Marek’s disease :

It is a lymphoproliferative disease of chickens characterised by mononuclear infiltration  of PNS , other tissues and visceral organs.

• Due to neuronal involvement , synonyms used are : Polyneuritis , Fowl paralysis , Range paralysis and Neurolymphomatosis .
• Mainly found in chickens , but occasionally affect pheasants , quail , game fowl and turkeys .

Etiology :

          Herpes virus [ enveloped , Ds DNA virus ]

• Classified into 3 serotypes :
• Serotype 1 : includes oncogenic strains of MDV
• Serotype 2 : includes naturally non – pathogenic strains
• Serotype 3 : antigenically related to herpes virus of turkey (HVT) . Used for production of vaccine k/a HVT126
• Serotype 1 again subclassified into :
• Mildly virulent (mMDV)
• Virulent (vMDV)
• Very virulent (vvMDV)
• Virus infectivity is strictly cell associated except in feather follicles epithelium where cell free viruses are produced .

Transmission :

• Virus is present in desquamated feather follicle epithelial cells , oral , nasal , tracheal secretion .
• Feather follicle : main source of infectivity of dander
• Air borne route is the most important route
• Virus is not transmitted through eggs .
• Infected birds continue to contaminate the environment by shedding viruses .

• Females are more susceptible than men .
• Stress is the main environmental factor .

Pathogenesis :

Four phases of infection are recognized :

1. Early cytolytic infection ( productive – restrictive )
2. Latent infection : immune development and T-cell ( mainly) affected ; B-cell are also affected .
3. Second phase of cytolytic ( productive – restrictive infection coincident with permanent immunosuppression )
4. Proliferative phase ( involving non-productively infected lymphoid cells that may or maynot progress to point of lymphoma formation )

Inhalation of Virus particles

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Growth of virus particles within cells of lungs

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Within 3-4 days ; cytolytic phase occur in lymphoid organs

[ destruction of lymphocytes mainly in Bursa of fabricius , thymus & spleen ]

 Primary target cell in all these organ – B-lymphocytes 

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After 6-7 days ; latency ( latent phase )

Interference with immune response and infect T-lymphocytes

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Virus spread throughout body by infected lymphocytes and is present in blood in cell associated form [ viremia]

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After 2 weeks of primary infection ; second phase of cytolytic occurs 9in feather follicle and remains in free form ( set free virus ) capable of producing infection and shed in the environment in feather debris and danders  .

Here dander like material is produced which acts as source of infection

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Proliferation of lymphocytes – final response and progress to tumor formation .

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T- lymphocytes transform into tumor cells and proliferate in nerve , other tissue and organs.

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This results in infiltration of these cells in nerves and lymphoma .

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Death of birds from lymphoma at any time 3 weeks onward .

 Various forms of disease :

a. Neurological :

• Paralysis
• Dropping wings
• Extended legs
• Torticollis

b. Ocular  :

• Blindness
• Distorted iris shape
• Discoloured iris ( iris may become whitish – Grey eye syndrome

c. Cutaneous :

• Enlarged feather follicles ,
• Leg lesion

d. Visceral :

• Nodular diffuse visceral lymphoid tumors

Clinical signs :

• Affects birds from about 6 weeks of age , mainly 12-14 weeks
• IP = 3-4 weeks

a. Acute MD :

• Mortality : 10-30%
• Birds may die suddenly or,

   Showing signs of dullness , depression or respiratory distress if heart is involved.

b. Classical MD :

• Mortality : 10-15 %
• Signs depends on peripheral nerves affected :

c. Brachial and sciatic nerves : 

• Progressoive spastic paralysis ( i.e. paralysis accompanied by muscular rigidity ) of wings and legs.
• Purplish discolouration of leg skin ( Red leg syndrome )
• Inco-ordination is a common early sign : one leg is held forward and the other backward because of unilateral paresis or paralysis .

d. Cervical nerves :  torticollis ( twisting of neck )

e. Vagus and intercostal nerve : respiratory signs

fig: Asymmetrical paralysis

fig: torticollis in marek disease

fig: Ulcer in feather and follicle

Gross lesions :

• Enlargement of one or more peripheral nerves, affected nerves are upto 2-3 times the normal thickness .
• Nerves commonly affected are brachial and sciatic plexus , coeliac plexus , abdominal vagus and intercostal nerves .
• Lymphoma / tumor ( cauliflower like growth ) usually in ovary but also in lungs , kidney , heart , liver , skin , feather follicles ( skin leukosis and muscles)
• Diffuse lymphomatous involvement and enlargement of liver , gonads , spleen , kidney , lungs , proventriculus and heart .
• Bursa : usually atrophic or some do develop diffuse thickening .

Microscopic lesions :

• There types of lesions in peripheral nerves :
• Cellular infiltration of nerves with mature lymphocytes
• Separation of nerve fibre associated with edema
• Nerves are infiltrated with lymphoblasts
• CNS : myelin degeneration of nerves , perivascular cuffing , schwann cell proliferation
• Necrosis of follicles of bursa of fabricius is commonly seen and cyst are also seen
• Diffuse infiltration of lymphocytes and plasma cells in affected organs
• Mainly , T-lymphocytes are present .

Diagnosis :

• History
• Clinical signs and lesions
• Age affected
• PCR
• Serological test : AGPT , Viral neutralization test , FAT
• Radial immunodiffusion test
• Histochemical staining
• MATSA ( MD associated Transforming Surface Antigen )

Non specific MD virus antigen ; also present in HVT vaccinated chicken . It is diagnostic antigen.

DDx :

• Lymphoid leukosis
• Botulism
• Deficiency of Thiamine
• Deficiency of Ca / P / Vit D , especially at start of lay