Mating patterns in malaria parasite populations of Papua New Guinea

Description of the genetic structure of malaria parasite populations is central to an understanding of the spread of multiple-locus drug and vaccine resistance. The Plasmodium falciparum mating patterns from madang, Papua New Guinea, where intense transmission of malaria occurs, are described here. A high degree of inbreeding occurs in the absence of detectable linkage disequilibrium. This contrasts with other studies, indicating that the genetic structure of malaria parasite populations is neither clonal nor panmictic but will vary according to the transmission characteristics of the region.

Emerging viral diseases of Southeast Asia and the Western Pacific

Over the past 6 years, a number of zoonotic and vectorborne viral diseases have emerged in Southeast Asia and the Western Pacific. Vectorborne disease agents discussed in this article include Japanese encephalitis, Barmah Forest, Ross River, and Chikungunya viruses. However, most emerging viruses have been zoonotic, with fruit bats, including flying fox species as the probable wildlife hosts, and these will be discussed as well. The first of these disease agents to emerge was Hendra virus, formerly called equine morbillivirus. This was followed by outbreaks caused by a rabies-related virus, Australian bat lyssavirus, and a virus associated with porcine stillbirths and malformations, Menangle virus. Nipah virus caused an outbreak of fatal pneumonia in pigs and encephalitis in humans in the Malay Peninsula. Most recently, Tioman virus has been isolated from flying foxes, but it has not yet been associated with animal or human disease. Of nonzoonotic viruses, the most important regionally have been enterovirus 71 and HIV.

Human malaria transmission studies in the Anopheles punctulatus complex in Papua New Guinea: sporozoite rates, inoculation rates, and sporozoite densities

Malaria sporozoite rates and inoculation rates were measured over periods up to 25 months in the different anopheline species biting humans in 13 villages in Madang Province, Papua New Guinea. Analysis of three members of the Anopheles punctulatus complex, 68,458 An. farauti, 36,779 An. koliensis, and 11,667 An. punctulatus caught in landing catches was made using monoclonal antibody based ELISAs to detect sporozoites of Plasmodium falciparum and P. vivax. Sporozoite rates ranged from 0%-5.5% in An. farauti, 0.2%-3.8% in An. koliensis, and 0%-3.3% in An. punctulatus. In addition, over 3,000 An. longirostris were analyzed and sporozoites were not detected in this species. No significant differences were observed between the three vector species in the densities of P. falciparum sporozoites (geometric mean 2,320). However, the geometric mean P. vivax sporozoite density was significantly higher in An. punctulatus (350) than in either An. koliensis (160) or An. farauti (150). An. koliensis was less susceptible to infections of P. falciparum or P. vivax than either An. farauti or An. punctulatus. Variations in average sporozoite and inoculation rates were found among different villages, despite their close geographic proximity. Sporozoite and inoculation rates varied greatly within a village over time, but malaria transmission was perennial with a higher transmission during the wet season by An. koliensis and An. punctulatus.

Isolation of Japanese encephalitis virus from mosquitoes (Diptera: Culicidae) collected in the Western Province of Papua New Guinea, 1997-1998

After Japanese encephalitis (JE) virus emerged in the Torres Strait in Australia in 1995, investigations were initiated into the origin of the incursion. New Guinea was considered the most likely source, given its proximity to islands of the Torres Strait. Almost 400,000 adult mosquitoes were processed for virus isolation from 26 locations in the Western Province of Papua New Guinea (PNG) between February 1996 and February 1998, yielding three isolates of JE virus. Two isolates of Murray Valley encephalitis, 17 isolates of Sindbis, and 1 each of Sepik and Ross River viruses were also obtained. Nucleic acid sequences of the PNG JE isolates were determined in the prM region, and in a region overlapping a part of the fifth nonstructural protein and the 3' untranslated region. The PNG isolates belonged to genotype II, and shared > 99.2% identity with isolates from humans and mosquitoes from the Torres Strait, suggesting that PNG is the source of incursions of JE virus into Australia.

Effects of untreated bed nets on the transmission of Plasmodium falciparum, P. vivax and Wuchereria bancrofti in Papua New Guinea

The impact of untreated bed nets on the transmission of human malaria and filariasis in a village in a hyperendemic area of Papua New Guinea was studied. In anopheline mosquitoes, the Plasmodium falciparum sporozoite antigen positivity rate, filarial infection rates and human blood indices dropped significantly after bed nets were introduced. This reduction in human-vector contact did not affect mosquito density as no significant difference in either landing rates or indoor resting catches was found. The number of bed nets in a house and ownership of dogs were factors significantly associated with a reduction in the number of indoor resting mosquitoes. However, the reduction in the P. falciparum sporozoite antigen rate in mosquitoes was not accompanied by a reduction in either malaria parasite or antibody prevalences or titres against the P. falciparum circumsporozoite protein.

Mosquito host-feeding patterns and implications for Japanese encephalitis virus transmission in northern Australia and Papua New Guinea

Japanese encephalitis (JE) virus spread to northern Australia during the 1990s, transmitted by Culex annulirostris Skuse and other mosquitoes (Diptera: Culicidae). To determine the relative importance of various hosts for potential vectors of JE virus, we investigated the host-feeding patterns of mosquitoes in northern Australia and Western Province of Papua New Guinea, with particular attention to pigs, Sus scrofa L. - the main amplifying host of JE virus in South-east Asia. Mosquitoes were collected by CDC light traps baited with dry ice and 1-octen-3-ol, run 16.00-08.00 hours, mostly set away from human habitations, if possible in places frequented by feral pigs. Bloodmeals of 2569 mosquitoes, representing 15 species, were identified by gel diffusion assay. All species had fed mostly on mammals: only <10% of bloodmeals were from birds. The predominant species was Cx. annulirostris (88%), with relatively few (4.4%) bloodmeals obtained from humans. From all 12 locations sampled, the mean proportion of Cx. annulirostris fed on pigs (9.1%) was considerably lower than fed on other animals (90.9%). Highest rates of pig-fed mosquitoes (>30%) were trapped where domestic pigs were kept close to human habitation. From seven of eight locations on the Australian mainland, the majority of Cx. annulirostris had obtained their bloodmeals from marsupials, probably the Agile wallaby Macropus agilis (Gould). Overall proportions of mosquito bloodmeals identified as marsupial were 60% from the Gulf Plains region of Australia, 78% from the Cape York Peninsula and 64% from the Daru area of Papua New Guinea. Thus, despite the abundance of feral pigs in northern Australia, our findings suggest that marsupials divert host-seeking Cx. annulirostris away from pigs. As marsupials are poor JE virus hosts, the prevalence of marsupials may impede the establishment of JE virus in Australia.

The prevalence of naturally acquired multiple infections of Wuchereria bancrofti and human malarias in anophelines

Malaria and filaria infection rates were determined for anopheline mosquitoes collected whilst biting and resting in village houses in Papua New Guinea. The number of anophelines infected with both parasites was greater than expected from the infection rates of each parasite and this difference was significant in resting collections. The excess of multiply infected mosquitoes is probably a result of a vector population composed of individuals with differing numbers of opportunities to become infected. Malaria-positive Anopheles punctulatus from resting catches had a significantly greater number of Stage 3 Wuchereria bancrofti larvae than malaria-negative mosquitoes. However, multiply infected mosquitoes appear to suffer greater mortality than non-infected or singly infected mosquitoes when the filarial worm reaches the third stage. Any potential increase in transmission resulting from multiple infections is thereby offset by a greater mortality rate in these mosquitoes.

The use of untreated bednets and malaria infection, morbidity and immunity

The effect of bednet use was investigated, without undertaking a specific intervention, in four cross-sectional community-based surveys in 10 villages of a highly endemic area of Papua New Guinea. Over half (55%) of the villagers interviewed reported that they had used a bednet on the previous night. In general and after adjustment for age, village and housing characteristics, bednet users, particularly children, had lower parasite prevalences and spleen rates and less enlarged spleens than non-users. However, users were similar to non-users in terms of fever reported for the previous week, axillary temperature, parasite density and haemoglobin level. The prevalence of antibody to the ring erythrocyte surface antigen and the major merozoite surface antigen 2 was lower in users than non-users. The association with malariometric indices and immune responses remained significant when bednet users were compared with non-users in houses without bednets. Thus, untreated bednets do not reduce malaria transmission sufficiently to decrease morbidity. They might paradoxically increase the risk of clinical malaria by lowering the development of humoral immunity.

Allozyme analysis reveals six species within the Anopheles punctulatus complex of mosquitoes in Papua New Guinea

Among samples collected from nineteen localities in Papua New Guinea, we have identified six species within the Anopheles punctulatus complex of mosquitoes, by means of cellulose acetate allozyme electrophoresis. An.punctulatus Dönitz sensu stricto was collected from seven villages in the Madang area and from Buksak, Sausi Mission and an area 18 km SW of Tari; An.koliensis Owen from eight villages in the Madang area, from Popondetta and Brown River near Karema; and An.farauti No. 1 from ten coastal areas including Madang, Lorengau, Popondetta, Port Moresby, Rabaul and Wewak. Three newly recognized species, reported here for the first time, are designated as An.farauti No. 4 from Gonoa and Hudini, Madang area; An.farauti No. 5 from Ketarabo near Goroka; and An.farauti No. 6 from Hiwanda near Tari. Three other known members of the complex, An.clowi Rozeboom & Knight, An.farauti No. 2 (Bryan, 1973) and An.farauti No. 3 (Mahon & Meithke, 1982) were not detected in Papua New Guinea. Problems arising with morphological characters for the identification of species in this group are discussed.

Variations in malaria transmission rates are not related to anopheline survivorship per feeding cycle

Anopheline survivorship, vectorial capacity, and mosquito infection probability estimates from mosquito infection rates were determined 4 times in 1 year in a Papua New Guinea village. Estimates of survivorship over the length of the extrinsic incubation period differed significantly during the year. However, survivorship per feeding cycle, individual mosquito vectorial capacity, and mosquito infection probability did not vary significantly. Estimates of these parameters were then compared to estimates of survivorship, individual vectorial capacity, and mosquito infection probability in mosquito populations in other villages in the study area. Since survivorship per feeding cycle did not vary significantly among the mosquito populations in these villages, changes in malaria transmission potential can be better gauged from estimates of survivorship over the length of the extrinsic incubation period. However, as measurements of relative inoculation rates are easier to perform and have been related to parasite prevalences in children in this area, estimates of inoculation rates are a preferred option for estimating malaria transmission in the Madang area of Papua New Guinea.

Human host selection by anophelines: no evidence for preferential selection of malaria or microfilariae-infected individuals in a hyperendemic area

Host selection among humans by Anopheles punctulatus was studied in an area of Papua New Guinea endemic for malaria and filariasis. Blood films were made from the stomach contents of freshly engorged mosquitoes found resting on the walls of houses in which the parasite status of the occupants was known. Engorgement rates on humans were non-random but could not be consistently related to the parasite status of individuals in the houses for either malaria or filaria. In some households, anophelines preferentially fed on parasitaemic individuals while in other households aparasitaemic individuals were significantly more often selected. This finding is believed to reflect the fact that malaria and filarial infections in this endemic area are predominantly asymptomatic. There were no significant differences in axillary temperatures between malaria or microfilariae positive or negative individuals.

The population dynamics in mosquitoes and humans of two Plasmodium vivax polymorphs distinguished by different circumsporozoite protein repeat regions

The population dynamics of two Plasmodium vivax polymorphs were studied over a two-year period in a village in a hyperendemic area of Papua New Guinea in both the mosquito and human populations. Strains of P. vivax were distinguished by different circumsporozoite (CS) protein repeats, the VK210 (classic) and the VK247 (variant) polymorphs. In 1986, 34% of P. vivax CS protein-positive mosquitoes were of the VK247 type. Although the proportion of P. vivax sporozoite antigen-positive mosquitoes compared with all sporozoite-positive mosquitoes did not change from 1986 to 1987, the proportion of P. vivax-positive mosquitoes of the VK247 polymorph decreased significantly from 34% to 11% (5 of 45) in 1987. In 1986, 61% (47 of 77) of humans tested had IgGs that recognized the VK247 CS repeat, while only 26% (22 of 84) had IgGs that recognized the VK210 CS repeat. The observed fluctuation in the proportion of the two P. vivax CS protein polymorphs recorded in the mosquito population from 1986 to 1987 is consistent with a hypothesis of selection by humoral immune pressure on the VK247 strain.

The Malaria Vaccine Epidemiology and Evaluation Project of Papua New Guinea: rationale and baseline studies

The range of possible malaria vaccines, against different species of Plasmodium and various stages in the life cycle of the parasite in both human host and mosquito vector, is reviewed. The importance, in a malaria-endemic area, of protection by a malaria vaccine against disease rather than infection is emphasized, and the ways by which disease prevention may be achieved are discussed. Mechanisms of production and presentation of vaccines are considered, including the importance of appropriate and more effective adjuvants. The variety of immune responses to malaria is set out and linked to both human and plasmodial genetic factors. Host genetics may also modify susceptibility to malaria through mechanisms which are not immunological. There is a need for entomological studies of the Anopheles vectors, especially but not only in preparation for transmission-blocking vaccines. This overall complexity justifies a multidimensional approach to epidemiology and field-site preparation. An iterative procedure is proposed for initial field evaluation, through adult male volunteers to community studies in immune adults and then to semi-immune school children, before evaluation in the principal target population of nonimmune young children. The outcome variables for epidemiological evaluation are specified. After this brief review of malaria vaccines, the baseline studies being undertaken by the Malaria Vaccine Epidemiology and Evaluation Project of the Papua New Guinea Institute of Medical Research in the Wosera area of East Sepik Province are discussed in some detail, and their rationale linked to the range and complexity of the malaria vaccines that have been reviewed. These studies are described under the headings of their principal components of epidemiology, parasitology, immunology, genetics and entomology.

Relative repellency of woodsmoke and topical applications of plant products against mosquitoes

The repellent action of various plant products was evaluated against anopheline and culicine mosquitoes in a rural village in the Wosera area, Papua New Guinea. A 5 x 5 Latin square design was used. Wood from four home-grown plant species was burned outdoors in the first experiment, and mosquitoes attracted to human bait were collected from 1800 to 2400 hours. In the second experiment, bruised leaves from another four plant species were rubbed on to the legs of human baits followed by mosquito collections. Woodsmoke and topical applications reduced biting of human volunteers by 79% and 51%, respectively. This low-technology control method may be included in the range of options for householders in order to reduce mosquito nuisance and improve their standard of health.

Can mosquitoes transmit AIDS?

Surveys to determine knowledge regarding AIDS have shown in many countries, including Papua New Guinea, that a large proportion of the literate population still mistakenly believe that mosquitoes can transmit the AIDS virus from one person to another. In this paper we review the theoretical mechanisms which would allow blood-sucking insects such as mosquitoes to transmit virus and discuss the evidence against transmission of HIV by mosquitoes. AIDS is a sexually transmitted disease with no scientific evidence for arthropod transmission.