By Mariah Quintanilla
Some mosquitoes would rather not suck your blood but they do it anyway because you are the closest warm-blooded animal around. So why do some mosquitoes prefer biting humans to biting other animals? The scientific answer: it’s complicated.
The same factors that regularly influence what you eat for lunch also affect whether a species of mosquito tends to bite humans or prefers other warm-blooded vertebrates. Say you most often eat sandwiches for lunch. Are you eating them because you always crave sandwiches? Or do you eat sandwiches all the time because there are only sandwich shops close to you? The answer to that question might be useful for scientists studying mosquitoes and their dietary habits. Understanding the biological and genetic components causing a mosquito to bite one animal over another is important when attempting to control the spread of diseases including malaria, West Nile virus, and Zika virus.
“Aedes aegypti prefers to feed on humans,” said Rebekah Kading, a professor in the arthropod-borne and infectious diseases laboratory at Colorado State University. This makes it a more efficient spreader of Zika virus than the other Zika-carrying mosquito, Aedes albopictus, which feeds both on humans and animals.
“In general, mosquitoes are going to cue in on carbon dioxide and heat,” said Kading in a phone interview. All warm-blooded animals give off both carbon dioxide and heat. Scientists have yet to confirm whether different levels of either element explain why a species of mosquito might suck the blood of, for example, humans more often than birds.
Monitoring the location, either indoors or outdoors, and time of day that disease-carrying mosquitoes tend to bite may lead to more effective control measures. In 2015, about 214 million cases of malaria were recorded worldwide, according to the Centers for Disease Control and Prevention. The parasite that causes malaria, which greatly affects children and pregnant women, is transmitted by Anopheles mosquitoes, and is still a leading cause of death in many African countries.
If a mosquito that can transmit malaria tends to bite humans indoors rather than outdoors, health officials may distribute insecticide-treated bed nets or personal bug spray instead of spraying large quantities of pesticides outdoors.
Perhaps a more obvious influential factor for a mosquito is the local availability of a host animal. If there are more cattle than humans in an area, as in the rural villages of East Africa, mosquitoes will simply bite cows more frequently. These concepts may seem simple, but scientists are finding that genetic components may be complicating our overall understanding of host selection among mosquitoes.
A recent field study by researchers at the University of California, Davis, suggests there may be a genetic difference between mosquitoes that tend to feed on humans and those that tend to feed on cattle. They studied Anopheles arabiensis, one of the last remaining vectors (carriers) of malaria in East Africa. After collecting and sequencing the genome of mosquitoes that fed on either cattle or humans, they found an inversion, or “a huge region cut out and flipped on the chromosome” in the mosquitoes that preferred cattle, said Bradley Main, the head researcher for the study .
“The bed nets are not totally working,” said Main, mostly because An. arabiensis does not solely feed on humans, and tends to bite outdoors. This remaining population of malaria-transmitting mosquitoes is essentially avoiding control measures targeted at humans, like insecticide-treated bed nets, by surviving off cattle blood, he said in a phone interview. Being able to discern from a mosquito’s genome whether An. arabiensis is more likely to bite humans may be critical knowledge for scientists and health officials developing novel malaria control measures.
More tests in a controlled setting are needed before an actual genetic basis for host preference can be identified, said Main. Bruce Eldridge, professor emeritus in UC Davis department of entomology and nematology is skeptical that genetics plays a major role in mosquito host-selection.
“It’s very difficult to make a persuasive argument that mosquitoes that feed on large mammals prefer one mammal over the other,” said Eldridge, in a phone interview. There are so many biological and environmental factors that could be causing this inversion in some An. arabiensis mosquito populations that a direct genetic link to host-preference might be hard to find, he said.
The next phase of research, said Main, is developing “a genotyping assay where you can track the inversion, and its association with cattle biting.” If a genetic marker is discovered, it may warrant the use of a “cattle-based” insecticide when controlling for malaria in those areas of sub-Saharan Africa, he said.