CHAPTER 36 Clostridial Diseases

The Clostridium genus encompasses a wide range of commensal and opportunistic pathogens that can be found in a wide range of animal species (Table 36-1). All members of this genus are anaerobic gram-positive spore-forming bacteria. Although all are anaerobes, Clostridium spp. have variable tolerance to oxygen; some, such as Clostridium perfringens, are very tolerant, whereas others, such as Clostridium difficile, are very fastidious. Regardless, an important aspect of clostridia is their ability to exist in both vegetative and spore forms. The vegetative forms are the active forms of the organism; the spore form is dormant and highly resistant to oxygen, environmental effects, and disinfectants. Because of this, many clostridia can be found in the environment and can persist there for prolonged periods.

Table 36-1 Important Clostridial Pathogens

Organism	Disease
C. difficile	Enterocolitis, duodenitis/proximal jejunitis
C. perfringens	Enterocolitis, myonecrosis, soft tissue/wound infections
C. tetani	Tetanus
C. botulinum	Botulism, grass sickness
C. piliforme	Tyzzer’s disease
C. sordellii	Enterocolitis, septicemia, acute pasture rhabdomyolysis, myonecrosis
C. bifermentans	Acute pasture rhabdomyolysis
C. sporogenes	Myonecrosis
C. septicum	Myonecrosis
C. chauvoei	Myonecrosis

Clostridia can cause an impressive range of diseases involving the gastrointestinal, musculoskeletal, and nervous systems, and certain species can produce a variety of different diseases. Our knowledge of this genus is somewhat limited by the difficulties in working with some fastidious species, the presence of many of these as commensal microorganisms, and our poor overall understanding of the endogenous microflora of the gut and other organs. Regardless, it is clear that certain clostridia are important equine pathogens.

CLOSTRIDIUM DIFFICILE COLITIS

In some regions, C. difficile is the most commonly identified cause of diarrhea, although there appears to be significant geographic variation. Outbreaks can occur, and most affect foals; however, disease is most often sporadic. It has been implicated as the cause of up to 28% of sporadic cases of colitis and up to 42% of cases of antimicrobial-associated diarrhea. C. difficile is a common cause of antimicrobial-associated diarrhea, but most cases in most areas are not antimicrobial-associated; and, according to one study, prior antimicrobial therapy is not a risk factor for C. difficile compared with other enteropathogens. In Sweden, there have been reports of fatal C. difficile colitis in mares with foals that were being treated with erythromycin, and disease was being experimentally induced with administration of low doses of erythromycin to mares in that country. This phenomenon appears to be geographically variable.

Clostridium difficile can also be identified in the intestinal tract of healthy horses. Figures of the prevalence of colonization in adult horses are variable and range from less than 1% to 10%. The variation in reported colonization prevalence may relate to the population studied or to testing methods. Because vegetative C. difficile cells are poorly aerotolerant, testing methods that do not efficiently recover the more tolerant spore form may underestimate the prevalence. In general, colonization rates are higher in foals and in horses treated with antimicrobials. Colonization of 29% of healthy foals less than 14 days of age has been reported. In human infants, C. difficile is not considered a pathogen; however, this appears to be different in foals because C. difficile–associated diarrhea (CDAD) has been experimentally reproduced in foals in the absence of antimicrobial treatment.

Clinical Presentation

Clinical presentation is highly variable and can range from mild diarrhea with no signs of systemic illness to peracute and rapidly fatal necrohemorrhagic enterocolitis. Typically, affected horses have diarrhea plus varying degrees of depression, dehydration, fever, anorexia, abdominal pain, and abdominal distension. Signs of severe systemic illness may be associated with endotoxemia and systemic inflammatory response syndrome.

Clinical Pathology

Clinical pathology results are variable and nonspecific. There are no indicators that can be used to suggest or confirm a clostridial cause. Leukopenia and neutropenia, often with a left shift, are common. Affected animals may exhibit either hypoproteinemia resulting from protein loss or hyperproteinemia as a consequence of hemoconcentration. Prerenal azotemia is common in dehydrated animals.

Diagnosis

Isolation of C. difficile alone is not diagnostic because it can be found in normal horses. Detection of C. difficile toxins is the preferred approach but is not straightforward (Table 36-2). The gold standard for CDAD is detection of toxin B by use of a cell cytotoxicity assay, but this test is rarely available. Commercial enzyme-linked immunosorbent assay (ELISA) tests are available for detection of C. difficile toxin A, C. difficile toxin A or B, and “common” antigen. None of these has been validated for use in horses, and the sensitivity and specificity of these tests are variable in different animal species. Concern has been expressed about the usefulness of all these tests. Although tests are still useful clinically, their limitations should be acknowledged. Polymerase chain reaction (PCR) testing is also of limited usefulness because the presence of toxigenic C. difficile is not necessarily clinically relevant.

Table 36-2 Diagnostic Testing Options for Clostridium difficile–Associated Colitis

At this point diagnosis should involve detection of clostridial toxins in feces. Tests that detect both toxin A and toxin B are preferred because some equine strains will produce only toxin B. The positive predictive value of detection of toxin can be strengthened by concurrent detection of C. difficile by antigen ELISA or PCR. In some human laboratories, samples are screened using an antigen ELISA, and positive samples are then tested for toxins. This rapid and cost-effective approach is reasonable for horses as well.

Treatment

Supportive care is the most important aspect of treatment. Depending on the severity of disease, this may include intravenous (IV) fluids (crystalloids or colloids), plasma transfusion, nutritional support, and measures to combat endotoxemia, such as administration of polymyxin B (1000-6000 units/kg slowly IV every 8 to 12 hours) or low doses of flunixin meglumine (0.25 mg/kg IV every 8 hours). Di-tri-octahedral smectite (1.5 kg loading dose administered orally [PO] followed by 450 g PO every 6 to 8 hours) is commonly used. It binds to clostridial toxins in vitro, although clinical efficacy is unclear. Probiotics are sometimes used, but there is minimal evidence regarding efficacy, and results of trials in humans have been mixed. Administration of the yeast Saccharomyces boulardii (1 × 10¹⁰ yeast cells PO every 12 hours for 14 days) has been reported to decrease the duration of diarrhea (but not outcome) in a small trial of horses with acute colitis. This yeast produces a protease that might be able to cleave clostridial toxins in the intestinal tract; however, further efficacy data are required. The potential role of other probiotics is unclear. If diarrhea is considered to be associated with antimicrobial treatment, the treatment should be stopped, if possible.

Metronidazole (15 mg/kg PO every 8 hours for 3 to 5 days) is currently the drug of choice for C. difficile colitis. Poor response to metronidazole does not indicate metronidazole resistance. Metronidazole resistance is rare but has been reported in California. Vancomycin is used in some cases of CDAD in humans and has been used in horses with suspected metronidazole-resistant CDAD. Concerns have been expressed about the use of vancomycin in horses, and consensus has not been reached whether it is appropriate, particularly in the absence of confirmed metronidazole resistance. Because of concerns regarding the use of vancomycin and the emergence of vancomycin resistance in commensal organisms such as enterococci, it should be avoided.

Affected horses should be isolated and handled with contact barrier precautions because outbreaks can occur. Overboots, an impermeable gown, or coveralls and gloves should be worn when handling affected horses or when in contact with the horse’s environment. Hands should be washed after glove removal. Handwashing is preferable to the use of alcohol-based hand sanitizers because C. difficile spores are alcohol resistant. Affected animals should not have direct or indirect contact with other horses and should not be turned out onto, or allowed to graze, areas to which healthy horses have access. Clostridial spores likely survive for years in soil, although the clinical relevance of this is unclear because C. difficile spores can be found in the farm and hospital environment in the absence of evidence of disease transmission.

Prognosis

The prognosis is variable and depends on the severity of disease. Mortality rates of up to 50% have been reported, but these results are likely biased toward more severe cases because they involve horses referred to academic hospitals. One study reported no difference in mortality rate between C. difficile-associated and non-C. difficile-associated colitis. Complications such as laminitis, venous thrombosis, and disseminated intravascular coagulation can develop.

Prevention

There are no specific preventive measures. Judicious use of antimicrobials and other general practices such as providing a proper diet, minimizing diet and management changes, and reducing stressors may be important. A vaccine is not currently available.

Public Health Considerations

C. difficile is an important pathogen in humans, particularly hospitalized individuals and those being treated with antimicrobials, chemotherapeutics, and proton pump inhibitors. Strains of C. difficile isolated from horses are often indistinguishable from pathogenic human types, and the concern for zoonotic transmission has been raised. At this point, it is unclear whether C. difficile can be transmitted from horses to humans, but it is prudent to treat affected horses with the assumption that thay are carrying a potentially zoonotic disease.

CLOSTRIDIUM PERFRINGENS COLITIS

C. perfringens is a ubiquitous anaerobic spore-forming organism that is commonly found in the intestinal tract of many species and in the environment. Although strains of C. perfringens can be classified into five types by their ability to produce major toxins (Table 36-3), the role of different types in enterocolitis is poorly understood. Enterotoxin-producing strains have been evaluated most, and studies have demonstrated an association between the presence of enterotoxin in feces and diarrhea. β₂-toxin–producing strains have received attention recently; this may also be an important virulence factor. Type C strains have been implicated in severe disease in foals. Type A strains are most commonly isolated from horses with diarrhea, but there is some debate over the clinical relevance of these strains in the absence of β₂ or enterotoxin.

Table 36-3 Clostridium perfringens Types and Toxins