Marek's disease
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Marek's disease is a highly contagious viral neoplastic disease in chickens. Occasionally misdiagnosed as an abtissue pathology it is caused by an alphaherpesvirus known as Marek's disease virus (MDV) or gallid herpesvirus 2 (GaHV-2). The disease is characterized by presence of T cell lymphoma as well as infiltration of nerves and organs by lymphocytes.[1] Viruses related to MDV appear to be benign and can be used as vaccine strains to prevent Marek's disease. For example, the related Herpesvirus of Turkeys (HVT), causes no apparent disease in turkeys and continues to be used as a vaccine strain for prevention of Marek's diease (see below). Birds infected with GaHV-2 can be carriers and shedders of the virus for life. Newborn chicks are protected by maternal antibodies for a few weeks. After infection, microscopic lesions are present after one to two weeks, and gross lesions are present after three to four weeks. The virus is spread in dander from feather follicles and transmitted by inhalation.[2]
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[edit] Syndromes
There are five syndromes known to occur after infection with Marek's disease. These syndromes may overlap.
- Classical Marek's disease or neurolymphomatosis causes asymmetric paralysis of one or more limbs. 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, visceral organs. Organs that are commonly affected include the ovary, spleen, liver, kidneys, lungs, heart, proventriculus and adrenals.
- 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.
- Ocular lymphomatosis causes lymphocyte infiltration of the iris (making the iris turn grey), anisocoria, and blindness.
- Cutaneous Marek's disease causes round, firm lesions at the feather follicles.[2]
- Atherosclerosis is induced in experimentally infected chickens. [3]
- Immunosuppression Imparement of the T-lymphocytes prevent competent immunological response against pathogenic challenge and the affected birds become more succeptible to disease conditions such as coccidiosis. Furthermore, without stimulation by cell-mediated immunity, the humoral immunity confered by the B-cell lines from the Bursa of Fabricius also shuts down. Thus resulting in birds that are totally immunocompromised.
[edit] Diagnosis
The demonstration of nerve enlargement, especially with the ischiatic nerve along with suggestive clinical signs in a bird that is around three to four months old is highly suggestive of Marek's Disease. The presence of nodules on the internal organs may also suggest Marek's disease but further testing is required for confirmation. This is done through histological demonstration of lymphomatous infiltration into the affected tissue. A range of leukocytes can be involved, including lyphocytic cell lines such as large lymphocyte, lymphoblast, primitive reticular cells and occasional plasma cells as well as macrophage and plasma cells. The T-cells are involved in the malignancy, showing neoplastic changes with evidence of mitosis.
The lymphomatous infiltrates need to be differentiated with another condition that affects poultry known as Lymphoid Leukosis as well as an inflammatory event associated with hyperplastic changes of the affected tissue.
[edit] Prevention
Vaccination is the only known method to prevent the development of tumors when chickens are infected with the virus. However, administration of vaccine does not prevent transmission of the virus; i.e., the vaccine is non-sterilizing.[1] However, it does reduce the amount of virus shed in the dander and hence reduce horizontal spread of the disease. Marek's Disease does not spread vertically. The vaccine was introduced in 1970. Before that, Marek's disease caused substantial revenue loss in the poultry industries of the United States and the United Kingdom. The vaccine is administered to one day old chicks through sub-cutaneous innoculation, and immunity develops within two weeks.[2] It usually contains the antigenically similar turkey herpesvirus, which is serotype 3 of MDV.[4] Currently, there is a push for in-ovo vaccination to reduce the need for handling the day-old chicks.
[edit] Eponym
The disease is named after Dr. Josef Marek.
[edit] References
- ^ a b (2001) in Hirai, K (Ed.): Current Topics in Microbiology and Immunology: Marek's Disease (Current Topics in Microbiology and Immunology). Springer: Berlin. ISBN 3-540-67798-4.
- ^ a b c Fenner, Frank J.; Gibbs, E. Paul J.; Murphy, Frederick A.; Rott, Rudolph; Studdert, Michael J.; White, David O. (1993). Veterinary Virology (2nd ed.). Academic Press, Inc. ISBN 0-12-253056-X.
- ^ Fabricant CG, Fabricant J. "Atherosclerosis induced by infection with Marek's disease herpesvirus in chickens". Am Heart J 1999 Nov;138(5 Pt 2):S465-8. PMID 10539849.
- ^ Carter, G.R.; Flores, E.F.; Wise, D.J. (2006). Herpesviridae. A Concise Review of Veterinary Virology. Retrieved on 2006-06-10.
