Currently, virulent serotype 1 strains are further divided into pathotypes (classification based on the severity of disease caused by that particular strain of virus), which are often referred to as mild (m), virulent (v), very virulent (vv), and very virulent plus (vv+) MD virus strains. Serotypes are identified by reaction with serotype-specific monoclonal (clones from a single cell) antibodies or by biological characteristics such as host range, pathogenicity (severity of disease), growth rate, and plaque morphology (the physical appearance of laboratory grown viral cultures). Grey iris and irregular pupil from Marek’s disease Source: United States Department of Agriculture The avirulent virus that is commonly found in turkeys is designated serotype 3. The serotypes that exist are ‘virulent’ (disease causing) chicken isolates (serotype 1) and ‘avirulent’ (non-disease causing) chicken isolates (serotype 2). The route of infection is usually respiratory. MD is caused by a highly cell-associated (virus particles that remain attached to or within the host cell after replication) but readily transmitted herpesvirus.
Reticuloendotheliosis, although rare, can easily be confused with MD because both diseases feature enlarged nerves and T-cell lymphomas (a type of tumour that involves white blood cells called T-cells, which are part of the active acquired immunity system) in visceral (soft internal) organs. Also, MD can develop in chickens as young as 3 weeks old, whereas Lymphoid Leukosis is typically seen in chickens older than 14 weeks old. The clinical signs associated with MD can look similar to those caused by Lymphoid Leukosis and Reticuloendotheliosis, however, the rareness of bursal tumours with MD helps distinguish this disease from Lymphoid Leukosis. Affected birds are immunosuppressed and as a consequence are more susceptible to other infectious diseases. Clinical signs include paralysis of the legs, wings and neck, weight loss, grey iris or irregular pupil, vision impairment and the skin around feather follicles can be raised and roughened. Marek’s Disease (MD) results in enlarged nerves and in tumour formation in nerve, organ, muscle and epithelial (cells that line the internal and external surfaces of the body) tissue.
Mortality rates can be very high in susceptible birds. Clinical disease is not always apparent in infected flocks, however subclinical disease is often more economically important as it reduces weight gain and egg production. It is one of the most common diseases that affect poultry flocks worldwide. Marek’s Disease Virus (MDV) is a highly contagious viral infection that predominantly affects chickens but can also affect pheasants, quail, gamefowl and turkeys.
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