Cause / Transmission: A herpes-virus. Highly contagious. Spreads by bird-to-bird contact, by contact with infected dust and dander, and by darkling beetles and mealworms that live in the chicken house.Morbidity is 10-50% and mortality up to 100%. Mortality in an affected flock typically continues at a moderate or high rate for quite a few weeks.
Occurs mainly in chickens under 16 weeks of age. In late Marek's the mortality can extend to 40 weeks of age. Affected birds are more susceptible to other diseases, both parasitic and bacterial. The virus doesn’t survive the incubation process well and is not spread by hatching eggs. Immune transfer from the hen to the chick provides some protection to the chick for the first few days of life.
Different forms of Marek’s Disease:
Cutaneous form (skin form): Enlarged reddened feather follicles and white bumps on the skin that form brown crusty scabs.
Neural (nerve form): Progressive leg or wing paralysis - a typical leg-paralysis victim will have one leg extended forward and one leg extended back. A swelling of the sciatic nerve is the cause.. Other possible symptoms: Twisted neck, weight loss, labored breathing, eye lesions, vision impairment, diarrhea, starvation and death due to an inability to reach feed and water and to trampling by other chickens.
Visceral (internal-organ form), i.e. tumors on visceral organs,- such as heart, ovary, tests, muscles, lungs
Treatment: None. High mortality.
Control: Vaccination of chicks at one day of age. Hygiene is of utmost importance. Please note that Marek’s disease-causing virus particles can survive for months in chicken-house dust and litter.
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. 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 disease (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.
There are five syndromes known to occur after infection with Marek's disease. These syndromes may overlap.
Classical Marek's disease or neurolymphomatosiscauses 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 diseaseis 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 lymphomatosiscauses lymphocyte infiltration of the iris (making the iris turn grey), anisocoria, and blindness.
Cutaneous Marek's diseasecauses round, firm lesions at the feather follicles.
Atherosclerosisis induced in experimentally infected chickens.
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 and "Escherichia coli" infection Furthermore, without stimulation by cell-mediated immunity, the humoral immunity conferred by the B-cell lines from the Bursa of Fabricius also shuts down. Thus resulting in birds that are totally immunocompromised.
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.
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 non-sterilizing. 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 can be administered to one day old chicks through sub-cutaneous inoculation or by in-ovo vaccination when the eggs are transferred from the incubator to the hatcher. In-ovo vaccination is the preferred method, as in does not require handling of the chicks and can be done rapidly by automated methods. Immunity develops within two weeks.
The vaccine originally contained the antigenically similar turkey herpesvirus, which is serotype 3 of MDV. However, because vaccination does not prevent infection with the virus, the Marek's Disease virus has evolved increased virulence and resistance to this vaccine. As a result, current vaccines use a combination of vaccines consisting of HVT and gallid herpesvirus type 3 or an attenuated MDV strain, CVI988-Rispens (ATCvet code: QI01AD03).
The disease is named after Dr. Jozef Marek.
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