Sudden Death in Sheep: Key Differential Diagnoses for the Veterinarian

A sheep found dead on pasture may leave little warning. One animal may have been affected by a rapidly progressive disease, while the rest of the flock appears normal. The challenge is not simply to identify a lesion after death, but to connect the flock circumstances, likely disease process and postmortem findings. 

Several causes described in the source are particularly relevant when investigating sudden death in sheep. 

Enterotoxemia: Think Beyond an Apparently Healthy Animal 

Enterotoxemia can be acute or peracute and is a recognised cause of sudden death in apparently healthy ruminants. Clostridium perfringens toxinotypes can produce severe toxin-mediated disease, with systemic effects that may include vascular and tissue injury^[1],[2]. 

At necropsy, acute hemorrhagic enteritis, particularly involving the jejunum and ileum, should raise suspicion of clostridial disease. However, these findings are not sufficient on their own. Clinical history, epidemiological information, necropsy, and histological findings should be considered together^[3]. 

Could Hypomagnesemia Be the Explanation? 

A flock with apparently unexplained deaths on pasture also warrants consideration of hypomagnesemia. 

The risk increases when the diet is low in magnesium and high in potassium, particularly with fast-growing lush grasses. Clinical signs may be absent, and abrupt mortality can occur on pasture^[4]. 

This makes the feeding and pasture history particularly valuable. Hypomagnesemia is characterised by low serum magnesium, with concentrations below 0.7 mmol/L. Because postmortem changes can complicate diagnosis, biochemical investigation is important; magnesium measurement in appropriate postmortem samples can support confirmation^[3]. 

When the Intestine is the Problem 

Red gut is another important cause of rapid death in sheep. It has been associated particularly with weaned lambs and adult sheep grazing lush pastures, including lucerne, white clover, and ryegrass^[3]. 

The proposed mechanism involves increased intestinal mass and mobility following high intake of digestible forage. Intestinal twisting can obstruct mesenteric blood vessels, resulting in shock and death^[3]. 

At necropsy, an intensely reddened or dark-red intestinal mass, often displaced towards the anterior abdomen, provides an important clue^[3]. 

Do Not Ignore Respiratory Causes 

Respiratory disease can also progress rapidly enough that death is the first obvious indication. 

Mannheimia haemolytica is particularly associated with sudden death in sheep and can also cause septicemia in young lambs and kids. Histophilus somni pneumonia in sheep and goats may similarly present with death as the first recognised sign, with septicemia and thromboembolic meningoencephalitis contributing to fatal disease^[3],[4]. 

Thus, an absence of previously observed respiratory illness should not automatically exclude an infectious respiratory differential. 

What About Parasitism? 

Heavy parasitic burdens can occasionally produce a peracute picture. Hyperacute haemonchosis, for example, can result from severe Haemonchus infection and may be associated with anemia, dark-coloured feces, weakness and submandibular edema or “bottle jaw”^[5],[6]. 

These findings can help connect an apparently sudden flock loss with an underlying parasitic process. 

Clinical Bottom Line 

When a sheep is found dead unexpectedly, the most useful question is not simply “What lesion is present?” but “Does the lesion fit the flock history and circumstances?” 

Pasture and feeding changes, age group, concurrent illness, gastrointestinal or pulmonary lesions, and targeted laboratory confirmation can help distinguish among enterotoxemia, hypomagnesemia, red gut, respiratory disease and severe parasitism. A systematic approach is especially important when clinical signs were not observed before death.

References:

1. Uzal FA, Freedman JC, Shrestha A, Theoret JR, Garcia J, Awad MM, Adams V, Moore RJ, Rood JI, McClane BA. Towards an understanding of the role of Clostridium perfringens toxins in human and animal disease. Future microbiology. 2014 Mar 1;9(3):361-77. https://pmc.ncbi.nlm.nih.gov/articles/PMC4155746/pdf/nihms617524.pdf
2. Simpson KM, Callan RJ, Van Metre DC. Clostridial abomasitis and enteritis in ruminants. The Veterinary Clinics of North America. Food Animal Practice. 2018 Feb 1;34(1):155. https://pmc.ncbi.nlm.nih.gov/articles/PMC7127689/pdf/main.pdf
3. Benchohra M, Hemida H, Ali-Nehari A, Adnane M, Boumezrag A, Chikhaoui M. Sudden death syndrome in domestic ruminants: a review. Comparative Clinical Pathology. 2024 Jun;33(3):489-502. https://www.researchgate.net/profile/Mira-Chikhaoui-3/publication/379908486
4. Underwood WJ, Blauwiekel R, Delano ML, Gillesby R, Mischler SA, Schoell A. Biology and diseases of ruminants (sheep, goats, and cattle). InLaboratory animal medicine 2015 Jan 1 (pp. 623-694). Academic Press. https://pmc.ncbi.nlm.nih.gov/articles/PMC7149867/pdf/main.pdf
5. Besier RB, Kahn LP, Sargison ND, Van Wyk JA. Diagnosis, treatment and management of Haemonchus contortus in small ruminants. Advances in parasitology. 2016 Jan 1;93:181-238. https://www.researchgate.net/profile/Jan-Van-Wyk/publication/302912593
6. Roeber F, Jex AR, Gasser RB. Impact of gastrointestinal parasitic nematodes of sheep, and the role of advanced molecular tools for exploring epidemiology and drug resistance-an Australian perspective. Parasites & vectors. 2013 May 27;6(1):153. https://link.springer.com/content/pdf/10.1186/1756-3305-6-153.pdf