Page 864 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Gastrointestinal system: 4.2 The lower gastrointestinal tr act 839

VetBooks.ir Differential diagnosis combination with toxin A testing. The common
antigen test also reacts with non-toxigenic strains
Non-infectious causes of colitis such as right dor-
sal colitis and sand enteropathy should be consid-
tive common antigen results in the absence of
ered. Intestinal accident is a differential diagnosis and some non-pathogenic bacteria, therefore posi-
in severe cases. Cantharidin toxicosis and cyathos- detectable toxins in faeces should be interpreted
tominosis should be considered depending on the with caution. Culture is usually reserved for epi-
geographic location, time of year and clinical pre- demiological purposes.
sentation. In foals, rotavirus and Lawsonia intracel-
lularis infection should also be considered. Other Clostridium perfringens-associated diarrhoea
toxic causes of colitis such as arsenic toxicosis Diagnosis of C. perfringens-associated diarrhoea can
are uncommon but should be considered in some be difficult because C. perfringens can be isolated
situations. from the faeces of a large percentage of normal
horses. No correlation between numbers of C. per-
Diagnosis fringens or bacterial spores in faeces and diarrhoea
General has been reported. Typing of bacterial isolates to
Clinical signs and haematology are non-specific determine what toxin genes they possess can be
and not diagnostic. Leucopenia, neutropenia, left- useful, but the relevance of identification of toxi-
shift and toxic changes in neutrophils are common. genic strains in the absence of demonstrable toxin is
Leucocytosis and monocytosis may occur more unclear. Type C strains are uncommonly identified
commonly with PHF. Urinalysis should be per- in normal animals and detection of these strains is
formed, particularly if elevations in urea and creati- highly suggestive of disease. Similarly, identifica-
nine levels are present. Total plasma protein levels tion of genes coding for the production of beta-2
should be evaluated. Monitoring of plasma electro- toxin and C. perfringens enterotoxin (CPE) is sug-
lyte concentrations is useful to guide treatment. gestive. The significance of identification of type A,
Tests for specific pathogens are described below. the most common strain, in the absence of beta-2
Coinfection can occur, so a broad range of testing toxin or CPE genes is debatable. Diagnosis is best
is recommended. if based on identification of toxins in faecal samples.
Detection of CPE in faeces is highly suggestive of
Salmonellosis disease. If rapid tests to detect other C. perfringens
Faecal culture is the most common diagnostic tool. toxins become available, the ability to diagnose this
Because of intermittent shedding and relatively low condition will improve.
sensitivity of testing, a single negative culture can-
not rule out salmonellosis and 3–5 negative samples Potomac horse fever
are required. Rectal mucosal biopsy specimens can A presumptive diagnosis is often made based on
also be submitted for culture although this is not appropriate clinical signs during the appropriate
commonly performed. This is perhaps more useful time of year in an endemic area; however, this is not
in cases with chronic colic or fever of unknown ori- definitive, as other sporadic causes of diarrhoea can-
gin versus fulminant colitis. The utility of PCR test- not be differentiated clinically. Response to empiri-
ing of faeces remains unclear, but it may be a useful cal treatment with oxytetracycline is suggestive,
screening test. but not diagnostic. Serological testing is available.
A four-fold change in antibody titre between acute
Clostridium difficile infection and convalescent samples is supportive, but often
Detection of C. difficile toxins in faecal samples does not occur. PCR testing is becoming more com-
is diagnostic for CDI. Faecal samples should be monly available and appears to be more useful. Early
tested for the presence of both C. difficile toxins A in the disease, blood samples may be PCR posi-
and B. Fresh, refrigerated samples are preferred; tive and faecal samples PCR negative. This may be
however, C. difficile toxins are stable in vitro. reversed later in the disease, so both blood and faecal
‘Common antigen’ testing is available, usually in samples should be submitted.