ISOLATION AND CHARACTERIZATION OF MAREK’S DISEASE VIRUS (MDV) SEROTYPE-1
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Date
2008-01
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SRI VENKATESWARA VETERINARY UNIVERSITY TIRUPATI - 517 502. (A.P.) INDIA
Abstract
ABSTRACT :
Poultry farms in India are experiencing excessive losses due to MD, in spite of vaccination with the serotype 3 (HVT) and serotype 2 (SB1) vaccines. Present study was undertaken with a view to isolate and identify Marek’s disease serotype 1 from the samples collected during field outbreaks; study the pathogenicity of the field isolates in chickens and determine the immunosuppressive effects of MDV field isolate. Three MDV serotype-1 isolates were recovered from flocks in which mortality due to tumors was observed. One each of the viruses was isolated from HVT vaccinated layer breeders (12 weeks), broiler breeders (39 weeks) and unvaccinated commercial broilers (42 days). MDV isolates were identified as serotype 1 by PCR using MDV serotype 1 specific primers. Of these isolates, DM-01 was selected for further characterization of pathogenicity and immunosuppressive potential in HVT vaccinated and unvaccinated white Leghorn chickens. To determine the pathogenicity and immunosuppressive potential of MDV isolate, and in-vivo experiment was conducted using white Leghorn male chicks. One-day old chicks were divided into three groups. Group 1 and 2 received HVT vaccine subcutaneously at neck region on day 1. On day 6th group 2 and 3 were inoculated intraperitoneally with 500 pfu/chick of MDV- field isolate. Presence of virus was detected in blood, spleen and feather follicles from MDV inoculated chickens using MDV PCR. The over all mortality rate was 15.6% in MDV infected, unvaccinated group and 9.4% in HVT vaccinated MDV challenged group. The frequency of MD lesions observed was 56% in MDV challenged unvaccinated group and 33.3% in HVT vaccinated challenged group. The protection index of HVT vaccine against challenge with field isolate of MDV was 40.5% and the virulence rank calculated was 59.5. The body weight was 13% less in unvaccinated challenge group and 10% less in vaccinated challenge group as compared to control group (vaccinated unchallenged). Relative bursal weight was significantly (≤0.05) lower in group 3 (MDV alone) compared to those of HVT + MDV and HVT groups. The effect of MDV on humoral immunity was assessed by measuring antibody levels to ND vaccination and SRBC inoculation. At 14th and 28th day post challenge there was significant reduction in HI antibody titers to ND vaccine and SRBC antigen respectively. Cell mediated immunity was assessed by measuring CBH response to PHA-P and Lymphocyte proliferation index to Con-A. At 42 days post challenge both the CBH response and lymphocyte proliferation index were significantly decreased in both HVT vaccinated MDV challenged group and unvaccinated MDV challenged groups as compared to HVT alone control group.
The results of the present study showed that the DM-01 isolate of MDV recovered during this study was able to cause mortality, MD lesions and immunosuppression in chickens. HVT vaccination partially prevented the effects of MDV serotype 1 challenge. Further, based on the results the MDV isolate can be categorized as vvMDV and there is a need to improve the present vaccines by incorporating attenuated serotype 1 strains of MDV to induce better protection and reduce economic losses to poultry farmers. 16
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