Causative agent 
- MD virus, family Herpesviridae
- Three types identified varied widely in their
  oncogenic potential from highly oncogenic to non-
  oncogenic



Differential diagnosis 
- Avian lymphoid leukosis
- Avian encephalomyelitis
- Infectious bursal disease
- Transient paralysis
- Newcastle disease
- Avian botulism



1. History taking 
- In the flock without vaccination, outbreak is in high
  percentage
- In the vaccinated flock, an outbreak is occasional
- Sporadical and also continuous outbreaks occur
  after 6 weeks old



2. Clinical examination 
- Partial or complete paralysis of legs and wings
- Delayed development
- Sometimes skeletal muscle and skin are tumorous
- Rarely ocular lesions (diffuse greyish opacity of iris)
- Enlargement of liver



3. Necropsy 
- Swelling accompanied with loss of cross-striation
  of peripheral nerves
- Lymphoid tumor in liver, spleen, kidney, gonad,
  lung, intestine, iris, heart, proventriculus, skeletal
  muscle and skin

4. Histopathological observation 
- Lymphoid tumour cells with various morphology
  infiltration in peripheral nerves and other tissues
  or organs
- Demyelination and Schwaan cells proliferation of
  peripheral nerves
- Proliferative lesions : "Marek's disease cell" (an
  unusual cell with basophilic, and vacuolated
  cytoplasm and a nucleus with little detail)
- Perivascular cuffing of lymphocytes in brain
- Atrophy and necrosis of bursa of Fabricius, and
  interfollicular lymphoid : tumour cells infiltration
- lntranuclear inclusions in degenerative lesions



5. Virus isolation 
Specimen : buffy coat, tumor cell, spleen,kidney
Method    :
1. CK inoculation or direct kidney cell culture from
    infected chicken and CPE observation (clusters of
    rounded refractile cells with plaque formation)
2. Chick embryo inoculation at 4-5 days of age by
    yolk sac route and observation of small necrotic
    plagues on the CAM with enlarged spleen and
    discolored greenish brown liver (sometimes with
    necrotic foci)



6. Virus Identification 
- FA test
- AGID test

Control 
- Vaccination
- Management and hygiene
- Breeding for resistant stock