The virus responsible for Marek's disease is thought to be responsible for greater mortality in chickens than any other disease. Every flock, except for those maintained under strict pathogen-free conditions, can be assumed to be infected. Flocks that do not show any symptoms may still be infected. Stress may weaken the immune systems and result in an outbreak of this disease. Although symptoms may not be apparent, there may be a reduced growth rate and a decrease in egg production.
The virus is concentrated in the feather follicles and shed in sloughed skin and feather cells. It has a long survival time and viable virus can be isolated from houses that have not been occupied for many months. Transmission is by aerosols containing infected dust. Younger birds are most susceptible to infection.
There is no treatment for Marek's disease. The main method of control is by vaccination of day-old chickens as a subcutaneous injection. Results are improved by strict sanitation to reduce or delay exposure and also by breeding for genetic resistance.
Marek's disease may produce a variety of clinical responses: visceral, neural, ocular, skin or combinations of these.
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