Quantifying the role of importation on sustained malaria transmission in a low-to-moderate burden region of Southwest Uganda.

BACKGROUND

Parasite importation remains a critical challenge to malaria elimination efforts. The extent to which human travel contributes to sustained transmission in low-to-moderate burden areas of highly endemic African countries is poorly understood.

METHODS

We conducted a 14-month longitudinal cohort study in Kamwezi subcounty in Southwest Uganda, an area targeted for malaria elimination where sustained transmission poses challenges. A total of 1,918 individuals from 400 households were enrolled and followed bi-monthly. Travel histories, malaria episodes, and household characteristics were collected through structured surveys. Incident malaria was captured via passive case detection at the local health facility and self-report. Multilevel logistic regression models estimated the association between overnight travel and incident malaria, adjusting for demographic and household factors. Population attributable fractions (PAF) quantified the contribution of travel to malaria burden, stratified by transmission intensity, season, and village of residence.

RESULTS

Over the study period, 244 malaria episodes were recorded, with incidence highest in villages with greater baseline transmission. Nearly one-third of participants reported at least one overnight trip. We found that associations between travel and malaria varied spatially and temporally, with positive associations in villages of lower transmission (odds ratio=5.65, 95% CI: 2.06-15.45) and during periods of low overall malaria transmission. We found these associations to be strongest for short-distance trips to nearby areas of higher incidence. PAF analyses suggested that travel accounted directly for 17% of malaria cases in low-transmission villages overall, rising to 44.6% during periods of low transmission.

CONCLUSION

Overnight travel contributed meaningfully to malaria burden, especially in villages with low local burden and during periods of seasonally low transmission. These effects were evident even at small geographic and temporal scales. These findings underscore the importance of malaria surveillance and control strategies that account for both local transmission and travel-related importation, even outside of elimination contexts.