Marek's disease In Poultry

Marek's disease (MD) is a tumour-causing viral disease of chickens. It is characterized by marked

enlargement of the nerves, or marked enlargement of the liver, spleen, and kidneys due to diffuse

growth of certain cells. It is an economically important disease. Although Marek's disease has been

effectively controlled by vaccine, sporadic and sometimes serious losses still continue to occur from it. The disease therefore needs adequate attention. Marek's disease appears almost exclusively confined to female birds. The author has observed that the disease usually begins in growers when they approach sexual maturity, that is, between 17-20 weeks of age. The disease may then continue to inflict mortality even up to 40 weeks (a grower-to-Iayer disease).

Cause

A virus - called herpesvirus. The virus is of three different types: (I) mildly harmful, (2) harmful, and (3) very harmful. Birds may become infected early in life, and remain infected until death.

Spread

I. The virus spreads rapidly from infected to uninfected birds. Cells from the feather follicles are the

most important source of infection. The virus is present in a free form in cells shed from the feather

follicles.

2. Marek's disease is highly contagious. Virus spreads through the air. Inhalation through the respiratory tract is the most important route of infection.

3. Once contracted, the infection persists throughout the life of the chicken, and infected birds

continue to contaminate the environment by shedding the virus. Continued shedding of the virus

by infected birds and hardiness of the virus are responsible for prevalence of the infection. The virus

survives for months outside the birds.

Symptoms

Marek's disease affects chickens from about 6 weeks of age. It occurs usually between 12 and 24 weeks of age, but older birds may also be affected. Clinical disease occurs mainly in two forms: (I) Classical Marek's disease, and (2) Acute Marek's disease.

Classical Marek's Disease

In our country this form is now rarely seen. It used to be common once upon a time. The symptoms

depend on which nerve is affected. Involvement of brachial and sciatic nerves is common, and leads to paralysis of the wings and legs. A particularly characteristic posture is that in which the bird lies on its side with one leg stretched forward and the other backward as a result of leg nerve involvement. Mortality varies, but is rarely more than 10-15%.

Acute Marek's Disease

This is the most common form of Marek's disease encountered in our country. Mortality in this form is usually much higher than in the classical form. Mortality of 10-30% of the flock is common, and outbreaks involving up to 80% of the flock are recorded. Many birds die suddenly without preceding symptoms. Others appear depressed before death, and some show paralytiC symptoms similar to those seen in the classical form.

A particularly characteristic posture is that in which the bird lies on its side with one leg stretched forward and the other backward (Fig. 18, 19). Non-specific signs such as weight loss, paleness, shrunken combs, loss of appetite, and diarrhoea may be observed, especially in birds in which the course is prolonged.

Postmortem Findings

I. In classical Marek's disease, the characteristic finding is marked enlargement of one or more nerves.Nerves commonly affected are sciatic and brachial. Affected nerves are up to 2-3 times the normal thickness

2. Acute Marek's disease is characterized by marked enlargement of the liver, spleen, kidneys, lungs,

gonads (ovary, testes), proventriculus, and heart. In younger birds, liver enlargement is moderate, but in adult birds the liver is greatly enlarged, which is similar to that in lymphoid leukosis. A characteristic postmortem finding of Marek's disease is marked enlargement of the liver and spleen , several times their normal size, showing white spots of cancerous tissue on their surface. The other typical finding in our country is significant enlargement of the proventriculus. When opened, its wall is greatly thickened and the internal lining shows irregular, somewhat diffuse, blotchy haemorrhages, quite different from those seen in Ranikhet disease .

Diagnosis

Diagnosis is based on the characteristic postmortem findings. That is, from the markedly enlarged liver and spleen, and the presence of tumours in various other internal organs.

Treatment

There is no treatment for Marek's disease.

Control

Control is based on management methods, which include: (I) isolation of growing chickens from sources of infection, (2) vaccination, (3) use of genetically resistant stock.