Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 29, 1996
Publication Date: N/A
Interpretive Summary: Marek's disease (MD) increasingly causes excessive economic loss in poultry flocks, in spite of vaccination. It is important to learn what factors may be responsible for this increase. One possibility is that the virus that causes MD is becoming stronger or more virulent. In this study, 31 viruses obtained from commercial chickens during the past 6 years were evaluated. Seven of these viruses caused more disease than earlier viruses and may be considered a new category of stronger viruses. These findings show that MD virus is continuing to evolve or change. This suggests that current vaccines may not remain fully effective and that alternative measures will be needed for long term control of the disease.
Technical Abstract: The continuation of an apparent evolutionary trend of Marek's disease virus (MDV) towards greater virulence may explain recent increased losses from Marek's disease (MD) in vaccinated flocks. To address this question, the virulence of 31 isolates of serotype 1 MDV obtained from layer or broiler flocks during 1989-1995 were characterized. Each isolate was cultured in duck embryo fibroblasts for 4-6 passages, and ascertained to be free from contamination with avian retroviruses, chicken anemia virus and MDVs of other serotypes. The viruses, along with prototype viruses JM/102W and Md5, were tested for virulence by inoculation at 6 days into laboratory strain 15I5 x 71 chickens of three types: nonvaccinated, HVT-vaccinated and bivalent (HVT+SB-1)-vaccinated. The results showed that three isolates did not differ from JM/102W and were classified in the vMDV pathotype. Twenty-one isolates produced significantly higher levels of MD in HVT- vaccinated chickens than the JM/102W control and were classified in the vvMDV pathotype. Seven isolates, five of which were isolated in 1994 or 1995, produced significantly higher levels of MD in bivalent-vaccinated chickens than the Md5 (vvMDV) control. These isolates, provisionally designated as the vv+MDV pathotype, appeared to be at the high end of a virulence continuum. Several MD response parameters, including lymphoma mortality, early mortality with bursal/thymic atrophy, and frequency of visceral lymphomas or ocular lesions in nonvaccinated chickens were positively correlated with virulence. These findings support the continued evolution of MDV towards greater virulence. The implication of this trend to strategies for long term control of MD is discussed.