Due to a question from one of my readers, I am writing this article to address the question. The reader sought to find out what was ailing her chickens which were exhibiting symptoms of being paralyzed. She sent the the picture. My research identified the disease as the Marek’s disease and so I would like to provide the following information:
Causes Marek’s disease
This disease is caused by a herpes virus which both gravitates towards lymphoid tissue (lymphoid tissue is spread throughout a chicken, unlike a mammal where it is confined mostly to lymph nodes) and also causes demyelination of peripheral nerves (leg and wing). There are two strains of the virus; one which lives in the cell and one which is cell-free and lives in the feather follicle. The virus was discovered in 1907 by Jozsef Marek, a Hungarian vet and thus the name
Life cycle of the herpes virus
The virus replicates in the host chicken’s lymphoid tissue and is shed in feather dander (dust). The virus can remain viable for at least one year in feather dander and henhouse dust.
Chickens from 6 weeks of age are affected, symptoms are most frequently seen 12-24 weeks of age with the hormonal stress of point of lay being a classic time for the signs to appear. Older birds can sometimes be affected if stressors (changes in weather, food, handling, environment) are not minimised. Females are more susceptible than males.
Mortality is variable and depends on which of the peripheral nerves is affected but leads to progressive spastic paralysis of the legs and wings. Sometimes, if the neck nerves are affected, the neck can twist around. There is an acute form where birds may die suddenly with no symptoms and tumours may be found in the liver, gonads, spleen, kidneys, lungs, proventriculus, heart, muscle and skin.
The virus is ubiquitous in poultry worldwide. Infection in chicks occurs by inhalation and two weeks later, the virus is shed by the infected chick in feather dander and by oral and nasal secretions. The virus is NOT passed on through the egg. Infected and recovered birds continue to shed the virus for life.
Good biosecurity is important, quarantine any new stock for 4 weeks. Rear chicks for 2-3 months away from adult feather dander if adult birds have shown symptoms. Ask vendors if stock has been vaccinated. Genetically resistant breeds include the Fayomi.
Treatment and Control
There is no treatment yet, they appear to return to normal, but frequently die from internal tumors a short time later. The following measures can help; Cull any affected birds. This increases the resistance to the disease in the surviving birds. Vaccination is feasible, especially if Silkies or Sebrights are kept. These are very susceptible to clinical signs of Marek’s and there would be few of these breeds seen at exhibitions if vaccination was not used. The vaccine is administered twice, once in the water (live vaccine) and once by injection when chicks are slightly older. In other breeds, using vaccine can hide the virus and so the whole stock gets progressively more susceptible (weaker) without any symptoms and if birds are sold without the recipient being told of the vaccination, the birds can pass on the virus to unvaccinated chicks, thereby bringing the disease to a flock which may have been free of it before.
Diagnosis: Clinical signs of paralysis and postmortem lesions.