Challenging successive mosquito generations with a densonucleosis virus yields progressive survival improvement but persistent, innocuous infections.

Research on cultivated shrimp suggests that they have the capability to tolerate viral pathogens in a highly specific manner by mechanisms currently unknown. The phenomenon is difficult to study in detail because they have a generation time of 1-2yr and lack continuous cell lines. Thus, we developed a mosquito-densovirus model to examine whether similar phenomena occur in insects. Serial challenge of five generations with a stock densovirus (AThDNV) resulted in progressive survival increases from 15% to 58%. Prevalence of AThDNV infection in surviving mosquito larvae (confirmed by PCR, histology, in situ hybridization and transmission electron microscopy) was relatively high (e.g. 36% in F2) but they grew normally to establish each succeeding generation. At the end of five generations, comparison of deduced amino acid sequences from genome fragments revealed a significantly higher (p=0.02) estimated prevalence of defective targets in the survivor virus population (29.7%+/-10.0 SD) than in the stored viral population (3.3%+/-5.8 SD). The results paralleled those reported for serially passaged C6/36 mosquito cell cultures infected with a densovirus. There, reduced infection rates are ascribed to the production of defective interfering particles (DIP). Thus, it is possible that the presence of prior AThDNV infections with a high level of DIP contributed to improved survival in our challenged F4 mosquito population. If so, it suggests that persistent viral infections in arthropods may serve in a specific, adaptive manner to reduce the incidence and severity of disease.