Samoa’s triple-drug filariasis campaign still left transmission in place
Triple-drug mass drug administration has not yet broken the lymphatic filariasis cycle in Samoa. After two rounds of therapy spaced five years apart, human antigen prevalence stayed essentially flat across the study period, while mosquito-based molecular testing moved earlier and more clearly than the conventional blood-based marker.
The contrast is biologically plausible. Antigen can remain detectable long after treatment has reduced active transmission, while microfilariae and infected mosquitoes are more immediate reflections of whether the parasite is still circulating through households and vector populations. In an endemic setting like Samoa, where mosquito exposure can rapidly reseed transmission, a slow-moving serologic or antigen signal can miss an early post-MDA shift that mosquito surveillance catches first.
Human antigen prevalence stayed stubbornly stable
Survey teams sampled eight primary sampling units in 2019, 2023, and 2024. Participants aged 5 years and older from randomly selected households were tested for antigen and microfilariae, creating a three-point view of the same endemic communities before and after the second triple-drug campaign.
In 2024, antigen prevalence was 10.3% (95% CI: 7.3-14.5), compared with 9.9% (95% CI: 3.5-21.0) in 2023 and 9.8% (95% CI: 5.6-15.5) in 2019. The odds ratio for a positive antigen test in 2024 versus 2023 was 1.0 (95% CI: 0.7-1.6), showing no meaningful decline. In practical terms, the community-level antigen signal did not improve after the more recent treatment round.
Microfilariae told a somewhat different story, though the confidence limits leave room for caution. Mf prevalence was 3.1% (95% CI: 1.8-5.5) in 2024, compared with 5.1% (95% CI: 1.3-12.4) in 2023 and 3.1% (95% CI: 1.3-5.9) in 2019. The odds ratio for the proportion of antigen-positive participants who were also Mf-positive in 2024 versus 2023 was 0.6 (95% CI: 0.2-1.4), a directionally favorable result that did not reach statistical certainty.
That pattern fits the biology of post-treatment surveillance. Antigen may persist even when circulating microfilariae start to fall, because the adult worm burden and its antigenic footprint do not disappear at the same pace as blood-stage transmission potential.
Mosquito testing registered a clearer post-treatment signal
The strongest shift emerged from molecular xenomonitoring. Mosquitoes were collected with BG-Sentinel traps at households, sorted into pools by species, and tested by qPCR for filarial DNA. In 2024, the prevalence of qPCR-positive mosquitoes dropped relative to 2023, with an odds ratio of 0.4 (95% CrI: 0.2-0.7).
This is the kind of signal public health programs need in the months after MDA. Mosquitoes integrate what is happening in the human reservoir and the local vector ecology, so when filarial DNA falls in the insect population, it implies fewer successful transmission events are occurring even if human antigen prevalence still looks unchanged. In a setting where the goal is elimination, a lagging human biomarker can obscure progress that is already taking shape in the vector.
From 2019 to 2024, qPCR-positive Aedes species also declined, with an odds ratio of 0.5 (95% CrI: 0.3-1.0), and Aedes polynesiensis showed a reduction with an odds ratio of 0.4 (95% CrI: 0.2-0.8). Because Aedes mosquitoes are central to local transmission dynamics, a drop in infection positivity in these vectors suggests the intervention pressure did reach the parasite’s transmission pathway, even if not deeply enough to extinguish it.
What the 2019 to 2024 pattern says about elimination pressure
The three survey points bracket a long interval between treatment rounds: 2018 MDA, then community assessment in 2019, another gap through 2023, and a post-2023 survey in 2024. That schedule makes the 2024 findings particularly instructive. They show that the second triple-drug round produced measurable change in mosquitoes within 10 months, yet the same window was too short to shift antigen prevalence in humans.
This matters because program decisions often depend on the biomarker being used. If antigen is treated as the only meaningful yardstick, early transmission suppression can be missed. If mosquito testing is added, program managers get a more time-sensitive read on whether local transmission is tightening or still active.
The study also shows why elimination in a mosquito-borne disease is rarely a single-intervention event. Even highly effective drug combinations depend on coverage, timing, and the degree to which remaining adult worms continue to seed microfilariae into the blood. Any surviving reservoir can sustain transmission once vector contact resumes.
Why mosquito-based surveillance may be the sharper early marker
Molecular xenomonitoring does not replace human testing, but it solves a different surveillance problem. Blood antigen surveys describe cumulative exposure and residual infection burden, while mosquito qPCR measures the present-day success of the transmission cycle. Those are related, but not interchangeable, states.
In Samoa, that distinction appears operationally important. The mosquito signal improved after the second triple-drug MDA, but the human antigen signal did not. Microfilariae moved in the expected direction, yet the change was modest and imprecise. The combined picture suggests partial suppression rather than elimination.
That is the central bedside and public health lesson here: parasites can stop moving efficiently through vectors before a community-level antigen marker shows a visible fall. For a program trying to decide whether to continue, intensify, or adapt MDA strategy, the mosquito readout provides earlier evidence of whether the intervention is pushing transmission in the right direction.
Persistent transmission still defines the endpoint
Despite two rounds of triple-drug MDA five years apart, LF transmission continues in Samoa. The study does not show eradication, and it does not support relaxing surveillance based on antigen trends alone.
The strongest limitation is also the most important one for interpretation: human antigen prevalence remained stable across time points, while the mosquito assay detected a decline after the 2023 intervention. Those results point in the same broad direction but operate on different time scales, which is exactly why the authors argue for combining indicators. Future surveillance will need to keep human and mosquito measurements paired if elimination status is to be assessed with any confidence.