Single mosquito bite enough to spread Rift Valley fever virus

Rift Valley fever virus (RVFV) is a (re)emerging mosquito-borne pathogen impacting human and animal health. How RVFV spreads through a population depends on population-level and individual-level interactions between vector, host and pathogen. Researcher Bieneke Bron and a multidisciplinary team of co-workers from within Wageningen University & Research designed a research set-up to assess if and how RVFV infections will develop in exposed animals. In this particular experimental design, the focus was on Aedes aegypti mosquitoes as a vector to transmit this virus to lambs.

Exposure

The animals were exposed to different numbers of RVFV infected mosquitoes. Exposure to 1 to 3 mosquitos was regarded as ‘low exposure’, where 7 to 9 mosquitos qualified as ‘high exposure’. All the lambs in the latter group became infected with the virus and showed characteristic signs of the disease. In the ‘low exposure’ group three out of twelve animals developed viremia and disease. “These results suggest that the likelihood for successful infection of a ruminant host is affected by the number of infectious mosquitoes biting, but also highlights that a single bite of an infectious mosquito can result in disease”, explains Bron.

Transmission efficacy

The results of the experiment were used to estimate the transmission efficacy. The per bite mosquito-to-host transmission efficiency was estimated to be 28 percent. “This means the bite of one infectious mosquito gives a 28% chance of a lamb developing viremia”, explains Bron.

A Ross-McDonald-type mathematical model was used to combine the transmission efficiency with estimates for life traits of Aedes aegypti or related mosquitoes. The mathematical model was subsequently used to illustrate scenarios that are likely to create large RVFV outbreaks in a previously unaffected population. Based on the model two factors are of importance for rapid spread of the disease (R₀ exceed to 1). These are: a relatively high number of mosquitoes per animal and the mosquitoes need to preferably feed on animals that can transmit the virus. Both factors are necessary for the spread of the disease.

Model

“This study has underlined the importance of mimicking natural exposure to diseases to better understand its natural progression. Studies like this contribute to obtaining epidemiological parameters for mathematical models”, according to Ten Bosch. The research team welcomes the extension of this research and the model for similar experiments, for example using other mosquito species.