Affected animals may display one or a combination of signs that include abnormal limb movement, swaying or staggering, head tilt, circling, recumbency, blindness, rapid lateral eye movements, convulsions, and coma.
Differential diagnoses in the live export process include:
- Metabolic conditions (pregnancy toxaemia, hypocalcaemia or hypomagnesaemia).
- Meningitis as an extension of bacterial pneumonia, i.e. thrombotic meningoencephalitis.
- Listeriosis.
- Hepatic encephalopathy.
- Nervous ketosis.
- Annual ryegrass toxicity.
- Tick fever, especially Babesia bovis.
- Trauma.
- Grain poisoning.
- Poisoning with lubricants, fumigants, exhaust gases or lead.
- Polioencephalomalacia from dietary changes.
- Salt poisoning (water intoxication).
Differentiation is by examination of history, clinical and necropsy findings.
Listeriosis and middle ear infection both cause circling and are likely to be recognised at assembly. Listeriosis is most commonly seen as outbreaks associated with feeding spoiled silage in cold weather. Middle ear infections are sporadic and can happen anytime and anywhere.
Annual rye grass toxicity (ARGT), hepatic encephalopathy, salt poisoning, and polioencephalomalacia (PEM) are convulsive diseases. Diseases such as pregnancy toxaemia and hypocalcaemia may terminate in convulsions or coma. Traumatic injuries should also be considered.
Complete necropsy of affected animals will help to rule out some causes. Laboratory samples likely to be of value include serum for biochemistry, brain in buffered formalin for histology, blood smears, spleen for haematology and rumen content for toxicology.