Marek's DiseaseClinical Signs
Methods of Spread
Marek’s disease (MD or fowl paralysis) is a very common disease of chickens caused by a herpes virus. Marek’s disease affects both commercial and backyard poultry and may result in death or severe production loss. The disease causes changes in many of the nerves and may cause tumours in major internal organs.
Chickens are the main species affected. The disease occurs rarely in some other types of birds.
Typically, Marek’s disease occurs as the nervous form, appearing as a progressive paralysis of one or more of the limbs or, less often, the neck or wings. The sciatic nerve (the main nerve to the leg) is commonly affected The birds are unable to stand, become paralysed, appear uncoordinated and slowly waste away from lack of food and water. In most cases the paralysis comes on quickly. In some cases the eyes may be affected, resulting in blindness.
In the visceral form, Marek’s disease occurs as tumours in internal organs, including the ovaries, liver, spleen, kidney and heart. Sometimes the liver and spleen are swollen without distinct tumours being present. Birds may show signs of depression, paralysis, loss of appetite, loss of weight, anaemia (pale combs), dehydration (shrunken combs), and sometimes diarrhoea. Some birds die without any clinical signs being noticed.
Most birds that develop Marek’s disease usually die.
Infected chickens carry the virus for life whether they develop the disease or not, and continue to shed the virus for long periods. The virus is shed from the feather follicles and spreads readily in fluff and dust, gaining entry when the bird breathes infected dust particles. This material can also be carried by people and equipment.
The virus can survive in the environment for as long as several months at room temperature. It is not spread from the hen to the chicken through the egg.
Diseased birds should be promptly removed from the flock and humanely destroyed. Other birds in the flock are likely to be infected at this stage also, so close monitoring of all birds is important.
For backyard flocks, the best protection against Marek’s disease is obtained by buying, from a commercial source, birds that have been correctly vaccinated.
Vaccination alone will not prevent Marek's disease. Particularly for commercial flocks, it is important to have good biosecurity to ensure that vaccinated chicks will develop immunity before they are subjected to a severe challenge of virus. For example, chicks need to be reared separately so that they are free from the infected fluff and dust of older birds. Standard hygiene measures are also important, including a thorough clean-out and disinfection of sheds and equipment between batches of chicks with a disinfectant effective against viruses. Good nutrition and maintenance of freedom from other diseases and parasites are also very important. These practices will help maintain the flock’s health and to ensure that the birds have optimum resistance against Marek’s disease infection.
For general advice on biosecurity, see the biosecurity checklist for bird keepers.
The breeding of genetically resistant strains of chickens, combined with the use of vaccination and good hygiene, can also be used to help control Marek’s disease.
For flocks with a serious Marek’s disease problem, the only solution is to depopulate, clean and disinfect all sheds and equipment, and spell for several months. Vaccinated chicks from a reputable hatchery would then need to be sought as replacements.
Contact: Animal Health and Welfare BranchAnimal Health and Welfare Branch
Biosecurity and Product Integrity Division
13 St Johns Avenue
NEW TOWN TAS 7008
Phone: 03 6233 6875
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