Mareks disease – an ever present threat

Mareks disease is named after the Hungarian veterinary surgeon, Jozsef Marek, who first described the disease in 1907. The disease was initially known as fowl paralysis.

In the late 1960s, the agent responsible for Mareks Disease was identified as a herpes virus, and by the 1970s, vaccines were available based on a naturally avirulent herpesvirus of turkeys which was related to Mareks disease virus. Since that time, further vaccines have been developed and layer chicks hatched today are usually vaccinated with a combination of turkey herpes virus (THV) and Rispens vaccine strains to achieve best protection against the disease.

What is Mareks disease?

Mareks disease is caused by a highly contagious herpesvirus. The virus is very common and is widespread on poultry farms.

The Mareks disease virus affects nerves and internal organs, particularly the liver, spleen and reproductive tract. Losses due to Mareks disease in an infected flock can be very variable – often between 10 and 15 per cent but can be higher.

What are the signs of Mareks disease in a chicken flock?

In layers, signs of Mareks disease are most often seen from 12 weeks up until around peak production. The incubation period can vary from weeks to months.

One of the most common signs seen in laying birds is paralysis of legs or wings. Depending on which nerves are affected, birds may also show twisted necks or dilated pendulous crops. Less commonly, birds may show a transient paralysis which may last for only 24 - 48 hours. Birds with tumours in the internal organs may show loss of condition.

Post mortem examination of affected birds may show obvious enlargement of nerves and tumours may also be seen affecting internal organs. A likely diagnosis of Mareks Disease can be made from the history of the flock and from the distribution and appearance of tumours and nerve lesions found on post mortem examination. Histological examination is usually required for confirmation of the diagnosis as there are other less common tumour-causing viruses of poultry.

How are birds infected?

As mentioned above, the virus is ubiquitous and is present in poultry flocks throughout the world. The virus is present in dust from feather follicles and feathers and can survive for several months in infected litter.

Vertical transmission of Mareks Disease virus does not occur so day old chicks are free from infection. It is likely that chicks are exposed to virus very soon after they arrive on farm. It is thought that most infection occurs following inhalation of infected material by the birds.

Birds can succumb to infection due to:

• poor administration of vaccine

• faulty vaccine

• immunosuppression due to other diseases (for example Gumboro disease) such that birds do not respond fully to vaccination

• overwhelming viral challenge on farm

• challenge by new Mareks virus strains

How do I protect my flocks from Mareks disease?

1. Your chick supplier will ensure that birds are vaccinated at day old at the hatchery with appropriate vaccine, usually a combination of turkey herpesvirus and Rispens strains.

2. Reduce the exposure of your birds to Mareks disease virus on farm by thorough cleaning and disinfection between flocks. This is easier to achieve on a single age, all in, all out site.

3. Make sure that when the chick arrives at day old on the rearing site that all aspects of the environment are optimal. If chicks are stressed, the immune system function will be impaired and birds will be more susceptible to infection.

4. Good high quality food from day old will promote a healthy immune system.

What should I do if Mareks disease is diagnosed in my flock?

It is not uncommon to see the occasional case of Mareks disease from layer flocks. Where larger numbers of birds are affected, then it is important to investigate what has gone wrong. Investigations will involve reviewing vaccination type and technique, checking biosecurity on farm and investigating any other concurrent disease problems, for example, Gumboro disease which might be causing immunosuppression.

Mareks disease is still very much with us. Despite knowing a lot more about the disease than we did 100 years ago and having good vaccines available, the Mareks disease virus still continues to cause economic losses in the layer industry.

In the past, before vaccines were available, control relied on very good biosecurity and attempts (still ongoing) to develop breeds of chicken which were less susceptible to the virus.

Good hygiene and biosecurity on farm still remain the cornerstone of good Mareks disease control.