B310 - MAREK'S DISEASE

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B310 - MAREK'S DISEASE

Nature of the disease
Marek's disease is a lymphomatous and neuropathic disease of domestic fowl caused by a herpesvirus. It causes important economical losses. 
Classification
OIE, List B disease
Susceptible species
Chicken, quails and turkeys. 
Distribution
Worldwide.
Clinical signs 
Marek's disease usually occur under a neural form but sometimes a visceral form is observed.

In the nervous form it is particularly the sciatic nerves that are affected. Signs include:

  • Birds are unable to stand, become paralysed and slowly waste away from lack of food and water. 
  • Wings or neck may be involved. 
  • Iris may be involved and it is associated with blindness.

In the visceral form signs are less specific and include:

  • Depression,
  • Paralysis, 
  • Loss of appetite, 
  • Loss of weight, 
  • Anaemia (pale combs), 
  • Dehydration (shrunken combs), 
  • Diarrhoea. 
  • Death without any clinical signs being noticed
Post-mortem findings 
In the nervous form there is no gross lesions part from starvation, lesions on nerves are only visible under microscope.

In the viscerous form, greyish-white tumours are found in the ovaries, liver, spleen, kidney, heart and other organs. Sometimes the liver and spleen are swollen without distinct tumours being present.

Differential diagnosis 
Lymphoid leucosis 
Specimens required for diagnosis 
Identification of agent can be done by inoculation. For that purpose specimens can be heparinised blood, lymph nodes or spleen. MD viral antigen can also be detected in the feather tips of infected birds using a radial precipitin test.
 
Serological diagnosis is possible 1 to 2 after infection. Tests include agar gel immunodiffusion test, the indirect fluorescent antibody test, and ELISA.
Transmission   
The virus can remain alive in the environment for as long as eight months. It is shed from the feather follicles and spreads in fluff and dust, gaining entry when the bird breathes infected dust particles. It is not spread from the hen to the chicken through the egg.

The virus is highly infectious and, once it is present in a flock, it spreads rapidly to unvaccinated poultry. Healthy birds can be carriers and infect others.
Risk of introduction   
If not already present Marek's disease could be introduced by importation of live infected birds, hatching eggs and poultry products such as meat and feathers. Appropriate heat treatment can inactivate the virus. 
Control / vaccines  
Control of the disease is difficult, hygienic measures and isolation of infected flocks are essential. If endemic vaccination can be used to reduce the economical consequences of the disease.
References
  • Marek's disease, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p. 1934-1936
  • Marek’s disease, Queensland Government, Department of Primary Industry, file APH0170,  revised February 2003 
  • Office International des Epizooties, 2002