Marek´s disease (MD) is caused by a herpes virus that can result in death or severe production loss in both layer and meat chickens. Vaccination reduces the losses. However, in recent years, the incidence of Marek´s disease has increased due to new strains of the virus and faster growing, more susceptible birds.

Marek´s disease is a problem in many household flocks, especially those that are not vaccinated. It causes changes in many of the birds' nerves and may cause tumours in major internal organs, such as ovary, liver, kidney, heart and spleen.

Marek´s disease occurs in two main forms - nervous and visceral - depending on which body parts are affected by the tumours.

Nervous form
In this form, the nerves are affected, particularly the sciatic nerves which are the main nerves to the legs. The birds cannot stand, become paralysed and slowly waste away from lack of food and water. In most cases, the paralysis appears quickly. Sometimes the wings or neck are involved. In some cases, the iris is involved, which can lead to blindness.

During an autopsy on a bird, the affected nerves are sometimes visibly swollen. However, in many cases, changes are only visible under a microscope.

Visceral form
In this form, greyish-white tumours appear in the ovaries, liver, spleen, kidney, heart and other organs. Sometimes the liver and spleen may be swollen without distinct tumours being present.

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 clinical signs being noticed.

Young birds are the most susceptible to infection. Most deaths from MD occur between 10 and 24 weeks of age; however, in some cases, the disease may not appear until later in life. Most birds that develop MD die.

Lymphoid leucosis also causes tumours in organs but does not cause paralysis. It is usually seen in birds over 16 weeks of age, whereas MD is commonly seen in birds younger than that.

The virus can remain alive in the environment for up to 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 does not spread from the hen to the chicken through the egg.

The virus is highly infectious and, once present in a flock, spreads rapidly to unvaccinated poultry. Healthy birds can be carriers and infect others.

Treatment is not effective. Diseased birds should be removed from the flock and humanely destroyed. Other birds in the flock are likely to be infected at this stage - it is too late to protect them.

Vaccination
The introduction of more potent vaccines is providing better control. These are made either from the Marek´s disease virus or from herpes virus of turkeys (HVT), a closely related virus. However, these new cell-associated vaccines are only available in packs of 1000 doses and must be stored in liquid nitrogen, so special equipment is needed. This makes it costly to vaccinate small flocks.

A freeze-dried, cell-free, HVT vaccine is available, which is stored in ordinary refrigeration and less costly. This is also available in packs of 1000. The vaccine must be reconstituted with a sterile diluent before use.

For backyard operations, the best way to ensure protection is to buy birds from a commercial source that have been correctly vaccinated either into 18- or 19-day-old embryonated eggs (before hatching) or at one day old.

Breeders who wish to vaccinate their own day-old chicks should follow the manufacturer´s directions for storing and administering the vaccine. The vaccine's effectiveness depends on this.

Management
Vaccination alone will not prevent Marek's disease. It is important to be attentive to the following management and hygiene procedures.

Vaccinated chicks must be isolated during their first two weeks of life to enable their immunity to develop before they are subjected to a severe challenge of virus. Chicks reared separately are free from the infected fluff and dust of older birds. This material can be carried by wind, people and equipment.

Hygiene is important. Sheds and equipment must be thoroughly cleaned out and disinfected between batches of chicks. To control MD, a disinfectant should be used that is effective against viruses, not just bacteria. Users must take care to protect themselves from spray by wearing suitable protective clothing and breathing apparatus.

Exposing the sheds and runs to sunlight helps the disinfection process.

Good nutrition, and freedom from other diseases and parasites, are necessary to maintain the flock´s health and ensure that the birds have optimum resistance against MD infection.

Breeding
Breeding programs to select birds for resistance to Marek´s disease are important to provide for better control in the future.

In summary
In summary, Marek´s disease control requires correct vaccination, the isolation of young chicks, hygiene, good health and genetically resistant bird strains.

For flocks with a serious MD problem, the only solution is to depopulate, clean and disinfect all sheds and equipment, spell for several months and then replace with vaccinated chicks from a reputable hatchery.