The effective dispersal distance of the dengue virus-carrying mosquito Aedes aegypti, the primary vector in southern Taiwan, was found to be about 150m, supporting the 200m spraying radius around confirmed cases set by authorities, a National Health Research Institutes (NHRI) study found.
“When an outbreak occurs, we need to eliminate virus-carrying mosquitoes,” National Mosquito-Borne Diseases Control Research Center assistant investigator Huang Chin-gi (黃旌集) said on Thursday. “How far they can fly is crucial, as it determines the scope of our control measures.”
More than 98 percent of local dengue fever cases occur in southern Taiwan, NHRI data showed, while the Centers for Disease Control (CDC) recorded 12 local cases this year as of Monday, all in Kaohsiung.
Photo: CNA
On why Kaohsiung still saw a cluster of 12 cases if the mosquitoes’ effective dispersal distance is about 150m and spraying is done over 200m, Huang said outbreaks could persist due to factors such as insecticide selection and operator technique.
Inappropriate insecticide selection and subpar field operations could allow transmission to continue, even when the control radius is sufficient, he said.
“The control radius is only one part of a successful response,” Huang said, adding that other factors, such as the timeliness of the intervention, also determine whether an outbreak is contained.
Researchers collected Aedes aegypti eggs from ovitraps — egg-laying traps for mosquitos — set across Tainan, Kaohsiung and Pingtung County, reared them to adults and analyzed their genomes to find how far closely related mosquitoes dispersed.
“For example, we can identify full-sibling relationships — offspring from the same mother and father — or half-sibling relationships, or those that share only one parent,” Huang said. “That lets us infer how far a mother might have flown to lay those eggs, or how far a father might have flown to mate with another female.”
Analyzing genomes from 192 Aedes aegypti reared from ovitrap-collected eggs, Huang’s team found that the median dispersal distance among closely related mosquitoes was 154m, with most values between 126m and 180m.
While counties and cities in Taiwan set different outdoor chemical-spraying ranges around a confirmed patient’s residence, most use a 200m minimum radius, which is sufficient to contain Aedes aegypti’s 154m spread, Huang said.
One “special” finding is that roads do not act as barriers — genomic data showed that mosquitoes cross streets freely — and administrative district boundaries within a city or county were linked to genetic differentiation among mosquito populations, he said.
“We found the separation [between administrative districts] is largely human driven,” he said, adding that control measures such as source reduction and spraying create “pressure” on mosquito populations, which pushes them apart.
Since districts carry out control measures independently and at different times, mosquitoes that cross into a neighboring district are often killed before they could reproduce, reducing gene flow across the boundary and gradually creating genetically distinct populations even over short distances, Huang added.
This genetic differentiation could lead to differing insecticide resistance among Aedes aegypti in neighboring districts, he added.
If an intercepted virus-carrying mosquito could be genomically traced to its home district, responders could look at that district’s effective chemicals and deploy those agents immediately, Huang added.
On whether districts could synchronize control measures, Huang said that administrative districts are large and it was impossible to complete all spraying and source reduction in a single day.
Huang’s team has since 2020 collected Aedes aegypti eggs and conducted the study over at least three years.
An academic article from the research, titled “The Impact of Administrative Districts and Urban Landscape on the Dispersal of Aedes aegypti via Genetic Differentiation,” was published as a cover article in the international journal Molecular Ecology in February.
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