Leovet and Veterinary Medicine

                       (Fowl Paralysis, Pearl eye, Range paralysis, Polyneuritis, Grey eye/Pearl eye)
Transmissible lympho proliferative Herpes viral disease which affects domestic fowl characterised by mononuclear infiltration around peripheral nerves and to lesser extent in skin, muscle, iris and internal organs. Named after Joseph Marak a Hungarian veterinarian. Most common in young bird of 2 – 5 months old. Apart from poultry disease is reported from turkeys, ducks, quail and pigeon. First description of the disease was by Mareks in 1907 from Germany. This was the first incidence where science prepared a successful vaccine against a tumour causing disease. Herpes virus of Turkey is antigenically related. 
	Virus can be cultivated in susceptible chicken, chicken embryo (on CAM via yolk sac route) and cell cultures (chicken embryo fibroblast culture or chicken embryo kidney culture). Virus can be isolated from tumour, spleen, kidney, buffy coat or whole blood from affected birds. Dander from feather follicle remains infective for one year.
There are three serotypes of MDV which have many common antigens and are distinguished by serologic tests.
                             

Transmission:
	Inhalation and oral route are main mode of transmission. Also transmitted through eggs (transovarian). Certain strains of White leghorns are resistant to MD.

Pathogenesis:
	Outcome depends on virus strain, route of infection, dose, age, sex, immune status and genetic susceptibility of chickens. Disease occurs in six forms. These syndromes may overlap.

1. Classical Marek's disease or neurolymphomatosis causes asymmetric paralysis of one or more limbs. One leg kept forward and another backward. With vagus nerve involvement, difficulty breathing or dilation of the crop may occur. Besides lesions in the peripheral nerves, there are frequently lymphomatous infiltration/tumours in the skin, skeletal muscle, and visceral organs. Organs that are commonly affected include the ovary, spleen, liver, kidneys, lungs, heart, proventriculus and adrenals.

2. Acute Marek's disease is an epidemic in a previously uninfected or unvaccinated flock, causing depression, paralysis, and death in a large number of birds (up to 80 percent). The age of onset is much earlier than the classic form; birds are four to eight weeks old when affected. Infiltration into multiple organs/tissue is observed.

3. Ocular lymphomatosis causes lymphocyte infiltration of the iris (making the iris turn grey), unequal size of the pupils, and blindness.

4. Cutaneous Marek's disease causes round, firm lesions at the feather follicles.
 
5. Atherosclerosis is induced in experimentally infected chickens. 

6. Immunosuppression – Impairment of the T-lymphocytes prevent competent immunological response against pathogenic challenge and the affected birds become more susceptible to disease conditions such as coccidiosis and "Escherichia coli" infection.
There are four phases of infection: 1) degenerative changes caused by early productive-restrictive virus infection, 2) latent infection, 3) another phase of cytolytic infection associated with permanent immunosuppression, and 4) nonproductive infected lymphoid cells that may or may not progress to lymphoma formation, a "proliferative" phase. 

Macroscopic lesions: Nerve lesions can be seen as grayish, edematous, two or three times the normal thickness, and loss of the normal striated white glistening appearance.  Nerves commonly affected include the brachial and sciatic plexi, celiac plexus, abdominal vagus and intercostals nerves.  Also, tumors such as lymphoma occur in the ovary along with the nerve lesions.  Organs are enlarged with diffuse grayish discoloration.

Microscopic lesions:  There are two main types of lesions in peripheral nerves.  Type A is interpreted as neoplastic in character, consisting of masses of proliferating lymphoblastic cells.  Type B is inflammatory in nature and is characterized by diffuse infiltration of lymphocytes and plasma cells, edema, and sometimes demyelination. In visceral organs, a type of blast cells called as MD cells can be seen which degenerating lymphoblast are.
	Herpesviruses replicate in the bursa of Fabricius and the thymus which results in degenerative changes in these organs.  Atrophy of the thymus can be severe and involve the cortex and medulla.  In some cases, lymphoid proliferation in the thymus was seen. 
 
Clinical signs:  MD commonly affects pullets between 12-24 weeks of age, but can infect broilers as early as 6 weeks of age.  The incubation period ranges from 3-4 weeks to several months.  Signs may vary according to the nerve or nerves affected.  Asymmetric progressive paralysis of one or more of the extremities can be seen.  Wing involvement is characterized by drooping of the limb.  Torticollis of nerves controlling the neck are affected.  Vagal involvement will lead to dilatation of the crop and/or gasping.  If the iris is involved, eyes will lose their ability to accommodate light intensity and blindness may occur (once called "grey eye").  Many birds die suddenly without symptoms.  There are nonspecific signs such as weight loss, paleness, anorexia, and diarrhoea.

Diagnosis:
	Isolation of the virus, symptoms, and studies based on day old chickens. Here a fine suspension of feather follicle or small pieces of skin, blood or buffy coat, dander or any lymphoid organ can be inoculated IP. Observe for 10-14 days. By about 14th day microscopic lesion will develop and by 3-4 weeks macroscopic lesion can be seen. Demonstration of IN IB also can do. 

Control:
Vaccination using HVT. 0.2 mL IM on day old chicks.