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Marek's disease
Marek's disease is a ubiquitous endemic pathogen of poultry first identified by Josef Marek in 1907. The causative agent, Marek's Disease Virus (MDV), is an alpha-herpesvirus that is highly collinear with Herpes Simplex Virus I. Like most herpesviruses, the early stages of infection with MDV are characterised by a semi-productive cytolytic infection in immune organs followed by a period of latency. Serotype 1 MDVs are oncogenic, and infection of susceptible birds with serotype 1 MDVs eventually leads to reactivation of the virus and the dissemination of infected and transformed CD4+ T-cells to visceral organs and nervous tissue.
Marek's disease was the first
herpesvirus and also the first oncogenic disease against which a successful vaccination strategy was achieved. Vaccination
in commercial settings occurs in ovo or at one day post-hatch using live attenuated or non-pathogenic strains. These
vaccines are classic examples of "leaky" vaccines, preventing morbidity and mortality in vaccinated birds but not preventing
the replication or transmission of the virus.
Since the original description of MDV, it has been shown unequivocally that more virulent pathotypes of the virus have emerged, particularly where chickens are farmed intensively. Current theory is that the use of leaky vaccines has driven the evolution of these virulent pathotypes.
This system therefore provides a useful model to study vaccine-driven evolution of pathogens and to build on the work already carried out in malaria. The system has several advantages including the ability to work in the natural host animal and the fact that evolution of the pathogen is known to occur in the field.
Group member involved: Sue Baigent