Marek's disease (MD) is a herpesvirus infection in chickens that induces the formation of tumors in nerve, organ, muscle and epithelial tissue. MD affects young chickens 2-16 weeks of age in all parts of the world. It is often seen in conjunction with other diseases, such as ILT and ND. Unfortunately, the stresses from other diseases can increase the severity of MD.
The MD virus is airborne and highly contagious, so chickens are usually infected through the respiratory system. However, infection also can be spread by feather dander, chicken house dust, feces and saliva. Infected birds carry the virus in blood for life and are a source of infection to susceptible birds. Transmission by egg is of no significance.
MD can manifest in different ways:
- Neurological — Acute infiltration of the CNS and nerves results in “floppy broiler syndrome” and transient paralysis, as well as long-term paralysis of legs or wings and eye lesions
- Visceral — Can cause tumors in the heart, ovary, testes, muscles and lungs
- Cutaneous — Can cause tumors in feather follicles
- Paralysis of legs, wings and neck
- Weight loss
- Grey iris or irregular pupil
- Impaired vision
- Raised, roughened skin around feather follicles
Some chickens with MD die without any clinical signs. Most affected birds will have some degree of paralysis, although chickens with the acute form may not show this condition. Those with paralysis may die because they are unable to reach feed and water. The first indication of infection is a variation in the growth rate and degree of feathering.
Morbidity ranges from 10%-50% and mortality can reach 100%. Mortality in an infected flock typically continues at a moderate or high rate for quite a few weeks. In “late” Marek's, mortality can extend to 40 weeks of age. Affected birds are more susceptible to other diseases, both parasitic and bacterial.
Unfortunately, there is no treatment for MD. Prevention strategies include hygiene, all-in/all-out production and vaccination.