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Large animal section: No. 10A NEGATIVE OR NON-SPECIFIC POST MORTEM EXAMINATION IN SHEEP : A DIAGNOSTIC CHALLENGEA necropsy examination is a valuable tool in the armentarium of the diagnostician, but may equally lead to frustration when no definite conclusions can be reached. In sheep, and for that matter also in other domestic ruminants, this may possibly be explained by the relatively large number of infectious and toxic conditions that do not produce characteristic gross lesions at necropsy, or by the fact that lesions are often subtle and need confirmation by histopathological examination. This is particularly true for diseases and syndromes that affect the central nervous system (CNS) in sheep: a meaningful interpretation of clinical signs is often difficult; clinical signs are frequently linked to musculoskeletal dysfunction or systemic illness; and macroscopical lesions are absent or difficult to detect at routine necropsy examination. A sound history and careful observation of clinical signs in affected sheep is even more important with negative or non-specific necropsy results: take note of the age and breed (falling disease in Dorper lambs), pregnancy (hypocalcaemia and hypomagnesaemia), blindness (poisoning by Helicrysum argyrosphaerum, rafoxanide and closantel, and polioencephalomalacia) and possible contamination of feed or dips (pesticide poisoning and urea poisoning). Tables 1-4 list differential diagnoses of diseases that affect the nervous system of sheep (and goats) on the basis of neurological signs. The post mortem examination should be thorough and a complete set of tissues be collected in formalin for histology. The brain and spinal cord should be included. The entire brain can be submitted or, alternatively, cut through the head longitudinally and fix both halves of the brain in an ample volume of formalin. Before fixation of the spinal cord, incise the dura mater to enhance penetration of formalin. If it is not possible to remove the entire spinal cord, cut transversely through the thoracolumbar spine and fix an entire vertebral body. A brain smear is a definite must - we have diagnosed heartwater in sheep with no distinctive gross lesions in the carcase! Botulism in sheep is a typical example of a condition that may be misdiagnosed at necropsy due to the non-specific nature of the clinical signs (sheep seldomly develop the typical flaccid paralysis seen in cattle), the lack of significant pathological lesions, and the inability to identify the source of the toxin. Confirmation of this disease will depend on identification of the toxin with the mouse bio-assay. Some carcasses in an outbreak of botulism may manifest with lesions suggesting heart failure such as pulmonary oedema and congestion, accumulation of fluid in body cavities including hydropericardium with fibrin clots and epicardial petechiation. When considering sudden mortality and lesions indication heart failure, differential diagnoses include plants poisonings caused by cardiac glycoside- and monofluoroacetate-containing plants, gousiekte, ionophore toxicity, and enterotoxaemia. In cases of botulism characterised principally by paresis, paralysis or muscular weakness, differential diagnosis to consider would be toxicoses such as krimpsiekte, diploidiosis, and the paralytic phase of Cynanchum or Sarcostemma poisoning (the latter two poisonings are rare). Differentiation of the above conditions should therefore be based on history, clinical signs, a search of the property for toxic plants, toxicological tests such as the determination of ionophore levels in the feed and cardiac glycosides in tissues, and a detailed necropsy. Also collect tissues for histology since several of these conditions produce myocardial lesions of diagnostic significance. For further information contact:
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