Mereks disease is caused by a herpes virus and is a common problem in rare breed poultry flocks.
An outbreak is often caused by stress and any unvaccinated birds that are free ranged are at high risk as wild birds can carry the disease.
It is a savage disease that will cause birds to die between the ages of six weeks and twenty weeks but the signs of the disease vary vastly. The classic signs of Mereks disease are birds showing paralysis on one side with the bird unable to stand with a leg stretched out and the bird using its wing to balance. There is no cure for Mereks disease and mortality is high.
Personally having suffered the devastating ravages of this disease several years ago I have since vaccinated against it. I have found it a cheap, easy and reliable way to halt the disease in my flock and since then have not had to wake up to my valuable growers dying in succession and the worry of receiving calls from customers wondering why their new birds are dropping like flies. Some breeders debate that you should breed for resistance but my view is that the Orpington has been bred now for well over 100 years and I have not heard of a Mereks resistant flock. Even the commercial poultry industry with billions of pounds at stake has decided that the resistance theory is unviable and vaccinate. I am not saying that resistance to this disease is not a reality in some birds and that this resistance will not be passed onto their progeny but as far as I know no rare breed flock or even any commercial flock in the UK has been proved to be so.