The susceptibility of Culicoides variipennis Coq. (Diptera: Ceratopogonidae) to laboratory infection with Rift Valley fever virus.

The vector potential of Culicoides variipennis for Rift Valley fever virus (RVFV) was investigated. Insects from a colony maintained at the Animal Virus Research Institute, Pirbright, were fed through a membrane on a mixture of mouse blood and RVFV (virus concentration of blood meal 7 X 95 log10 MLD50ml). Engorged insects were maintained at 25 degrees C +/- 1 degrees C. Samples of insects were tested at daily intervals to determine their virus content. Four of the five females tested immediately after feeding contained virus. The mean virus concentration of these infected flies was 2 X 7 log10MLD50. The virus concentration per fly decreased to 0 by day 2 post infection. On day 3, a virus concentration of 2 X 4 log10 MLD50 per fly was recorded from a pool of 17 flies but between day 4 and day 12 when the experiment terminated no virus was detected in any of the 135 females tested. Because of the pathogenic nature of RVFV, this work was carried out under the stringent security regulations at the PHLS Centre for Applied Microbiology and Research, Porton Down. The problems arising from experiments requiring the handling and infection of insects under such conditions are described.