Avian encephalomyelitis is a viral disease of chicks. It is characterized by muscular incoordination and rapid tremors, especially of the head and neck. Because of the trembling and shaking, it is also called 'epidemic tremor'. Its economic importance is mainly due to the mortality it causes in chickens.
Cause
A virus called enterovirus.
Spread
I. Transmission through hatching egg (vertical transmission) is a very important means of spread.
2. Contaminated water and feed are sources of spread from bird to bird and house to house.
3. Disease can be spread by people and equipment contaminated with virus-containing faeces. Ingestion is the usual route of entry. Exposure through the respiratory tract is unimportant.
Symptoms
I. The disease usually occurs when chicks are 1-2 weeks of age. Symptoms include depression, muscular incoordination, and tremors. The nervous symptoms may be seen soon after hatching, but are usually seen at one week of age. The muscular incoordination varies from slight incoordination to sitting on the hocks, or lying down on one side . The birds are unable to move. In such a condition, death occurs from not reaching to the feed (starvation), or getting crushed by other members of the flock. Birds over 4 weeks of age rarely show avian encephalomyelitis.
2. In layers, there is some drop of egg production, with reduced hatchability.
3. Very mildly affected birds recover completely.
Postmortem Findings
There are no gross les ions (changes) in the young or older birds. Therefore, the disease cannot be diagnosed from postmortem.
Diagnosis
I. The symptoms in young birds, absence of gross changes at postmortem together with the absence of other infectious and nutritional deficiencies affecting the nervous system, are strongly suggestive of avian encephalomyelitis.
2. However, a definitive diagnosis requires help of laboratory tests.
3. In differential diagnosis, it is necessary to rule out vitamin E deficiency (nutritional encephalomalacia; see 'vitamin E deficiency').
Treatment
Treatment is of no value.
Control
I. Under commercial conditions, it is not possible to eliminate infection by high standards of hygiene alone. Thus, control depends on the vaccination of birds, and using hatchery eggs from breeder flocks that are free from avian encephalomyelitis.
2. Live and killed vaccines can be given which produce a lasting and adequate degree of protection.
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