There are two forms of Marek’s Disease:
‘Classical’ denoting splayed legs, caused by the cells on the surface of the nerve becoming cancerous and leading to swelling of the sciatic nerve. Birds go off their legs due to partial paralysis and are unable to feed and drink; eventually dying of starvation and dehydration.
‘Acute’ tends to form tumours on the internal organs, causing enlargement of the organ involved and replacement of functional tissue with non functional cells until a point is reached when they can no longer function and the bird therefore dies.
- Marek’s is caused by a chicken Herpes Virus which affects chickens and turkeys.
- Incubation Period is 6-8 days, post challenge; clinical signs can take weeks to appear.
- The virus is shed via the feather follicles and can remain in the environment for long periods of time. Birds are at risk as soon as they hatch, hence vaccination usually takes place in the hatchery. A vaccinated bird also sheds Marek’s virus at varying degrees throughout its life—adult poultry, especially free range layers in close proximity to other ages of birds, can therefore be the causal factor of infection.
- Vaccination does not solely prevent Marek’s Disease– the chick, even when vaccinated, can pick up Marek’s Disease if it meets a severe challenge—with added stresses such as unstable temperatures, other disease challenge or overcrowding, the likelihood is greater.
- The chick needs 7-10 days post vaccination for immunity to establish. This also means that the chick needs the right environment for this immunity to develop-no extra stresses and no exposure to the virus, otherwise the vaccine may not be effective.
- If the chick is exposed to other viral challenges such as Gumboro virus, then the bird can become immunosuppressed. This can then lead to mortality, runts, poor doers and very uneven flocks. Stress will contribute to this condition.
- Multi age sites are at higher risk because the bird is being challenged at different times of its growth by other birds.
- Research has shown that vaccinated birds if kept under stress will develop Marek’s tumours with up to 50% of the birds at slaughter with clinical changes to the organs and tumours. Post Mortem Inspection (PMI) reports will show nodules on the feather follicles especially around the thighs, therefore giving large numbers of rejects.
- A Marek’s challenge lowers your profits! Marek’s and other disease will cause uneven growth and runts.
Prevention
- Good hygiene and management practice during the brood stage.
- Keep stress on the chick to the minimum.
- Keep age groups separate as far as practically possible.
- Dedicate a member of staff to tend to particular tasks to specific age groups & wear separate clothing and footwear between ages.
Equipment and machinery carries disease challenge as well as housing—keep it clean!
Use your boot dips and other biosecurity measures effectively.
A healthy chick, vaccinated correctly, kept in the right environment, will withstand field challenge from Marek’s disesase