Marek's disease

Marek's disease is caused by a herpes virus.

The virus can survive in the environment for about a year. Birds may become infected in the first few weeks of life, but may not start to display symptoms until about 12 weeks of age.

Marek's disease has a number of phases that affect different parts of the bird, which display different symptoms. Initially when a bird is infected, it will display some paralysis. It will then progress after a few weeks and internal tumours develop. Areas of the body affected by Marek's disease are the:

• nervous system – symptoms include paralysis of the legs, wings and neck; loss of weight; occasionally eye lesions and vision impairment.
• organs – greyish-white tumours appear in the ovaries, liver, spleen, kidney, heart and other organs. Birds may show signs of depression, paralysis, appetite loss, weight loss, anaemia (pale combs), dehydration (shrunken combs) and sometimes diarrhoea. Some birds die without any signs being noticed.
• skin – tumours appear in the feather follicles and the skin around feather follicles are raised and roughened. Birds become contagious once the skin is infected.

Often results in death or severe production loss in both layer and meat chickens.

In meat chickens, the disease can develop within 3-4 weeks. In layers, most deaths occur between 12 and 24 weeks of age however, in some cases, the disease may not appear until later in life. Birds affected later in life are more likely to be affected by lymphoid leucosis, which has very similar presentation.

Losses due to Marek's can be as low 10-15% or above 70% in large outbreaks.

Marek's disease is highly contagious and spreads quickly between birds. The virus resides in the feather follicles and can be shed into the environment in the feathers and dander. The main route of infection is respiratory, where birds breathe in infected dust and dander (i.e. tiny flakes shed from skin and feathers).

Not all birds die from the disease. Once infected, the disease will remain in the bird for life and it will continue to be infectious. Birds that are healthy in appearance can be carriers of the disease.

The virus is highly infectious and, once present in a flock, spreads rapidly to unvaccinated poultry. Other means of spread can be from insects that live in the litter, plus contamination of clothing, equipment and vehicles.

Young birds are the most susceptible as it takes a few weeks following vaccination for them to develop immunity. Unvaccinated birds and those that are challenged with nutritional deficiencies, other disease and parasites are also at risk.

Vaccination

Vaccination is available for Marek's disease. However, this does not always prevent the disease and should not be the only form of control.

Vaccination may be given at the hatchery within the chicks first day of life. Purchase chicks and pullets from reputable hatcheries and request proof of vaccination from your supplier. Breeders who vaccinate their own day-old chicks must follow the manufacturer's instructions for storage and administration to ensure the effectiveness of the vaccine. The vaccine will only work if administered before the chick is infected with the disease. In situations when chicks are hatched under the hen – and not in an incubator – the chick may already be infected.

Biosecurity measures

Practising good biosecurity measures is also important to help reduce the incidence of Marek's disease, particularly in young birds. Standard hygiene procedures such as regular cleaning and disinfection of housing facilities will help to reduce the incidence of Marek's disease. In commercial operations, practice an all-in and all-out system, with a thorough shed and equipment clean and disinfection between batches of chicks.

Farm practices

Good nutrition, parasite control and maintaining a clean environment is also very important. Poor farming practices can increase stress of birds which can lead to an outbreak.

There is no treatment for birds are that are affected with Marek's disease. They should be removed from the flock and humanely euthanased.