Marek’s disease in birds 

Pashu Sandesh, 29 November 2021

Dr. Ragini Mishra¹ and Dr. Vikas Bhardwaj²

Marek’s disease (MD) is a lymphomatous and neuropathic disease of domestic fowl caused by an alphaherpesvirus, designated Marek’s disease virus (MDV), genus Marpirus. The genus includes three species (serotypes) designated as Gallid herpesvirus 2 (serotype 1), Gallid herpesvirus 3 (serotype 2) and Meleagrid herpesvirus 1 or herpesvirus of turkeys (HVT) (serotype 3). Serotype 1 includes all the virulent strains and some attenuated vaccine strains. Serotype 2 includes the naturally avirulent strains, some of which are used as vaccines.

Clinical features

In chickens, MD can occur at any time, beginning at 3–4 weeks of age or older, sometimes even well after the onset of egg production. Marek’s disease is a progressive disease with variable signs and several overlapping pathologic syndromes. Lymphoproliferative syndromes are most frequent with Marek’s disease, lymphoma being most common, with involvement of several visceral organs and, usually, paralysis of one or both legs or wings. Incoordination is a common early sign: one leg is held forward and the other backward when the bird is stationary, because of unilateral paresis or paralysis, usually involving the sciatic nerve. Wing dropping and lowering of the head and neck are common. If the vagus nerve is involved, there may be dilation of the crop and gasping. Marek’s disease lymphoma sometimes may occur without neurologic signs, and present only as depression and comatose state, with visceral lymphomas. MDV strains of higher virulence may also cause increased mortality in young birds of 1–2 weeks of age, especially if they lack maternal antibodies. Depending on the strain of MDV, lymphomatous lesions can occur in multiple organs such as the ovary, liver, spleen, kidneys, lungs, heart, proventriculus and skin. 

Most common clinical forms of Marek's disease

Acute Marek’s disease or fowl paralysis occurs in explosive outbreaks in young chickens. Ocular lymphomatosis is a rare syndrome that leads to greying of the iris of one or both eyes as a result of infiltration of transformed (neoplastic) lymphocytes. Cutaneous Marek’s disease is recognized readily after plucking, when round, nodular lesions up to 1 cm in diameter occur, particularly at feather follicles of young birds. The nonfeathered area of the legs may have distinct red colouration, and Marek’s disease is therefore sometimes called “red leg syndrome.” Other syndromes include immunosuppression and transient paralysis from brain oedema.

Diagnosis 

If sufficient numbers of birds are examined, history, age, clinical signs, and gross necropsy findings are adequate for the diagnosis, which can be confirmed by histopathology and quantitative PCR methods. Detection of viral antigen by immunofluorescence is the simplest reliable laboratory diagnostic procedure. Gel diffusion, indirect immunofluorescence, or virus neutralization is used for the detection of virus-specific antibodies but rarely is necessary or done. A variety of inoculation methods can be used for virus isolation: inoculation of cell cultures, preferably chicken kidney cells as well as chicken or duck embryo fibroblasts. The presence of the virus can be demonstrated by immunofluorescence or immunohistochemistry on tissues or cultures using monospecific antisera or specific antibodies to Marek’s disease virus, demonstration of a specific antigen in agar gel immunodiffusion tests, detection of parts of Marek’s disease viral genome by quantitative PCR assays, or by electron microscopy to demonstrate the presence of characteristic herpesvirus virions.

Prevention and control

Vaccination is the principal method of control. The standard method has been to vaccinate 1-day-old chicks. Parenterally; however, more than 80% of the 8 billion birds vaccinated annually in the United States are vaccinated in ovo at 18 days, by robotic machines. The vaccine is available as either a heterotypic lyophilized cell-free preparation (herpesvirus of turkeys) or a cell-associated preparation using Gallid herpesvirus 3 or live-attenuated Marek’s disease virus. The cell-free vaccine is not effective in immunizing chicks with maternal antibodies, whereas cell-associated vaccines are. Protective immunity develops within about 2 weeks. A further level of control can be achieved if flocks are built up with birds with increased genetic resistance. It is possible to establish flocks free of Marek’s disease, but commercially it is extremely difficult to maintain that disease-free status. The production of chickens on the “all-in-all-out” principle, whereby they are hatched, started, raised, and dispersed as a unit, improves the efficacy of vaccination as a control measure. In some countries, reduction of Marek’s disease virus load in the environment by removal of litter and cleaning/disinfection of the housing after each production cycle has reduced or even eliminated the requirement for vaccine use.

Dr Ragini Mishra¹ (PhD Scholar- Division of Veterinary Microbiology)

Dr Vikash Bhardwaj² (M.V.Sc. Scholar- Division of Poultry Science) 

ICAR-IVRI, Izatnagar-243122, Bareilly (U.P.)