The present study evaluated the durability of Yahe ITNs distributed in late 2021 in Papua New Guinea (PNG). This is the first longitudinal ITN durability study conducted in PNG. Over 1,401,000 Yahe ITNs were distributed in the country to protect > 2.8 million people from malaria for 3 years (2021–2023).
With > 60% of study nets still present at 24 months after distribution, and a median half-life of around 2.5 years, the data indicate that ITN retention in the 4 survey communities was higher than observed in previous studies. A range of such previous studies were summarised in a recent review, which found that > 75% previous durability monitoring studies reported median retention times of < 2.4 years [17]. The present study shows that around 34% of ITNs had either never been utilised by 6 months (14%) or were already lost to follow-up (20%) after 6 months. However, of the nets lost to follow-up > 60% were indicated to have been given away to other users, or used as intended elsewhere, which indicates that these ITNs were likely not lost to the overall control effort. Only a very small proportion of nets were reported to have been destroyed or lost but there was a reporter bias occluding the reason for around 16% of ITNs lost to follow up. These ITNs were reported as ‘never received’, yet receipt had been confirmed by the study teams.
The present study indicates that ITNs were washed relatively infrequently in the study sites in PNG, as compared to other studies [18–20], with over 90% of ITNs never having been washed in the first 6 months and a median number of washes of 1 (IQR 1–3) at 24 months after distribution. Most people used laundry powder or laundry soap bars to wash the ITNs but only a small percentage (8%) resorted to adding bleach. The majority of people (> 65%) indicated drying of ITNs in the sun, and it appears that including more information around the need to minimize exposure of ITNs to sunlight into behavioural change communication may be beneficial [21].
With only < 10% of the ITNs experiencing any washes within the first 6 months after distribution, it is all the more surprising that the insecticidal efficacy, as measured in cone bioassays with fully susceptible An. farauti colony mosquitoes decreased rapidly to less than 50% within this period (Fig. 3). This indicates that the Yahe ITNs received were inherently unable to retain their insecticidal efficacy under the conditions prevailing in these PNG sites, which are alike to those in many tropical settings where ITNs are used, with average temperatures ranging from 23.3°C to 29.3°C and humidity > 85%. The strong correlation between bromine as a proxy for deltamethrin, and 24 h mortality in the net samples collected during follow-up (Fig. 4) indicates that at least some of the observed rapid decay of insecticidal efficacy is due to the loss of insecticide. Given most of the nets had been washed less than 3 times at the end of the 24 months period, the mechanism of this insecticide loss remains elusive. It would be important to better understand this mechanism as it appears that simply washing ITNs is not enough to simulate this form of ITN decay or aging.
Yahe ITNs accumulated physical damage such that 35% of nets were too torn to protect from mosquito bites at 24 months. While this is context-specific and not easily generalisable, this proportion seems to be within the range of observations made in similar studies on pyrethroid-only nets, [22–26]. The number of net users (i.e., the number of people regularly using the same net) was significantly associated with physical degradation. This correlation emphasises the need to maximise ITN coverage and reconsider current assumptions regarding target net per person rates in distribution campaigns [16].
Simulated washing of Yahe ITNs lead to unacceptably rapid decay in insecticidal efficacy of the ITNs in the present study, with a median 24h mortality of < 20% after 10 washes. These findings were complemented by XRF measurements indicating a rapid decrease in the insecticide concentration over the first few washes, which could be described well by a biphasic exponential decay function. The XRF measurements indicated that the ITNs had a wash resistance index of around 86% (the wash resistance index being the average loss of insecticide over the first 4 washes [27]).
The current specifications for Yahe LN require a wash resistance index of only 85% [28], which, in general, seems unacceptably low as it means that after 20 washes only 4.6% of the original active ingredient concentration is expected to remain (i.e., around 2.6 mg/m2), which is far below the minimum effective concentration of 10 mg/m2 estimated in previous studies for deltamethrin coated nets [29].
The present study had important limitations. It is conceivable that the need to label the study nets due to the absence of net-specific barcodes on the label of Yahe ITNs may have led to a bias in the attrition rate as study participants may have been more aware of these nets being part of a study. However, there is no evidence from previous durability studies that would suggest that such a bias exists. The present study purposefully did not remove the nets from their original plastic packaging at the time of distribution as this is not done during the programmatic distributions in PNG. This may also have contributed to the relatively long observed median half-life. However, it also enabled a more realistic observation of the gradual uptake of the newly distributed campaign nets in the communities. This may be useful for mathematical modelling studies endeavouring to model the impact of ITN distributions, and which frequently assume that all ITNs are accessible immediately to all potential users at the time of distribution, which may be an unrealistic assumption [30, 31].
While the XRF technique employed in the present study is not equivalent to the methodology developed by the Collaborative International Pesticides Analytical Council (CIPAC) to determine the wash resistance index, it can be considered a very good approximation. In addition, this study did not conduct further chemical analyses such as HPLC to determine changes in the isomeric state of the deltamethrin.