Disease processes affecting the nervous system may be infectious or inflammatory, toxic, metabolic, traumatic or vascular. There are others such as degenerative, genetic and neoplastic conditions but these will be extremely rare. Specific viruses and bacteria cause infectious conditions. Insecticides, fungicides and rodenticides cause toxicities. Hypoglycaemia, hypoxia, hepatic dysfunction, hypocalcaemia and hypomagnesaemia cause metabolic nervous diseases. Haemorrhage, oedema and direct physical damage to the central nervous system cause a range of focal or multifocal neurological deficits. Septicaemia and bacterial embolisation cause vascular lesions in the central nervous system. With inflammatory, metabolic and toxic processes there will be evidence of disease in other body systems. With traumatic and toxic exposures there may be obvious external signs or clues. Toxic and metabolic processes are unlikely to produce asymmetric deficits.
Diseases of the nervous system generally result in changes in posture and gait or mental state, depending on the location and extent of the pathology.
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:
Differentiation is by examination of history, clinical and necropsy findings. Differentiation in the laboratory may be assisted by specimens of serum for biochemistry, brain in buffered formalin for histology, smears of blood and spleen for haematology, and rumen content for toxicology.
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, water intoxication 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, smears of blood and spleen for haematology, and rumen content for toxicology.
Choice of treatment is dictated by the diagnosis.
Careful screening for early stage disease during selection may prevent many of the nervous diseases of sheep and goats becoming a problem in the export process. Specific preventative measures will depend on the disease(s) involved.