The Effect of Laboratory Colonization on the Vector-pathogen Interactions of Egyptian

Field and laboratory findings implicated Culex pipiens as a vector of Rift Valley fever (RVF) virus during the 1977-1978 epizootics/epidemics in Egypt. This study evaluated changes in infection and transmission rates, and viral titers in F, through F,6 generation Cx. pipiens mosquitoes orally infected with RVF virus. Infection and transmission rates of RVF S virus by this species changed significantly during the colonization process. However, the ultimate viral titers of either the transmitting or the infected nontransmitting mosquitoes were not affected by the colonization process. Following ingestion of virus, Cx. pipiens could be separated into three distinct subpopulations, an uninfected group and two types of infected mosquitoes-transmitters and nontransmitters. Transmitters contained significantly more virus (approximately 100-fold) than nontransmitters. These results demonstrated that not every infected female mosquito should be considered a competent vector, even if the species (population) is known to be a primary vector. Transmission was also accomplished by probing * ,mosquitoes which were unsuccessful in obtaining a blood meal. These data document the long-held suspicion that vector competence studies based upon laboratory-colonized specimens may not represent the field situation. Until recently, Rift Valley fever (RVF) was conwas to examine variation, if any, in the vector N -. sidered to be primarily a disease of domestic anpotential of different geographic populations of imals in sub-Saharan Africa, while in humans it Egyptian Cx. pipiens for RVF virus. However, it was considered serious, but rarely fatal. The virus was discovered that this objective could be was first reported north of the Sahara Desert in achieved only by using field-collected and not lab1977 when it caused an epizootic/epidemic in the oratory-reared material. Because of the nonavailNile Delta of Egypt which resulted in considerable ability of a continuous supply of field-collected animal and human mortality." Field and labomosquitoes, this study focused on the effects of ratory findings implicated Culex pipiens L. as a laboratory colonization on the vector-pathogen invector of RVF virus• teractions of subsequent filial generations of fieldOne of the original objectives of this research collected Egyptian Cx. pipiens. Specifically, we examined the infection and transmission rates, and Accepted 14 February 1983. various parameters of viral replication in these hmosquitoes. *In conducting the research described in this report, the investigators adhered to the "Guide for the Care and MATERIALS AND METHODS Use of Laboratory Animals," as promulgated by the Committee on the Revision of the Guide for Laboratory Mosquitoes j Animal Facilities and Care of the Institute of Laboratory Animal Resources, National Research Council. The faThe Cx. pipiens used in this study originated cilities are fully accredited by the American Association from approximately 500 adults and larvae colfor Accreditation of Laboratory Animal Care. The views of the authors do not purport to reflect the lected on 10-11 March 1981 from several sites in positions of the Department of the Army or the Departthe village of Abu Heif, Sharqiya governorate, ment of Defense. Egypt. Adult mosquitoes were collected by aspiAddress reprint requests to: Thomas P. Gargan, 11, ration during night human and animal bait colDeportment of Arboviral Entomology, U.S. Army Medlections, while larvae were collected from small -" ical Research Institute of Infectious Diseases, Fort Detrick, Frederick, Maryland 21701. stagnant pools of water. Both stages were trans-