Marek’s disease is a highly contagious viral disease of chickens that occurs throughout the world. The virus is readily transmitted between chickens and can survive for months in poultry house litter or dust. Dust or dander from infected chickens is particularly effective in transmission. Marek’s disease infection can result in tumour formation, paralysis and increases in mortality.
THE VIRUS
Marek’s disease virus is a member of the genus Mardivirus within the subfamily Alphaherpesvirinae. Within the genus Mardivirus are three closely related species previously designated as three serotypes of Marek’s disease virus. Marek’s disease serotypes and pathotypes:
| Genus | Classification | Serotype | Examples |
|---|---|---|---|
| Mardivirus | Gallid alphaherpesvirus 2 | 1 | CVI/988 (Rispens) |
| Mardivirus | Gallid alphaherpesvirus 3 | 2 | SB-1 |
| Mardivirus | Meleagrid alphaherpesvirus 1 | 3 | Herpesvirus of Turkeys (HVT) |
- First Image. Tumour formation in the liver, 25-week-old layer.
- Second Image. Paralysis in a bird infected with vMDV.
- Third Image. Sciatic plexus enlargement in a bird infected with vMDV.
Photos courtesy of Seung-Hwan Jung.
MAREK’S DISEASE CONTROL
There is no treatment for Marek’s disease. Methods of prevention include biosecurity, vaccination and breeding of birds with genetic resistance to Marek’s disease. The object of vaccination is to induce protection by establishing vaccine viraemia as early as possible before field exposure. Once the virus is introduced into a chicken flock, regardless of vaccination status, infection spreads quickly from bird to bird. Shedding of infectious virus can be reduced, but not prevented, by prior vaccination. Vaccination forms the backbone of the prevention strategy and may include any of the following vaccine types:
- CVI988/Rispens (attenuated Gallid alphaherpesvirus 2)
- SB-1 or 301B/1 (naturally avirulent Gallid alphaherpesvirus 3)
- Herpesvirus (HVT, naturally avirulent Meleagrid alphaherpesvirus 1)
Bivalent vaccines are also available which include 2 strains from different serotypes for example a vaccine which includes HVT and Rispens. Many recombinant vaccines also make use of HVT as a carrier with a protein insert of another disease, for example Newcastle disease or Infectious Laryngotracheitis. These vaccines provide protection against Marek’s disease and the disease inserted into the carrier.
CVI988/Rispens appears to be the most protective commercial vaccine currently available, an attenuated Marek’s disease virus strain that is also commonly mixed with HVT at vaccination. It should be noted that 2 vaccines containing HVT strains should not be administered concurrently.
