Blood-sucking mosquitoes have their uses. An innovative approach analysing their last blood meals can reveal evidence of infection in the people or animals that the flying insects feasted on.
Scientists say that the method, presented at an infectious-disease conference in Malaysia last week, could be used to study people’s and animals’ past exposure to a range of pathogens, while avoiding the ethical and practical issues of testing them directly.
“This is a novel and fascinating approach, which demonstrates innovative ways to use the environment around us to learn more about exposure to infection,” says Shelly Bolotin, a vaccine scientist at the University of Toronto in Canada.
It could also aid early detection in animals of diseases such as Ebola and SARS-CoV-2, says Niels Verhulst, who studies pathogens transmitted by insects, at the University of Zurich, Switzerland. And it could help scientists to identify the animal host of a new virus, adds Verhulst, who has tested the approach.
People, cows, kangaroos
Previous studies have detected past exposure to pathogens by testing blood for antibodies, markers of past infection that can circulate in blood for months to years, from specific animal hosts1. The technique used by Carla Vieira, a disease ecologist at the QIMR Berghofer Medical Research Institute in Brisbane, Australia, can detect antibodies in blood from a range of animals and people.
Vieira focused on Ross River virus, a potentially debilitating mosquito-borne disease that is endemic to Australia and islands in the southern Pacific Ocean. The virus spreads through insects like dengue, Japanese encephalitis and yellow fever.
Vieira and her colleagues trapped roughly 55,000 mosquitoes in parks in Brisbane in 2021 and 2022. From those insects that had recently had a meal, they squeezed out a couple microlitres of blood and tested it for antibodies that can bind Ross River virus. They also sequenced DNA fragments in the blood to identify the animal host that the insect had feasted on.
In preliminary results presented at the International Congress of Infectious Diseases in Kuala Lumpur on 20 November, Vieira reported that 480 of the trapped mosquitoes were engorged with blood. More than half of those had fed on people, some 9% on cows, and 6% on kangaroos, among other animals. Of the 253 samples from people, more than half had antibodies against Ross River virus — “a really high population”, she says. Close to three-quarters of the cows and kangaroos also had evidence of past exposure.
In a separate study2 published in January, Verhulst and his colleagues detected antibodies against SARS-CoV-2 and the parasite Toxoplasma gondii in blood sucked by mosquitoes from animals, including alpacas and cats.
In theory, the approach could be used “for pretty much any pathogen that elicits an immune response in its host”, says Carl Lowenberger, an entomologist and parasitologist at Simon Fraser University in Vancouver, Canada.
The technique is exciting and could help researchers to study certain diseases about which little is known, such as Japanese encephalitis in Australia, says Eloise Skinner, a disease ecologist at Griffith University in the Gold Coast. “But it comes with some heavy limitations as well.” For instance, the data lack details about the location of exposed animals and people, and when they became infected. This limits the method’s use in reducing the risk of virus transmission, she says.
But it could be used to study past infections in specific neighbourhoods, because mosquitoes tend not to travel far, says Bolotin.
Another drawback of the technique is that it’s not clear how well the proportion of blood samples with antibodies reflects the true proportion of people infected, says David Harley, a medical epidemiologist at the University of Queensland in Brisbane. Multiple mosquitoes could have fed on the same person.
Blood-fed mosquitoes are also notoriously difficult to catch, which limits the technique’s use in outbreak surveillance, says Skinner. After filling up on blood, female mosquitoes typically try to find a dark, moist spot to hide in and digest it, says Lowenberger.
Verhulst and his colleagues have created a concoction — based on mosquitoes’ attraction to carbon dioxide produced by fermenting molasses — to capture more blood-engorged mosquitoes than without the concoction3. They hope to test it outside the lab soon.