Nonsystemic transmission, where a pathogen is transmitted between infected and uninfected vectors without the vertebrate host becoming viremic, may provide an explanation for transmission in systems where the vertebrate hosts have been difficult to identify. This transmission pathway had been previously demonstrated for tick-borne viruses and bacteria, but the recent demonstration for Simulium and vesicular stomatitis virus is the first for a blood-feeding insect. The epidemiology of vesicular stomatitis viruses has been difficult to understand, and nonsystemic transmission may be important. We use mathematical formulations of the basic reproduction number, R0, to compare systemic and nonsystemic transmission. The absence of a latent period before host infectiousness in nonsystemic transmission may allow a more rapid increase in prevalence in the biting flies early in the development of a new outbreak. Aggregation of flies between hosts and at favored feeding sites on hosts will be important, but further data on nonsystemic transmission as a function of space and time are required to fully assess this pathway. The data needed to compare the two pathways and their relative roles in virus epidemiology are discussed.
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