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threshold. The time interval between the ingestion of an infectious blood meal andthe salivary secretion of the virus (extrinsic incubation period) must not exceed thelife span of the arthropod (Monath and Heinz, 1996). In that report increasedtemperature shortens the extrinsic incubation period, and may significantly increasethe rate of virus transmission in nature. Other extrinsic factors affecting Flavivirustransmission by arthropod vectors include:I. Mosquito larval crowding and nutritional deprivation which couldincrease transmission by adult vectors2. Rearing temperature (Kay et al., 1989)3. Co - infection with other infectious agents such as microfilaria, .whichmay include lesions in the midgut epithelial barrier and therebyfacilitate viral dissemination to the hemocoele (Turell, 1988).Transfer of virus in mosquito saliva occurs during the process of probinghost tissues with the piercing mouthparts in an attempt to canulate a small vessel(Monath and Heinz, 1996). The probing process often results in micro hematomaformation, which facilitates location and canulation of the vessel (Edman andspeilman, 1988). Salivary enzymes play important role in feeding, particularlyapyrase, which prevents ADP-dependent platelet aggregation and coagulation(Mamotti et al., 1990). Salivary virus is deposited principally in the extravasculartissues of the host during probing and the mosquito rapidly reingests saliva that isinjected intravascularly during blood feeding (Turell and Spielman, 1992; Turell andTammarielo, 1993). This results in virus replication at the site of inoculation andrelatively slows the spread ofinitial infection through lymphatic channels to regionallymph nodes, rather than a rapid viremic dissemination of the inoculums to theblood stream. The delay engendered in this initial phase of virus infection may beimportant in the initiation of the immune response and eventually the abrogation ofinfection. Conversely, if the virus does enter the vascular space during mosquitofeeding and is disseminated early in infection, the incubation period may beabbreviated and infection accelerated (Monath and Heinz, 1996). In that report, thedifferences in these early events in infection could partially explain variations in thecourse and outcome of infections in individual hosts.The vector must find and successfully feed on a vertebrate host for virustransmission to occur (Monath and Heinz, 1996). Factors that influence the search19