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Let It Be B: In the Search for Blood, One Mosquito Species Has a Type

Anopheles stephensi mosquito

A new study shows the malaria-transmitting Anopheles stephensi has a strong preference for type B human blood. The finding has implications for identifying individuals at high risk of mosquito-borne disease, but those with other blood types shouldn’t put away the insect repellent. (Photo by Jim Gathany, CDC Public Health Image Library)

By Cameron Webb, Ph.D., and Nur Faeza Abu Kassim, Ph.D.

Nur Faeza Abu Kassim, Ph.D.

Nur Faeza Abu Kassim, Ph.D.

Cameron Webb, Ph.D.

Cameron Webb, Ph.D.

Mosquitoes need blood. While they can get blood from a range of animals, it’s their propensity to bite people that causes so many pest and public health risks around the world. Over half a million people die each year due to a mosquito-borne disease and, even during the COVID pandemic, mosquito bites will infect hundreds of millions of people across the globe with pathogens, such as arboviruses and malaria parasites.

We know mosquitoes bite people, but why do they pick you out from the crowd? It’s a phenomenon commonly observed, especially by scientists who need to ensure their volunteers attract enough mosquito bites to ensure the reliability of their insect repellent research and assessments. But what is going on with those people more likely to be bitten by mosquitoes? Is it really because they have “sweet blood,” or is that just the kindest thing we can say to our itchy, bite-covered friends and family?

Mosquitoes pick up on a cocktail of cues when searching for a blood meal. The carbon dioxide we exhale, the temperature of our bodies, the chemicals found our skin, and perhaps even our choices of perfume may all determine our likelihood of being bitten. But what about our blood type? It may make sense that, as mosquitoes are after blood, perhaps our attractiveness to mosquito bites is influenced by our blood type.

Newly published collaborative work between scientists in Pakistan, Malaysia, China, and Australia has demonstrated that mosquitoes may have a taste for B-type blood. But the results don’t mean that those with other blood types can put away the insect repellent.

The research, published in December in Scientific Reports, sought to determine how human blood types influenced the host-seeking behaviour and fecundity of the mosquito Anopheles stephensi. This mosquito is one of the most important vectors of malaria parasites throughout many countries stretching from southern Asia to Africa. It is well adapted to urban environments and is a driving factor in outbreaks of malaria in Pakistan, Iran, Afghanistan, and India.  Its recent spread into Africa is a major concern for authorities where many countries already suffer under the burden of mosquito-borne disease.

Understanding the host-seeking behavior of this mosquito may provide opportunities for identifying individuals at high risk of mosquito-borne disease and consequently targeted public health actions, or it may also provide new perspectives on the development on mosquito control or surveillance technologies.

In a series of laboratory studies, we and our fellow researchers investigated how different human blood types (i.e. A, B, AB, O) were digested by the mosquito and how those blood meals influenced the number of eggs developed and survivorship of subsequent immature stages. However, it was the use of wind tunnel bioassays that provided the most fascinating result. Groups of 100 mosquitoes were released into chambers with a choice to fly up five individual tubes, each containing one of the four blood types, in addition to a control, all provided by an artificial blood feeding device.

The result was surprising given the strong preference for a particular blood type: Over 70 percent of the mosquitoes were attracted to type B blood. This result was backed up by some electroantennography experiments, in which tiny electrodes are attached to the mosquito’s antennae to record their response to blood type, which demonstrated mosquitoes exhibited a significantly stronger response to type B blood.

This study was the first to demonstrate that Anopheles stephensi exhibits a preference for type B blood, at least in a laboratory setting. The result may pave the way for further investigations to translate this observed mosquito behavior to better public health outcomes for communities in countries where An. stephensi is present.

There also needs to be some caution in extrapolating these results from the laboratory to the field. This result assists our understanding of An. stephensi and their host-seeking behaviour, but there are thousands of other mosquito species buzzing about the globe, and it is highly unlikely that such a preference exists across them all. There are already well-documented differences in the host-seeking behaviour of some of the world’s most important pest mosquitoes. Some prefer mammals, birds, frogs, or even earthworms. However, even for those mosquitoes that prefer to feed on humans, there may still be a much more complex puzzle to solve than simply determining an individual’s blood type.

The attraction of mosquitoes to people is likely driven by a wide range of factors, and it is clear that the chemical cocktail of smells produced on out skins is likely to play an important role, too. The host-seeking preferences of other medical important mosquitoes, such as Aedes aegypti and Anopheles gambiae have demonstrated that there may be novel combinations of chemicals that could be used to improve the effectiveness of mosquito surveillance. But, it could take scientists quite a while to solve the host-seeking behavior of just our most “dangerous” mosquitoes. There is still so much to learn about the ecology of so many other mosquitoes, and working out who or what they’re most likely to bite may still be a little low on our list of research priorities.

What does this all mean if you’re heading out on a hike? The important thing to remember is there is unlikely to be just one type of mosquito biting you. So, no matter how much of a “mosquito magnet” you think you are, there probably isn’t anyone completely immune to mosquito bites, so you should still take those precautions to cover up and wear insect repellent to prevent mosquito bites.

Cameron Webb, Ph.D., is an associate professor in the Department of Medical Entomology at The University of Sydney and NSW Health Pathology. With more than 25 years’ experience as a mosquito researcher, Webb provides expert advice to local, state, and federal government agencies in Australia on the management of mosquitoes and mosquito-borne disease. Twitter: @mozziebites. Facebook: www.facebook.com/mozziebites. Email: cameron.webb@sydney.edu.au. Nur Faeza Abu Kassim, Ph.D., is an associate professor in the School of Biological Sciences, at Universiti Sains Malaysia and COMBI Penang in Penang, Malaysia. An expert in medical and epidemiological entomology, her research focuses on mosquito-borne disease and the role of vectors in transmission of disease. She is also actively involved with community awareness programs on public health issues. Twitter: @aija81. Email: nurfaeza@usm.my.

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