Several papers have appeared in the literature in recent years that use the levels of viremia in different vertebrate host species to determine which hosts are important in arboviral transmission. For example, birds with low viremias, below some threshold needed to infect mosquitoes, have been considered as not being very important in West Nile transmission compared to American crows which have high viremias. This is not necessarily so and the relationship between host viremia and vector susceptibility may be quite complex. In a paper, that will appear in the Journal of Medical Entomology, we provide a model that explores the relationships between host viremia and vector susceptibility for arboviruses.

A categorical threshold model, where a minimum level of host viremia is necessary to infect a vector, impacts the determination of the relative importance of different host species in the transmission and spread of these pathogens. This threshold model shows that the intensity and duration of the viremia above the threshold level is critical in determining the potential numbers of infected mosquitoes. Other models may provide more accurate descriptions of the relationship between host viremia and vector infection. In particular, in comparison to the simple threshold model for vector infection, a probabilistic approach to vector infection may be more appropriate and realistic. A probabilistic model relating host viremia to the probability distribution of virions in the mosquito blood meal shows that the threshold model will underestimate the significance of hosts with low viremias. A probabilistic model that includes avian mortality shows that the maximum number of mosquitoes are infected by feeding on hosts whose viremia peaks just below the lethal level. The relationship between host viremia and vector infection is complex, and there is little experimental information to determine the most accurate model for different arthropod vector-host systems. Until there is more information, the ability to distinguish the relative importance of different hosts in infecting vectors will remain problematic.

Cynthia C. Lord, Associate Professor
C. Roxanne Connelly, Assistant Professor
Walter J. Tabachnick, Professor
Florida Medical Entomology Laboratory,
University of Florida, Vero Beach

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