From Mississippi State Extension:
Marek’s disease is one of the most widespread poultry diseases in the world. The first report of the disease was in 1907 by József Marek, for whom the disease is named. It is a highly contagious viral disease caused by a herpes virus called Alphaherpesvirinae.
Marek’s disease can occur in chickens 3 to 4 weeks of age but is most commonly seen in birds between 12 and 30 weeks of age. Female birds are more often affected than are males.
Transmission of the virus occurs by direct and indirect contact between chickens. Once the virus is introduced into a chicken flock, infection spreads quickly from bird to bird, even if the chickens are vaccinated. Seemingly healthy birds may be infected and, if so, will regularly shed the virus into the surrounding environment. Both infected and recovered birds are lifelong carriers of the disease; they will continue to shed the virus in their feather dander and through oral and nasal secretions for the remainder of their lives.
If you have infected birds in your flock and you bring in new birds, the new birds are at risk of becoming infected if they are housed with your birds and did not receive Marek’s vaccination at hatch. The disease is not transmitted through the parents to the egg. Unfortunately, there is no effective treatment for the disease and infected birds never recover.
Birds that develop clinical symptoms of the disease usually do so because of some form of stress trigger. This could be normal hormonal changes associated with the onset of egg-laying in hens or crowing in roosters, flock fighting and peck order establishment, extreme weather conditions, predator attack, change in management or flock ownership, high parasite load, or rough handling. Birds may suddenly go lame, and this initial lameness may be mistaken for an injury. However, the lameness worsens until the bird is unable to walk. These birds may develop the classic Marek’s paralysis pose in which one leg is positioned straight out in front of the body and the other straight out behind.
There are four different forms of Marek’s disease, and infected birds may exhibit one or more forms:
- Skin (cutaneous)
- Nerve (neural)
- Eye (ocular)
- Internal organ (visceral)