Occurrence and Economic Significance
Marek's disease affects chickens from 5 to 35 weeks of age throughout the world.

Highly pathogenic (vvMD) strains of the virus are responsible for acute outbreaks of mortality which may attain 50% in exposed, non-immunized flocks up to 60 weeks of age.

Marek's disease virus is responsible for neural and visceral tumors. The agent is immunosuppressive and affected flocks are susceptible to a wide range of viral and bacterial infections.

Transmission
Exposure to MDV occurs by horizontal infection. The virus is resistant to environmental exposure and can remain viable for long periods in houses especially if units are not decontaminated between cycles. Infected birds shed "dander" (feather dust) contaminated with virus which can be distributed by wind, equipment, and personnel.

Clinical Signs
Involvement of the peripheral nerves results in paresis (weakness) of the legs or wings which progresses to paralysis.

Pathology
Enlargement of the feather follicles is observed on the skin of de-feathered broilers and results in condemnation of carcasses in the USA, Canada, and Europe.

The characteristic lesion comprises enlargement of the peripheral nerves of the sciatic or brachial plexus. Occasionally visceral lesions are observed and the kidney, eye, proventriculus or other organs may be affected.

Diagnosis and Confirmation
The gross appearance of neural lesions is generally diagnostic. Histological examination of nerve and visceral lesions will show characteristic lymphocytic proliferation.

The causal virus may be isolated and identified by submitting tissues to a suitably equipped laboratory using specific tissue culture techniques.

Prevention
Vaccination of embryos using the in ovo technique at the 18th day of incubation or by subcutaneous administration of vaccine to chicks at day old.

Three vaccine types are available:

• Type 1: attenuated chicken strain (e.g. Rispen's)
• Type 2: apathogenic chicken strain (e.g. SB1)
• Type 3: apathogenic turkey, strain (e.g. HVT)

Due to the interfering effect of maternal antibody to HVT it is advisable to alternate vaccine types in successive generations. In countries where highly pathogenic MDV occurs, parents should be vaccinated with Rispen's strain, allowing commercial progeny to be immunized using the less expensive HVT strain alone or in combination with the potentiating SB1 strain.

Cell-associated, frozen vaccines require special storage in a liquid nitrogen canister. Careful reconstitution using the diluents supplied by the vaccine manufacturer is necessary to maintain viability of the vaccine virus. Improper vaccination technique may lead to defective immunization with resulting "breaks".

To allow vaccinated flocks to develop immunity, it is essential to place day old chicks in houses which have been thoroughly decontaminated. Farms should be operated as single-age units with all-in-all-out cycles.