Lymphoid leukosis and Marek’s disease virus (MDV) are two diseases of the avian leukosis complex. These conditions are commercially important diseases of poultry seen worldwide (Payne 2000) (Fig 1). Lymphoid leukosis and MDV were the first neoplastic diseases shown to be transmitted and caused by viruses (Payne 2012).
Lymphoid leukosis is a retroviral infection that causes tumors in chickens 14 weeks of age and older. The incidence of avian leukosis is typically low (<4%), however, mortality can be high in affected birds. Lymphoid leukosis is transmitted transovarially. Hens shed virus into eggs. Chicks are infected at hatch and shed virus their entire lives. Necropsy findings include profound hepatomegaly, bursal enlargement, and neoplastic nodules. Unlike Marek’s disease virus, neural involvement does not occur in lymphoid leukosis.
Marek’s disease virus is a highly infectious disease caused by a herpesvirus. Commercial poultry are vaccinated against Marek’s disease virus (MDV) at 1 day of age in hatcheries. There are four forms of MDV (Fig 2). The visceral form is difficult to distinguish from lymphoid leukosis. Marek’s disease virus is an important cause of neurologic disease in backyard poultry. In the neurologic form, MDV is often first noticed as gait disturbances like ataxia or stilted movements. As MDV progresses, birds can exhibit range paralysis in which one leg is stretched forward and the other leg is stretched back. Unilateral enlargement of the ischiatic nerve is identified at necropsy.
|Feature||Lymphoid Leukosis||Marek’s Disease|
|Etiologic agent||Retrovirus (RNA)||Herpesvirus (DNA)|
|Age||4 months or older||2-5 months old (sexually immature, usually < 14 weeks)|
|Course of disease in a flock||Insidious onset|
No obvious peak
Low persistent mortality
|Acute or distinct onset
|Paralysis observed?||Rare||Common (important to distinguish from debilitation)|
|Neoplastic lesions involving the:|
|Nerve trunks||-||+ (ischiatic plexus)|
|Bursa of Fabricius||(nodular tumors)||(atrophy or neoplastic involvement)|
|Characteristic cells||Lymphoblastic lymphocytes||Pleomorphic lymphocytes|
|Control||Elimination of carriers, primarily at the primary breeding level||Vaccination|