathies, vasculitis, or meningitis. Serologic titers generated by ehrlichiosis are not protective, and reinfection may occur. Platelet counts may decrease despite rapid clinical improvement in response to doxycycline. Serum antibodies may not be useful because they remain elevated for months after therapy. Monitoring PCR may be the best method of evaluating therapeutic success; because it cannot differentiate dead from live organisms, a strong positive the week after completing doxycycline suggests persistent infection.²⁶⁰

Long-term prophylaxis might be considered in endemic areas or kennels and consists of tetracycline (3 to 6 mg/kg once daily orally) or reposital tetracycline (200 mg intramuscularly twice weekly). Davoust and coworkers²⁶⁴ reported a prophylactic program based on French military dogs (614; average weight 29 kg) returning after 4 months of stay in an area highly endemic for ehrlichiosis. The study was coupled with detection of plasma doxycycline concentrations (n = 124 dogs) and its association with chemoprevention. Dogs were treated with doxycycline (100 mg [approximately 3 mg/kg] by mouth qd). In 10 of these dogs, the time course of doxycycline was determined. Peak doxycycline (approximately 5 hours) in these dogs ranged from about 1 to 1.7 μg/mL; half-life approximated 9 hours. The drug was assayed by high-performance liquid chromatography; the peak concentrations were based on total drug. At 24 hours, concentrations ranged from 0.26 to 0.4 μg/mL. Spot checks on 114 dogs in the field yielded concentrations that were above 2 μg/mL. Of the 614 dogs, 4% (n = 24) were seropositive; concentrations were not determined in these dogs. They were asymptomatic. Based on a previous report that demonstrated an MIC of equal to or less than 0.03 μg/mL for ehrlichia and doxycycline, the authors concluded that the chemoprevention program was appropriate.

Rocky Mountain Spotted Fever

Causative Organism

The causative agent of Rocky Mountain spotted fever is R. rickettsii. It is transmitted primarily by Dermacentor species, with the American dog tick being the principal vector in the eastern United States and the wood tick the principal vector in the western United States. Transmission of disease requires tick attachment to the host for at least 5 hours and up to 20 hours; thus the disease might be prevented by routine checks of the animal’s body.²⁶⁵ The incidence of infection in dogs caused by this organism only now is being appreciated. Unrecognized and untreated illness can lead to death, although the severity of clinical signs depends, in part, on the degree and location of the initial vascular damage induced by the organisms. Diagnosis is complicated by cross-reactivity to several nonpathogenic members of Rickettsia and should be based on both acute and convalescent serum titers or direct immunofluorescence in skin biopsy material.²⁶⁵

Pathophysiology

Organisms are transmitted through the saliva of the tick into the bloodstream, where they replicate in endothelial cells of small blood vessels and capillaries. Damaged endothelial cells become inflamed. Vessels become permeable, causing extravasation of fluid into perivascular spaces. Depending on the severity of infection, clinical signs may indicate edema, hemorrhage, hypotension, or shock. Infection in vessels of the CNS may lead to neurologic signs and a more rapid deterioration of clinical signs.

Other clinical signs vary and may require medical management, depending on which organs are infected: Cardiac abnormalities may include conduction abnormalities or other life-threatening arrhythmias; the respiratory system may be characterized by clinical signs reflecting pulmonary edema, which is minimally responsive to diuretic therapy; ocular abnormalities range from subconjunctival inflammation to retinal detachment; and, in severe cases, acute renal failure occurs as a result of decreased renal perfusion. Severe inflammation causing vascular obstruction can lead to gangrene of peripheral limbs, ears, lips, scrotum, or mammary glands. Other less specific clinical abnormalities include fever, anorexia, depression, muscle pain, polyarthritis, and weight loss.²⁶⁵ In addition to increases in liver enzymes and serum bilirubin, clinical laboratory tests may reveal the need to treat thrombocytopenia; severe acute infection may also cause leukopenia and anemia. Hypoproteinemia, azotemia, hyponatremia, and hypocalcemia may also be present. Treatment is focused on both eradication of the organisms and supportive therapy of derangements in the infected body system.

Antimicrobial and Supportive Therapy

Tetracyclines remain the treatment of choice for rickettsial infections. Although tetracycline may be sufficient, drugs characterized by better lipophilicity (e.g., doxycycline, minocycline) may be more advantageous. Chloramphenicol is also effective, but less so than tetracyclines. Fluoroquinolones are also effective experimentally,²⁶⁵ although clinical efficacy has not been established. Combinations of both doxycycline and enrofloxacin may be additive to synergistic, but the effects of this antimicrobial combination on rickettsial organisms has not been established. Clinical response should be rapid except in cases with severe vascular sequelae (i.e., neurologic or renal damage). Supportive therapy should target abnormalities previously described. These include electrolyte abnormalities and replacement of colloid (protein). Vascular permeability, however, will complicate volume replacement (with either crystalloids or colloids) and, if too intensive, may contribute to peripheral (including pulmonary) edema.

Breitschwerdt et al.²⁶⁶ reported the efficacy of azithromycin (3 mg/kg by mouth qd) and trovofloxacin (5 mg/kg by mouth bid) compared with doxycycline (5 mg/kg by mouth bid) in dogs (n = 16; 4- to 5-month-old beagles; four per group, including untreated control) experimentally infected with R. rickettsii. Treatment duration was for 7 days (starting day 5 post infection), except for azithromycin, which was used for only 3 days. Drug concentrations were measured after the last treatment, with peak concentrations (μg/mL) evident at 1 hour: 0.15 (azithromycin), 3.5 (doxycycline), and 1 (trovafloxacin). Whereas all three antimicrobials caused rapid improvement in a number of outcome measures, ocular lesions were rare with doxycycline or trovafloxacin but present in all azithromycin-treated dogs. Based on PCR, DNA was present though day 21 after infection, although organisms were not isolated. The authors concluded that fluoroquinolones might be a reasonable alternative to doxycycline. Although azithromycin was discouraged as a first-line treatment by the authors, the half-life of the drug was not known at the time of the study, leading to the shorter duration of therapy. As such, azithromycin (which has a 29-hour half-life in dogs) would not have yet reached steady state at the time that it was discontinued. Further, a dose used was lower than that currently recommended; as such, azithromycin might be reconsidered through controlled studies.

Other Rickettsial-Like Diseases

Canine cyclic thrombocytopenia is caused by E. platys, an organism that replicates in platelets. Cyclic thrombocytopenia occurs at 10- to 14-day intervals. Both platelet numbers (as low as 20,000) and aggregation are impaired. Treatment should be with tetracyclines as described for E. canis. Neorickettsia helminthoeca is one of the causative agents of salmon poisoning disease, which is transmitted after ingestion of fish containing the trematode vector. Like other rickettsial diseases, it is treated with tetracyclines. Hemobartonellosis is caused by a hemotrophic organism that causes acute or chronic anemia in dogs or cats. Damaged red blood cells are removed by the host’s immune system. The host is not able to resolve infection without treatment. Tetracyclines are the drug of choice. Supportive therapy may include glucocorticoids at immunosuppressive doses. Metronidazole (40 mg/kg orally once daily for 3 weeks) has been used to treat resistant infections.²⁶⁵

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