Mareks disease is one of the most common problems we see in a flock of small birds. It causes great distress as it is often seen in pullets between 12 and 20 weeks of age and it 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.
Mareks is caused by a Herpes virus which can survive for long periods of time in the environment. It is thought that in feather dust it can survive on a farm 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 1 day old and initially circulates in the blood stream causing no clear symptoms. 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 recognize 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 Mareks virus you will have a bird that, as long as it survives, will have a compromised immune system. This will make the chicken more susceptible to other diseases such as Respiratory disease and Coccidiosis.
At about the same time of the initial infection and when the B cells are destroyed, the viruis will also infect the T cells. This type of cell is involved in controlling the bird’s response to infection and in a way moderates the effects on the immune system. It is also involved in the control of viral infections. The T cell is not killed by the Mareks 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 from the virus and lose their efficacy, resulting innerve failure and paralysis in the bird. This is commonly the sciatic nerve supplying the legs and leads to a bird with a classic one legged paralysis.
Feather follicles, another cell type, in the skin can also become infected. This will cause small tumours in the feather follicles and is a major problem when meat birds become infected. Unfortunately this feather follicle infection leads to the virus being present in feather dust which infects more birds.
Mareks virus has been with us for a very long time and continues to evolve, meaning it is now becoming more resistant to the effects of various vaccines and, due to this evolution, more virulent. It is now common to see older infected birds which have been vaccinated previously; also the classic paralysed bird is a lot less common. At SoFresh Chickens, we frequently see thin, weak, young adult birds with severe weight loss; it can be very traumatic for the owner to be told that their new pet chicken is already dying and has an extremely poor prognosis.
Treatment and Diagnosis
If a bird has Mareks there is no treatment and infected birds will eventually die. Try not to buy chickens from unknown sources, and if possible buy vaccinated birds. However, very stressed chicks subjected to a high field challenge can still get Mareks disease. Use recommended disinfectants and follow strict biosecurity rules to try and prevent spread of the disease.
- use footdips on your site.
- vehicles and equipment can always be a cause to spreading disease, disinfect regularly.
- chickens can only be diagnosed through post mortem and samples sent off for histopathological examination.
- chickens that are suffering with this problem will often succumb to other ongoing problems and infections such as respiratory disease and diarrhoea because their immune system has been compromised.