Marek’s disease is a highly contagious, tumour-causing virus. It is shed from the feather follicles of infected birds and, when inhaled, crosses into the bloodstream and into the white blood cells that fight infection. It then causes devastating cell damage.
Birds aged between three and six months are most susceptible: it’s a disease unlikely to be seen in older birds. In field conditions, Marek’s disease outbreaks can occur in unvaccinated layers from three weeks old, but most serious cases occur between eight and nine weeks of age. Later outbreaks may occur in broiler breeders, or after moulting.
The virus prevents the cells from functioning and reduces a bird’s ability to fight off infection, leaving it vulnerable to other infections and reducing growth rate. Secondary diseases, such as coccidiosis or bacterial infections, can then take advantage of the dampened-down immune system, again reducing growth and resulting in increased mortality rates.
The virus causes the white blood cells to divide uncontrollably and form tumours. Tumours can occur in the feather follicles, presenting as large numbers of raised areas of the skin. Changes in the appearance of the eye can also indicate Marek’s disease – infected birds have poorly pigmented irises and less well-defined pupils.
Tumours which have reduced growth rates and increased the incidence of diarrhoea – as can be found in the liver, spleen, kidneys and reproductive tract – can be found post mortem. This is referred to as acute form.
Marek’s disease can also present in a classical form. Tumours can also develop in the nervous system, leading to paralysis and abnormal head and neck movements, as well as causing splaying of the legs. These birds are unable to eat and drink and, as there is no cure, the only option is culling the affected birds.
The key to prevention is to eliminate the virus so that young chick’s exposure is minimised. Often older birds are the source of infection. These do not necessarily show clinical signs if they are infected. Prevention tactics are: