Arbovirus evolution in vivo is constrained by host alternation

Abstract

The intrinsic plasticity of RNA viruses can facilitate host range changes that lead to epidemics. However, evolutionary processes promoting cross-species transfers are poorly defined, especially for arthropod-borne viruses (arboviruses). In theory, cross species transfers by arboviruses may be constrained by their alternating infection of disparate hosts, where optimal replication in one host involves a fitness tradeoff for the other. Accordingly, freeing arboviruses from alternate replication via specialization in a single host should accelerate adaptation. This hypothesis has been tested by using cell culture model systems with inconclusive results. Therefore, we tested it using an in vivo system with Venezuelan equine encephalitis virus (VEEV), an emerging alphavirus of the Americas. VEEV serially passaged in mosquitoes exhibited increased mosquito infectivity and vertebrate-specialized strains produced higher viremias. Conversely, alternately passaged VEEV experienced no detectable fitness gains in either host. These results suggest that arbovirus adaptation and evolution is limited by obligate host alternation and predict that arboviral emergence via host range changes may be less frequent than that of single host animal RNA viruses.

Fig. 1.

Experimental design for VEEV in vivo adaptation studies. VEEV subtype IC or ID strains were serially passaged 10 times in vertebrates (Left) or A. aegypti mosquitoes (Right) to artificially bypass one host, or alternately passaged (Center) to simulate natural transmission. The fitness of VEEV derived from these passage series compared to parent VEEV was determined by direct replication comparison or competition assay.

Fig. 2.

Mean viremias in National Institutes of Health Swiss mice (VEEV ID strain 8131) (a) or Syrian golden hamsters (VEEV IC strain 3908) (b) inoculated with doses ranging from 3.2 to 3.5 log₁₀ PFU/ml (ID) or 2.9–3.1 log₁₀ PFU/ml (IC) of parent, alternating p10, mouse/hamster p10, mosquito p10, or BHK p10. Mice (n = 6) and hamsters (n = 3) were bled at each time point; mean viremias are presented. Error bars indicate standard deviations.