It is well known that the domestic house fly, Musca domestica spp., and some other fly species can transmit many kinds of pathogens mechanically [21] [22] [23] [24] [25] . In particular, M. domestica spp. are the most important fly species at poultry farms [26] with regard to mechanical transmission of >30 different pathogens [13] , for example, bacteria, protozoa, viruses, and parasite oocysts and eggs. Some viruses can be transported to animals by contact with contaminated body surfaces of flies. In the case of the house fly, it has been shown that rotavirus can be mechanically transported by contaminated fly surfaces [21] . House flies frequently defecate while feeding and resting on food surfaces [27] . However, in studies of C. nigribarbis, neither defecation nor vomiting was observed within 24 h after feeding (data not shown). The body surface of the house fly could be contaminated by viruses easier than that of blow flies. This would suggest that the mechanisms of virus transmission by blow flies could be different from those of house flies. Therefore, to evaluate virus transmission mechanisms that are more complex than contact with a contaminated fly surface, blow fly intestinal organs, crop, and gut were analyzed for their possible role in transmission of avian influenza virus. A seasonal consideration is that M. domestica vicina populations are generally highest in the summer in Japan. In fact, no house fly was found around any poultry farm or pigpen in Tamba Town during our survey in March. Therefore, it seems reasonable that winter blow flies may be involved in transmission of winter pathogens, like influenza virus, by maintaining minimum infectious titers.