Aetiology
The causative agent is a cell associated DNA herpes virus. Isolates of marek’s disease virus vary greatly in pathogenicity. A herpes virus antigenically related to marek’s disease virus is present in commercial turkey. This virus which is not able to cause disease in turkey and chicken is now extensively used as vaccine for marek’s disease.
Host
Marek’s disease affect chicks of about 6 weeks of age although it occur most frequently in 12-24 weeks of age. Chicken may rarely be affected later in life. Affected flock remains so throughout its life and continue to shed virus throughout its commercial life. The virus is an ubiquitous infection of poultry worldwide.
Transmission
The source of the infection include oral, nasal, tracheal discharge and cloacal excrements. These materials may retain infectivity for over one year. Poultry house dust and litter may also remain infectious for one year. The route of infection is mainly by airborne route. Fowl ticks have also been incriminative in transmission of the disease following blood meal.
Clinical signs and post-mortem lesions
There are two major forms of marek’s disease and more than one form can occur together.
1. Classical marek’s disease: This is the neural form affecting the peripheral nerves especially the celiac, mesenteric, vagus, intercostal, brachial and sciatic nerves. The cardinal signs in affected nerves include diffused or nodular enlargement which is usually 2-3 times its normal size which is unilateral. There is discoloration or loss of whiteness or loss of cross striations. Mortality in classical marek’s disease is variable but is varies above 10-15%. The signs observed depends on the peripheral nerve involvement. Involvement of brachial and sciatic nerve is common and results in progressive paralysis of wing and leg. Involvement of cervical nerve results in torticolis while involvement of vagus and intercostals nerve result in dyspnea. When celiac and mesenteric nerve are involved, diarrhea is observed.
2. Acute mareks disease: This is the visceral form affecting the soft tissues like the liver, spleen, kidney, bursa, heart, gonads and occasionally proventriculus and lungs. Mortality is higher usually 10-30% or even as high as 70% in some cases. Many birds die suddenly without preceding clinical signs and others appear depressed before death and some show paralytic signs similar to those of classical marek’s disease. Affected organs are enlarged to several times than normal and show greyish discoloration.
3. Other forms: These include the ocular and cutaneous forms. Gross ocular changes include loss of pigmentation in the iris(grey eye) and irregularity of the pupils. In the cutaneous form, whitish nodules are seen in the feather follicle. Transient paralysis which is an uncommon encephalitic manifestation of the infection of marek’s disease virus occurring between 5-18 weeks of age. Birds suddenly develop varying degrees of paralysis of the legs, wing and neck and is characterized by recovery andsigns disappear after 24-48 hours.
Histopathology
In the peripheral nerve, marek’s disease is both neoplastic in nature with proliferation of lymphoblastic cells and inflammatory in nature with diffused light to moderately infiltration with small lymphocytes and plasma cells normally with edema. These are referred to as A and B response respectively. Lymphomatous lesions in the visceral organs are most uniformly proliferative in nature.
Diagnosis
This is based on a combination of clinical signs, post-mortem lesions, age at infection and histology. Because of the ubiquitous nature of marek’s disease virus, diagnosis is not based on isolation and recognition of the virus alone. Marek’s disease virus is present in most flocks.
Control
It is based on breeding and stocking resistant breeds of chicken.
Vaccination using 3 different types of vaccines which are: attenuated pathogenic, example rispeus strain; A-pathogenic, example 3B-1 strain; Herpes virus of turkey, example HV5. These vaccines are given at day old by intramuscular or subcutaneous route. They come in two forms: the cell associated form stored in liquid nitrogen AND lyophilized/freeze dried form.