Diarrhoea

Diarrhoea in horses and foals
10g Faeces
Diarrhoea is common in equines of all ages but the percentage of horses with diarrhoea in which an
ante mortem diagnosis is made has been variably reported to be as low as 10 to 20%. In many cases
the cause may be undeterminable as the inciting factors may no longer be present by the time the
horse is examined or may be related to dietary or other factors. However, infectious agents and/or
their associated toxins are of major pathogenic importance in equine diarrhoeas and there are
several test procedures now available to improve our diagnostic rate and help select more targeted
therapy and prevention strategies.

NEMATODE EGGS AND LARVAE

An adult parasite burden is an uncommon cause of weight loss (especially as owners will invariably
have dewormed a thin horse!) but cyathostomin larval emergence is a common cause of acute (and
sometimes chronic) weight loss and in many cases, diarrhoea. Increasing prevalence of resistance
and shorter egg reappearance periods are leading to a significant parasite problem in horses which

are reportedly ‘well wormed’ therefore the de-worming protocol, including non-anthelminitic-
dependent approaches, should be critically appraised.

CLOSTRIDIAL TOXINS

Testing is available for toxins of Clostridium difficile (toxins A&B) and also Clostridium perfringens
enterotoxin and beta toxin. These toxins are readily detectable in faecal samples using enzyme
immunoassay tests and positive results infer either Clostridial enterocolitis or areas of severely
compromised or necrotic bowel with secondary Clostridial infection (e.g. neoplasia). Clostridia are a
common cause of post-antimicrobial diarrhoeas but are often also seen in equids of all ages in the
absence of prior antimicrobial treatment.

SALMONELLA

Salmonella can be a primary cause of diarrhoea in horses although is not common in the UK.
Intermittent shedding of Salmonellae may lead to false negative results and repeat samples are
always advisable. Confirmation of Salmonella can be made by either PCR or culture and to ensure a
horse is negative the convention is to collect 5 samples from different defaecations over a period of
up to 5 days. A positive result in an otherwise healthy horse can be due to shedding of the bacteria
and does not always confirm its involvement in the diarrhoea.

BACTERIAL CULTURE

Anaerobic culture for Clostridia is often uninformative as Clostridium difficile is very difficult to
culture in vitro and Clostridium perfringens (non-toxigenic) is a normal gut constituent of horses.
Hence immunoassays for clostridial toxins are preferred diagnostic methods. Similarly, many other
bacteria such as E. coli, Bacteroides and Enterococcus are also of highly little relevance when
cultured. By contrast, aerobic culture of Aeromonas, Campylobacter and Salmonella spp. are
probably relevant to diarrhoea. The gastrointestinal tract is normally very heavily populated by an
extensive and diverse bacterial population creating significant difficulty and confusion when faeces
are subject to culture or PCR in suspected bacterial enteritis cases. Even unequivocal
enteropathogens such as Salmonella spp. may sometimes be shed secondary to other primary
disease processes such as cyathostominosis.

SAND

The significance of finding sand in faecal samples is difficult to determine and probably simply
indicates the horse is grazing sandy pasture! Sand enteropathy is commonly suspected in certain
parts of the country as a cause of weight loss syndromes, diarrhoea and colic. However, the
presence of fairly large quantities of sand may be normal in some horses on certain pastures and the
finding of sand in faecal samples only really indicates that the horse is ingesting large quantities of
sand and doesn’t always necessarily imply aetiologic significance. To further diagnose sand as a
cause of colic radiography of the abdomen should be performed.

FAECAL OCCULT BLOOD

Faecal occult blood tests can be performed in horses to investigate distal intestinal bleeding;
however, blood loss in the proximal intestinal tract such as stomach and duodenum is highly unlikely
to be detected in faecal samples due to degradation in the colon. Claims that horse-side test kits for
faecal blood and protein can differentiate gastric and colonic bleeding or even detect bleeding from
any gastrointestinal source have little evidence-base and our experiences of using such kits have
been very disappointing. There seems to be little, if any, relationship between test kit results and
gastroscopy findings when diagnosing gastric ulcers. If a rectal examination has been performed
then the test should not be run afterwards as it will always be positive.