The mosquito scourge might somehow be even worse than we thought. Intriguing research out this month finds it isn’t just females that are capable of sucking our blood—males can apparently do it, too.

Scientists from Pennsylvania State University and other institutions made their case in a preprint released last week on the website bioRxiv. Across various experiments and situations, they found evidence that some male mosquitoes will reliably feed on blood, at least under the right circumstances. The findings could overturn a long-held belief about the threat these insects pose to humans, the researchers say, though further research will be needed to better understand the phenomenon.

“We need to actually look at what’s happening in the field rigorously,” senior study author Jason Rasgon, an entomologist specializing in vector-borne diseases at Penn State, told Gizmodo.

An accidental discovery

Male adult mosquitoes have long been thought to feed on nectar and other sugary sources for their nourishment, whereas most female mosquitoes will regularly feed on other animals’ blood for their survival. And Rasgon and his colleagues weren’t initially looking to challenge these widely held assumptions.

A few years ago, however, one of Rasgon’s graduate students was conducting blood-feeding experiments with mosquitoes in the lab when she noticed some of the fed insects were male. Rasgon stopped the student from throwing out these mosquitoes. He had remembered a 2016 paper where researchers enticed Culex quinquefasciatus males into feeding on blood; the blood proved toxic, killing the males within days. Given the opportunity, he wanted to see if the same thing would happen with a different species, so the team simply kept an eye on them. To everyone’s surprise, their blood-drinking males lived just as long as males on a typical sugar diet.

“A lot of science is like that. I always say that most of the big advances we’ve made in my 20 years as a scientist have been accidents. And in general, scientific things start out as ‘Oh, that’s weird. What’s happening there?'” Rasgon said. “And most people don’t follow up, but you need to follow up on them.”

At first, Rasgon thought their finding was nothing more than a cool scientific footnote. But he and his team continued to pursue the research with their spare resources. In one experiment, for instance, they found they could get the males of certain species to routinely suck blood from an artificial feeder if the insects were first dehydrated—something they already showed will happen with females. They then created CRISPR-edited mosquitoes that lacked the ability to sense humidity. These males didn’t feed on blood, further hinting that dehydration is key to provoking blood-feeding in males.

Jason Rasgon opportunistically used his own open wound to show that male mosquitoes will feed on a person’s blood. © Jovana Bozic et al/bioRxiv

Rasgon also started to discuss his work with other scientists in the field, who provided their own contributions to the now growing project. Some of the co-authors had collected male, blood-fed mosquitoes from the wild (hailing from Texas in the U.S. and Mallorca in Spain), for instance. These wild mosquitoes tested positive for vertebrate DNA (dog and human), indicating they had successfully fed on animal hosts.

Additionally, the team’s experiments demonstrated that some male mosquitoes can be infected with the blood-borne germ West Nile virus. What’s worst, the virus can replicate enough inside these males to potentially cause a new infection if transmitted to another host via blood feeding, they found.

In the trippiest part of this whole thing, Rasgon used himself as bait. As luck would have it, he had been scratched by his cat a day earlier, leaving behind a wound that was starting to scab over. He pulled the scab off and exposed his hand to a cage of 20 male dehydrated mosquitoes. Five of the males probed around the wound and one successfully fed off his blood. In another experiment, a male mosquito even successfully pierced Rasgon’s skin and caused a typical immune reaction to the bite, though only for ten minutes (this mosquito didn’t manage to find a blood-carrying capillary, however).

All in all, the team found that at least five species of male mosquitoes (Cx. tarsalis, Cx. quinquefasciatus, Ae. aegypti, Ae. notoscriptus, and An. stephensi) can feed on blood. And what first started as a small, quirky study written by three authors has now expanded to 17 authors from Germany, Spain, Australia, and the U.S. (sadly, Rasgon’s cat doesn’t seem to have gotten a credit for their contribution).

What happens next?

As interesting as this is, the researchers caution there’s still a lot we don’t know about what’s going on here.

The team’s research does suggest male mosquito blood feeding requires specific criteria, namely dehydration. That could mean this behavior only rarely happens in the wild. At the same time, Rasgon notes the real world is often harsh. So perhaps plenty of males get thirsty enough to try their proboscis at blood feeding.

Another crucial unanswered question is whether male mosquitoes are regularly spreading diseases to their hosts. Several of the species they studied are common vectors for germs that sicken people. And even if males only play a small part in the transmission cycle, that role could be significant enough to warrant attention. The researchers note that sterile male mosquitoes are being now used to combat mosquito-borne diseases like malaria—a strategy that could be riskier than currently assumed.

“I got into this thinking it was a stupid little whatever. And now I’m not so sure. I actually think this might be more important than we previously gave it credit for,” Rasgon said.

Rasgon and his team have submitted their study for peer-review, a vital part of vetting any research. And they’re optimistic that their findings can drive more interest into solving the mysteries surrounding the vampiric habits of male mosquitoes.

“We need to do this in a rigorous manner. We need to get some real funding. Some pretty hardcore field work needs to be done,” Rasgon said. “Hopefully next year we’ll be able to do that.