27 May 2019
Jamie Prutton
Job Title
Image: © Timur Abasov / Adobe Stock
Gastrointestinal (GI) disorders in horses are too numerous to discuss in detail in this piece. Two articles have reviewed the diagnostic and treatment modalities for horses presenting with colic, as well as how to make the decision to refer for surgery.
As such, colic will not be discussed in this article – rather, an overview of gastric ulceration treatment, weight loss investigation and reviews of recent literature shall be discussed, with emphasis placed on the increasing knowledge base of the GI microbiota and the effects veterinary intervention can have on this.
The horse’s GI tract is incredibly sensitive to non-specific systemic disturbances as well as GI-specific insults. Some research has shown, within the adult horse, distinct bacterial and protozoal compositional divides exist between the stomach, small and large intestines (Ericsson et al, 2016).
A difference also exists in the bacterial populations residing within the lumen and gut contents versus that on the mucosal surface. This information indicates analysis of faeces may not be truly representative of the gut microbiota and this could affect the diagnostic sensitivity of many faecal tests.
Various studies have looked at the effect of pharmacological and stressful intervention on the microbiota of the equine intestine. One study assessed the impact anthelmintic administration (moxidectin and praziquantel) may have and showed the individual GI microbiota was altered, but that the changes at a group level did not influence the individual horse. Researchers suggested, though, that in some individual cases, the effect could be more serious (Kunz et al, 2019) and lead to GI disturbances.
Antibiotics have been directly linked with the induction of diarrhoea, which, in some cases, can become fatal. It has been assumed this is due to a dysbiosis and subsequent overgrowth of pathogenic or abnormal bacteria, leading to maldigestion or direct damage to the intestine itself. Various antibiotics and their effects on the microbiome were assessed, and all were shown to have a dramatic effect on the microbiota (Costa et al, 2015).
The researchers found oral trimethoprim sulfadiazine had the most severe effect when compared with IV ceftiofur and IM penicillin. This may be due to the route of administration rather than the direct antimicrobial effect of each antibiotic. The bacterial population returned to normal 20 days after cessation of the medications. This highlights the importance of considering the side effects of all antibiotics in equids and, therefore, the use of antibiotics should always be carefully considered.
Rapid dietary modification is often cited as a cause of GI disruption and subsequent diarrhoea, gas production or colic, although the aetiology behind this change is relatively unknown. Some work has shown change from a forage-based diet to spring grass, with or without supplements, leads to a change in the microbiota of the intestine (Snalune et al, 2019).
None of the horses in the study showed abnormalities in their faecal production or were affected by colic; however, a 95% change in the volatile metabolome occurred when placed on grass.
It is relatively easy to speculate a small disruption in this change could lead to a dysbiosis that was not favourable to digestion of the forage. These changes could lead to overproduction of gas or overgrowth of pathogenic bacteria leading to the clinical signs often seen during transitions to different forage sources.
The current work on the microbiome opens up a huge potential for diagnosis and treatment of a large number of diseases. Leng et al (2018) showed a severe dysbiosis in equine grass sickness horses due to an increase in Bacteroidetes and a decrease in Firmicutes. They were also able to assess the urinary excretion of various markers that could predict the likelihood of equine grass sickness being present in the patient. This paper confirms the potential use of these scientific techniques to help our diagnostic strike rate in horses.
The World Health Organization recognises probiotics as “live microorganisms that, when administered orally at adequate concentrations, provide a beneficial effect beyond that of their nutritional value”. The majority of GI disturbances in horses are associated with microbiome changes occurring in the caecum and colon. As such, any treatment should, ideally, be aimed at returning this region to normality.
The concern with most probiotic products is that their constituents are only found in very low numbers in the equine colon and caecum. As an example, Lactobacillus species, Bifidobacterium species and Enterococcus species are frequently found in probiotics, but make up only 1% of the large intestinal microbiota in horses. Saccharomyces boulardii survived for less than 20 days following cessation of medication, indicating ongoing treatment is likely required to have any beneficial effect.
Whether these products do exert a positive effect on horses is still relatively unknown as the strength of evidence in research is weak. In assessing their effect on foals, some studies showed an increased risk of diarrhoea, while others a decreased risk. In adult horses, many studies have shown no improvement in diarrhoea rate, severity, duration or shedding of infectious aetiologies, while the most positive studies have only shown a reduction in the severity of diarrhoea, but not duration or outcome of cases. Therefore, their use should be undertaken in light of the lack of definitive research, as well as consideration of the poor regulatory control of their production.
Given evidence the equine microbiota is highly sensitive to change, it is unsurprising diarrhoea of unknown aetiology remains a common problem. That said, in all cases of adult diarrhoea it is important to assess the faeces for any infectious aetiologies, including Salmonella, Clostridium difficile, Clostridium perfringens or parasitism, depending on the age of the animal.
Gastric ulceration – be it within the squamous or glandular portions of the stomach – continues to be a frequently diagnosed syndrome affecting horses. A higher prevalence is seen in horses involved in performance disciplines, and ulceration has been shown to affect performance and have the most effect on maximal oxygen uptake during intense exercise.
The only reliable diagnostic method is gastroscopy with its associated costs, time and sometimes limited availability. The use of sucrose absorption was assessed as a possible diagnostics modality due to an inability to absorb sucrose as a whole molecule when the mucosa is intact (Hewetson et al, 2017). Frustratingly, the study found this was not a suitable test and, therefore, diagnosis must continue to be made using gastroscopy as no other biochemical or haematological markers exist.
Treatment has become a topic of debate with the introduction of new preparations of drugs. The most appropriate umbrella advice is to follow the cascade to chose medications while using the literature to guide which drugs might be the most applicable. Oral omeprazole remains the only licensed product for the treatment and prevention of gastric ulcers with good treatment success seen in squamous ulcers, but a more limited response noted in glandular ulcers.
Many other treatments have been investigated – some with limited or no response and others with some improvement in the clinical outcome. Misoprostol (5µg/kg every 12 hours) is more efficacious than combined oral omeprazole and sucralfate in treating glandular ulcers, but its use must be tempered by the human health implications of this medication, in the author’s opinion (Varley et al, 2016). Aloe vera was investigated and shown to have no effect on the healing of gastric ulceration (Bush et al, 2018) while the addition of corn oil has been shown to increase prostaglandin production and increase the gastric pH (Cargile et al, 2004).
A large number of papers have looked at the optimal dosing time, duration of action and absorptive capacity for omeprazole, but a review of this is beyond the scope of this article. The more novel preparations include a “specials” formulation of injectable omeprazole, which has been shown to have good efficacy in a small study when compared to the reported results for oral omeprazole (Sykes et al, 2017).
Weight loss can be a frustrating syndrome in the horse – often with limited definitive outcomes in the diagnostic procedures. When assessing the recent literature, very little has been published to help in the diagnosis or treatment of cases and, therefore, a sensible approach to each case should be methodical diagnostics to rule out common diseases.
Initial work-up should include blood work as hypoalbuminaemia and hypoproteinaemia have been shown to correlate with the survival of patients. Survivors were seen to have a total protein of 66.2g/L (44g/L to 81.7g/L) compared with 53.9g/L (39.7g/L to 93g/L) in non-survivors, while the albumin in survivors was 29.5g/L (17g/L to 38g/L) compared with 21.6g/L (13.6g/L to 34g/L) in non-survivors (Metcalfe et al, 2013). Any hypoalbuminaemia/hypoproteinaemia should be thoroughly investigated for further evidence of parasitism (encysted cyathostomins), right dorsal colitis or infiltrative bowel disease. A complete review of the haematological and biochemical parameters may reveal an ongoing hepatopathy, renal disease or even myopathies that could be contributing to the weight loss.
Faecal analysis can be highly frustrating, with a negative faecal worm egg count not definitively ruling out an encysted cyathostominosis as the encysted larvae do not produce eggs. Therefore, if the clinical suspicion leads to a diagnosis of encysted cyathostominosis and the blood work backs this up, treatment should be instigated in the face of a negative faecal worm egg count.
Ultrasonography is essential for a complete abdominal examination, with diseases such as gastric impactions, infiltrative bowel disease and peritonitis (with subsequent abdominocentesis performed) all relatively easily noted, and then further diagnostics performed to confirm the aetiology. Often, ultrasonography will allow a more guided diagnostic approach, even if it does not give a definitive diagnosis.
If an infiltrative bowel disease – be it inflammatory or neoplastic – is suspected, biopsies can be taken. When the horse presents with diarrhoea, a rectal biopsy would be ideal, but this should not be taken when no large intestinal disease is suspected as interpretation of the results is very difficult. If duodenal thickening is noted on ultrasound, biopsies can then be taken when performing a gastroscopy – although, due to the small size of endoscopic biopsy forceps, samples can be non-representative or damaged during sampling.
Biopsies can also be taken during a laparotomy or under standing sedation and laparoscopy. The latter has been performed at Liphook Equine Hospital, with an excellent success rate of sample collection and diagnosis. Treatment will then be based on the exact aetiology, but often, in infiltrative bowel disease cases, revolve around immunosuppressive medications including dexamethasone (0.1mg/kg every 24 hours), prednisolone (1mg/kg every 24 hours) or azathioprine (2mg/kg every 24 hours).
