Acute Equine Diarrhoea: Why Are We Still Missing the Diagnosis?

Acute diarrhoea remains one of the leading causes of hospitalisation and mortality in horses worldwide. Despite advances in diagnostic testing, identifying the exact aetiological agent continues to be challenging in equine practice¹ . Common causes include Salmonella enterica, Clostridioides difficile, Equine coronavirus (ECoV), Neorickettsia risticii (Potomac horse fever), cyathostominosis, sand enteropathy, antimicrobial-associated diarrhoea (AAD), and NSAID-induced right dorsal colitis. However, even after extensive diagnostic investigations, the causative agent is identified in fewer than 50% of horses^1,2. 

A recent multicentre retrospective study evaluating diarrhoeic horses across referral institutions worldwide highlighted major inconsistencies in diagnostic approaches and pathogen testing. Surprisingly, an enteropathogen was detected in only 16% of horses included in the study. This low detection rate likely reflects the limited and inconsistent use of diagnostic testing rather than the true absence of infectious disease¹. 

Diagnostic Testing Still Highly Variable 

One of the most important findings was the variation in testing protocols among institutions and even among clinicians within the same hospital. For example, only 44% of horses were tested for Salmonella, and many underwent just a single faecal culture despite current recommendations advocating serial collection of 3–5 samples using enrichment techniques to improve sensitivity^1,3. 

Similarly, Neorickettsia risticii testing was performed in only 42% of horses, Clostridioides difficile in 40%, and ECoV in just 29% of cases. Availability of diagnostic tests, geographical disease prevalence, financial limitations, and clinician perceptions regarding the usefulness of testing were all considered likely contributors to this variability¹. 

Importantly, the study demonstrated that developing guidelines does not necessarily translate into consistent implementation in clinical practice. Some horses within the same institution were tested extensively, while others received minimal or no aetiological investigation. 

Salmonella: Serial Sampling Still Matters 

The overall prevalence of Salmonella was 13%, consistent with previous reports in hospitalised horses worldwide. However, detection rates varied significantly between institutions, ranging from 0% to 22% ⁴. 

Geographic and climatic influences appeared important. Warmer and wetter regions demonstrated the highest detection rates¹. 

Crucially, the probability of detecting Salmonella increased with each additional faecal sample submitted for culture. This reinforces the importance of serial sampling in suspected cases. PCR assays also showed good agreement with faecal culture and may improve future diagnostic efficiency, particularly when combined with enrichment methods¹. 

Emerging Role of PCR Panels 

Because traditional diagnostic methods frequently fail to identify pathogens, in-house PCR-based diarrhoea panels have been proposed as a strategy to improve detection rates and identify co-infections⁵. These panels may also help identify emerging pathogens that could be overlooked using conventional techniques. 

Recently proposed organisms associated with equine diarrhoea include NetF-producing Clostridium perfringens, Paeniclostridium sordellii, and Clostridium innocuum^1,6,7,8. Metagenomic studies have also repeatedly identified increased abundance of Fusobacteria in diarrhoeic horses⁹. 

As equine practitioners continue to face diagnostic uncertainty in acute diarrhoea cases, this study highlights the need for more standardised testing protocols, improved accessibility to diagnostics, and continued investigation into emerging enteric pathogens. 

References  

1. Gomez DE, Arroyo LG, Schoster A, Renaud DL, Kopper JJ, Dunkel B, Byrne D, MEDS group, Toribio RE, Mykkanen A, Gilsenan WF. Diagnostic approaches, aetiological agents and their associations with short‐term survival and laminitis in horses with acute diarrhoea admitted to referral institutions. Equine veterinary journal. 2024 Sep;56(5):959-69. https://beva.onlinelibrary.wiley.com/doi/pdf/10.1111/evj.14024  

2. International Equine Colitis Research Group. Science‐in‐brief: report on the Havemeyer Foundation workshop on acute colitis of the adult horse. Equine veterinary journal. 2020 Feb 8;52(2):163. https://doi.org/10.1111/evj.13223  

3. Gomez DE, Leclere M, Arroyo LG, Li L, John E, Afonso T, Payette F, Darby S. Acute diarrhea in horses: A multicenter Canadian retrospective study (2015 to 2019). The Canadian Veterinary Journal. 2022 Oct;63(10):1033. https://pmc.ncbi.nlm.nih.gov/articles/PMC9484212/  

4. Burgess BA, Morley PS. Risk factors for shedding of Salmonella enterica among hospitalized large animals over a 10‐year period in a veterinary teaching hospital. Journal of veterinary internal medicine. 2019 Sep;33(5):2239-48. https://doi.org/10.1111/jvim.15579  

5. Bierbaum M, Rapport F, Arnolda G, Nic Giolla Easpaig B, Lamprell K, Hutchinson K, Delaney GP, Liauw W, Kefford R, Olver I, Braithwaite J. Clinicians’ attitudes and perceived barriers and facilitators to cancer treatment clinical practice guideline adherence: a systematic review of qualitative and quantitative literature. Implementation Science. 2020 May 27;15(1):39. https://doi.org/10.1186/s13012-020-00991-3  

6. Gohari IM, Arroyo L, MacInnes JI, Timoney JF, Parreira VR, Prescott JF. Characterization of Clostridium perfringens in the feces of adult horses and foals with acute enterocolitis. Canadian journal of veterinary research. 2014 Jan 1;78(1):1-7. https://pmc.ncbi.nlm.nih.gov/articles/PMC3878003/  

7. Mehdizadeh Gohari I, Unterer S, Whitehead AE, Prescott JF. NetF-producing Clostridium perfringens and its associated diseases in dogs and foals. Journal of veterinary diagnostic investigation. 2020 Mar;32(2):230-8. https://doi.org/10.1177/1040638720904714  

8. Uzal FA, Arroyo LG, Navarro MA, Gomez DE, Asín J, Henderson E. Bacterial and viral enterocolitis in horses: a review. Journal of Veterinary Diagnostic Investigation. 2022 May;34(3):354-75. https://doi.org/10.1177/10406387211057469  
9. Zakia LS, Gomez DE, Caddey BB, Boerlin P, Surette MG, Arroyo LG. Direct and culture-enriched 16S rRNA sequencing of cecal content of healthy horses and horses with typhlocolitis. PLoS One. 2023 Apr 13;18(4):e0284193. https://doi.org/10.1177/10406387211057469