Occurrence: Worldwide.
Species affected: Chickens (all breeds), pheasants and occasionally quail.
Age affected: Usually under 16 weeks, but birds can die near the onset of egg production.
Causes: Marek’s disease virus is a cell-associated herpes virus containing double-stranded DNA, of which there are three serotypes.
Effects: It is immunosuppressive and causes increased susceptibility to other diseases. Signs include weakness, paleness, feed refusal, diarrhoea, poor performance culls and blindness. There is paralysis or perisis (partial paralysis), which can be unilateral or bilateral in wings and/or legs, which causes one leg to stretch forward and the other backwards. Tumours and tremors occur. Mortality ensues.
Marek’s disease
It was named after a Hungarian pathologist (Josef Marek) and also range paralysis because it caused paralysis in chickens, which use to be reared on the range. It affects chickens (all breeds), occasionally pheasants and quail. It occurs worldwide in commercial flocks. Disease is chronic. It takes 4-6 weeks for tumours to form.
Classical type (nervous form) is common in white layers between 6-16 weeks. Visceral type with tumours in various internal organs usually occurs between 16-35 weeks. Infection takes place at very young age, but birds can die of Marek’s disease (MD) near the onset of egg production.
It is caused by a cell-associated Herpes virus containing double-stranded DNA. It has hexagonal naked particles or nucleocapsids of 85 or 100 nM. Enveloped particles of 150-400 nM are occasionally seen in the feather follicle epithelium. There are 3 serotypes. Serotype 1 viruses can be oncogenic (causes tumors). Serotype 2 virus (SB1) and serotype 3 (herpes virus of turkeys -HVT), do not cause tumors and are used as vaccines. One attenuated serotype 1 virus (Rispen’s strain) is used as a vaccine.