The mosquito Aedes aegypti has been considered the main vector of Zika virus, estimated to have infected 49,000 people by June 2016, although 139,000 cases were notified in this period. Zika is also widely considered as the cause of microcephaly in 1,600 babies born in 582 Brazilian cities.
However, Brazilian scientists believe Zika may have other vectors besides A. aegypti, such as Culex quinquefasciatus, the southern house mosquito. Indeed, there is growing evidence that peridomestic mosquitoes may be involved in spreading Zika.
One such warning has recently come from Constância Flávia Junqueira Ayres Lopes, a researcher at the Aggeu Magalhães Institute, a research center belonging to the Oswaldo Cruz Foundation (Fiocruz) in Recife, the capital of Pernambuco State. Lopes was speaking during a panel session entitled “The mosquito, the virus, and what we must do to combat them” at the 68th Annual Meeting of the Brazilian Society for the Advancement of Science (SBPC), held on July 3-9 at the Porto Seguro campus of the Federal University of Southern Bahia (UFSB).
“There are grounds for serious doubts as to whether A. aegypti is the only Zika virus vector,” Lopes said. “In the wild, several species ofAedes are involved in the transmission process. Why should a single species be involved in urban environments?”
According to Lopes, A. aegypti started receiving blame for transmitting Zika in 1947, when researchers funded by the Rockefeller Foundation identified its transmission in Zika Forest near Entebbe in Uganda, Africa.
The scientists were attempting to isolate yellow fever virus and were studying mosquitoes of the genus Aedes, long known to transmit the disease.
Since then, the virus has been found in other species of the same genus, such as A. africanus, Lopes recalled.
In 1966, when Zika was first isolated in Malaysia, several mosquito pools were analyzed in the Southeast Asian country and only one was identified as A. aegypti.
According to Lopes, no pools of A. aegypti have been associated with the more recent outbreaks of Zika, such as the 2007 epidemic in Micronesia, in the western Pacific, when approximately 70% of Yap Island’s population of 7,300 was infected.
“Actually, there are very few A. aegypti mosquitoes in Micronesia,” she said. “There are other species of Aedes in the region, but A. aegypti is very rare on most of the islands and is completely absent from the islands where the vast majority of cases of Zika occurred.”
At the time of the epidemic, Lopes contacted researchers in the region to identify which mosquito species was most abundant there. Their answer was C. quinquefasciatus, which had not been investigated as a Zika vector.
“The problem is that until now everyone who studied the circulation of Zika virus only looked at species of Aedes,” Lopes said. “It was assumed these mosquitoes must be Zika vectors because they were already well-known vectors of dengue, chikungunya and yellow fever.”
At the start of Brazil’s Zika outbreak, Lopes decided to investigate whether C. quinquefasciatus could also transmit the virus. This peridomestic mosquito is 20% more abundant than A. aegypti in urban environments and is a vector of other arboviruses (transmitted mainly by arthropods), such as West Nile virus and Japanese encephalitis virus, both of which are closely related to Zika virus.
Lopes also began to draw the attention of the scientific community and the World Health Organization (WHO) in letters published by journals such as The Lancet on the urgent need to investigate other species of mosquito that may also be Zika vectors in addition to A. aegypti.
“Until then, the scientific community and the WHO had unfortunately focused only on A. aegypti,” Lopes said. “All efforts to combat Zika targeted this species alone, neglecting several others, such as C. quinquefasciatus.”
The results of assays performed by the researchers, who infected C. quinquefasciatus and A. aegypti mosquitos with Zika in the lab to compare their capacity to transmit the virus, showed very similar transmission performances.
They observed the presence of Zika virus in the salivary glands of both C. quinquefasciatus and A. aegypti three days after infection.
“This cycle is shorter than that for dengue virus, which takes 10-15 days to surmount resistance barriers and reach the mosquito’s salivary glands. In the case of Zika, it took only seven days after infection for the viral load to peak in the salivary glands of our insects,” Lopes said.
To understand whether Zika virus was capable of leaving the salivary glands and could be found in mosquito saliva, the researchers performed a test in which insects were exposed to a sheet of filter paper covered with honey and an antibiotic. When they fed on the honey, the mosquitoes deposited saliva on the filter paper. Their RNA was then collected and analyzed.
According to a paper describing the results of the assay, which is in press, Zika was found to be present with similar viral loads in the saliva of both C. quinquefasciatus and A. aegypti.
“Because C. quinquefasciatus is more abundant in urban environments than A. aegypti, we now want to find out which of these two species is the more important Zika vector,” Lopes said.
The results of the studies performed by Lopes and her research group at Fiocruz were presented to the WHO, which asked the Pan American Health Organization (PAHO) to have other mosquito species, especially C. quinquefasciatus, investigated in regions of the world where cases of infection by Zika virus have been recorded.
Change in mosquito control methods
According to Lopes, one of the implications of the involvement of other species in the transmission of Zika virus, if proven, is that the approach to mosquito control would have to change drastically. Vector control is currently confined to A. aegypti, but the habits of this species are quite different from those of others such as C. quinquefasciatus.
A. aegypti bites during the day, while C. quinquefasciatus prefers to feed at night, so it would not be sufficient for pregnant women to use insect repellent only during the day, for example.
Females of A. aegypti typically lay their eggs in clean standing water such as rain puddles or old tires, whereas C. quinquefasciatusprefers to breed in sewers, septic tanks and other highly polluted water containers. “To combat house mosquitoes, Brazil will have to invest in basic sanitation, where it has a massive historic deficit,” Lopes said.
Nevertheless, she added, it would be less difficult to control C. quinquefasciatus than it is proving to combat A. aegypti, largely because females of the latter species prefer to spread eggs in relatively small clumps so their offspring have the best possible chances of survival, whereas C. quinquefasciatus lays all of its eggs in one place.
“The house mosquito’s breeding sites are more concentrated and display high levels of infestation, whereas A. aegypti’s oviposition behavior favors distribution at several breeding sites,” Lopes said.
“Control programs are far more likely to succeed against C. quinquefasciatus than A. aegypti. The fact that even today Brazil hasn’t managed to control dengue effectively is evidence of this difficulty.”
She also noted that besides being the epicenter of the Zika outbreak in Brazil, Recife is also the only Brazilian city with a significant prevalence of filariasis, an infectious tropical disease caused by thread-like parasitic roundworms (filariae), which in this region are transmitted solely by C. quinquefasciatus.
A study by colleagues of Lopes at Fiocruz Recife showed that the cases of microcephaly reported in Recife occurred precisely in areas where filariasis has also been notified.
“About 85% of the mothers that have given birth to babies with microcephaly live in neighborhoods where there have been cases of filariasis,” Lopes said.
These neighborhoods lack basic sanitation and have open sewers, among other relevant features. “If Zika were transmitted solely by A. aegypti, it would be a democratic virus like dengue, in the sense that anyone can catch the disease, not just very poor people who are especially vulnerable and exposed to dense infestations of these mosquitoes,” she stressed.