Occurrence: Worldwide.   Species affected: Chickens (all breeds), pheasants, turkeys 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. Only serotype 1.

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.

It was named after a Hungarian pathologist (Josef Marek) and also because it caused paralysis is chickens, which use to be reared on the range (range paralysis). 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. Marek’s disease viruses and Marek’s disease vaccines are classified into 3 serotypes. Serotype 1 viruses can be oncogenis (cause tumours).

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 is immunosuppressive. Tumours are a leading cause of condemnation in broilers and MD is the leading cause of tumours in US broilers. There has been an increase in the incidence of MD in 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.

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 is 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.

The peripheral nerves are often enlarged (vagus, sacaral, sciatic and brachial) with a loss of striations. They can also have grey or yellow discolouration and be edematous (fluid filled).

The bursae are sometimes enlarged with tumours, but most often is strophic. There are enlarged organs (gonads, spleen, heart, lungs, liver, kidneys, proventriculus, intestines etc) with focal to nodular, palpable tumours.

The skeletal muscles have tumours. The eye (iris and pupil) may have diffuse depigmentation, diffuse bluish fading or diffuse grey opacity of the iris. The pupil may be irregular and tiny.

The diagnosis is by observation of gross tumours in immature birds. Ocular or skin leucosis and nerve involvement are diagnostic for MD. Histopathologically, small to medium lymphocytes comprise tumours. Lymphoid cells occur in peripheral nerves.

It simulates riboflavin deficiency, lymphoid leucosis, reticuloendotheliosis and colibaccillosis.

Vaccinate (HVT-serotype 3 alone, or HVT and SB1-serotype 2) chicks at day-old subcutaneously in the hatchery. Some injections are done at 18 days or embryonation (in ovo).

HVT and SB1 are used for broiler breeders and white leghorns at full dosage. HVT is used alone or HVT and SB1 at 1/3-1/4 dosage for broilers if they are to be kept over 50 days, although broiler vaccination is common in the US.

SB1 may cause immunosuppression and increase leucosis is some Leghorn strains. HVT = Herpes virus of turkeys. SB1 = S (susceptible strain) and B1 (pen B1). HVT + SB1 or HVT + Rispens vaccine can be used.

Rispens strain (Serotype 1) may be used alone or in combination with HVT. Outside the USA, Rispens strain is the most commonly used vaccine, usually given by intramuscular injection at day old. In areas with a high infection risk bivalent vaccine (Rispens + HVT) is used.

None.

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