Mosquito Survey in Myanmar Reveals Hidden Filarial Worms Slipping Past Bed Nets

In the remote villages of Karen state, eastern Myanmar, a quiet hunt for malaria has turned up something unexpected. Researchers conducting routine blood surveys as part of a malaria elimination program stumbled upon focal transmission of lymphatic filariasis, a disfiguring parasitic disease caused by thread-like nematode worms. That accidental discovery prompted a dedicated entomological investigation, now published in the journal Parasites & Vectors, which set out to answer a deceptively simple question: which mosquito species in these villages are carrying filarial worms, when do they bite, and how intense is the transmission they sustain?

The study, led by Patcharamai Rongthong and colleagues at the Shoklo Malaria Research Unit, part of the Mahidol-Oxford Tropical Medicine Research Unit, took the form of a cross-sectional survey conducted in September 2019 across six villages. Rather than relying on the traditional method of dissecting mosquitoes under a microscope to look for worm larvae, the team turned to molecular tools. A polymerase chain reaction assay targeting the parasites’ 5S ribosomal RNA gene allowed them to detect filarial DNA even in specimens where infection would be easy to miss morphologically. For positive Aedes specimens, the researchers went a step further, sequencing both the filarial PCR products and the mosquito cytochrome c oxidase subunit 1 gene to confirm exactly which worm species and which vector species they were dealing with.

Collecting the mosquitoes was itself a logistical feat. The team captured insects over full 24-hour diel cycles using two complementary methods: the human-landing catch, in which trained collectors allow mosquitoes to land on their exposed limbs before being trapped, and cow-baited traps, which exploit the attraction of many mosquito species to livestock. Specimens were then identified to species level using dichotomous morphological keys. This combination of approaches proved crucial, because it revealed not just which mosquitoes were infected but where and when those infected insects were actively seeking blood meals.

The headline finding was striking: 5.2 percent of the 772 mosquito specimens tested, 40 individuals in total, produced a positive filariasis PCR result. Half of those positive specimens carried DNA of Wuchereria bancrofti, the principal cause of lymphatic filariasis worldwide and the worm responsible for the vast majority of elephantiasis cases. But the survey did not stop there. Three additional zoonotic filarial species, belonging to the genera Brugia and Dirofilaria, were also identified, underscoring that these villages host a richer community of mosquito-borne nematodes than anyone had suspected. Zoonotic filariae normally circulate between animals and mosquitoes, but their presence in human-landing catches signals that people are being exposed to them as well.

The list of mosquito species carrying W. bancrofti DNA reads like a roll call of known and suspected vectors. The researchers detected the parasite in Anopheles barbirostris and Anopheles hyrcanus, both established vectors in Southeast Asia, as well as in Downsiomyia harinasutai, Finlaya poicilia and Culex sitiens. More provocatively, W. bancrofti DNA also turned up in Aedes albopictus/pseudoalbopictus, the aggressive day-biting mosquito better known globally as a dengue and chikungunya vector, and in Armigeres flavus. Finding the human filarial parasite in such a diverse array of genera, spanning Anopheles, Aedes, Culex and Armigeres, suggests that transmission in this setting is not the work of a single specialist vector but of a broad ecological network of opportunistic biters.

To quantify the risk, the team calculated the entomological inoculation rate for W. bancrofti, a standard measure of transmission intensity expressed as the number of infective bites a person receives per unit time. Their estimate came to 0.094 infective bites per person per day, with a 95 percent confidence interval of 0.049 to 0.164. While that figure may sound modest, it implies that over the course of a year a villager could plausibly receive dozens of infective bites, more than enough to sustain transmission of a parasite that requires only a handful of successful inoculations to establish infection in a susceptible host.

Perhaps the most consequential finding concerns biting behavior. Only 7.5 percent of the infected specimens, three out of forty, were captured indoors with the human-landing catch method during the classic overnight window between 9 PM and 5 AM. In other words, the overwhelming majority of infected mosquitoes were biting outdoors, or at hours outside the window that insecticide-treated bed nets are designed to protect. Bed nets have been one of the great success stories of malaria control, but this study suggests they may offer only limited protection against the filariasis vectors of Karen state, many of which appear to bite in the early evening, at dawn, or during daylight hours in the peridomestic environment.

The cow-baited traps added another layer of insight. Infected specimens were collected from these livestock-oriented traps, which points toward an intriguing intervention strategy: vector control measures that target mosquitoes at the animal interface. Because many of the vector species in this region feed opportunistically on both cattle and humans, treating livestock with insecticides or deploying traps around animal shelters could knock down mosquito populations before they ever reach their human hosts. Such veterinary-based approaches have been deployed successfully against other vector-borne diseases, and the authors argue they deserve consideration for filariasis in this setting, potentially alongside or even in place of methods that depend on indoor protection.

The detection of zoonotic filariae in specimens captured by human-landing catch deserves particular attention. Species of Brugia and Dirofilaria that normally parasitize animals were clearly biting and attempting to infect people in these villages. While zoonotic filarial infections in humans are often abortive or clinically milder than W. bancrofti infection, they can still cause disease, and their presence complicates the epidemiological picture. Any elimination campaign that focuses solely on the human parasite risks overlooking a reservoir of related worms circulating in the environment, and the molecular methods used here provide a template for surveillance programs elsewhere in Southeast Asia that may face similar hidden burdens.

The broader implications reach well beyond six villages in eastern Myanmar. Lymphatic filariasis is one of the World Health Organization’s targeted neglected tropical diseases, and global elimination efforts rest heavily on mass drug administration coupled with vector control. This study demonstrates that in ecologically complex settings, where multiple vector species with diverse biting habits coexist, the standard toolkit may need to be expanded. Outdoor and early-evening transmission, vectors attracted to livestock, and a background of zoonotic parasites all argue for interventions tailored to local entomology. The authors conclude that trials of vector control measures designed to prevent outdoor and early transmission are warranted, a call that will resonate with elimination programs across the region. For the communities of Karen state, the message is clear: the worms are there, the mosquitoes carrying them bite when and where nets cannot follow, and the next phase of the fight against filariasis will have to move outdoors.

Subject of Research: Natural filarial infections in mosquitoes and their transmission dynamics in Karen state, Myanmar

Article Title: Assessment of natural filariae infections in the mosquitoes of Karen state, Myanmar: a cross-sectional PCR survey in six villages

Article References: Rongthong, P., Kularbkeeree, T., Gloria, N., Jaruwan, N., Lee, N. Y., Trackoolchengkaew, M., Chaumeau, V., & Nosten, F. (2026). Assessment of natural filariae infections in the mosquitoes of Karen state, Myanmar: a cross-sectional PCR survey in six villages. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07649-x

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07649-x

Keywords: lymphatic filariasis, Wuchereria bancrofti, mosquito vectors, Myanmar, Karen state, PCR, Brugia, Dirofilaria, Aedes albopictus, vector control, entomological inoculation rate, bed nets