We report the pathotyping of six Australian isolates of Marek's disease virus-1 (MDV1) isolated between 1992 and 2004 and association of virulence with meq gene polymorphism. Unvaccinated and herpesvirus of turkeys (HVT)-vaccinated specific pathogen free chickens were challenged at day 5 with 500 plaque forming units of Marek's disease virus. The isolates induced gross Marek's disease lesions in 53 to 94% of unvaccinated chickens, and HVT induced a protective index ranging from 38 to 100% by 56 days post challenge. This experiment provides evidence that current Australian isolates of MDV1 vary significantly in pathogenicity. However, there was no clear evidence that the most virulent recent isolates were more pathogenic than isolates from the 1980s or that any of the isolates belong to the highest pathotype category of very virulent plus. Evidence is presented that virulence can be predicted by measurements taken as early as 13 days post challenge. The meq gene sequences of five of the isolates used in the experiment were determined. When compared with the very virulent US isolate Md5, there was a 177 base-pair insertion and distinct point mutations in each of the five isolates. There were no individual mutations in the meq sequences that correlated with levels of virulence. However, amino acid alignment of the five Australian and 14 international isolates revealed that the number of repeat sequences of four prolines (PPPP repeats) in the meq gene (overall range 2 to 8) was strongly associated with virulence across all isolates, with the most pathogenic isolates having the fewest number of repeats. The results suggest that the presence of the 177 base-pair insertion alone is not an indicator of attenuation. Rather, the number of PPPP repeats, independent of the presence of the insertion, is a better indicator of pathogenicity.

Various antimicrobial factors present in human milk were tested for in-vitro antiviral activity against three rhinoviruses (two clinical isolates and rhinovirus 2) and an isolate of cytomegalovirus (CMV) from human milk. These factors included the gangliosides GM1, 2 and 3, sialyl-lactose, chondroitin sulphates A, B and C, prostaglandins E2 and F2á, monolaurin, vitamin A and the protein lactoferrin. All were tested for their ability to inhibit growth of the viruses in cell culture. Human milk was also tested for antiviral activity against these viruses. Only vitamin A, monolaurin and lactoferrin inhibited the growth of CMV, whereas both prostaglandins enhanced the growth of this virus at least four-fold. CMV infects infants from milk but, nevertheless, the milk-borne CMV isolate showed no special resistance to any of the antiviral factors tested. None of the compounds inhibited or enhanced the growth of the rhinoviruses. However, human milk decreased the growth of some of the rhinoviruses and speci®c secretory immunoglobulin A (sIgA) neutralised the virus.

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A real-time quantitative polymerase chain reaction was utilized to study the in vivo replication of Marek's disease vaccine viruses and of virulent oncogenic strains. In the first of four experiments, the growth of the herpes virus of turkeys (HVT) vaccine was detectable in various organs of infected chicken embryos, with the highest viral loads being present in the spleen. No evidence was obtained for replication of serotype-1 Marek's disease viruses in embryos. In the second experiment, viral loads were measured in several organs of chickens after administration of the Rispens and HVT vaccines immediately after hatching. Lowest levels were noted for the Rispens strain after 1 to 8 weeks. By contrast, HVT vaccine grew well in all tested organs, with the highest loads being present in the spleen. Highest loads were observed in unvaccinated birds after challenge with the highly virulent strain MPF57 at day 8, especially in lymphoid organs. A positive relationship was observed between viral load and clinical signs, including tumour formation. In a third study, viral loads were measured in the organs of chickens administered the Rispens vaccine on the day of hatch and challenged at day 8 with MPF57. High levels of clinical protection were afforded against MPF57 by the Rispens vaccine but, in confirmation of earlier findings, sterilizing immunity was not induced. In a fourth study, two experiments were conducted--in which viral loads were measured after challenge of chickens vaccinated with HVT in ovo or at day 1 after hatching. Similar protection was achieved in birds vaccinated in ovo on embryonic days 11 and 17, although protection was slightly, but not significantly, lower than for birds vaccinated at day 1.