Are insecticide-treated bednets more protective against Plasmodium falciparum than Plasmodium vivax- infected mosquitoes?
© Bockarie and Dagoro; licensee BioMed Central Ltd. 2006
Received: 24 October 2005
Accepted: 21 February 2006
Published: 21 February 2006
The outcomes of insecticide-treated bednet (ITN) interventions for malaria control in Papua New Guinea tend to suggest a differential protective effect against Plasmodium falciparum and Plasmodium vivax. Little is known about the impact of ITNs on the relative abundance of mosquitoes infected with either P. falciparum or P. vivax. This paper describes the biting cycle of P. falciparum and P. vivax-infected mosquitoes and the impact of an ITN intervention on the proportion of mosquitoes infected with either parasite species.
Entomological investigations were performed in East Sepik (ESP) and New Ireland Provinces (NIP) of PNG. Mosquitoes were collected using the all-night (18:00 - 06:00) landing catch and CDC light-trap methods and species specific malaria sporozoite rates were determined by ELISA.
Results and discussion
The distribution of sporozoite positive mosquitoes in three four-hour periods (18:00-22:00, 22:00-02:00 & 02:00-06:00) showed that a higher proportion of P. vivax-infected mosquitoes were biting before people retired to bed under the protection of bednets. In the intervention village, the 308 mosquitoes collected before ITNs were introduced included eight (2.0%) P. falciparum-positive and four (1.0%) P. vivax- positive specimens, giving a parasite ratio of 2:1. The sporozoite rate determined from 908 mosquitoes caught after ITNs were introduced showed a significant decrease for P. falciparum (0.7%) and a slight increase for P. vivax (1.3%), resulting in a post intervention parasite ratio of 1:2. In the East Sepik Province, where ITNs were not used, P. falciparum remained the dominant species in 12 monthly mosquito collections and monthly P. falciparum:P. vivax formula varied from 8:1 to 1.2:1.
These findings suggest that people sleeping under treated bednets may be more exposed to P. vivax than P. falciparum-infected mosquitoes before going to sleep under the protection of bednets. This difference in the biting behaviour of mosquitoes infected with different malaria parasites may partly explain the change in the P. falciparum:P. vivax formula after the introduction of ITNs.
All four human malaria species are found in Papua New Guinea (PNG), with a predominace of Plasmodium falciparum and Plasmodium vivax . The outcomes of insecticide-treated bednet (ITN) interventions for malaria control in PNG tend to suggest a differential protective effect against P. falciparum and P. vivax. Millen  working in the coastal areas of Madang Province showed a significant reduction in the prevalence of P. falciparum among children between one to four years of age, after treated bednets were introduced, but there was no effect on P. vivax prevalence. Graves et al  who introduced ITNs in different villages in the same province reported a significant reduction in the incidence of P. falciparum in children 0–4 years old, 4–10 weeks after the ITNs were introduced; however, they also found no effect on the incidence of P. vivax. In the Highlands Region, the P. falciparum:P. vivax ratio in all age-groups changed from 3.8:1, before ITNs were introduced, to 1:1 six months afterwards . Other phenomena such as P. vivax relapses may also lead to a change in the prevalence of P. vivax in humans independent of ITN interventions. Relapses are a feature of vivax malaria that may occur 8–10 weeks after a previous attack (short-term relapses) or about 30–40 weeks later (long term relapses). The form and frequency of relapses depend on the infecting strain and occur due to the activation of hypnozoites in liver cells .
Little is known about the impact of ITNs on the relative abundance of mosquitoes infected with either P. falciparum or P. vivax. One study on the biting pattern of Plasmodium-infected Anopheles punctulatus mosquitoes, the main vector in PNG, suggests a differential feeding behaviour of mosquitoes infected with P. vivax and P. falciparum, with P. vivax-infected mosquitoes having a tendency to bite earlier than P. falciparum-infected mosquitoes . If indeed P. vivax-positive mosquitoes have a greater tendency to bite earlier than P. falciparum-positive ones, people may be more exposed to P. vivax than P. falciparum before going to sleep under the protection of bednets. This could result in changes in the proportion of people infected with the two species following a bednet intervention. This paper describes the biting cycle of P. vivax- and P. falciparum-infected mosquitoes and the impact of an ITN intervention on the relative abundance of mosquitoes infected with either parasite species.
The impact of ITNs on the parasite ratio in mosquitoes was investigated on Lihir Island in New Ireland Province. The island, about 240 km2, is bounded by 70 km of coastline with savannah grassland in the north-east and swampy vegetation with thick mangrove shores in the south-west. The average annual rainfall is about 4,000 mm with no distinct dry and wet seasons. The inhabitants live in coastal villages, usually adjacent to a sandy beach or coral reef. Most of the people of Lihir Island do not normally go to bed before 22:00. In September 1993, the PNG government started the sale of permethrin-treated bednets in all 25 villages on the island, at a subsidized price of US$ 3.00 per net. By October 1993, over 50% of the island population reported that they were sleeping under treated bednets. Pre-treatment blood slides obtained from 471 randomly selected residents (all age-groups) of five indicator villages in September 1993 revealed prevalence rates of 31% for P. falciparum and 6.3% for P. vivax, hence a parasite formula (P. falciparum:P. vivax) of 5:1. Follow-up blood slide surveys were not carried out by the Health Department team, but entomological monitoring of the bednet intervention was performed by the Papua New Guinea Institute of Medical Research. Mosquito sampling was conducted on a fly-in fly-out basis as the entomology laboratories were located about 700 km from Lihir island. Sampling could, therefore, only be carried out during four months; one month before and three months after intervention. However, mosquitoes were processed individually to determine infection with malaria parasites.
Mosquitoes were collected using the all-night (18:00 - 06:00) landing catch method and CDC light-traps placed in bedrooms between 18:00 and 06:00. Human biting mosquitoes were caught by two male adults seated outside houses and sporozoite rates were determined for P. falciparum and P. vivax as described previously [6, 7]. Differences in sporozoite rates were evaluated using the Chi-square test. To investigate the extent of variation in monthly sporozoite rates in the An. punctulatus complex in the absence of bednets, sporozoite rates were analysed for P. vivax and P. falciparum for 12 consecutive months (January – December 1997), in the highly malarious village of Yauatong in the East Sepik Province. This village, where less than 3% of the population slept under bednets, has already been described by Bockarie et al .
Results and discussion
Plasmodium falciparum and P. vivax sporozoite antigen positivity rates and parasite ratios (P. falciparum: P. vivax) for monthly catches of human-biting An. punctulatus mosquitoes in East Sepik Province, Papua New Guinea.
Number infected with
P. falciparum (%)
P. vivax (%)
These findings suggest that people sleeping under treated bednets in the Lihir villages may be more exposed to P. vivax- than P. falciparum-infected mosquitoes before going to sleep under the protection of bednets. This difference in the biting behaviour of mosquitoes infected with different malaria parasites may partly explain the change in the P. falciparum:P. vivax formula after the introduction of bednets. A similar differential feeding behaviour of mosquitoes infected with the two species have been reported in the East Sepik Province of Papua New Guinea . Other studies in Papua New Guinea have shown that the use of untreated bednets to protect against mosquito bites can also lead to an increase in the proportion of P. vivax infections in mosquitoes  and humans [4, 8]. Recent studies in Madang Province showed that P. vivax is becoming more prevalent relative to P. falciparum with the increasing use of ITNs in the Province. On Bagabag island, where over 60% of the inhabitants slept under bednets, there was no significant difference (P = 0.129) in the prevalence of P. falciparum (19.5%) and P. vivax (17.1%) parasites . On the other hand, the prevalence of P. vivax (9%) was significantly lower (P < 0.0001) than P. falciparum (23%) in Liksul village where less than 15% of the population were sleeping under bednets .
This work was supported jointly by the Lihir Management Company, The New Ireland Province Department of Health and the Papua New Guinea Institute of Medical Research
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