Marek’s disease (MD) is a common lymphoproliferative disease in chickens, usually characterized by mononuclear cells’ infiltration in peripheral nerves and various other organs and tissues, including iris and skin.
The incidence is quite variable in commercial birds and has declined in general since the worldwide introduction of vaccines. The majority of birds that develop the clinical picture of the disease die. Some birds can recover from clinical disease, but recovery is rarely permanent.
Grouping of Virus
According to the recent classification by the International Committee for Virus Taxonomy (ICTV), all serotypes of Marek’s disease are grouped into the genus Mardivirus and family Herpesviridae. Members of the genus Mardivirus are grouped as three species: Gallid herpesvirus 2 (serotype 1, single oncogenic), Gallid herpesvirus 3 (serotype 2, isolated from chickens), and a Meleagridi herpesvirus (serotype 3, isolated from turkeys [HVT]).
Based on their virulence, strains of serotype one are further divided into pathotypes, which are often referred to as mild (m), virulent (v), very virulent (vv), and very virulent plus (vv +).
How does Mareck’s disease spread in chickens
It is easily transmitted by direct or indirect contact between chickens by air. Viruses are present in the feather pulp, being the main source of contamination of chickens’ environment and infection. The contaminated site remains with an infectious potential for several months at 20 ° -25 ° C, and for years at 4 °, C.
Marek’s disease occurs mainly in young birds (1-16 weeks). Early studies, based on infection from contact, showed that virus excretion starts about two weeks after infection and continues indefinitely. Transmission by rattlesnakes (Alphitobius diaperinus) has been reported but in free-living birds.
Signs of Mareck’s disease
The signs associated with the visceral form are mainly depression, anorexia, weight loss, pallor, and diarrhea. Nervous form, incoordination, torticollis, unilateral spastic paralysis or paralysis of the limbs, dilation of the crop, and flaccid paralysis of the neck. The signs of the disease are nonspecific and include weight loss, pallor, anorexia, and diarrhea. Under commercial conditions, death often results from hunger and dehydration, due to the inability to reach food and water.
Symptoms of Mareck’s disease in chickens
Associated signs vary according to the specific syndrome. In general, signs related to nerve damage are paralysis and complete spastic paralysis of one or more of the extremities. Locomotor disorders are easily recognized, and incoordination may be the first sign observed. The characteristic particular clinical presentation is a bird with one leg stretched forward and the other back due to unilateral paresis or paralysis of the leg.
However, chickens with lymphomas can appear clinically normal, even though they have extensive neoplastic involvement in the heart, ovaries among other organs, and the lesions are detected only in the post-mortem examination. In contrast, other birds can become depressed before death.
Detailed Analysis
The main macroscopic lesions are thickened nerves, with a higher incidence of sciatica and, in cases of oncogenic serotype involvement, tumors involving the feather follicles, lymphoma in the ovaries, and other organs. Eye injuries such as pupil and iris irregularities with changes in pigmentation occur due to the infiltration of mononuclear cells. Some studies report the occurrence of atherosclerosis. Under microscopy, many lymphocytes infiltrate into tissues associated with edema, especially in organs and tissues presenting alterations such as nerves.
How to diagnose Marek’s disease in chickens
The diagnosis is made through a careful analysis of the necropsy associated with the histopathological diagnosis. However, as the lesions are not pathognomonic, confirmation must be made through PCR or immunohistochemistry. The main differential diagnoses are avian leukosis and reticuloendotheliosis.
How to treat Marek’s disease in chickens?
There is no treatment, and control is based on biosafety measures such as eliminating organic matter, fire broom, and vaccination of birds in the hatchery. Due to the great difficulty of controlling the disease when it is already installed on the property, the best method of control is the introduction of vaccinated birds to avoid future contamination.
