Nervous Disease in Sheep
· Winter-Spring disease, associated with silage feeding.
· Caused by Listeria monocytogenes bacterium that lives in plant-soil environment, in particular, the less acidic soil of spoiled silage; which enhances multiplication of the bacteria.
· Usually clinical outbreak only affects around 2% of a flock. Outbreaks occur around 14-21 days after ingestion of poor silage.
· Typical occurrence includes: sheep aged 18-24 months; poorly stored silage; rarely higher than 2% incidence, 14-21 days after silage ingestion.
· Clinical presentation is based on the infection usually only limited to one side of the brain; hence the one-sided appearance of the nerve paralysis. Initially, sheep do not eat/come to food trough, are depressed, disorientated and show erratic movements into corners, fences or feed troughs. Often leaning against objects is seen, due to weakness affecting one side of the body, with knuckling of the foreleg. Profuse salivation and food impacted in the cheek of the affected side. Drooping of the ear, deviated muzzle, flaccid lip and lowered eyelid on the affected side.
· KEY SIGNS: not eating, depressed, disoriented, propelling into corners, leaning, salivation, food impaction, drooping ear, deviated muzzle, flaccid lip and lowered eyelid.
· Differential diagnosis include: pregnancy toxaemia, peripheral vestibular lesions, brain abscesses or Gid.
· Treatment depends on early detection and aggressive anti-biotic use. High doses are needed in order to achieve high conc. of anti-biotic in the brain tissue. Side treatments include sugar drench to avoid severe energy deficit, palatable fresh food, clean water and topical antibiotic eye ointment.
Polioencephalomalacia (Cerebrocortical necrosis/CCN)
· Most commonly seen, but not limited to, weaned lambs aged 4-8 months. Usually seen about 2 weeks after pasture or dietary change.
· Early stages appear that sheep are blind and isolated from the group, wandering aimlessly and star-gazing when stationary. Condition deteriorates within 12-24 hours to lateral recumbency with seizure activity during handling. Without treatment, death follows within 3-5 days.
· Clinical signs include: blind; isolated; wander aimlessly; star-gazing; recumbency with seizure activity.
· Caused by brain swelling and laminar necrosis of the cerebral cortex, resulting from interference with brain metabolism. This metabolic fault is related to thiamine deficiency at cell level. In cells, thiamine pyrophosphate is needed for decarboxylation of pyruvate during the conversion of pyruvate to AcCoA. When thiamine deficiency occurs, membrane repolarization occurs (as lack of energy produced), pyrophosphate is reduced and pyruvate and lactate accumulate. Some bacteria in some forages, produce thiaminase which breaks down thiamine. These bacteria are often found in the rumen of CCN affected sheep.
· Treatment of high doses of thiamine (10mg/kg) intravenously for first occasion is good. Normal vision may not return for up to a week, but should be able to stand within 24 hours. Treatment should be continued for 3 consecutive days. Intravenous thiamine (Vitamin B1).
· Uncommon disease however still occurs in some areas. Treatment of farm dogs with anthelmintics and correct disposal of sheep carcasses have broken the sheep/dog cycle.
· Dogs act as a reservoir for the disease; Coenurus cerebralis is the larval stage of the Taenia multiceps tapeworm, which infests the small intestine of dogs. Defecation on sheep pastures, which are then grazed by sheep, can lead to larval invasion of the CNS, forming a cyst in the brain and clinical disease. The life cycle is complete when the larvae develop into tapeworm and ingest the infested sheep brain.
· Clinical presentation of chronic coenurosis is more commonly reported in growing sheep aged 6-18 months, where the signs develop slowly and progressively. Compulsive circling behaviour, depression and head-pressing may occur.
· Treatment is usually successful, with 85% surgical success rate for removal of coenurus cyst from the brain. Economic reasons cause most farmers to euthanize affected sheep.
Vestibular disease (middle ear infections)
· Unilateral peripheral vestibular lesions are commonly associated with middle ear infections and ascending infection of the Eustachian tube.
· Clinical presentation consists of head tilted and loss of balance.