The virus remains alive in the environment for as long as eight months. It is shed from the feather follicles and spread in fluff. The virus gains entry when the bird breathes infected dust particles. Marek's disease is not spread through eggs. The virus is highly infectious and, once it is present in a flock, it spreads rapidly to unvaccinated poultry.
Healthy birds can be carriers and infect others.
Age of birds Young birds are the most susceptible to infection Most deaths from Marek's disease occur between 10 and 24 weeks of age, although in some cases the disease may not appear until later in life. Most birds that develop MD will die. The birds are unable to stand, become paralysed and slowly waste away from lack of food and water. In most cases the paralysis comes on quickly. Sometimes the wings or neck are involved.