About Marek's Disease

Marek’s disease is a common lymphoproliferative disease affecting poultry populations worldwide. Without any control measures, it is capable of causing significant economic losses in commercial poultry flocks. Even in the absence of clinical signs, the immunosuppressive effects of the disease may lead to poor growth and performance, significantly affecting production and economics in the poultry industry. It cannot be treated, but vaccination can aid in the prevention of disease caused by the virus.

Cause, transmission, & evolution

Marek's disease is caused by an alphaherpesvirus that affects many different types of poultry, but chickens are the most severely affected. Young chickens, whose immune systems are still under development, are most susceptible to infection. The virus is highly contagious, transmitted through the dust and dander of infected birds, and can survive for long periods in farm and field environments.

Named after the Hungarian veterinarian József Marek who discovered it, the disease was first described in 1907, and is widely found in chickens all over the world. Vaccination was first introduced in 1970 and helped aid in disease prevention, although in the 1980s and '90s more virulent forms of the virus began to appear, especially in North American and Europe.

Symptoms & disease impact

The clinical manifestations of Marek's disease vary widely, depending on virus strain, bird age and condition, and other factors. It can be difficult to differentiate from other diseases, manifesting as a variety of syndromes with wide-ranging signs, including:

• Peripheral nerve lesions, which may cause limb paralysis or persistent neurologic disease
• Blindness or other eye abnormalities, caused by lymphocytic infiltration of the iris
• Visceral lymphomas
• Skin lesions around the feather follicles
• Listlessness, wasting, and early mortality

Even in the absence of external symptoms, Marek's disease can significantly damage chickens' immune systems as the virus attacks T lymphocytes. The resulting immunosuppression results in poor growth and performance, causing major economic loss:

• Reduced broiler carcass quality and lack of uniformity increases production costs
• Egg production is reduced in layers
• Birds are more prone to secondary infections and less responsive to other vaccines, leading to increased medication costs

Protection and control

Fortunately, while Marek's disease cannot be treated, vaccination can aid in its prevention and control. Vaccines against the Marek’s virus have been widely used in the poultry industry since the 1970s. While vaccination can prevent infection in chickens, it does not prevent spread of the virus—vaccinated birds may still be carriers, so good hygiene practices are also important.

The most widely used Marek's vaccine is the herpesvirus of turkeys (HVT), which is closely enough related to the Marek's virus to stimulate an immune response that, as shown in published research, experiences minimal interference from maternal antibodies.¹² HVT is also non-pathogenic in chickens and therefore a safe vaccine option. A more complete Marek’s disease vaccination program for longer-living birds must include at least one other strain of another Marek’s serotype, 1 or 2.²⁰