Mareks disease in Chickens

Marek's disease The virus responsible for Marek's disease is thought to be responsible for greater mortality in chickens than any other disease. Every flock, except for those maintained under strict pathogen-free conditions, can be assumed to be infected. Flocks that do not show any symptoms may still be infected. Stress may weaken the immune systems and result in an outbreak of this disease. Although symptoms may not be apparent, there may be a reduced growth rate and a decrease in egg production. The virus is concentrated in the feather follicles and shed in sloughed skin and feather cells. It has a long survival time and viable virus can be isolated from houses that have not been occupied for many months. Transmission is by aerosols containing infected dust. Younger birds are most susceptible to infection. There is no treatment for Marek's disease. The main method of control is by vaccination of day-old chickens as a subcutaneous injection. Results are improved by strict sanitation to reduce or delay exposure and also by breeding for genetic resistance. Marek's disease may produce a variety of clinical responses: visceral, neural, ocular, skin or combinations of these. Visceral Marek's of the visceral type is characterized by widespread involvement with lesions commonly seen in gonads, liver, spleen, kidney and occasionally heart, lungs and muscles. The disease is often acute, with apparently healthy birds dying very rapidly with massive internal tumours. Neural The neural type is characterised by progressive paralysis of the wings, legs and neck. Loss of body weight, anaemia, laboured breathing and diarrhoea are common symptoms. Ocular Ocular leukosis (gray-eye) is usually seen in early maturity. Morbidity and mortality are usually low but may approach twenty-five percent in some flocks. It is characterized by the spotty depigmentation or diffuse graying of the iris in the eye. The pupil develops an irregular shape and fails to react to light. Emaciation diarrhoea and death follow. Skin Skin leukosis produces the most severe losses in intensively reared broilers, as a result of carcasses being condemned at the processing stage. Enlargement of the feather follicles due to accumulations of lymphocytes is the typical lesion. This is potentially the most infective version of Marek's because it is produced around the feather follicles and is shed with the skin particles and with feathers.