10 THINGS EVERY CHICKEN OWNER SHOULD KNOW ABOUT MAREK’S DISEASE

Just the mention of Marek's disease arouses dread in many poultry keepers, but there is a lot of misinformation around which only serves to enhance those fears. Words

BEFORE SCIENCE knew its name or what the causative agent was, Marek's was known by various terms, among them new pullet disease, range paralysis and grey eye. Today we know a lot more about this nasty virus.

1 THESE BIRDS ARE MOST AT RISK

It typically affects young birds from about 10 to 25 weeks old and 25% mortality is not uncommon in unvaccinated birds during the rearing stages. Chicks are most susceptible from one-day-old and for the first month of life, however the disease symptoms may not manifest themselves for many weeks, most commonly 10-25 weeks.

2 MAREK’S IS CAUSED BY A HERPES VIRUS

The disease was named after József Marek, a Hungarian veterinarian, and is caused by a highly contagious viral disease of the Herpes family. It is primarily only a disease of domestic chickens, although infection has been reported in other avian species and a related Herpes virus found in turkeys which causes no apparent disease in turkeys can be used as a vaccine (HVT) for chickens.

3

VACCINES ARE FAIRLY NEW

A vaccine for Marek's disease has only been available since 1970. Prior to this it was normal for world poultry farmers to have significant losses from Marek's disease, so almost all commercial laying hens were vaccinated once it was discovered. However, there was a resurgence of the disease in the mid-1980s and a new vaccination programme was developed to cope with new virulent strains. It is now common practice to vaccinate the parent breeding stock of commercial laying birds with a different vaccine to that which is used on their offspring.

4 VACCINES CAN’T WORK FOR THE AVERAGE BACKYARD FLOCK

Marek's vaccine is readily available but it comes in vials with a 1000 dose minimum for one-day-old chicks. Once mixed with the diluent, it must be swirled gently every five minutes, and the bottle must be completely used up within 24 hours of being opened, making use of vaccine by the small poultry keeper virtually impossible. There are a few other restrictions which makes Marek's vaccinations an unsuitable process for small flock owners (see box at 58).

5 MAREK’S DISEASE IS MUCH MORE COMMON THAN YOU THINK

There is a saying ‘if your chicken breathes it has been in contact with Marek's disease' so no matter how stringent your cleaning or processes, you can't stop birds getting Marek's disease.

6 IT’S A DOUBLE WHAMMY FOR AN INFECTED BIRD

Marek's supresses the immunity of a bird, increasing its susceptibility to other diseases such as coccidiosis.

7 IT IS NOT EGGTRANSMITTED

Marek's cannot be passed by the mother through the shell. It is shed in skin particles and in cells associated with the feathers.

8 THE DISEASE HAS A LONG INCUBATION PERIOD

This can be up to three months or longer, and it can manifest itself in three different ways:

1. NERVOUS (NEUROLOGICAL) - paralysis of the legs/wings,eye lesions. 2. VISCERAL - tumours of the organs and muscles.

3. CUTANEOUS - tumours of the skin and feather follicles.

The vaccine prevents the formation of tumours but it does not prevent the infection and shedding of the field virus by the chicken. Even vaccinated birds may exhibit other clinical symptoms unless

attention is made to good hygiene. It is essential to place the vaccinated day old chicks into a clean environment where there is no challenge by the virus.

9 SOME BREEDS ARE MORE SUSCEPTIBLE

Some breeds and even strains within a breed may be more susceptible to Marek's disease and certain blood groups may also genetically have better

resistance, depending on:

• the level of maternal antibodies in their

system;

• their genetic resistance or susceptibility

- Silkies and White Leghorns appear to be

more prone to Marek’s disease, but some

strains will be more susceptible;

• the contamination in their environment;

• early challenge to diseases like

coccidiosis, infectious bronchitis or other

immune-lowering conditions.

10 THERE IS NO EFFECTIVE CURE

Support your birds with a warm, dry, airy (but draught-free) living area, appetising food (eg, their usual pellets and something high in protein like dog roll or cat food), a vitamin supplement like Poly Aid Plus (see your vet or pet store), and hand-feeding a bird that is paralysed. A small percentage of birds may survive, and if they do they will be immune to Marek’s.