Marek’s Disease

INTRODUCTION Marek’s disease (MD) is a common lymphoproliferative disease of chickens, usually characterized by mononuclear cellular infiltrates in peripheral nerves and various other organs and tissues including iris and skin. The disease is caused by a herpesvirus, is transmissible, and can be distinguished etiologically from other lymphoid neoplasms of birds.

ETIOLOGY MDV is a cell-associated herpesvirus Serotype 1 MDV. Serotype 1 strains are further divided into pathotypes, which are often referred to as mild (m)MDV, virulent (v)MDV, very virulent (vv)MDV, and very virulent plus (vv+)MDV strains Serotype 2: Apathogenic Serotype 3: HVT

Transmission MDV is transmitted readily by direct or indirect contact between chickens, apparently by the airborne route Virus associated with feathers and dander is infectious, and contaminated poultry house dust remains infectious for at least several months at 20— 25°C and for years at 4°C Vertical transmission of MDV does not occur

Incubation Period Under field conditions, MD outbreaks sometimes occur in unvaccinated layer chickens as young as 3—4 weeks. Most of the serious cases begin after week 8 or 9 but also sometimes commence well after the onset of egg production.

Clinical Signs Signs associated with MD vary according to the specific Syndrome. In general, signs related to peripheral nerve dysfunction are those associated with asymmetric progressive paresis and, later, complete spastic paralysis of one or more of the extremities. Involvement of the vagus nerve can result in paralysis and dilation of the crop and/or gasping.

Because locomotory disturbances are easily recognized, incoordination or stilted gait may be the first observed sign. A particularly characteristic clinical presentation is a bird with one leg stretched forward and the other back as a result of unilateral paresis or paralysis of the leg

Fowl paralysis. Spastic paralysis of limbs associated with peripheral nerve involvement in Marek’s disease.

Nonspecific signs such as weight loss, paleness, anorexia, and diarrhea may be observed, especially in birds in which the course is prolonged. Under commercial conditions, death often results from starvation and dehydration because of the birds’ inability to reach food and water or, in many cases, from trampling by penmates.

Birds with ocular involvement may show evidence of blindness, which can be unilateral or bilateral, although recognition of clinical blindness requires careful observation. Affected eyes gradually lose their ability to accommodate to light intensity.

Chickens become depressed and comatose prior to death, which occurs within 48 hours of the onset of signs. Some affected chickens may also exhibit flaccid neck paralysis prior to death

Two forms exist, classical and acute Two forms exist, classical and acute. In the classical form, affected chickens display varying degrees of ataxia and flaccid paralysis of the neck or limbs beginning 8—12 days after exposure to the virus by inoculation or contact. Signs typically last 1—2 days followed by a rapid and complete recovery, although recovered chickens may succumb a few weeks later with MD lymphomas. The acute (fatal) form results in death within 24—72 hours following the onset of paralytic signs.

C. Ocular lesions of MD. Note that the normal eye (left) has a sharply defined pupil and well-pigmented iris. Affected eye (right) has a discolored iris and very irregular pupil as a result of mononuclear cell infiltration.

E. Multiple lymphomas in lungs. F. Multiple lymphomas in heart.

Gross Pathology Pathologic changes in MD have been reviewed and consist mainly of nerve lesions and visceral lymphomas. Enlarged peripheral nerves are a frequent finding in affected birds.

Nerves Severely affected peripheral nerves may show loss of cross-striations, gray or yellow discoloration, and sometimes an edematous appearance.

Enlarged sciatic plexus (left) and normal plexus (right).

Visceral Organs Lymphomatous lesions can be found in the gonad (especially the ovary), lung, heart, mesentery, kidney, liver, spleen, bursa, thymus, adrenal gland, pancreas, proventriculus, intestine, iris, skeletal muscle, and skin. Probably no tissue or organ is without occasional involvement.

Integument Skin lesions, probably the most important cause of condemnation in broiler chickens, usually are associated with feather follicles. The nodular lesions may involve few scattered follicles, or they may be numerous and coalesce.

Eye Gross ocular changes include loss of pigmentation in the iris (“gray eye”) and irregularity of the pupil, both the result of mononuclear infiltration of the iris

DIAGNOSIS Virus Isolation Polymerase Chain Reaction (PCR) Assays Antibody Demonstration

INTERVENTION STRATEGIES Vaccination Genetic Resistance Management Procedures