Differential antibody responses to Plasmodium falciparum and P. vivax circumsporozoite proteins in a human population.

Papua New Guineans exposed to hyperendemic malaria in the Madang area showed different antibody responses to Plasmodium falciparum and Plasmodium vivax sporozoites despite comparable entomological inoculation rates. Although there was a significant trend of increasing prevalence of anti-P. falciparum circumsporozoite (CS) protein immunoglobulin G (IgG) with age, there was no significant increase in the antibody units of IgG recognizing P. falciparum CS proteins. Antibodies recognizing P. vivax CS proteins steadily increased in prevalence and antibody units with age. Significant trends of increasing prevalence of antibody responders (both IgG and IgM) with increasing splenic enlargement were found in the younger age groups for P. falciparum CS proteins but not for P. vivax CS proteins. When antibody responders were analyzed by quartiles, there was a trend of increasing antibody response with age against P. vivax CS peptide, but not for P. falciparum CS protein. There was no evidence for increasing protection against blood-stage infections with increasing antibody levels for either P. falciparum or P. vivax. Neither were any significant relationships found between entomological inoculation rates and either CS antibody prevalence or concentration among the villages studied.

Small area variation in prevalence of an S-antigen serotype of Plasmodium falciparum in villages of Madang, Papua New Guinea.

Cross-sectional and longitudinal village-based studies of the transmission dynamics of an S-antigen serotype of the asexual erythrocyte stages of Plasmodium falciparum have been carried out in Madang, Papua New Guinea (PNG). Sera collected from village residents were screened for circulating S-antigen of the FC27 serotype by enzyme-linked immunosorbent assay (ELISA). The prevalence of the FC27 S-antigen was found to vary between villages at a given point in time, as well as within a village over time. Residents of villages 2-5 km apart were infected with P. falciparum of different S-antigen serotypes. This study documents the periodic nature of transmission of a sub-population of P. falciparum defined by the FC27 S-antigen. The variation in a small area in the prevalence of this serotype of P. falciparum in Madang illustrates the complexities of malaria transmission which must be considered in the design of malaria vaccine trials.

Malaria vaccines: results of human trials and directions of current research.

Recent human trials of candidate vaccines against the sporozoite and asexual blood stages of the malaria parasite have produced encouraging evidence that synthetic peptides and recombinant proteins can stimulate an antibody response and provide at least limited protection from malaria infection. Current research is focused on the continued evaluation of asexual blood stage antigens and on the improvement of the immunogenicity of candidate antigens through stimulation of more effective cellular and humoral responses. Efforts are also being directed toward the development of transmission-blocking vaccines based on antigens of sexual stages of the parasite.

Measurement of malarial infectivity of human populations to mosquitoes in the Madang area, Papua, New Guinea.

The proportion of blood meals taken on humans which are infectious to mosquitoes in the Madang area, Papua New Guinea was estimated by two methods. In the first, laboratory reared Anopheles farauti were fed on individuals of all ages at village surveys. The results showed that 3.8% of people were infectious and that the mean percentage of mosquitoes which became infected by feeding on these people was 37.9%. From the average proportion of mosquitoes infected, the probability that a mosquito feeding on a human would pick up infection was 0.013 +/- 0.005. In the second approach mosquitoes were fed on identified Plasmodium falciparum, P. vivax and P. malariae gametocyte carriers. The results indicated that 46% of gametocyte carriers were infectious and that the mean probability of a mosquito becoming infected after feeding on a gametocyte carrier was 0.151 +/- 0.029. Gametocyte prevalence rates in all ages measured over 18 months in three villages averaged 3.3% P. falciparum, 4.0% P. vivax and 0.7% P. malariae, totalling 8.0 +/- 0.7%. Combining gametocyte prevalence rates with the probability of a mosquito becoming infected from a gametocyte carrier, the probability of a mosquito becoming infected following a blood meal on a member of the human population was estimated to be 0.012 +/- 0.003.

Reduction in incidence and prevalence of Plasmodium falciparum in under-5-year-old children by permethrin impregnation of mosquito nets.

The malaria incidence and prevalence rates among children who slept under permethrin-impregnated mosquito nets in four villages near Madang, Papua New Guinea, were compared with the rates among children who slept under unimpregnated nets in four paired control villages. Immediately following a parasitological survey in the eight villages, malaria parasites were cleared from the children with chemotherapy, and the mosquito nets in the four experimental villages were impregnated with permethrin. Follow-up parasitological surveys were performed 4 and 10 weeks later. Sporozoite rates in female mosquitos of the Anopheles punctulatus complex decreased significantly in two of the experimental villages after impregnation. Also, the incidence of Plasmodium falciparum between the 4-week and 10-week surveys was significantly lower among the 0-4-year olds in villages with impregnated nets than in those with unimpregnated nets, leading to reduced prevalence of P. falciparum in this age group. Use of permethrin-impregnated nets had no effect on the incidence or prevalence of P. falciparum among 5-9-year olds or on that of P. vivax among the 0-4- or 5-9-year olds.

Hereditary ovalocytosis and reduced susceptibility to malaria in Papua New Guinea.

Ovalocytosis, an hereditary condition in which most erythrocytes are oval in shape, is a polymorphism that occurs in up to 20% or more of the population in Papua New Guinea and Malaysia. Due to the geographical correlation of the trait with endemic malaria, the possibility of a selective advantage in resistance to malaria has been raised. In a study of 202 individuals with greater than or equal to 50% oval red cells matched by age, sex and village of residence with controls having less than or equal to 30% oval cells, ovalocytic subjects had blood films negative for Plasmodium vivax (P = 0.009), for P. falciparum (P = 0.044), and for all species of malaria parasites (P = 0.013), more often than controls. Among individuals parasitaemic at any time there were no clear differences in density of parasitaemia. However, in children 2 to 4 years old, parasite densities of both species were lower in ovalocytic subjects than in controls (0.01 less than P less than 0.025). The differential susceptibility to malaria infection suggested by this study has implications for the evaluation of interventions, including possible future vaccine field trials, in populations where high-frequency ovalocytosis is present.

The epidemiology of malaria in a population surrounding Madang, Papua New Guinea.

Malaria is prevalent throughout coastal and lowland Papua New Guinea. Recent changes, including a shift from predominance of Plasmodium vivax to Plasmodium falciparum, appearance of chloroquine-resistant P. falciparum and decreased effectiveness of vector control programs have been observed. Epidemiological features of malaria were studied through four six-month surveys of a population of 16,500 in Madang Province from 1981-1983. Baseline data on parasitology, splenic enlargement, serology, hemoglobin levels, prevalence of 4-aminoquinolines, utilization of mosquito nets and incidence of fever were collected for use in future evaluation of malaria control measures including possible field trials of an antimalarial vaccine. Prevalence of parasitemia (all species, all ages) varied from 35.0% to 42.7% over the four surveys each of which covered a random sample of 25% of the population. The ratio of parasite species was: P. falciparum 70:P. vivax 25:P. malariae 5 in the dry seasons, shifting slightly in favor of P. falciparum during the wet seasons. Intense year-round transmission was indicated by decreasing parasite prevalence and splenic enlargement with age, low density asymptomatic parasitemias and high prevalence of antimalarial antibodies (i.e., greater than 80% of the population over five years of age was ELISA-positive). Levels of endemicity varied geographically, presence of 4-aminoquinolines in urine samples was relatively common (12.7% positive) and chloroquine resistance was widespread (81.6% in vitro, 46.6% in vivo).

Water contact behavior and schistosomiasis in an upper Egyptian village.

Preliminary findings of a study of the role of microspatial behavior and local ecological circumstances in the persistence of Schistosoma haematobium infection in El Ayaisha village, Upper Egypt, are presented. Twelve types of water contact activities were studied in three cohorts of 274,324 and 392 male students aged 5-16 during a 2-year period. Swimming and playing resulted in more frequent and intensive contact with potentially infective water and in more pollution of snail habitats with schistosome eggs than any other type of activity. Irrigation was probably not a major cause of S. haematobium infection in schoolboys and drinking water and fishing carried the smallest risk for this age group. Mapping and discriminate analysis of infection and transmission indicators revealed spatial associations between water contact, schistosome transmission and infection among the various age groups. The highest egg excretion rates in individuals and the highest prevalence rates were noted in the neighborhoods nearest to the canals. Water contact and contamination of water by the heavily infected school age boys is seasonally concentrated during the summer, when intermediate host snails and infective cercariae are also most common in the water. Frequency, type and duration of water contact change with age together with types of water bodies used, causing corresponding changes in exposure risk. Nevertheless, other factors, including acquired immunity, preexisting infections, chemotherapy and the effect of the study on the normal water contact behavior of the study population must also be evaluated. Contact with the Nile and the canals by the general nonschool population of El Ayaisha was studied to verify the observations among the schoolboys and to study socially linked activities and the transmission potential of the various transmission sites. Results are evaluated in regard to the feasibility of various schistosomiasis control measures in El Ayaisha.