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Transovarial transmission of St. Louis encephalitis virus by Culex pipiens complex mosquitoes
Abstract:
Experiments were conducted to determine whether transovarial transmission (TOT) of St. Louis encephalitis (SLE) virus occurs in Culex pipiens complex mosquitoes, the principal vectors of SLE virus in the central-eastern United States. In 1978, field-collected mosquitoes from Memphis, Tennessee, and McLeansboro, Illinois, were used; during 1979, colonized mosquitoes from Chicago, Illinois, and Memphis, Tennessee, were used. Mosquitoes were infected by feeding on viremic chicks inoculated with an SLE virus strain isolated from Cx. pipiens complex mosquitoes collected from Memphis, Tennessee, in 1976. During the 1979 experiments, progeny larval and adult mosquitoes were held at two temperatures, 18 and 25 degrees C. Progeny were tested for virus by plaque assay in duck embryo cell cultures and by inoculation of Aedes albopictus C6/36 cells and examination by immunofluorescence. In 1978, most of the progeny tested were from the first ovarian cycle, and a single occurrence of TOT was documented. In 1979, a single TOT occurred from 46,856 first ovarian cycle progeny, whereas 7 of 9,234 progeny of the second ovarian cycle were infected. The rate of TOT was higher for progeny of Memphis than Chicago mosquitoes, and for mosquitoes held at 18 degrees C than at 25 degrees C; however, these differences were not statistically significant. Four positive pools were females, and three were fed on chicks for transmission attempts. The positive Chicago mosquito pool failed to transmit, but both Memphis pools successfully transmitted virus. The overall rates of TOT of SLE virus in progeny of the first and second ovarian cycle were, respectively, 1/45, 151 and 1/1,460. The significance of these results as they relate to the natural history of SLE virus is discussed.
Insights
Transovarial transmission (TOT) of St. Louis encephalitis (SLE) virus was investigated in Culex pipiens mosquitoes. SLE virus TOT was documented in both first and second ovarian cycles, with higher rates observed in the second cycle.
Area of Science:
- Arthropod-borne viruses
- Medical entomology
- Viral ecology
Background:
- St. Louis encephalitis (SLE) virus is a significant public health concern in the central-eastern United States.
- Culex pipiens complex mosquitoes are the primary vectors responsible for SLE virus transmission.
- Understanding the role of transovarial transmission (TOT) is crucial for comprehending SLE virus's natural history and epidemiology.
Purpose of the Study:
- To determine the occurrence and efficiency of transovarial transmission (TOT) of St. Louis encephalitis (SLE) virus in Culex pipiens complex mosquitoes.
- To evaluate the influence of mosquito ovarian cycle and environmental temperature on SLE virus TOT.
- To assess the potential for SLE virus transmission by mosquitoes that acquired the virus vertically.
Main Methods:
- Experiments utilized field-collected and colonized Culex pipiens mosquitoes infected with an SLE virus strain.
- Mosquitoes were maintained at different temperatures (18°C and 25°C) across two ovarian cycles.
- Viral detection in progeny involved plaque assays and immunofluorescence on cell cultures.
Main Results:
- Transovarial transmission (TOT) of SLE virus was confirmed in both first and second ovarian cycles of Culex pipiens.
- The rate of TOT was significantly higher in the second ovarian cycle (1/1,460) compared to the first (1/45,151).
- While not statistically significant, higher TOT rates were observed in mosquitoes from Memphis and at 18°C.
Conclusions:
- Transovarial transmission (TOT) is a viable mechanism for SLE virus maintenance and dissemination in Culex pipiens populations.
- The second ovarian cycle plays a more significant role in vertical transmission of SLE virus.
- These findings contribute to understanding the enzootic cycle and potential for SLE virus epidemics.