by KJ Theodore – A discussion on Mareks Disease….

MAREKS DISEASE (MD), is a common virus that causes internal lesions (tumours), and kills more birds than any other disease. It is so common that you should assume you have it in your flock, even if you detect no evidence.

Mareks is a member of the herpesvirus family of viruses. It is also known as ‘Range Paralysis’. Mareks is spread through airborne feather dander so microscopic that it can spread from one farm to another via the wind, even when no human or bird contact is made between the two farms. The virus enters through the bird’s respiratory tract.

The turkey version is Herpes Virus Turkey (HVT), and the waterfowl version is known as Duck Virus Enteritis (or DVE). All three are from the same family of viruses.

It’s not common for MD, HVT, and DVE to cross over between chickens, turkeys, and ducks kept together, but it has happened.

The study of Mareks Disease in poultry is exciting because it has had a profound effect on cancer research in all species, including human. And the Mareks vaccine for chickens was the first time medical science was able to produce an effective cancer vaccine for any species.

There are a few different types of Mareks in chickens. The most common are eye, visceral (tumor producing), and nerve.

The nerve version is known by some Fanciers as ‘down in the leg’, and symptoms range from slight to severe paralysis in the wings, legs, or neck, and usually results in death from trampling by other chickens, and/or the inability to get to food and water. There can be ‘transient’ paralysis that disappears after a few days, such as a dropped wing that suddenly corrects itself. In the eye version, you’ll detect an irregularly shaped pupil, cloudy eye (‘gray eye’), or sensitivity to light. It can result in blindness. The visceral version should be considered when a bird is just generally ‘wasting’.

Mareks is extremely contagious but has not been shown to spread vertically (from hen to the egg). It can, however, survive on fomites on the surface of the egg ( faecal matter on the egg itself ). This could be added to one of the reasons to only select clean eggs to incubate.Youngsters should develop a natural immunity (called ‘age resistance’), by the time they’re five months old. This is one of the reasons it is important to raise your youngsters separately from your oldsters. The older birds that have encountered Mareks and have managed to survive are carriers. New birds coming in from other flocks are always potential carriers.

Mareks usually hits between 5 and 25 weeks of age, but can appear even later if the bird had ‘latent’ MD and is substantially stressed. However, if the bird is a few years old, I would suspect a similar disease called Lymphoid Leukosis (which does pass through the egg). Both diseases will produce internal lesions (or tumours), detectable upon post mortem examination, but LL does not produce paralysis.

Since it is so difficult to control your birds’ exposure to Mareks, (showing, bringing in new birds, airborne spread from other farms), the best course is prevention.


There are certain ‘B factors’ contained in the blood of some chickens that make them resistant to Mareks.

Overall, the easiest way to keep Mareks out, is to promote ‘age resistance’ by keeping your youngsters separate from the adults and away from the poultry shows until they’re over 5 months old.

For larger hatcheries, a suitable vaccination program is essential coupled with strict bio-security control measures around brooding and rearing times.

Vaccination is useless if strict brooder hygiene is not in place as it is critical the chicks are not exposed to the virus within the first 2-3 weeks of life Hence, if chicks are raised under hens – reducing or inhibiting exposure is virtually impossible. The chicks are hatched in a virus present area, exposed to adult chickens from day 0. The home hatcher has the best chance by hatching clean eggs into a clean and sterilized brooder area for 3-4 weeks. Once old enough, heading outside to clean pasture area with no adults in contact. Protection is even better if vaccinated from hatching day.

The Vaccination is only available in NZ in a 1000 dose vial. Once reconstituted ( freeze dried vaccine ), it must be administered within 1.5-2 hours before rapidly deteriorating and becoming inactive.

Each Chick is individually vaccinated in the subcutaneous tissues at the back of the neck. Cleanliness is definitely next to godliness in this case and all areas must be sterilized with disinfectants that kill 99.9% of viruses otherwise the vaccine will not be effective in protecting your chickens.

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