Occurrence: Worldwide. Species affected: Chickens, pheasants and occasionally quail Age affected: All.

Causes: Marek’s disease virus is a cell-associated herpes virus containing double-stranded DNA of which there are three serotypes. Only serotype 1 is pathogenic.

Effects: Large scabby lesions on the surface of the skin are usually only seen in the processing plant, resulting in carcass downgrading. Other signs of Marek’s disease include weakness, paleness, feed refusal, diarrhoea, poor performance, culls and blindness. There is paralysis or paresis (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.

Skin leukosis was once recognised as a distinct disease. It is now recognised as a clinical sign associated with Marek’s disease.

Marek’s disease was named after the Hungarian pathologist Josef Marek. It is also know as range paralysis because it caused paralysis in chickens, which use to be reared on the range.

Chickens (all breeds), pheasants and occasionally quail are susceptible to this chronic disease. It occurs worldwide in commercial flocks and takes 4-6 weeks for tumours to form.

The 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 a very young age, but birds can die of Marek’s disease (MD) near the onset of egg production.

The agent involved in the aetiology of this disease is 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 tumours).

It is spread by contamination 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 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 were 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 paresis (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, sacral, sciatic and brachial) with a loss of striations. They can also have grey or yellow discoloration and be oedematous (fluid filled).

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

The skin has 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 bone is never involved and the blood and fat sometimes have tumours.

The diagnosis is by observation of gross tumours in immature birds. Ocular or skin leukosis and nerve involvement are diagnostic for MD.

Histopathologically, small to medium lymphoid leukosis, reticuloendotheliosis and colibaccillosis.

Vaccinate (HVT-serotype 3 alone or HVT and SB1-serotype 2) chicks at a day-old SQ in hatchery. Some injections are done at 18 days of 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. Broilers vaccination is common practice in USA. Outside USA, this is generally only done if broilers are kept over 50 days.

SB1 may cause immunosuppression and increase leukosis in some leghorn strains. HVT=Herpes virus of turkeys. SB1=S (susceptible strain) and B1 (pen B1). HVT + SB1 or HVT + Respins 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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