ARTICLE Marek’s Disease: Tumor-causing viral disease of chickens Introduction Dheeraj Chaudhary, Sanjeevna K. Minhas Division of Virology Indian Veterinary Research Institute, Mukteswar-263 138 Marek’s disease, a widespread, highly contagious viral disease, primarily of domestic fowls (chickens), paralyses the wings and legs and/or causes tumors in various organs. It can also affect the bird’s nervous system. The disease is named after Jozsef Marek, a Hungarian vet who first discovered the Marek disease virus (MDV) in1907. The fowls are more susceptible to MDV between 3 and 30 weeks of age. Mortality may increase rapidly over a few weeks and then cease, or can continue at a steady or slowly falling rate for several months. There is no satisfactory treatment except vaccination which poses the disease as a threat for poultry industry in developing as well as developed nations. Causative agent The disease is caused by Gallid herpesvirus 2 (GaHV-2), a ds DNA virus, which belongs to the genus Mardivirus within the family Herpesviridae. Two serotypes of MDV are recognized, 1 and 2 and a third serotype is represented by the related herpesvirus of turkeys (HVT). Serotype 1 includes all the virulent strains and some attenuated vaccine strains. Serotype 2 includes the naturally avirulent strains, some of which are used as vaccines. Host Range Chickens are the most important natural host for Marek’s disease virus. Quail can be naturally infected, and turkeys can be infected experimentally. However, severe clinical outbreaks of Marek’s disease in commercial turkey flocks, with mortality from tumors reaching 40%–80% between 8 and 17 week of age, were reported in France, Israel, and Germany. In some of these cases, the affected turkey flocks were raised in proximity to broilers. Turkeys are also commonly infected with turkey herpesvirus (HVT), an avirulent strain related to Marek’s disease virus that is commonly used as a Marek’s disease vaccine in chickens. Other birds and mammals appear to be stubborn to the disease or infection. Transmission Virus spreads through inhalation of dust or dander from the feather follicle epithelium of infected birds. Once upon introduction into a flock, infection spreads quickly from bird to bird. The virus can remain viable for months in feather follicle and hen house dust. Infected and recovered birds continue to shed the virus in the environment. The virus is not transmitted through the eggs. Clinical Signs Chickens with Marek’s disease may show one or any combination of the following symptoms • Sudden death • Paralysis of leg(s) or wing(s) • Characteristic symptom of one leg forward and one back • Sometimes twisting of neck • Depression, weakness, diarrhea • Tumors on feather follicles, skin, skeletal muscle, ovary, spleen, liver, kidneys, lungs, heart, and proventriculus • Blindness often in one eye • Pale, shriveled comb Laboratory Diagnosis • Choice of clinical material is buffy coats, lymphoma cells and spleen cells, various internal organs or skin containing feather tips from infected birds. • Virus can be isolated, in chicken kidney cells or duck embryo fibroblast cells. • Various serotypes can be differentiated based on the plaque morphology, time of appearance, rate of development. • HVT plaques appear earlier and are larger than serotype 1 plaques, whereas serotype 2 plaques appear later and are smaller than serotype 1 plaques. 108 February, <strong>2019</strong>
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