Dynamics of La Crosse virus: Surveillance, Control and Effect on Vector Behavior

La Crosse virus (LACV) encephalitis is the most common and important endemic mosquito-borne disease of children in the U.S. with an estimated 300,000 annual infections. The disease is maintained in a zoonotic cycle involving the eastern treehole mosquito, Aedes triseriatus and small woodland mammals such as chipmunks and squirrels. The objectives of this study were 1) to conduct surveillance of LACV and other mosquito-borne viruses; 2) to evaluate the effect of virus infection on mosquito host-seeking and neurotransmitter levels, and 3) to determine the effectiveness of barrier sprays to control infected mosquito vectors.

Our surveillance study demonstrated the involvement of an invasive species, Aedes japonicus, in the transmission cycle of Cache Valley virus (CVV). CVV is a mosquito-borne virus that is closely related to LACV. Thus, surveillance is a critical step in public health, providing pathogen distribution and frequency data as well as identifying and incriminating new vectors.

LACV infection did not affect the host-seeking behavior of Ae. triseriatus females. Using high performance liquid chromatography with electrochemical detection (HPLC-ED), the levels of serotonin and dopamine were measured in infected and uninfected mosquitoes. Serotonin is known to affect blood-feeding and dopamine affects host-seeking. Serotonin levels were significantly lower in LACV-infected mosquitoes but dopamine levels were unaffected by virus. A previous study found that LACV infection caused an alteration in mosquito blood-feeding in a way that could enhance virus transmission. This work showed that LACV infection can reduce the level of serotonin in the mosquito, promoting virus transmission through altered blood-feeding without impairing the vector's ability to locate a host.

Standard CDC bottle assays were used to evaluate the efficacy of two pyrethroids and two essential oil sprays on LACV infected and uninfected mosquitoes. LACV-infected Ae. triseriatus females were more susceptible to both pyrethroids than uninfected ones. Infection status did not affect the susceptibility of Ae. albopictus to either pyrethroid. The essential oils were inconsistent in their effects. These results demonstrate that barrier sprays may be a viable part of a mosquito control program, not just to reduce the biting rate but to potentially reduce the virus-infected portion of the vector population.