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.
It is spread by contaminated litter, dust, down, or air-borne (bird to bird). Feather (dust or dander) epithelium contains virus. There is an incubation period of 2 weeks for virus shed and for clinical signs from 3-6 weeks.
It causes increased susceptibility (immunosuppression) to other diseases. Signs include weak, pale, off feed, diarrhoea, poor performance, culls and blindness. In white leghorn type birds, Classical Marek’s is most common: There is paralysis or perisis (partial paralysis). There can be unilateral or bilateral paralysis of wings and legs, mortality, tumours and central nervous system signs (tremors). One leg stretches forward and the other backwards due to leg paralysis.
Vaccination by in ovo or day of age by SQ injection with HVT, SB1/and or Rispens strains. Recombinant HVT vaccine now exits.
It is immunosuppressive. Tumors are a leading cause of condemnation in broilers and MD is the leading cause of tumors in US broilers. There has been an increase in the incidence of MD is broilers in the US since the change to dry cups and nipple drinkers. These systems make the house drier and dustier, which provides an environment where the virus is more easily spread.
Study: Poultry at risk from ‘leaky’ Marek’s vaccination
Scientific experiments with the herpes virus strain that causes Marek’s disease in poultry have shown, for the first time, that some types of vaccines allow for the evolution and survival of increasingly virulent versions of a virus that could put unvaccinated individuals at greater risk of severe illness.
Managing Marek’s disease through genomics
Scientists at the Agricultural Research Service (ARS) are working to decipher the genomes of several nonvirulent Marek’s disease (MDV) vaccine strains, in order to identify how to best fight virulent and other infectious strains of the disease.