This is an acute infectious disease of birds (3 weeks old), which is caused by a member of the genus Enterovirus in the family Picornaviridae.
Epidemiology: Virus is transmitted by fecal-oral route. In laying hens, it can be transmitted to the chicks through the eggs.
Clinical signs: The incubation period varies from 2 to 4 weeks. The virus affects particularly the central nervous system of young chicks. The disease is characterized by ataxia and rapid tremors of the head, neck and limbs, hence, the synonym epidemic tremor . Symptoms appear within the first 3 weeks of life, in the form of complete or partial paralysis, unsteady gait and resting on the hocks; a very rapid tremor of the head, neck and tail in some chicks and tremors of the body muscles. The disease in older birds is usually inapparent except for a slight decline of production in laying hens.
Diagnosis: Clinical signs and histopathology are suggestive of the disease. A dense aggregation of lymphocytes in the muscular wall of the proventriculus constitutes a pathognomonic symptom. Confirmatory diagnosis is made by serum neutralization, immunofluorescence and immunodiffusion tests. The virus may be isolated either in cell culture or by inoculating 5-7 day old embryonated hen eggs obtained from antibody free hens by the yolk sac route. The chicks are allowed to hatch and are observed for seven days for signs of encephalomyelitis. It has to be differentiated from Newcastle disease as well as from a range of non-viral causes of central nervous system disease in chickens.
Treatment, prevention and control: Attenuated virus vaccines administered in the drinking water are available. The vaccines are administered after chickens reach 10 weeks of age and are designed to provide protection for chicks during the first 21 days after hatching by ensuring that adequate levels of specific antibodies are transferred from hens to progeny chicks. They are not administered to chicks because they are not sufficiently attenuated nor is there sufficient time to provide protection for chicks hatched into heavily contaminated environment. Inactivated vaccines are also available and are preferred when immunized birds are housed in close proximity to non-immunize chickens.