Mosquitoes less likely than people to spread disease via air travel

For the current study, they estimated a 0.25 probability of any mosquito on a given flight, or an average of one mosquito on every fourth plane.

FILE: A feeding female 'Anopheles sinensis' mosquito on a human hand. Picture: CDC/James Gathany.

LONDON – Human travellers are much more likely than stowaway mosquitoes to import illnesses like Zika, yellow fever, malaria and dengue to a new part of the world via airplane, researchers say.

Based on calculations of how many mosquitoes get onto commercial aircraft, how many are infected and how many survive long enough at the destination to bite someone, the study estimates human travellers are 200 times more likely to spread dengue virus and 1,000 times more likely to introduce P. falciparum, the parasite that causes malaria.

“Pathogens such as the Zika virus and chikungunya have drawn a lot of interest lately and are showing up in travellers,” said senior study author Michael Johansson, a biologist at the US Centers for Disease Control and Prevention Division of Vector-Borne Diseases in San Juan, Puerto Rico.

Insects that transport disease, such as mosquitoes, have been introduced via airlines to geographic areas where they didn’t previous live, the authors write in PLoS Neglected Tropical Diseases. Airports have insect-elimination, or “disinsection,” policies mandated by the UN’s International Health Regulations. They typically use insecticides to try to kill insects in bags, cargo, containers and conveyor belts, but infections still occur.

“We’ve had disinsection policies for a long time, which are targeted at mosquito species and agricultural pests,” Johansson told Reuters Health in a phone interview. “But we need to focus on ways to prevent the spread through humans.”

The study team assessed scenarios for the spread of mosquito-borne diseases, focusing on areas where either malaria or dengue are “endemic,” or widespread, and places where the diseases are not endemic, but conditions are favourable for them to establish themselves.

Based on past studies, the researchers calculated how many mosquitoes would likely be on any given plane, and how many of them would be infected with disease. For instance, the largest number of mosquitoes ever detected on a plane in past research is 17 Anopheles gambiae mosquitoes, the primary transmitters of malaria, the researchers note. For the current study, they estimated a 0.25 probability of any mosquito on a given flight, or an average of one mosquito on every fourth plane.

They also calculated the infection rates for human travellers, the likelihood of mosquito bites at the destination and probability of disease transmission to a new person.

Even in the absence of mosquito control or disinsection on planes, humans were hundreds of times more likely to spread the diseases through travel. Overall, the probability that an airplane travelling from a mosquito-heavy area would lead to infection was extremely low.

“We expected to find this, but we were surprised by the magnitude,” Johansson said. “We didn’t think it would be so obvious, and this was focusing on the worst-case scenarios.”

Future studies should focus on preventing human transmission, he added. Policies around preventing the transportation of infected mosquitoes won’t likely work, the study authors conclude.

“There may be a misperception about how pathogens disperse globally,” said Moritz Kraemer of Harvard Medical School in Boston, who wasn’t involved in the study.

“With the increase in travel globally, infectious diseases spread quickly from one location to another,” he told Reuters Health by email.

Kraemer, who recently published a study in the journal Nature about the multiple introductions of Zika virus to Florida, suggests creating maps of the geographic areas where people are infected and their airports to better target disease. Pathways and directionality is also important, he added.

“One aspect that is still difficult to assess is which travelers are at highest risk of infection,” he said. “The most exciting thing about this study is that it shows the differences between dengue and malaria and the risk of mosquito versus human-driven importation.”