Marek's disease

 

Introduction : 

• Marek's disease ia a lymphoproliferative disease of chickens, caused by herpes virus and characterized by a mono-nuclear infiltration of peripheral nerves, gonads, iris, various viscera, muscle and skin.

• Marek's disease exists in poultry producing countries throughout the world, and chickens are by far the most important natural host for Marek's disease.

• The Marek's disease virus is present in desquamated feather follicle epithelial cells and in oral, nasal and tracheal secretions of infected birds.

• Main transmission is by infected premises, where day-old chicks will become infected by the oral and respiratory routes.

• Dander from feather follicles of Marek's disease infected chickens can remain infectious for more than a year.

• Marek's disease affects chickens from about 6 weeks of age, and it occurs most frequently between 12 and 24 weeks of age but older birds may also be affected.

Clinical Signs :

1. Classical Marek's disease :

• The signs depend on the peripheral nerves affected and involvement of the brachial and sciatic nerves is common, leading to progressive spastic paralysis of the wings and legs, respectively.

• Leg nerve involvement causes a bird to lie on its side with one leg stretched forward and the other backward.

• When the cervical nerves are involved, their may be torticollis and if the vagus and intercostal nerves are affected, respiratory signs may develop.

2. Acute Marek's disease :

• Many birds die suddenly without preceding symptoms.

• Some appears depressed before death and some show paralytic signs.

3. Transient paralysis :

• Birds suddenly develop various degrees of paresis or paralysis of the legs, wings and neck.

Diagnosis :

• 1. A tentative diagnosis of Marek's disease can be made on history, clinical signs and pathological changes.

• Leukocytic enlargement of peripheral nerves.

• Lymphoid tumors in various tissues in birds.

• Iris discoloration and pupil irregularity.

• Skin involvement often consists of tumours of feather follicles of in between follicles.

• 2. A definitive diagnosis can be made on isolation and identification of the causative virus.

Treatment : 

• There is no effective practical treatment for Marek's disease in either flocks or individual birds.

• Prevention and control of Marek's disease is based on 1. Management and hygiene, 2. Rearing of genetically resistant stock and vaccination.

Control :

• All commercially available vaccines are used most commonly ad a single dose in day-old chicks.

• A significant level of protection is developed withing about a week and protection lasts throughout the life of the chicken.