Diarrhoea is a common ailment in adult horses; though frequently innocuous, in a minority of cases it can become fatal. There have been no major changes or breakthroughs in the diagnosis and treatment of equine diarrhoea, yet it continues to plague both ambulatory and referral vets. More recent research has looked at the microbiome of the horse and hopefully this will shed light on the aetiology of the idiopathic cases, which make up the majority of cases. Only 30 percent of cases in one study had a definitive aetiology diagnosed antemortem, although with changes and advances in diagnostic techniques since the study was performed, this percentage might now be higher (Love et al., 1992).
Biosecurity should always be at the forefront of the attending veterinary surgeon’s mind to ensure there is not a zoonotic risk to the owners, particularly if there are any immunocompromised people involved in the care of the animal. Therefore, if infectious aetiologies are considered a risk, all personnel involved should wear protective personal equipment.
Simplistically, diarrhoea can be broken down into three main groups: idiopathic, acute colitis and protein losing enteropathies. This list is not exhaustive as each aetiology is not discussed in detail.
Idiopathic diarrhoea
Owners will often report that horses have a free water phase of faecal expulsion. There is no clear aetiology behind this, although speculation surrounding fibre digestion in the colon has been put forward. Changing the diet can help these clinical signs, with an improvement in the dietary fibre being helpful. Ideally, blood work should be performed and if this is unremarkable then ongoing monitoring is advisable. Faecal analysis should be performed to rule out any infectious aetiologies.
Sand enteropathies are location dependant but can lead to either chronic or acute diarrhoea depending on the severity of the sand accumulation.
Acute colitis
These are often the most severe of cases with marked disturbances in the haematological and biochemical parameters, particularly acid-base values, electrolytes and serum proteins. If the horse is presenting with sepsis and endotoxaemia, immediate empiric intensive care should be instigated to facilitate the best prognosis. During the initial diagnostic phase, the owners should be made aware of the high risk of secondary problems such as laminitis and thrombophlebitis.
The aetiological agent in these cases is frequently difficult to ascertain. Possible causes can include: antibiotic-induced diarrhoea (with secondary bacterial overgrowth), salmonellosis, encysted cyathostominosis with mass emergence (Figures 1 and 2), clostridial overgrowth with enterocolitis and carbohydrate overload when the horse had access to unusual or excessive food. The inciting cause can be something as innocuous as a colic episode that leads to a suspected imbalance in the microbiome within the colon.
Protein losing enteropathies
More frequently these cases will present with weight loss rather than diarrhoea due to the propensity for the small intestine to be affected rather than the large intestine (Figure 3). Some cases of right dorsal colitis due to NSAID toxicity will present with diarrhoea and a severe thickening of the right dorsal colon.
If a PLE is suspected, then diagnoses can be made if appropriate samples can be taken (rectal or duodenal biopsies) although in some cases a laparoscopic approach is required to retrieve a suitable sample for diagnosis.
Diagnostics
In all cases of diarrhoea, full haematology and biochemistry should be run. Haematology will allow:
- Evaluation of the white cell population: marked neutropaenia is seen in acute cases due to sequestration into the site of inflammation. In more chronic cases, you would expect to see a neutrophilia. The neutrophilia will become severe in cases of encysted cyathostominosis.
- Assessment of red cell indices for evidence of hypovolaemia but splenic contraction can lead to an elevation in PCV so other markers such as lactate can be used if concerned. Anaemia can be seen secondary to chronic inflammation as a negative acute phase reaction.
Biochemical analysis will often show several derangements in severe, acute colitis cases, but particular attention should be paid to:
- Serum proteins: The equipment used, and their associated reference ranges, can dramatically alter the interpretation of results. Some analysers have completely inappropriate reference ranges for horses. Most horses will have an albumin of 30-40g/L and when using a refractometer, the total solids will include globulins (25-40g/L) and therefore should have a range of 55-80g/L. Interpretation of results should be made using absolute numbers rather than just the highlighted abnormal results. The hydration status of the horse must be taken into account. If a horse is severely dehydrated with a “normal” serum protein, once it is rehydrated it will likely have a severe hypoalbuminaemia and may start to show clinical signs of hypoproteinaemia.
- Acute phase proteins: Serum amyloid A, fibrinogen and iron can all be analysed to assess the severity of the inflammation, giving some gauge as to the chronicity of the disease process.
- Azotaemia: It is important to distinguish between pre-renal, renal and post-renal elevations to give an appropriate prognosis. This is virtually impossible at presentation but you would expect to see a dramatic improvement/resolution of creatinine levels with appropriate fluid therapy in less than 24 hours.
- Lactate: This is a fundamental marker that can help monitor fluid therapy outcome and help predict prognosis once therapy has started. There can be a worsening of the lactate levels when fluid therapy is started due to reperfusion, but this should gradually resolve with ongoing fluid therapy.
Faecal analysis is essential to arrive at a diagnosis and there are multiple different tests that should be performed:
- Worm egg count: It should be noted that false negative results will frequently be seen, especially when considering encysted cyathostomins as they are not reproductively active. There is a large inter-day variability in the production of eggs which can again lead to false negatives. It is a useful adjunct to the diagnostics, but a negative result does not always rule out parasitism.
- Sand: Faecal analysis for sand is notoriously inaccurate as often it will remain in the colon without passing into the faeces or can pass through with no accumulation within the colon. If sand is considered a likely aetiology for the diarrhoea, abdominal radiographs should be taken. These cases will often present with a low-grade diarrhoea, intermittent colic or hypoalbuminaemia.
- Culture: The only pathogen that is important to culture for is Salmonella. A positive result does not definitively confirm Salmonella as the cause but is likely relevant and strict biosecurity should be implemented. Culture of anaerobic bacteria such as clostridia is rarely helpful as they are often a commensal of the gastrointestinal tract.
- PCR: Rapid and sensitive results can be achieved for Salmonella and research has shown it to be as sensitive and specific as culture. Positive results cannot be typed without having to culture them first. Often the speed of diagnosis heavily outweighs the delay in typing. More recently there has been interest in coronavirus as a cause. It has been diagnosed by PCR frequently in the United States, Japan and France with the first diagnosis in the UK in 2016. It seems to be an infrequent pathogen but should be considered in appropriate cases.
- ELISA: Analysis for toxins associated with C. difficile and C. perfringens can be the most rewarding diagnostic modality utilised in equine patients. It is important to test for the toxins themselves rather than the bacteria to confirm that they are active and likely causing the clinical signs. C. difficile is frequently associated with antibiotic-induced diarrhoea and should be tested for at the earliest opportunity in these cases.
Ultrasonography is often essential when considering PLE, encysted cyathostomins or other forms of diarrhoea. The most important aspect of the scan is to assess the intestinal wall thickness of both the small and large intestine. It can also be helpful to assess the gut content (Figure 4), although liquid faeces in the colon does not always correlate with clinical diarrhoea.
The next article will discuss treatment protocols.
