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. It 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. On rare occasions Marek's disease can affect other types of birds. Birds that develop Mark's disease usually die.
Clinical Signs
Young birds are most susceptible to infection. Most deaths from Marek's disease occur between 8 and 20 weeks of age. In some cases the disease affects birds as young as 3-4 weeks of age or as old as one year of age.
Typically, Marek's disease occurs as the nervous form:
Birds show 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 are 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.
Diagnosis
Veterinary examination is necessary to diagnose Marek's disease. The diagnosis can often be made through clinical signs and post-mortem findings. Enlargement of nerves such as the sciatic nerve are commonly seen at post-mortem. Changes in one or more internal organs may also be observed.
Lymphoid leucosis is another viral disease. It causes tumours in organs. It does not cause paralysis. It usually affects birds older than 16 weeks. Marek’s disease is more common in younger chickens.
Methods of Spread
Marek's disease virus occurs commonly wherever chickens are raised. The virus is highly infectious. Once introduced into a flock it spreads rapidly, infecting most of the unvaccinated birds.
The virus is shed from the feather follicles. It spreads readily in fluff and dust, gaining entry when the bird breathes infected dust particles.
The virus can also be moved by people or on equipment. It can survive in the environment for as long as several months at room temperature. It is not spread from the hen to the chicks through the egg.
Whether they develop Marek's disease or not, infected chickens carry the virus for life and continue to shed it for long periods.
Treatment
There is no treatment for Marek's disease.
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.
Prevention
Vaccines are commonly used in the commercial poultry industry. The vaccine is given into the egg or to chicks at one day of age. Unfortuntaly, small volumes of vaccine are not available for use in backyard flocks.
Vaccinated birds may be available for sale from commerical poultry farms or suppliers.
Vaccination alone will not prevent Marek's disease. Outbreaks can still occur in very young, vaccinated chicks.
In all flocks it is important to maintain good biosecurity:
Chicks need to be reared separately so that they are free from the infected fluff and dust of older birds.
Hygiene measures are also important between batches of chicks. Thoroughly, clean-out and disinfect sheds and equipment with a disinfectant effective against viruses.
Maintain good nutrition , parasite prevention and freedom from other poultry diseases.
For general advice on biosecurity, see the biosecurity checklist for bird keepers:
Biosecurity Checklist for Bird Keepers
(44Kb)
For flocks infected with Marek's disease it may be necessary to depopulate all birds. Clean and disinfect any sheds and equipment, and then spell (keep empty) for several months. Consider sourcing birds vaccinated aginst Marek's disease as replacements.