Insecticidal bed nets and filariasis transmission in Papua New Guinea

Abstract

Background: Global efforts to eliminate lymphatic filariasis are based on the annual mass administration of antifilarial drugs to reduce the microfilaria reservoir available to the mosquito vector. Insecticide-treated bed nets are being widely used in areas in which filariasis and malaria are coendemic.

Methods: We studied five villages in which five annual mass administrations of antifilarial drugs, which were completed in 1998, reduced the transmission of Wuchereria bancrofti, one of the nematodes that cause lymphatic filariasis. A total of 21,899 anopheles mosquitoes were collected for 26 months before and 11 to 36 months after bed nets treated with long-lasting insecticide were distributed in 2009. We evaluated the status of filarial infection and the presence of W. bancrofti DNA in anopheline mosquitoes before and after the introduction of insecticide-treated bed nets. We then used a model of population dynamics to estimate the probabilities of transmission cessation.

Results: Village-specific rates of bites from anopheline mosquitoes ranged from 6.4 to 61.3 bites per person per day before the bed-net distribution and from 1.1 to 9.4 bites for 11 months after distribution (P<0.001). During the same period, the rate of detection of W. bancrofti in anopheline mosquitoes decreased from 1.8% to 0.4% (P=0.005), and the rate of detection of filarial DNA decreased from 19.4% to 14.9% (P=0.13). The annual transmission potential was 5 to 325 infective larvae inoculated per person per year before the bed-net distribution and 0 after the distribution. Among all five villages with a prevalence of microfilariae of 2 to 38%, the probability of transmission cessation increased from less than 1.0% before the bed-net distribution to a range of 4.9 to 95% in the 11 months after distribution.

Conclusions: Vector control with insecticide-treated bed nets is a valuable tool for W. bancrofti elimination in areas in which anopheline mosquitoes transmit the parasite. (Funded by the U.S. Public Health Service and the National Institutes of Health.).

Figure 1. Location of Study Villages in East Sepik Province, Papua New Guinea

Figure 2. Proportion of Anopheles punctulatus Mosquitoes Carrying Nematodes Causing Lymphatic Filariasis, before and after the Distribution of Insecticide-Treated Bed Nets in Five Villages in Papua New Guinea

Shown are the percentage of mosquitoes that were found to be infected (solid colors) and the percentage that were found to be infective (hatched areas) on dissection before bed-net distribution (8181 mosquitoes) and after bed-net distribution (678 mosquitoes). The T bars indicate 95% confidence intervals.

Figure 3. Annual Transmission Potential for Lymphatic Filariasis in the Five Study Villages during the 26 Months before and 11 Months after Bed-Net Distribution

Shown are the estimated numbers of infective larvae that were inoculated per person per year during the periods of July 2007 through August 2008, September 2008 through August 2009, and September 2009 through July 2010. Insecticide-treated bed nets were distributed in the villages in August 2009.

Figure 4. Probabilities of the Cessation of Transmission of Lymphatic Filariasis before and after Bed-Net Distribution on the Basis of Village-Specific Goodness of Fit with the Anopheline Transmission Model

Panel A shows the probability of the cessation of transmission of lymphatic filariasis in the village of Peneng, according to mosquito-biting rates, which are expressed as the natural logarithm (log_e) on the x axis. The horizontal dashed lines show the biting thresholds associated with cessation probabilities of 50%, 75% and 95%. The bars at the bottom of the panel indicate the frequency distribution of the modelestimated biting thresholds for Peneng. Panels B through F show changes in the mosquito-biting rate and probabilities of cessation in the five study villages before the distribution of insecticide-treated bed nets and 1 or more years after bed-net distribution. (Mosquito-biting rates were available for Yauatong and Nanaha for years 2 and 3 after the distribution.) Shown are estimates (open circles) of the most likely 500 biting thresholds calculated by goodness of fit of the model with 2008 data regarding the prevalence of microfilariae, stratified according to the age of residents in the five villages in the study. The shaded bands between dashed lines denote the range and biting threshold values associated with 5%, 50%, and 95% probabilities of transmission cessation. The diamonds indicate measured biting rates, and the I bars 95% confidence intervals.