Mareks Disease in Chickens

Posted by Susan Lenz on

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.

Clinical Signs

Young birds are most susceptible to infection. Most deaths from Marek's disease occur between 8 and 20 weeks of age, although in some cases the disease may be seen in 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, 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.


Veterinary examination is necessary to diagnose Marek's disease. The clinical signs, combined with post-mortem findings, will confirm the diagnosis in most cases, and, most importantly, rule-out other diseases. 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.

Similar Disease

A different viral disease known as lymphoid leucosis also causes tumours in organs, but does not cause paralysis. It is usually seen in birds over 16 weeks of age, whereas Marek's disease is commonly seen in younger chickens.

Methods of Spread

Marek's disease virus occurs commonly wherever chickens are raised. The virus is highly infectious and once introduced into a flock it spreads rapidly to unvaccinated birds, so that most chickens in an unvaccinated flock become infected.

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.


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.


Although vaccines are commonly used in the commercial poultry industry, small numbers of doses cannot be purchased for use in backyard flocks.

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 exposed 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 ensure that the birds have optimum resistance to Marek's disease infection.

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.

The Importance of Quarantining New Flock Members:

It is always a good idea to quarantine new chickens before adding them to your existing flock for a few different reasons. Firstly, this gives you a chance to worm and treat/prevent mites and lice if you don’t know or suspect it wasn’t done recently.

Secondly, a new chicken can get stressed with the change of location and travel and as a result this can bring out a dormant disease in a chicken that looked healthy when you first bought it.

This is similar to how we may be fine and healthy, but stress from moving house can bring out a cold (or a cold sore, for those of us who are familiar with cold sores). Quarantining new chooks allows for them to recover or receive treatment in a less stressful environment and minimises the risk of your old established flock getting sick from a strain of disease that they don’t have any immunity to.

Sadly some birds can be carriers of disease but not show any symptoms, so quarantining new members can only protect your flock so much.

Should you choose not to quarantine your new chickens, because you don’t have the room/facilities in your backyard or some other reason, you need to be aware that you are putting your new girls and your existing girls at risk of getting sick. Therefore you should be more vigilant for signs of disease/illness and be ready to treat them if necessary…


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