The susceptibility of mosquito larvae and pupae to infection via excreta, emphasizes the potential for increased virus persistence within mosquito populations.
Understanding the transmission routes of arboviruses is important for controlling their epidemiology and reducing their global health burden. A recent study focused on the West Nile virus (WNV) and Culex mosquitoes, investigating the potential for arboviruses to be transmitted through mosquito excreta.
The findings revealed a concentration of infectious virions in mosquito excreta. Virion excretion began early after oral infection and remained consistent over time, irrespective of the infection level in the mosquitoes. Larve and pupae were found to be susceptible to infection, with pupae exhibiting higher permissiveness. The study established proof-of-concept that immature mosquitoes could become infected via infectious excreta.
In an exclusive online interview with Contagion, study researcher Julien Pompon emphasized the implications of this newly identified transmission route. “We have shown that there exists an excreta-mediated route of transmission for WNV and potentially other arboviruses. Epidemiologically, this means that WNV can be transmitted without circling through bird hosts and potentially persists, although at low levels, in mosquito populations when there are not enough birds. Because the excreta transmission occurs in water puddles, our findings emphasize the importance of managing water sources,” he explained.
Pompon highlighted two potential impacts of this study in the context of the recent surge in WNV cases and deaths. “The maintenance of WNV in mosquito reservoirs can favor emergence. Additionally, excreta-mediated transmission can indiscriminately infect aquatic stages of other species. Therefore, in theory, a Culex excreta can infect an Aedes pupae and thus facilitate bridging WNV from a mosquito that bites birds to a mosquito that mostly bites mammals and humans,” he said.
The researchers began by determining the presence of infectious virions in mosquito excreta and quantified the concentration of these infectious units. They then monitored the timing of virion excretion following oral infection and assessed the susceptibility of mosquito larvae and pupae.
Pompon also discussed specific factors that influence the susceptibility of mosquito larvae and pupae to infection through excreta. He noted, “We found that pupae are more susceptible to infection than larvae. This is surprising as pupae are not supposed to feed and therefore should not ingest viruses. However, the same pattern of differential susceptibility between larvae and pupae was found for ZIKV in another study.”
“It would now be of interest to understand what causes the higher susceptibility of pupae. We put forward several hypotheses in our papers, such as the lack of a protective peritrophic membrane in pupae, and the production of a new cuticle during the pupal stage that protects from exterior aggression. Nonetheless, we still need to determine the exact mechanisms,” he added.
Pompon reiterated the importance of water management in preventing the presence of aquatic-stage mosquitoes. “This reduction in aquatic-stage mosquitoes can be achieved by reducing water sources and using chemicals to kill larvae and pupae.”
By mathematically modeling excreta-mediated transmission in the field, the researchers demonstrated its significant potential impact on arbovirus epidemiology. This study uncovers a new transmission route for arboviruses, revealing mechanisms for viral maintenance in mosquito reservoirs and potential shifts in vector species that may contribute to zoonotic emergence.
Both articles emphasize the need for effective management strategies to control WNV, advocating for the integration of climate change adaptation into public health strategies, which resonates with the focus on water management to prevent mosquito breeding. Additionally, the the emphasis on forecasting disease dynamics through ecological models aligns with the call for further research into transmission mechanisms, underscoring the importance of understanding environmental shifts to prevent future outbreaks.
