SURVIVAL OF TYPE 1 AND TYPE 3 POLIO VACCINE VIRUS IN BLOWFLIES. (PHAENICIA SERICATA) AT 40 DEGREE C.

Discussion and summary Interest in the behavior of polioviruses in flies stems from two factors: (1) the potential importance of these insects as vectors of the virus under certain circumstances; and (2) the possibility that if polioviruses multiply in flies, they may undergo mutation during the process. Investigations of the genetic stability of polioviruses have been carried out largely in connection with the use of attenuated vaccine strains as immunizing agents for humans. It is apparent that wild virulent strains multiply readily at 40°C, while attenuated strains, either naturally occurring or laboratory manipulated vaccine strains, do not. In tropical areas, and in temperate climates during the summer, flies are exposed to high environmental temperatures at least for some time during the day. If this occurs while they are hosts of polioviruses, and if polioviruses undergo multiplication in flies, it is possible that attenuated strains might undergo changes in their genetic characters, with a shift toward the pattern of greater virulence. The results of the present investigation, which was undertaken to explore this possibility, indicate that under the conditions tested there was no evidence of multiplication in type 1 or type 3 attenuated vaccine strains (Sabin) in the fly species Phaenicia sericata. Poliovirus was recovered in gradually diminishing quantities for as long as 15 days from flies which had been fed type 1 virus and for at least 13 days from those which had received type 3. When certain genetic characters of the virus strains isolated from 2 to 15 days after virus feeding were compared with those of the type 1 and 3 vaccine strains, they were found to be identical; all isolates tested proved to be rct/40°C negative and vaccine-like in the intratypic serodifferentiation test of McBride. Thus the results do not provide quantitative evidence for poliovirus multiplication in the fly Phaenicia sericata, nor was it possible to demonstrate any shift in genetic characters suggesting changes toward a pattern of more virulent strains of virus.