Marek's disease (MD)
It is a lymphopoliferative disease of domestic chicken caused by a herpes virus. Of the known 3 serotypes of MDV, Serotype I includes all the oncogenic strains of MDV and their attenuated forms; serotype II is a group of naturally nonpathogenic strains and serotype III is a virulent and related to herpes virus of turkeys (HVT). MDV and HVT exist in either cell associated or cell free status and have different survival proprieties. Litter and feathers from infected chickens are infectious and contain cell free virus which survive in the environment for about 4-8 months.
Symptoms and lesions: The disease occurs most commonly in young birds of 6 weeks to 18 weeks of age. It causes various clinical conditions under 4 categories.
Classical form: Progressive paralysis of the wings and legs gives typical appearance of the affected bird as the extension of one leg forward and the other backward. This involves brachial and sciatic nerves. If the vagus nerve is affected, dilatation of the crop results in gasping. Torticollis may develop when the cervical nerves are affected. The affected birds are unable to reach the feeders or waterers resulting in death due to starvation and dehydration. Affected nerves are thickened, oedematous and degenerated.
Visceral form: This is considered as acute and virulent form with high morbidity. Mortality may be 10 to 30% but may go upto 80%. The clinical signs are depression followed by paralysis in some birds. Lymphoid nodular tumors are seen in the visceral organs viz. gonads, liver, spleen, lungs, kidney, bursa and thymus.
Ocular form: Infiltration of the tumor cells into iris results in blindness. The condition is knows as pearl eye or gray eye.
Cutaneous form: There is infiltration of lymphoid tumor cells in feather follicles and is characterized by the formation of nodular lesions at the base of the feathers.
From the clinical signs and lesions, the disease can be identified. More than one type can also be observed at a time in one flock. The detection of viral antigens by agar gel diffusion or fluorescent antibody tests can be confirmatory. For virus isolation, the specimens of choice are buffy coat of the blood, tumorous tissue and feather follicles. Antibody detection can be done by agar gel diffusion and ELISA.
Prevention and control: Vaccine derived from all the 3 serotypes is available.
They are the cell culture adapted MDV, naturally apathogenic strains of MDV and the HVT strains. All these vaccines are equally effective but when given singly they are not very effective against the very virulent from MD. A bivalent vaccine consisting of SB-1 strain of serotype 2 and FC-128 strain of serotype 3 (HVT) is reported to be very effective against the very virulent MD and is available in the cell associated form. Chickens are vaccinated at hatching and should be reared in isolation until after 4 weeks. The vaccine virus persists in the birds but does not prevent super infection, replication or shedding of virulent MDV. At the same time it protects the birds from clinical disease. High levels of biosecurity to reduce early exposure and use of genetic resistant strains are necessary for preventing and controlling the disease.