History
Marek's Disease (MD) was first described by Marek in Hungary (1970). Since then, it has been described by numerous authors in many countries. In the fifties and sixties of the 20th century, MD has been responsible for enormous losses in the poultry industry. Mortality up to 80% could be seen in some flocks due to the disease.
At the end of the sixties, vaccines for MD were developed. The THV and CVI 988, solved the problem almost instantly, although some incidental breaks were known to occur. Since then however, the incidence of Marek's has been occasionally increasing, causing sometimes high mortality in properly vaccinated flocks. Very virulent pathotypes and multiple age production systems are blamed for this.
Etiology
The disease is caused by a Herpesvirus. Several types of this Herpesvirus are known and they can be divided in to three different groups.
Group 1: This group consists of Herpesviruses, isolated from chickens that are pathogenic, which means that viruses in this group can cause clinical symptoms of MD.
Group 2: Viruses in this group are isolated from chickens, but they are not pathogenic.
Group 3: Herpes virus isolated from turkeys. Not pathogenic for chicken.
The group 1 Herpes viruses are isolated world-wide. Almost all flocks world-wide carry this type of virus and will shed the virus even if the flock is properly vaccinated and shows no clinical signs of Marek's disease.
Clinical symptoms and pathology
Classical type of MD:
This form of MD is nowadays almost only found in white egg layers. It occurs at an age of 2 - 4 months and is characterised by asymmetrical paralysis of the birds. They are not able to walk, and they die from starvation. At postmortem, thickened peripheral nerves can be observed, that lost their normal transversal stripes. Mainly the condition of the nervus ischiadicus is used for diagnosis, although other nerves are also involved.
Transient paralysis:
Here we see a sudden paralysis, especially of legs and neck. Normally, only a low number of birds in a flock are affected. A quick recovery, within a few days, is the normal pattern. Very low mortality.
Ocular form of MD:
Very rare. Occurs mainly in adult birds. The iris - normally having a bright orange colour -, is greyish, and the pupils form an irregular shape. The birds become blind but will not die, provided they are able to find feed and water.
Skin lesions
Can be seen in broilers and breeders. The feather follicles are swollen and haemorrhages in skin and follicles can be observed. This condition is mainly a severe problem for the processing plant.
Visceral form of MD
Although this form can be noticed already at an early age, nowadays it starts usually at the end of rearing and continues until 40-45 weeks of age. In several countries, it is the most common form of MD at the moment, causing high mortality - up to 30% - even in vaccinated flocks.
Lymphoid tumours can be found in liver, spleen, ovaries, kidneys, heart, proventriculus, mesenterium, muscles and skin. The birds are pale and look distressed. They lose body weight and die after several weeks. Production in affected flock is usually about 5% below standard, as many affected birds continue to produce eggs for some time. Differential diagnosis with other tumour-like diseases, especially Avian Leukosis, can be difficult and the final diagnosis can only be made by further laboratory investigation.
Pathogenesis:
Birds most of the time get infected by inhalation of the virus. The virus then is transmitted to the lymphoid organs, mostly B cells are affected. After the initial infection further spread to several lymphoid organs takes place. From here feather follicle cells are infected which take care of further spread into the environment. The development of lymphomas is the last step.
The entire incubation period takes several weeks.