The tiny body of the Aedes aegypti mosquito hosts a peculiar microbiome that can enhance its ability to carry certain viruses and transmit them to humans and other animals. According to a study published in the scientific journal Nature Microbiology in January, the chance that the dengue virus is present in Aedes aegypti is three times higher in mosquitoes simultaneously infected with two insect-specific viruses: Phasi Charoen-like virus (PCLV) and Humaita Tubiacanga virus (HTV).
Dengue viral load in mosquitoes infected with both of the insect viruses is, on average, five times greater than in A. aegypti without PCLV and HTV, neither of which are transmitted to vertebrate animals. This increased load accelerates the replication cycle of the dengue virus in mosquitoes, which thus transmit the disease more quickly and increase the risk of outbreaks in humans.
“Viruses formed of an RNA molecule, such as PCLV and HTV, normally compete with each other for resources in the insects they infect, but we observed the opposite effect in A. aegypti,” explains biologist João Trindade Marques of the Federal University of Minas Gerais (UFMG), head of the team that carried out the study. The results indicate that in this mosquito species, there appears to be a positive association between these two insect-specific viruses and the viruses that cause dengue and Zika.
The research began with an analysis of the types of RNA viruses found in 815 adult mosquitoes collected in six countries on four continents: Brazil, Suriname, France, Gabon, Senegal, and Singapore. The samples came from A. aegypti and Aedes albopictus, two species whose females transmit arboviruses, such as dengue, Zika, and chikungunya. The former is more prevalent in urban settings and the latter is more common in rural and transitional areas. In total, 12 viruses were identified in the genetic material of A. aegypti, seven of which were already known and five of which were completely new.
The two viruses found most frequently in the analysis—PCLV and HTV—are unique to A. aegypti and were present in more than half of the samples. Humaita-Tubiacanga virus was discovered in Brazil in 2015. The highest viral loads of both viruses came from mosquitoes collected in areas of South America (Brazil and Suriname) and Asia (Singapore), where there are high incidences of dengue and Zika. In the Americas, Brazil had more cases of dengue and Zika than any other country in 2022, at 2.3 million and 34,100 respectively, according to data from the Pan American Health Organization (PAHO). The researchers formulated a hypothesis based on this apparent coincidence: that the presence of PCLV and HTV may favor the transmission of arboviruses by the mosquitoes.
To test their theory, the team performed a new analysis of RNA samples from 515 specimens of A. aegypti and 24 of A. albopictus, collected in the Brazilian town of Caratinga, Minas Gerais, between 2010 and 2011. The results supported their hypothesis. Only the A. aegypti samples were found to be infected with PCLV and HTV. The former was present in 61% of the samples and the latter in 85%. “Mosquitoes with both viruses were around three times more likely to be infected with dengue than those that did not carry both PCLV and HTV,” says bioinformatician Roenick Olmo, lead author of the article, who defended his PhD at UFMG under the supervision of Marques and is currently on a postdoctoral fellowship at the University of Strasbourg in France. “During a dengue outbreak, around 2–3% of mosquitoes are usually infected with the virus. But in the samples from Caratinga, 5% of A. aegypti had dengue, which is more than normal.”
Mosquitoes from areas of South America and Asia with a high incidence of dengue and Zika tended to be infected with both PCLV and HTV
In the next stage of the study, the UFMG team attempted to produce laboratory evidence that dual infection with PCLV and HTV influences replication of the dengue and Zika viruses inside the mosquito. Different lineages of A. aegypti females were bred in the lab from a population of mosquitoes found in nature—one infected with both PCLV and HTV, the other with neither of the viruses—and then allowed to feed on the blood of mice infected with dengue and Zika. Compared to the group that was not infected with PCLV and HTV, the dengue and Zika viral loads in mosquitoes with both infections were about five times higher.
The research also showed that dual PCLV and HTV infection in the mosquito shortens the extrinsic incubation period of the Zika virus—the time required for the insect to become capable of infecting humans or animals with the pathogen—by between one and five days. According to mathematical models, a two-day reduction of this period leads to five times as many infections in the human population, making it a risk factor for outbreaks.
José Henrique Oliveira, a biochemist from the Federal University of Santa Catarina (UFSC) who did not participate in the study, says the work by his UFMG colleagues highlights the importance of determining the prevalence of dual infection by PCLV and HTV in A. aegypti populations in Brazil. “This way, we could forecast regions most likely to suffer arbovirus outbreaks and intensify surveillance to reduce the number of vector mosquitoes,” suggests Oliveira.
The next step for the UFMG team is to try to identify the mechanisms that lead to dual infection by PCLV and HTV in the mosquitoes, especially the genes involved in this process. It could then be possible to discover why the presence of both viruses makes A. aegypti more competent at transmitting dengue and Zika to humans. One initial finding in this regard has already been achieved. Marques’s team found that expression (activation) of histone H4, one of the proteins responsible for wrapping the A. aegypti DNA inside the nucleus of its cells, is altered in mosquitoes with dengue that present dual infection by PCLV and HTV. The group’s data suggest that when histone H4 is less expressed, the dengue virus appears to replicate more slowly in the mosquitoes. In future studies, histone H4 could be a target for testing as a means of modulating the mosquito’s ability to transmit dengue and other diseases.
OLMO, R. P. et al. Mosquito vector competence for dengue is modulated by insect-specific viruses. Nature Microbiology. Jan. 5, 2023.