The strategy of distributing mosquito nets impregnated with insecticides has been critical in the reduction of malaria worldwide and is one of the main interventions toward the goal of elimination of the disease . The present study followed the implementation of this strategy in an area of difficult geographical access, wherein mosquito nets were distributed to 100% of the residents in an intervention area. The evaluation of the use and retention of the nets were undertaken 1 and 5 years after the intervention, which was in contrast to most studies of this type wherein the evaluations usually occur only within the first 12 months after the distribution of the nets [7, 11, 13]. At 12 months after implementation of the strategy, in the intervention area, the coverage of 85.2% of the households and sufficient LLINs for all family members were observed. This coverage was slightly lower than that found by Alvarado et al.  in the Venezuelan Amazon, wherein the proportion of households with at least one mosquito net was 93.7%. A study conducted in Nigeria has shown a good coverage of households with at least one LLIN (74.5%), but the number of households owning sufficient LLINs for all family members was substantially low (27.2%) . After 5 years of intervention, the coverage of households owning at least one LLIN in the intervention group was 80%. Despite the fact that universal coverage (100%) was expected, it is easy to understand that there was an increase in the number of inhabitants in these localities, either by the birth of children or by the mobility of individuals who migrated to work of extract fibers from the plant Leopoldinia piassaba. This evolution was observed during the interviews conducted in 2014. However, in the intervention area, the coverage of mosquito nets, as well as owning sufficient LLINs for all members of the family, was greater than that in the control area, showing a positive result of the strategy of using mosquito nets. According to the WHO, in sub-Saharan Africa, the ownership of at least one LLIN in the family increased from 50% in 2010 to 80% in 2016. However, only 43% owned sufficient bed nets for all family members by that year . Similar findings were observed in southern Ethiopia  and in the Democratic Republic of Congo , wherein although a coverage of 80%–90% of households owning at least one mosquito net was found, sufficient LLINs for all family members were not available. In a study performed in a region of eastern Ethiopia, a coverage of just over half the households (57.9%) was found, although 68% of the inhabitants in this country live in malaria risk areas .
Use of the LLINs
The proportion of individuals who used mosquito net the previous night increased 1 year after the distribution of the LLINs in the intervention area; however, this was not maintained over time and decreased after 5 years. In all three evaluations, the proportion of individuals using the LLINs was higher in the control group than in the intervention group. This result could be explained because a majority of the inhabitants of the control group resided in the urban area wherein there is a greater variety and quantity of mosquitoes and more access to the LLINs. Furthermore, the use of the LLINs the urban area was at least four times greater than that the rural areas, in both the baseline pilot study and in 2014. In rural areas, individuals tend to use mosquito nets only when there are significant mosquitoes and malaria cases.
Moreover, it was observed that in the intervention group, after 5 years of strategy implementation, only 29.3% of the nets were hanging in the house. In this area, malaria is a seasonal disease with the highest number of cases occurring at the end of the rainy season when there is a greater presence of anophelines. Anopheles darlingi has a endophilic behaviour and is the main vector of malaria in the Amazon region. The anophelines exhibit a peak of activity in the evening and morning twilight and continued their activity throughout the night in this region . In Costa Marques, State of Rondonia, Brazil, a decreased number of anophelines collected intra domiciliary was observed . This result could reflect the repellent action of the LLINs. Although one of the main reasons associated with the residents preferring to sleep with mosquito nets in both the intervention and control groups is prevention against being bitten by anophelines, unfortunately, this is not reflected via a continued use of the LLINs. Further efforts are required to increase the perception of protection that mosquito nets can provide to individuals in endemic areas to create a “mosquito net culture,” despite variations in mosquito densities throughout the year, as Koenker et al.  have shown.
Another interesting finding was observed among individuals in the intervention group who slept with mosquito nets the previous night. In 2008, 71.4% shared the mosquito net with another family member; however, in 2014 this proportion had decreased to only 46.7% (p = 0.0009). It is possible that individuals are less frequently sharing mosquito nets because they currently have more access to them, with control programmes continuing to distribute the LLINs in this area. The greatest improvements in the use of mosquito nets have been observed following massive community distributions . Despite the decrease in the proportion of sharing, 57.1% of the individuals shared with two others; i.e., three people slept under the same LLIN, despite it being recommended that no more than two individuals should share the same net .
On comparing access to the nets with actual use, it was observed that although 85.1% of the individuals in the intervention group had access to an LLIN in 2014, only a remarkable 14.9% used one the previous night. Moreover, a similar relationship occurred among the individuals in control group, wherein although 63.8% had access, only 30.8% used them the previous night. These data demonstrate that despite the access to the LLINs in the intervention area was higher than in the control area, the gap between access and use was also higher in this group, showing that the lack of a mosquito net was not the reason for the net not being used the previous night. The reason appears to be cultural or psychologically oriented. In both cases, the estimate of use was less than the ownership estimate, suggesting a significant difference between owning and using . Some studies that specifically evaluated the use of mosquito nets have found that between 15 and 50% of distributed LLINs remain unused [26,27,28]. Therefore, ownership is not the sole obstacle to achieving reductions in malaria morbidity and mortality associated with the use of the LLINs. Individuals, who own mosquito nets [or to whom the nets are available], should use them to have an impact on malaria reduction . The results differ from those of Kilian et al. , wherein the proportion of the population that used a LLIN the previous night was 41.3%—only slightly lower than the access rate (50%)—indicating a high general level of use among those who have access. However, these studies do not include data on vector behaviour in the targeted areas, thereby inhibiting an analysis of this aspect of the issue.
In 2016 in sub-Saharan Africa, 54% of the at-risk population slept under an LLIN, which is a substantial increase from 30% in 2010 . It has already been demonstrated that LLINs are important for protecting all individuals in a community, including those who do not sleep under a mosquito net . Such a community effect from the LLINs is attributable to the fact that insecticides incorporated in their mesh kill the vectors, reducing their overall density in the community. In addition, it is known that the LLINs can prevent only up to 54% of malaria cases in a given area because their action occurs predominantly when people are inside the mosquito nets while sleeping. This happens in a variable way according to the age and behaviour of the residents , but the coverage seems to be an important factor.
The main reason reported by locals for not using mosquito nets is the heat. This finding is similar to that of the study by Cohee et al.  conducted in Uganda and that conducted in Bukoba and Zanzibar , wherein participants said they felt crammed, uncomfortable, hot, and itchy when they slept under a mosquito net. In a study conducted in eastern Ethiopia , the main reason reported for not using a mosquito net (69.9%) was “because there is no mosquito in the area.” In a study by Egrot et al.  in southern Benin that interviewed 91 individuals, 56 mentioned that a possible cause for the non-use of the LLINs is that they can ignite and cause serious material damages and bodily injuries or even death. Of these individuals, 34 narrated specific events that they heard or experienced, where fire was always related to the internal use of a lantern or candle that accidentally came into contact with a mosquito net. The review by Pulford et al.  has shown that the main reason for not using mosquito nets were discomfort, chiefly due to the heat, and the low density of mosquitoes. These authors have expressed that if a motive to use a mosquito net is the density of the mosquitoes, it seems apparent that in areas where this density falls as a result of increased LLINs coverage, indoor spraying, or by other measures, the motivation to use the nets may decrease. It may be possible to achieve greater use of nets among this population via behaviour-modifying education strategies. Regarding the personal discomfort, modifications to the mosquito nets to make them more comfortable would likely complement any educational campaign in a useful way.
Retention of the LLINs
Five years after the distribution of the LLINs, retention was high in the intervention group (83.7%). Similar results were found by Cohee et al. . A few individuals in the control group owned the LLINs delivered during the campaign probably because they relocated from the intervention area to live in one of the control areas. All participants in that study said sleeping with mosquito nets prevented them from contracting malaria and from mosquitoes biting them while sleeping without net. More broadly, in the study by Cohee et al. , 80% of the participants agreed that the LLINs are used to prevent malaria, and in another investigation conducted in Ethiopia, this proportion was 97.6% . Despite the existence of knowledge regarding the importance of mosquito nets to prevent malaria, this perception is not producing a change of behaviour, at least in the long term.
Concerning the maintenance of the nets, 93.9% were washed 1–5 times in the intervention area. The LLINs, under field conditions, have a duration of 3 years, depending on the form and frequency of washing, because their biological effectiveness without a new treatment is retained for at least 20 washes under laboratory conditions . Therefore, the inhabitants of the study area are following the standards of care for the LLINs, without exceeding the recommended washing frequency. Most (60.9%) used soap powder or detergent compared with only 23.9% that used the recommended bar soap. It should be noted that the LLINs were delivered following an educational strategy that was apparently not sufficient to produce adequate mosquito net washing practices. A similar result was found by Tomass et al. , wherein 44.2% of the respondents expressed that they dried the LLINs in the sun. According to the Centers for Disease Control and Prevention (CDC) , the insecticide pyrethroid does not decompose rapidly unless exposed to sunlight. The CDC instructs that the nets should be washed with neutral soap and cold water and dried in the shade for better conservation of the insecticide . Clearly, it was observed that this population was not properly washing the mosquito nets and that this may be harming the effectiveness of the insecticide. Studies to evaluate the retention of insecticides under field conditions are required.
Regarding the physical condition of the nets in the intervention group, it was observed that 5 years after the LLINs distribution, although most of the nets were clean, more than half contained holes and some were torn. In the study by Cohee et al. , 4 out of 32 mosquito nets were found torn and 3 of these were still assembled. In Ethiopia, only 10.3% of the LLINs contained holes that could allow mosquitoes to enter . Studies similar to this one, comparing the state of the LLINs after a long period of use could not be found. However, it has already been observed that in the impregnated mosquito nets, the irritant effect of the insecticide causes repellency, thereby decreasing the survival or changing the behaviour of mosquitoes coming into contact with the insecticide. Therefore, the repellent effect of the impregnated mosquito net would exert its protective action, despite being damaged and having tears, thereby continuing to reduce the possibility of infective bites .
Regarding possible adverse effects with the use of the nets, 57.1% of the individuals in the intervention group reported experiencing some symptom when they started to use the nets. The main symptoms cited were “blazing” and “itching.” These may be due to the insecticide contained in the mesh of the mosquito net, which in some individuals caused an allergic reaction or irritation. The results of this study differ from those found by Alvarado et al. , wherein a substantially low percentage (0.4%) of users reported mild discomfort that spontaneously disappeared after the initial days of use. In this study, after 5 years, only 2.4% of the individuals reported experiencing some symptoms. These individuals no longer experience these symptoms, probably because the mosquito nets have already lost part of the insecticide.
Finally, the results show that despite individuals having access to the LLINs, new strategies are necessary to increase long-term use. In numerous localities, there is a need for permanent education measures to ensure that individuals do not lose the practice of using the mosquito nets. Moreover, the need to develop communication strategies for behaviour change was observed in the studies with African populations [10, 32]. Assessing integrated disease control strategies in endemic areas is difficult. Some studies have used mathematical models to measure how far the effect of an intervention is due to the use of a new technology or product of the synergistic action of various strategies . In the present study, there was no attempt to measure the impact of the use of nets for the reduction of malaria, but only the behaviour of individuals regarding the use and retention of the LLINs.