Marek's disease is caused by a herpesvirus. The domestic fowl can be affected by it.
Main transmission is by infected premises, where day-old chicks will become infected by the oral and respiratory routes. Dander from feather follicles of MD-infected chickens can remain infectious for more than a year. Young chicks are particularly susceptible to horizontal transmission.
Susceptibiliy decreases rapidly after the first few days of age.
Infected birds show weight loss, or may exhibit some form of paralysis. Mortality varies from 5% to 50% in unvaccinated birds. The classical form (paralysis) with leg nerve involvement causes a bird to lie on its side with one leg stretched forward and the other backward. When the gizzard nerve is involved, the birds will have a very small gizzard and intestines and the bird will waste away. Mortality usually occurs between 10 and 20 weeks of age.
The presence of tumours in liver, spleen, kidneys, lungs, ovary, muscles, or other tissues is indicative of MD, but they can also be indicative of lymphoid leukosis. However, nerve involvement, either grossly (swelling of leg, wing or other nerves) or microscopically, is typical of MD. Eye involvement can be visible as an irregular constriction of the iris (ocular lymphomatosis). Skin involvement (skin leukosis) often consists of tumours which can be a reason of broiler condemnation in certain parts of the world. A proper diagnosis to differentiate MD from LL requires histological examination.
Treatment and control
Vaccination of day-old chicks is an effective means of control. It has been demonstrated that MD vaccine only prevents the appearance of Marek's disease tumours and paralysis. It does not prevent the birds from becoming infected with MD-virus. It is therefore of major importance to maintain high hygienic and sanitary measures by good management to avoid early exposure of young chickens.