1. Following bed net distribution, mosquitoes infected with any stage of W. bancrofti larvae decreased from 1.8% to 0.4% (p=0.005).
2. Based on the authors’ statistical model, probability of transmission cessation was less than 1% in all villages prior to bed net distribution and increased to probabilities ranging from 4.9% to 95.8% following bed net intervention.
Evidence Rating Level: 2 (Good)
Study Rundown: This prevalence study investigated the effect of insecticide-impregnated bed nets on measures of filariasis transmission in Papua New Guinea. Lymphatic filariasis, caused by W. bancrofti nematodes and affecting approximately 120 million people in Africa, Asia, the Pacific, and the Americas, has been targeted for global elimination by 2020 through the usage of mass administrations of antifilarial drugs. Recent attention has focused on the possibility of using vector control, potentially in conjunction with ongoing malaria control efforts, to augment filariasis elimination efforts.
In this study, data from before and after a 2009 nationwide mosquito bed-net effort in Papua New Guinea provide quantitative support for this notion. Following bed-net intervention in 2009, there were significantly decreased village-specific rates of mosquito bites as well as significantly decreased proportions of mosquitoes infected with the nematode causing filariasis. Due to a lack of detectable infective nematode in mosquitoes following bed net intervention, the probability of transmission potentials dropped to zero in the five villages following the 2009 intervention. These results represent compelling epidemiological data to support incorporation of vector control through insecticide bed nets into ongoing efforts to control filariasis with antifilarial medications. Drawbacks to the study include the lack of randomization and the challenge of detecting human microfilariae infection at progressively lower levels. Nevertheless, the results provide grounding for future studies with individual-level data and conducted in broader contexts than the villages in Papua New Guinea, as well as longer study lengths that encompass the 5-year reproductive life span of adult worms residing in human lymphatics.
In-Depth [prevalence study/entomologic survey]: This prevalence study and entomologic survey quantified the effect of deltamethrin-impregnated bed nets on measures of filariasis transmission in five villages in Papua New Guinea that had previously undergone mass antifilarial medication treatments between 1994 through 1998. A nationwide bed-net intervention in 2009 led to an increase in self-reported bed net usage from 3.8% to 12.4% to 75.0 to 90.6% of households. 20, 345 anopheline mosquitoes were collected in the 26 months before bed-net distribution, and 1,554 in the 11 months after distribution; 2 villages were selected for long-term assessment of mosquito biting rates up to 3 years following intervention. Following bed net distribution, mosquitoes infected with any stage of W. bancrofti larvae decreased from 1.8% to 0.4% (P=0.005); no mosquitoes with infective larvae were identified in any villages after bed-net distribution (P=0.07). Furthermore, the annual transmission potential for lymphatic filariasis to drop to zero in the post-bed net period.
Using a numerical-modeling and Bayesian analysis method based on the mosquito-biting rate and the prevalence of microfilariae measured in villagers prior to the bed-net intervention, the study authors calculated the probabilities of cessation of transmission of filariasis in the 5 villages. Daily biting rates of mosquitoes decreased significantly in all 5 villages (p<0.001 for all comparisons). According to the model, probability of transmission cessation was less than 1% in all villages prior to bed-net distribution and increased to probabilities ranging from 4.9% to 95.8% following the 2009 intervention. Probabilities of transmission cessation tended to be higher in those villages where levels of microfilariae detectable in human blood were lower from the outset, suggesting a role for mass medication interventions in tandem with vector control efforts.
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