by Dr. Amandeep Singh
Etiology or causative agent
Marek’s Disease Virus (MDV), a highly infectious and easily transmitted herpesvirus, is one of the most common viral infections worldwide and is capable of causing tumours and immunosuppression in broiler breeders. It also leads to nerve paralysis and tumour formation in chicks.
Transmission
The disease is highly contagious and readily transmitted among chickens.
The virus matures into a fully infective, enveloped form in the epithelium of the feather follicle, from which it is released into the environment.
It may survive for months in poultry house litter or dust.
Dust or dander from infected chickens is particularly effective in transmission.
Once the virus is introduced into a chicken flock, regardless of vaccination status, infection spreads quickly from bird to bird.
Schematic diagram showing the different stages of Marek’s Disease Virus pathogenesis including the virus shedding from the feather follicle epithelium and the transformation of T lymphocytes in susceptible birds.
Symptoms
Paralysis of legs, wings and neck.
Loss of weight.
Grey iris or irregular pupil.
Vision impairment.
Skin around feather follicles raised and roughened.
Post-mortem findings
Enlarged nerves are one of the most consistent gross lesions in affected birds. Various peripheral nerves, but particularly the vagus, brachial, and sciatic, become enlarged and lose their striations.
Diffuse or nodular lymphoid tumors may be seen in various organs, particularly the liver, spleen, gonads, heart, lung, kidney, muscle, and proventriculus.
Enlarged feather follicles (commonly termed skin leukosis) may be noted in broilers after defeathering during processing and are a cause for condemnation.
The bursa is only rarely tumorous and more frequently is atrophic.
Diagnosis
Based on history, clinical signs, distribution of lesions, age affected, histopathology.
Differentiate from Lymphoid leukosis, botulism, deficiency of thiamine, deficiency of Ca/Phosphorus/Vitamin D, especially at the start of lay.
Usually, diagnosis is based on enlarged nerves and lymphoid tumors in various viscera.
The absence of bursal tumors helps distinguish this disease from lymphoid leukosis, although the presence of bursal tumors does not exclude Marek’s disease.
Marek’s disease can develop in chickens as young as 3 wk old, whereas lymphoid leukosis typically is seen in chickens >14 wk old.
Treatment
There is no treatment.
Any broad spectrum antibiotic like enrofloxacin, levofloxacin, ciprofloxacin can be used to prevent secondary bacterial infections.
Vitamin supplements are also advised in this case.
Prevention
Maintaining strict hygiene
All-in/all-out production
Conforming with bio-security measures
Vaccination
Vaccination against Marek’s Disease
Vaccination is generally with 1500 PFU of HVT at day old (but increasingly by in-ovo application at transfer), association with other strains (SB1 Sero-type 2) and Rispen’s.
Vaccination is the only known method to prevent the development of tumors when chickens are infected with the virus.
However, administration of vaccines does not prevent transmission of the virus, i.e., the vaccine is not sterilizing. However, it does reduce the amount of virus shed in the dander, hence reduces horizontal spread of the disease.
Marek’s disease does not spread vertically.
Marek’s Disease Vaccine
Administering Marek’s Disease Virus vaccines
The administration of Marek’s Disease Virus vaccines in-ovo results in better protection against early Marek’s Disease.
Still the vaccination can be given in day old chicks or soon after hatching in hatchery itself.
Chicks are vaccinated through subcutaneous route.
Administering Marek’s disease vaccine subcutaneously in day old chick
