Acute colitis (adult horses)

Treatment

Horses with 2 of the 3 classical signs of colitis (fever, diarrhoea and leukopaenia) should be managed in isolation because of the risk of contagious and zoonotic diseases.

Referral of cases that have watery diarrhoea and signs of endotoxaemia (with or without fever), to a facility with 24 h care and isolation facilities is encouraged to provide intensive care and high-volume fluid therapy and improves the probability of a successful outcome.

Overarching treatments

The main treatment for horses with diarrhoea is symptomatic. Fluid therapy to correct dehydration, hypovolaemia and electrolyte loss is critical, and large volumes are usually required. Fluids can, in mild cases, be administered orally, but in moderate to severe cases oral fluid therapy is not sufficient or can even be contra-indicated (if ileus is present), and intravenous fluid therapy is essential. The water absorptive capacity of the colon (the section of the gastro-intestinal tract that absorbs water) is limited and compromised due to pathology and dysfunction of the colon and due to poor gastro-intestinal perfusion secondary to shock. Hartmann’s solution is preferred over normal saline (0.9% NaCl), as normal saline is acidifying, whereas Hartmann’s solution is alkalinising because it contains lactate, which is converted to bicarbonate in the liver. This is an advantage in colitis, where metabolic acidosis is the predominant acid-base abnormality.  Hartmanns is also polyionic and therefore causes less electrolyte disturbances when given in large volumes than normal saline. Hypertonic saline (7% NaCl at 4 ml/kg IV) can lead to rapid volume expansion in severely hypovolaemic cases, but must be followed with isotonic fluids, either administered orally (via a nasogastric tube or providing free access to them) or intravenously. A resuscitation bolus with isotonic fluids of up to 60-80 ml/kg can be administered if needed. Ongoing fluid requirements can be estimated from maintenance needs, the degree of dehydration, and ongoing losses through the diarrhoea. Ongoing losses are difficult to estimate and may be large. Frequent monitoring of PCV, total protein, electrolytes and lactate are recommended to guide fluid therapy.

Treatment to address endotoxaemia is addressed elsewhere (see Section 7).

Protein loss is a common feature of acute colitis and can be significant and cause decreasing colloid oncotic pressure. Therefore, in the face of hypoproteinaemia, fluid therapy can easily lead to formation of oedema. Even though hydration is a vital treatment for colitis, it is also important to avoid overhydration and not to prolong use of fluid therapy longer than needed to minimize formation of oedema. Colloids, ideally plasma (up to 20 ml/kg) can be required. The cost of plasma for adult horses is substantial. Synthetic colloids are also available, with Hetastarch (10 ml/kg/day) the most widely used, although Pentastarch and Tetrastarch have also been administered to horses with colitis, hypoproteinaemia and low colloid oncotic pressure. Evidence for their use is weak, but in a retrospective study evaluating the use of Hetastarch, a smaller proportion of cases (47%) receiving Hetastarch survived than cases treated with plasma (80% survival), even though they had similar disease severity on admission (12). There has been concern about adverse effects associated with treatment with synthetic colloids, but none were recorded in the 23 horses that received Hetastarch alone in this study. Further studies are needed to evaluate the benefits of these products. At this time, in colitis cases with severe hypoproteinaemia, when plasma is not available or cost-prohibitive, synthetic colloids can be considered to improve colloidal oncotic pressure. Administration by constant rate infusion is recommended when using synthetic colloids to increase colloidal oncotic pressure (13).

Non-steroidal anti-inflammatory drugs are integral to address endotoxaemia, SIRS and pain. Flunixin meglumine remains the most used NSAID at either the so-called ‘anti-endotoxic dose’ (0.25 mg/kg IV q 8 h), which has no anti-inflammatory effects, or at a higher dose (0.5 – 1.1 mg/kg IV q 12 h) (See Section 7 for more information on managing endotoxaemia). The authors recommended higher doses of flunixin meglumine for combatting inflammation and pain in addition to endotoxaemia. However, NSAIDs should be avoided in cases with right dorsal colitis. If pain cannot be managed with NSAIDS (or without NSAIDs, in right dorsal colitis), use of other analgesic medications is recommended (opioids, alpha2 agonists, lidocaine constant rate infusion). In cyathostominiasis cases, steroid therapy (prednisolone or dexamethasone) can be indicated, instead of NSAIDs, to reduce the intestinal inflammation (6).

In general, antimicrobials are not indicated in cases of colitis. Antimicrobial use results in intestinal dysbiosis, which may prolong recovery and shedding of pathogens in colitis cases. In addition, there is no evidence of a more rapid recovery in cases of salmonellosis after antimicrobial therapy. Translocation of bacteria across the damaged intestinal mucosa has been demonstrated, but there is no evidence that antimicrobial therapy improves the prognosis. Historically, clinicians have treated horses with broad-spectrum prophylactic antimicrobials when neutropaenia was severe. However, there are many clinicians managing colitis cases with severe neutropaenia without antimicrobial therapy with no adverse effects. If secondary infections develop, then antimicrobial therapy should be targeted to these infections.

It is appropriate to avoid potentially nephrotoxic drugs as much as possible until onset of fluid therapy or until creatinine normalises.

Cryotherapy for all four feet may reduce the incidence of laminitis. Ideally, cryotherapy should be applied for the duration of the developmental phase of laminitis, so horses exhibiting endotoxaemia should be treated from the first clinical examination (see Section 7). There is an excellent review by van Eps, (14) of mechanism and recommendations for cryotherapy.

Re-instauration of a healthy flora, by administration of probiotics, is often suggested, but evidence for this is lacking (15). Faecal microbiota transplantation (administration of faecal liquid from a healthy horse to a sick horse) can be performed and has been shown to be safe. Significant advantages to patients are yet to be demonstrated (16) and optimal methods and timing of this treatment remain unclear, however this treatment is common in horses. Donors should be healthy, have not been exposed to antimicrobials in the past 6 months, and free from potential gastrointestinal pathogens, such as Salmonella spp. and gastrointestinal parasites. Ideally donors should be from the same farm and housed under the same conditions as the recipient. Recipients should have antimicrobial therapy discontinued prior to transplant and be pretreated with a protein pump inhibitor (omeprazole). Faeces are removed from the rectum of the donor horse and mixed with warm water or isotonic saline then the mixture blended and strained. A 2-3 L volume is administered by nasogastric tube for an average 450 kg horse. Free choice long stem early cut hay should be offered as soon as practical following faecal microbiota transplantation. The procedure should be repeated daily until there is improvement in faecal consistency, or for up to 3 days. A fresh transplant should be prepared daily (17).

General feeding guidelines for horses with colitis are:

1. Withhold food if the horse is in severe shock, as the reduced perfusion of the gastro-intestinal tract and potential ileus limit the capacity to tolerate food.
2. Do not force an inappetant horse to eat; this is a sign that the gastro-intestinal tract is probably not ready to tolerate food.
3. If there is a substantial period of inappetence, intravenous glucose and/or amino acids can be administered to provide calories.
4. There is no evidence to guide refeeding, and refeeding strategies vary – some authors advocate the use of fractioned feeding when the appetite returns (small portions multiple times per day), and high fibre but low bulk foods (grass, chaff, lucerne cubes, beet pulp, psyllium, avoiding significant amounts of hay, no straw), whereas others offer free access to lucerne hay throughout the course of disease with a view to providing nutrition to enterocytes to promote healing.

Disease-specific treatments

Antimicrobial-associated diarrhoea: Antimicrobial therapy should be discontinued.

Clostridial diarrhoea: Metronidazole is indicated (15 mg/kg PO q 8 h or 25 mg/kg PO q 12 h) for 3 days. Resolution of diarrhoea is often rapid following metronidazole therapy. Resistance has not been documented in Australia. Treatment should be reserved for cases where clostridial infection has been documented with concurrent toxin gene detection. Because the action of the metronidazole is required within the gastrointestinal tract, oral metronidazole treatment is recommended. The intravenous and rectal routes have not been evaluated and transport into the gastrointestinal tract is unknown.

Cyathostominosis: The preferred treatment is with moxidectin (0.4 mg/kg PO). Fenbendazole (10 mg/kg PO q 24 h for 5 days) can also be used, but there is increasing resistance of strongyles to fenbendazole. Prednisolone at 1 mg/kg PO q 24 h for 24-48 h is recommended in clinically affected horses prior to and during administration of anthelmintics to control inflammation in the colon.

Right dorsal colitis: There are four principles of treatment: 1, avoid further NSAID use; 2, minimise stress; 3, dietary modification; and 4, targeted medication. Large-volume fibre should be eliminated from the diet to remove bulk from the colon and favour healing. Pelleted feed alone, or with small quantities of green grass, should be fed for 3-6 months. Oil can be used to increase calories in the diet, if necessary, and corn oil has been demonstrated to increase gastric PGE2 and promote mucosal healing. Misoprostol is a synthetic PGE1 analogue that can be administered at doses of 2-5 µg/kg every 6-12 h, but should be handled with caution, as exposure can cause abortion in people. Sucralfate and psyllium husks have also been used, but the efficacy of these treatments is unknown.

Sand enteropathy: Combination therapy with psyllium and MgSO₄ (Epsom salts) (both at 1 g/kg) via nasogastric tube once daily for multiple days is the preferred method of treatment. Further, the collective findings of published research suggest that administering psyllium in feed as a preventative has no advantage over removing access to sand (18).

Prognosis

The prognosis is guarded. Referral to a hospital that can provide 24 h intensive care is recommended, as retrospective studies have reported mortality rates of between 19 and 42%.

Serial blood lactate measurement can be used to guide the prognosis. In retrospective studies, non-survivors had the highest lactate concentrations (mean of 5.35 mmol/L) at admission and after 72 h (mean of 2.35 mmol/L), compared to survivors (means of 1.75 mmol/L at admission and 0.75 mmol/L after 72 h) (19). Another study found 4-8 h lactate concentration, 24 h lactate concentration and the percentage reduction in lactate concentration (≥ 30% at 4-8 h and ≥ 50% at 24 h) were significantly associated with survival. No clear “cut-off value” for non-survival has been elucidated, emphasising the importance of serial measurements and examination of trends in lactate concentration, and full assessment of all clinical aspects of the patient, rather than any one value in assessing the prognosis.

The prognosis for right dorsal colitis and larval cyathostominosis may be worse than for other aetiologies, with mortality rates of approximately 60% reported. Antimicrobial-associated diarrhoea has been shown in multiple studies to have a significantly higher mortality than other causes of diarrhoea.