Probably one of the most common problems we see is Marek’s disease in small flocks of birds. It causes great distress as it is often seen in pullets between 12 and 20 weeks of age and will be fatal. The control of the disease is difficult and you need to have a good understanding of the basic principles in order to reduce the chance of your birds becoming infected.
Marek’s is caused by a Herpes virus which can survive for long times in the environment. Experimentally it is thought that in feather dust it can survive for at least 12 months and it is this problem which leads to it being so common.
The virus infects the young bird as early as day old and initially circulates in the blood stream causing no clear symptoms in the bird. Within 7 days the virus will have infected the white blood cells and initially causes the death of the B Lymphocytes. These white blood cells are the type that produce antibodies and are essential for a fully working immune system. Antibodies are proteins that are specific for individual pathogens such as bacteria and viruses. The antibody attaches to the pathogen and it is this attachment which stimulates other immune cells in the body to recognise the pathogen as foreign material and to start a complicated process of destroying and removing it from the body.
So, if the B Lymphocytes are destroyed by the Marek’s virus you will have a bird that as long as it survives, will have a compromised immune system. This will show itself as more susceptibility to other diseases such as Respiratory disease and Coccidiosis.
At about the same time after infection that the B cells are destroyed, the virus also infects the T cells. This type of cell is involved in controlling the bird’s response to infection and in a way moderates the effects of the immune system. It is also involved in the control of viral infections.
Tumours in the spleen and liver.
The T cell is not killed by the Marek’s virus, but is modified in a way which will eventually kill the bird. The infected T cells stay dormant and when the bird is around 10 weeks post infection, start to multiply. These cells then form tumours in various organs including Liver, Kidney and Spleen. The bird slowly loses weight but outwardly appears normal, until at some stage it will become incapacitated and is normally presented to the vet as a sudden problem, which obviously is not the case.
Another more classic form of the disease is the infection of the cells making up the nerve sheath which is similar to the outside coating of an electrical wire. These cells multiply due to infection with the virus and lose their efficacy, with the result that the nerve fails and the bird shows paralysis depending on the nerve infected. This is most often the sciatic nerve supplying the legs and leads to a bird with a classic one-legged paralysis.
A paralysed Silkie with Marek’s disease.
Yet another cell type infected is the feather follicles in the skin, often in the leg. This will cause small swellings of tumours in the feather follicles and is a major problem when meat birds become infected. The problem for all birds is that it is this feather follicle infection which leads to the virus being present in feather dust which infects more birds.
Marek’s virus has been with us for a very long time and continues to evolve becoming resistant to the effects of various vaccines and more virulent. It is becoming more common to see infection in older birds which were vaccinated and the classical paralysed bird is a lot less common.
In our surgery the most common presentation is the thin weak young adult bird with severe weight loss, very traumatic for the young owner to be told that their new pet bird is already dying and has an extremely poor prognosis!