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Chicken Breeds

Cockerels in WinterAvian Encephalomyelitis (AE)

Avian encephalomyelitis (AE) is a viral disease of young chickens caused by a virus from the Hepatovirus family and characterised by central nervous system signs (Epidemic Tremors). Primarily a viral infection of poultry, chickens, turkey and pheasants. First reported in 1932, the virus grows in the yolk sac and brain of the chicken embryo in eggs from nonimmune hens. Most prevalent in chickens 1 to 6 weeks of age. Susceptible chickens more than 5 weeks old will develop antibodies to AE, but do not show clinical signs at the time of infection.

AE occurs world wide and occurs in all seasons of the year, but most cases are reported from January to June. Egg-passage transmission from infected hen to chick is the most common mode of spread, but direct contact of susceptible hatchlings with infected birds accounts for spread within the flock. Indirect spread via fecal contamination of feed and water also occurs. The virus can survive at least 4 weeks in droppings.

Clinical signs appear at 7 to 10 days of age. Tremors of the head and neck are presumptive of the disease in the flock hence the name "Epidemic tremor". Affected chicks first may show a dull expression of the eyes, followed by progressive in coordination, sitting on hocks, tremors of the head and neck, and finally paralysis or prostration.

Muscular tremors are best seen by exercising the bird. Affected birds are inactive; some may refuse to walk or walk on their hocks. Diagnosis is confirmed by fluorescent antibody test, virus isolation and agar gel precipitin test. AE must be differentiated from other encephalitic diseases such as ND, EEE, MD etc.

There is no treatment for acute outbreaks. Control is through prevention. Affected birds should be removed, killed and incinerated. Recovered chicks are unthrifty. Prove good nursing during outbreaks will help with mortality. Prevention is by selecting hatching eggs from immune breeder flocks. Lifetime immunity is acquired through vaccination or recovery from a natural outbreaks.

Chickens of all ages are susceptible, but clinical signs of encephalitis only develop in those younger than four weeks. The disease is similar in turkeys and chickens. Following initial dull expression of the eyes, the following signs are seen:

  • progressive ataxia with the chick losing control of legs, sitting on its haunches and falling onto its side;
  • tremor of the head and neck.

Ataxia progresses to paralysis and death results from inability to feed or drink, or through being trampled. Some birds recover, and others may survive with persistent clinical signs. In susceptible adult birds, infection is usually sub-clinical, although there may be a transient drop in egg production.

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