Marek’s disease is a significant cause of illness and death in chickens the world over and even if you’ve never seen the disease in your flock, it’s best to assume that the infection may well exist, sub-clinically, in carrier birds that your own. The disease can also be seen in turkeys and quail which can occasionally be infected too.
Marek’s disease was first described in 1907, by Jozsef Marek, a Hungarian veterinarian and although he could not identify the cause, he suspected that it was an infectious agent and more probably a virus. He was proved correct 60 years later when the virus was isolated using modern technology that vets had at their fingertips.
Marek’s disease is caused by a herpes virus, named Marek’s disease virus (MDV). The virus causes lymphomas, a type of tumour, which can spread throughout the body into many organs namely the kidneys, liver and spleen. The skin and eyes may also be affected and in the classic disease there is swelling of the brachial plexus and sciatic nerves which are the main nerves found in the wings and legs.
There are several strains of MDV some of which are harmless. The strains that cause disease can vary from those classed as mild through to virulent right through to very virulent strains.
The classic signs of Marek’s disease (which was originally called fowl paralysis) are leg weakness and paralysis and drooping of the wings which is caused by tumours infiltrating the main nerves supplying a chickens limbs.
Birds with tumours in internal organs may show very vague signs, such as loss of weight despite a good appetite and occasionally diarrhoea is seen. Chickens may become dehydrated and anaemic which means their wattles become pale almost white. It’s likely that there’ll be multiple cases in the affected age group and during an outbreak there’s nothing you can do to prevent new cases developing which can be very demoralising and upsetting.
If a flock is infected with a very virulent strain, large numbers of birds might die in a short period of time showing few, if any, signs of the illness first. Typical pale cream tumours will be seen in internal organs if an autopsy is carried out on any dead birds. An infection can damage the birds’ immune systems making them more likely to succumb to other infections, such as intestinal parasites, or respiratory infections.
You might suspect Marek’s disease if young birds are showing the classic signs of leg weakness and drooping wings. However, you can only confirm the disease after a post mortem examination. Birds with the typical paralysis will have thickening of the sciatic nerves in the legs, and/or the brachial plexus under the wings. In the visceral form of disease there’ll be pale tumours in several organs and numerous small tumours of the feather follicles are seen in the skin form of disease. Less commonly, in the eye form of disease, there are tumours inside the eyes. There are other causes of tumours in chickens, which can look very similar, so the only way to be sure of the diagnosis is by laboratory testing of tumour tissues. Your vet might perform a post mortem for you, or you may have to take your bird to a specialist centre.
The disease is extremely infectious. Chicks are infected very early in life, usually within the first few days, from virus found in dust in their environment. The virus multiplies in the bird’s lymphoid tissues and infected birds shed this virus in feather dust (dander). Birds which survive will shed the virus for life and present a risk to others in the flock. Because of this risk, it’s generally advised that you should cull any affected birds.
The virus is very resistant in the environment, and it can survive for at least a year in feather dander and dust in the hen house, so this is then a source of infection for the next generation of birds which means it is crucial that an environment be thoroughly disinfected using a potent yet poultry friendly product which a vet may be able to recommend you use.
After chicks are infected, the virus becomes dormant in the body and disease is not usually seen until the birds are older. It can occur at any age, but it’s most often diagnosed between 12 and 24 weeks of age, when the stress of approaching and entering lay can trigger the disease. Other stress factors which can trigger disease include moving or mixing groups of birds which may seen some chickens being bullied by other more dominant chickens.
Vaccination and good hygiene and husbandry practices are the mainstays of controlling Marek’s disease. Chicks need to be vaccinated before they come into contact with the virus in the environment, so in commercial hatcheries chicks are vaccinated at day old. If you hatch eggs at home, it’s essential to clean incubators and brood boxes thoroughly using disinfectants which are effective against herpes viruses.
It's also useful to know that rearing chicks away from adults will reduce their chances of becoming infected.
Vaccination doesn’t stop birds becoming infected with Marek’s virus later in life, but it will almost always prevent them developing the disease. There have, however, been cases where very virulent strains of virus have been able to cause disease in vaccinated birds.
Because MDV is evolving and more virulent strains are appearing, there is interest in breeding chickens which are genetically more resistant to the virus. It’s known that Silkies and Sebrights are very susceptible to MDV, whilst the Fayomi is genetically more resistant.
If you keep chickens, then it’s likely that at some time you’ll see Marek’s disease. If you’re aware of the different forms of the disease and you can confirm the diagnosis early in an outbreak, you’ll be better placed to reduce future losses in your flock. However, this is a devastating disease and one that can creep up on you which can be very distressing to your chickens because there is so little you can do to help your birds apart from making their lives more comfortable.