Do mosquitoes spread Lyme disease?

Lyme disease causing Borrelia spirochetes can survive in the tick midgut. Mosquitoes, lice and other insects have a different way of digesting their blood meal. This midgut environment of other insects is hostile to b. burgdorferi. The digestion process in the midgut of other insects involves more acidity and proteases against which b. burgdorferi spirochetes have no defense.

Relapsing fever Borrelia such as b. recurrentis can be transmitted by lice because in contrast with b. burgdorferi they do survive the harsh environment of the midgut of lice. Relapsing fever Borrelia are totally different spirochetes and cause a different disease which is accompanied by high fevers unlike Lyme disease, which in the first stages resembles a mild summer flu.

The myth that other insects could possibly spread Lyme disease stems from a study from Germany that evaluated the presence of b. burgdorferi sensu lato spirochetes in mosquitoes and from this study came the patient-culture myth that you could get Lyme disease from mosquito bites.

Summing up the German study

In less than 3,6% of the 682 mosquitoes evidence of borrelia burgdorferi sensu lato was found. The authors identified some of the spirochetes in the saliva glands of the mosquito.

After that they describe the transmission cycle of ticks and conclude that none of the conditions that would be needed for mosquitoes to transmit spirochetes are present as far as is known.

According to the authors the question whether mosquitoes can serve as a second mechanical vector remains unclear. They would like to further study how mosquito larva acquire spirochetes and if mosquitoes have function related proteins that could enable the attachment and survival of spirochetes after ingestion.

What are the requirements for Borrelia burgdorferi transmission?

There have been a fair amount of studies done that look for the presence of Lyme spirochetes in mosquitoes and most studies found that between 1-3% of the mosquitoes indeed carried the Lyme spirochete. In a patients mind this must mean mosquitoes are able to infect humans but it is not so simple.

In ticks b. burgdorferi upregulates outer surface protein A (OspA) and remain immobile while the tick molds into their next developmental stage. When the tick feeds in this next stage the feeding activates the dormant spirochetes. They proliferate and replace OspA with with other surface proteins, leave the midgut and disseminate into the saliva glands. From there they are inoculated into the vertebrate skin. These developmental changes in the spirochete take approximately 2 days to complete. Far longer than a mosquito feeds (about 90 seconds).

  • Because of this low attachment and feeding time of the mosquito they are unlikely to ingest many Lyme spirochetes.
  • We already established that Borrelia burgdorferi has no natural defenses against the proteases that are involved in the digestion of the bloodmeal in the midgut of other insects such as mosquitoes.
  • The spirochetes are unlikely to invade the saliva glands before the feeding of the mosquito is finished.
  • Studies have failed to show the ability of mosquitoes to transmit Lyme spirochetes to the vertebrate host.

Differences in feeding

Mosquitoes feed from vessels and cannulate dermal capilaries and rarely draw blood from the interstitial tissue. The destruction of skin by tick feeding facilitates the uptake of Lyme spirochetes whereas the cannulation of capilaries by mosquitoes does not.

Tick saliva

Besides the fact that mosquitoes do not meet the complex transmission requirements of Lyme spirochetes, ticks secrete saliva into the host. The components of this saliva assist the spirochetes within the new host. Even if mosquitoes were able to transmit the Lyme spirochete, the human immune system would kill off the Lyme spirochete immediately. Other insects don’t need this protein in their saliva because they feed in a much shorter time and thus don’t have to temporarily shut down the inflammation response around the bite site like ticks.

Borrelia afzelii

Recently a preprint suggested that Borrelia afzelii transmission is different from the model transmission cycle of Borrelia burgdorferi. The scientists suggest that tick saliva is not important for B. afzelii transmission and infectivity. Instead they suggest Borrelia afzelii relies on the rotation of outer surface proteins. They suggest a direct gut-to-mouth transmission instead of the route from the midgut, trough the hemolymph to the saliva glands. This could mean some of the arguments against mosquito transmission are not valid when it comes to Borrelia afzelii.

No case reports

There are no case reports of patients that develop an erythema migrans after a mosquito bite in the literature.

Conclusion

It seems impossible that mosquitoes can transmit Borrelia burgdorferi considering the complex transmission model. However, scientists have recently suggested that Borrelia afzelii has a different transmission cycle. But mosquito transmission is still an unlikely probability.

Some mosquitoes do transmit viruses such as West Nile Virus and parasites like Malaria so if you
are visiting endemic area for these diseases protect yourself and get the required travel vaccinations.

References