Chemical ecologist and mosquito researcher Walter Leal, distinguished professor in the UC Davis Department of Molecular and Cellular Biology, and his lab collaborated with scientists in Recife to ask “Does Zika Virus Infection Affect Mosquito Response to Repellents?”
The work, funded by the National Institutes of Health, was published Feb. 16 in Scientific Reports of the journal Nature. The researchers used mosquitoes originating from colonies reared by UC Davis medical entomologist Anthony Cornel and from colonies in Recife.
“We used assays mimicking the human arm to test the mosquitoes infected with the Zika virus,” Leal said, “and we asked whether the Zika infection affects mosquito response to repellents.” They tested DEET and Picaridin, considered the top two mosquito repellents. “We discovered that DEET works better than Picaridin against the southern house mosquito, Culex quinquefasciatus, and the yellow fever mosquito, Aedes aegypti, whether infected or not.”
The researchers also found that old mosquitoes that already had a blood meal “were less sensitive to repellents,” said Leal, adding “It was not clear whether this was due to the virus, but mostly likely because of age. ”
“Lower doses--normally used in commercial products--work well for young mosquitoes,” Leal said, “but the old ones are the dangerous ones because they may have had a blood meal infected with virus and there was enough time for the virus to replicate in the mosquito body.”
“The bottom line: to prevent bites of infected mosquitoes, higher doses of repellent are needed. The data suggest that 30 percent DEET should be used. Lower doses may repel nuisance young mosquitoes, but not the dangerous, infected, old females.”
Leal, a native of Brazil, collaborates with Rosangela Barbosa and Constancia Ayres of the Oswaldo Cruz Foundation (FIOCRUZ-PE), Recife, Brazil. The work with infected mosquitoes was conducted at FIOCRUZ-PE.
Leal, Barbosa and Ayres co-authored the paper with Fangfang Zeng and Kaiming Tan, both of the Leal lab; and Rosângela M. R. Barbosa, Gabriel B. Faierstein, Marcelo H. S. Paiva, Duschinka R. D. Guedes, and Mônica M. Crespo, all of Brazil.
The World Health Organization (WHO) recommends that people traveling to or living in areas with Zika virus (ZIKV) outbreaks or epidemics adopt preventive measures, including the use of insect repellents, to reduce or eliminate mosquito bites. Prior to the Feb. 16 published research, it was not known whether the most widely repellents are effective against ZIKV-infected mosquitoes, “in part because of the ethical concerns related to exposing a human subject's arm to infected mosquitoes in the standard arm-in-cage assay,” the researchers pointed out.
They used a previously developed, human subject-free behavioral assay, which mimics a human subject to evaluate the top two recommended insect repellents.
Scientists isolated the Zika virus (ZIKV) nearly seven decades ago from a sentinel rhesus monkey while they were trying to unravel the cycle of sylvan yellow fever virus in Uganda..
According to the Centers for Disease Control and Prevention:
- Zika is spread mostly by the bite of an infected Aedes species mosquito (Ae. aegypti and Ae. albopictus). These mosquitoes bite during the day and night. The virus can also be sexually transmitted.
- Zika can be passed from a pregnant woman to her fetus. Infection during pregnancy can cause certain birth defects.
- There is no vaccine or medicine for Zika.
- Local mosquito-borne Zika virus transmission has been reported in the continental United States.
In a groundbreaking discovery, a scientific team of Brazilians and Brazilian-born chemical ecologist Walter Leal of the University of California, Davis, has announced that the Zika virus has been detected in wild-caught Culex quinquefasciatus mosquitoes in Recife, the epicenter of the Zika epidemic.
Scientists from the Fiocruz Institute, Pernambuco, confirmed the discovery July 21. The detection could have widespread repercussions, as the Culex mosquitoes are more common and widespread than the yellow fever mosquito, Aedes aegypti, known as the primary carrier of the Zika virus.
Leal, who collaborates with Fiocruz Institute researcher Constancia Ayres in a National Institutes of Health-sponsored project on the investigation of Zika in the C. quinquefasciatus, said that the Brazilian lab earlier discovered that Culex had the capability of transmitting the virus. Although the scientists were able to infect the lab mosquitoes with the virus, they had not found the virus in wild-caught mosquitoes—until now.
“This could have major repercussions here in the United States and in other parts of the world,” said Leal, a distinguished professor in the UC Davis Department of Molecular and Cellular Biology who is co-chairing the International Congress of Entomology meeting Sept. 25-30 in Orlando, Fla. The conference is expected to draw some 7000 entomologists throughout the world.
Leal said more work needs to be done to see if Culex mosquitoes are playing a role in the current epidemic. In an interview July 21 with health reporter Jennifer Yang of the Toronto Star, Canada's largest daily, he commented: “It looks like there were more vectors than we thought, and this is one of them. We don't have to panic, but we have to know. And now that we know, we have to take care of the Culex.”
A. aegypti is already established in California; it has spread to at least seven counties since its discovery in Clovis, Fresno County, in June 2013, according to medical entomologist Anthony Cornel of the UC Davis Department of Entomology and Nematology and the UC Kearney Agricultural Research and Extension Center, Parlier.
The Zika virus, which can result in birth defects in pregnancy, can be transmitted through exposure to infected blood or sexual contact. The Centers for Disease Control estimates that between 400,000 and 1.3 million cases have been discovered across South, Central, and North America, where the disease was previously unknown.
Leal and a group of 18 students just hosted a Zika Public Awareness Symposium on May 26 at Giedt Hall, UC Davis campus. The podcast can be accessed at https://video.ucdavis.edu/media/Zika+Virus+Public+Awareness+Symposium/0_n3aupf5c
Native Brazilian Walter Leal, chemical ecologist and professor in the UC Davis Department of Molecular and Cellular Biology, is there.
So is the mosquito.
At the symposium today, Constancia Ayres, research coordinator of the Oswaldo Cruz Foundation (Centro de Pesquisa Aggeu Magalhaes/FIOCRUZ, considered one of the oldest and most prestigious scientific research institutes in South America), reported that the common southern house mosquito, Culex quinquefasciatus, may be a potential vector of the Zika virus.
Studies in the Ayres lab confirmed that Culex quinquefasciatus infected with Zika (isolated from local patients) showed high transmission rate (as determined by virus replication in the salivary glands).
Of course, these studies were done in the lab, not the field, and this is the beginning of the research.
We asked medical entomologist William Reisen, editor of the Journal of Medical Entomology, and professor emeritus, Department of Pathology, Microbiology and Immunology, UC Davis School of Veterinary Medicine, about this.
"In California less than 3 percent of the Culex including Cx. quinquefasciatus have been found to feed on humans, even in cities like Los Angeles, where humans are the most numerous host," Reisen said in an email. "Therefore, even if they are susceptible to infection, the probability of a female feeding on humans to acquire and then refeed on humans to transmit would be 0.03 x 0.03 = 0.0009 or a rare event indeed. That said, there are areas of the world where quinquefasciatus feeds predominantly on humans in domestic settings. (See his research paper, Host Selection Patterns of Some Pakistan Mosquitoes (U.S. National Library of Medicine, National Institutes of Health).
In his research paper on Pakistan mosquitoes, Reisen mentions Culex pipiens fatigans, now known as Culex quinquefasciatus. Its feeding patterns "varied opportunistically with host availability," he wrote in the abstract. Resting in cattle sheds during the winter, it "fed on birds and bovids, changing to man and bovids during the spring and then to man and birds during summer."
Medical entomologist Thomas Scott, distinguished professor of entomology (now emeritus) of the UC Davis Department of Entomology and Nematology, is a global authority on Aedes aegypti, which transmits dengue, yellow fever, chikungunya and the Zika viruses.
"Vector competence studies in the lab is valuable information, but before we come to the conclusion that Culex quinquefasciatus is an important contributor to transmission of Zika virus, the lab results would need to be confirmed," Scott told us. "Other laboratories and appropriate field studies would need to be carried out in areas where Zika virus is being transmitted to confirm that this species is naturally infected and is regularly biting people. Although it is a potentially important discovery that would change they way we think about Zika virus transmission, it would be wise to carefully explore all of the details necessary to incriminate a mosquito vector before coming to a strong conclusion."
UC Davis medical entomologist Anthony Cornel of the Kearney Agricultural Research and Extension Center and UC Davis Department of Entomology and Nematology, weighed in: "I have to examine the vector competence data carefully to determine its infectivity dose relative to Aedes aegypti. If Culex quinquefasciatus does play an additional role in transmission then it would be of even greater concern because the threat of autochthonous transmission will occur in areas other than where Aedes aegypti is found in California. Culex quinquefasciatus also feed on people but do so at dusk and at night."
"Brazil could be facing a greater fight against the zika virus than previously feared as researchers investigate whether the common mosquito is transmitting the disease," Moraes wrote in the article, headlined "Brazilian Experts Investigate if 'Common Mosquito' is Transmitting Zika Virus."
"The Aedes aegypti species of mosquito was thought to be solely responsible for spreading the virus," Moraes pointed out. "But scientists are now studying whether the Culex mosquito--the variety (species) most commonly found in Brazil--could also be passing on the infection."
Constancia Ayres was quoted as saying: "This may be the reason for the virus replicating faster. The interaction of the mosquito with the virus may explain the epidemiological profile of disease transmission.”
Meanwhile, the headlines continue as research proceeds.
"The concern is that the Culex mosquito--which is 20 times more prevalent than the Aedes variety in Brazil-- might also play some role in the rapid spread of these viruses," Outbreak News Today noted. "Researchers hope to have some answers in a few weeks,"
Valor.com.br is hot on the story as well. Reporter Marina Hawk emphasized that the work was done in the lab, but field collection is underway.