The Centers for Disease Control and Prevention calls upon an endemic in at least 100 countries in Asia, the Pacific, the Americas, Africa, and the Carribean. According to them, there are about 50 to 100 million Dengue cases every year with about 22,000 deaths.
Epidemics of these mosquito-borne diseases, such as Dengue, Malaria, and Zika, have been prevalent in the Western Hemisphere for more than 200 years and there have been increasing trends of dengue cases in the past 30 years.
Although this disease is more common in tropical countries like the Philippines, cases in the United States are also increasing due to modern transportation which makes traveling to different countries, especially to those areas where Dengue is endemic, relatively more straightforward and more accessible. With the increasing number of travelers in and out of these endemic countries, disease transmission rates are also escalating in other countries.
Aedes aegypti mosquitoes are the principal mosquito vector of dengue virus as well as Zika virus. In its normal life cycle, only the female mosquitoes feed on humans for blood while the males feed on nectar and other sources of sugar. To put it simply, females need protein from human blood in order to lay eggs. Therefore, measures to prevent and control transmission is usually directed at female mosquitoes since they have the ability to transmit the virus to a healthy person after feeding on an infected one.
Researchers in the United States discovered in a study that these female mosquitoes are attracted to certain sensory factors usually found in humans such as heat, body odor, moisture, visual cues, and carbon dioxide. These stimuli can help them locate and identify human hosts to bite and feed on.
Carbon dioxide was found out to be the best signal for a warm-blooded animal and this gas can be sensed from 30 feet away.
However, greater emphasis is given to body odor which is a complex of substances because this could distinguish humans from other vertebrate hosts. These substances include the following: lactic acid, ammonia, amine, carboxylic acid, ketones, sulfide, and 1-octen-3-ol which could be detected by the mosquitoes’ receptors.
In an attempt to discover possible ways to protect humans from mosquito bites and therefore reduce the transmission of mosquito-borne diseases, researchers from Florida International University in Miami have conducted a study which led them to identify a specific olfactory co-receptor called IR8a found in the female mosquitoes’ antenna and undetectable in other tissues. It was found out that this gene is responsible for the detection of lactic acid and acidic components in human odor.
By disrupting this IR8a gene through the use of CRISPR/Cas9 gene-editing system, their findings showed that the mosquitoes with mutant IR8a gene have decreased attraction to humans specifically due to their decreased detection of lactic acid which is a behaviorally active component of human sweat. In addition, these mutant mosquitoes were unable to detect other acidic components of human odor, therefore making them less likely to bite humans and transmit mosquito-borne diseases. However, reduced responses to heat and carbon dioxide were not observed in these mutant mosquitoes.
Dr. Matthew DeGennaro, who is a mosquito neurobiology researcher and the senior author of the study, said that removing the function of the IR8a removes approximately 50% of the host-seeking activity of these mosquitoes.
Furthermore, he added that odors which mask the IR8a pathway could be discovered that could enhance the efficacy of current repellents like N,N-Diethyl-m-toluamide (DEET) or picaridin. According to them, this is a potential avenue to create new solutions. They plan on doing the chemical screens soon using their knowledge from the newly discovered gene and its pathway to create potentially more effective mosquito attractants and repellants.
Since findings from their study showed that there were no reductions in mosquito response with the presence of heat and carbon dioxide, they stated that there are other receptors that researchers need to focus onto to completely eradicate humans as hosts of these mosquitoes.
Through the past decades, many government efforts in different countries have been directed towards vector control and transmission. However, the morbidity and mortality rates of these mosquito-borne diseases remain high prompting us to take a more efficient approach in dealing with these diseases. This research has revealed valuable information which could be used as baseline data for further research taking us several steps closer to having total control over these diseases.