Why mosquitoes don't spread Lyme disease

Why mosquitoes were ever suspected

Lyme disease is caused by spirochete bacteria in the Borrelia burgdorferi sensu lato group, and Ixodes ticks are its recognised vectors. A puzzle has always hung over that picture: many patients do not remember a tick bite. In one Canadian cohort cited by the researchers, only 56 of 210 patients, about 27 percent, recalled one. Tick bites are painless, so people tend to remember the itchy, noticeable bites of mosquitoes and flies instead.

Earlier field surveys added to the suspicion. Borrelia DNA has been detected in Culex and Aedes mosquitoes, and in horse flies and deer flies; one study found it in 10 mosquito species. But finding a microbe's DNA inside an insect only shows the insect drank infected blood at some point. It says nothing about whether the insect can pass the microbe on.

What it takes to be a vector

Entomologists call the ability to transmit a pathogen vector competence, and it has three parts. The insect must acquire the pathogen when it feeds, maintain it alive through digestion and the time between meals, and then transmit it to a new host. Failing any one step rules the insect out, no matter how often its DNA shows up in surveys.

A team led by Miriama Pekľanská at the University of South Bohemia, with collaborators including Florida International University, tested all three steps in the laboratory. They used three mosquito species, Aedes aegypti, Culex quinquefasciatus and Culex pipiens biotype molestus, and three Lyme species, B. afzelii, B. burgdorferi sensu stricto and B. garinii. A relapsing-fever spirochete, B. duttonii, which does circulate in blood, served as a positive control. The results were published in Parasites and Vectors in June 2025.

What the experiments found

Acquisition was poor. When mosquitoes fed on infected mice, only 3 of 20 Aedes aegypti that fed on B. burgdorferi-infected mice tested positive, none picked up B. afzelii, and neither Culex species acquired either Lyme species. By contrast, all 20 Aedes that fed on mice carrying the blood-borne relapsing-fever spirochete were positive.

Maintenance failed next. Using artificial membrane feeders, the team put spirochetes into every mosquito, then watched them disappear. Within 24 hours median spirochete counts dropped many-fold, and by 96 hours only a small fraction of mosquitoes still tested positive. Mosquito homogenates taken right after feeding infected all 5 test mice, but those taken at 48 and 72 hours infected none.

Transmission never happened. Mosquitoes that fed on spirochete-laden blood, digested it and then fed on clean mice infected 0 mice. Interrupting a meal halfway and letting the mosquito finish on a clean mouse, a test of mechanical transfer on contaminated mouthparts, also infected 0 mice, even though those mosquitoes were still PCR-positive. Infected ticks used as a control infected all 4 of their mice.

Why ticks succeed and mosquitoes fail

The authors point to biology on both sides. Lyme spirochetes live mainly in skin and other tissues and rarely reach high numbers in the bloodstream, so a quick sip of blood seldom contains many. A tick stays attached for days, giving spirochetes time to migrate from the surrounding tissue into its feeding site; a mosquito feeds in seconds to minutes.

Digestion matters too. Ticks digest blood inside the cells of their gut and can hold spirochetes between meals. Mosquitoes digest blood in the open gut using the enzyme trypsin, and the study showed trypsin is what kills the bacteria: when it was chemically blocked, all 20 Aedes still carried spirochetes at 72 hours, versus 1 of 20 normal mosquitoes. In a dish, active trypsin killed about 97 percent of spirochetes within 48 hours.

What it means

The authors conclude that mosquitoes lack the biological capacity to acquire and maintain Lyme spirochetes and cannot transmit them by natural or mechanical routes, reinforcing Ixodes ticks as the sole competent vectors. Their practical point is about focus: tick checks, tick control and accurate risk messages are where prevention effort belongs. The study's sample sizes were small, often four or five mice per group, but every route tested came back negative while the tick control worked every time. A forgotten tick bite remains the likelier explanation than a remembered mosquito.