THE RISK OF MALARIAL INFECTIONS AND DISEASE IN PAPUA NEW GUINEAN CHILDREN

PASCAL MICHON 0 JENNIFER L. COLE-TOBIAN 0 ELIJAH DABOD 0 SONJA SCHOEPFLIN 0 JENNIFER IGU 0 MELINDA SUSAPU 0 NANDAO TARONGKA 0 PETER A. ZIMMERMAN 0 JOHN C. REEDER 0 JAMES G. BEESON 0 LOUIS SCHOFIELD 0 CHRISTOPHER L. KING 0 IVO MUELLER 0 0 tute of Medical Research , PO Box 378, Madang, MAD 511 , Papua New Guinea In a treatment re-infection study of 206 Papua New Guinean school children, we examined risk of reinfection and symptomatic malaria caused by different Plasmodium species. Although children acquired a similar number of polymerase chain reaction-detectable Plasmodium falciparum and P. vivax infections in six months of active follow-up (P. falciparum 5.00, P. vivax 5.28), they were 21 times more likely to develop symptomatic P. falciparum malaria (1.17/year) than P. vivax malaria (0.06/year). Children greater than nine years of age had a reduced risk of acquiring P. vivax infections of low-to-moderate (>150/ L) density (adjusted hazard rate [AHR] 0.65 and 0.42), whereas similar reductions in risk with age of P. falciparum infection was only seen for parasitemias > 5,000/ L (AHR 0.49) and symptomatic episodes (AHR 0.51). Infection and symptomatic episodes with P. malariae and P. ovale were rare. By nine years of age, children have thus acquired almost complete clinical immunity to P. vivax characterized by a very tight control of parasite density, whereas the acquisition of immunity to symptomatic P. falciparum malaria remained incomplete. These observations suggest that different mechanisms of immunity may be important for protection from these malaria species. - The epidemiology of Plasmodium falciparum malaria suggests children first acquire immunity against severe disease after relatively few infections.1,2 However, uncomplicated P. falciparum malaria remains common throughout most of childhood, and a significant decrease in risk of infection is only seen in adolescence and early adulthood.1 Similar patterns have also been described in area of areas of Papua New Guinea highly endemic for malaria.3,4 It has therefore been argued that the mechanisms responsible for protection against severe disease may be distinct from those that protect against infections per se and mild episodes of disease,1,5 and that immunity might be acquired in stages. Although many potential targets and mechanisms of protective immunity have been identified,1,58 we still know little about mechanisms involved in the acquisition of protective immunity against P. falciparum. Even less is known about the acquisition of immunity to non-P. falciparum malarias. In highly endemic areas such as Papua New Guinea where the different species co-occur, prevalence of infection with P. vivax peaks at younger ages3,9,10 and contributes proportionally less to the burden of febrile illness11 than P. falciparum. Conversely, P. malariae reaches maximum prevalence only in adolescents.9,10 These data indicate that immunity to P. vivax may be acquired more quickly than immunity to P. falciparum despite lower transmission rates.12 Although a number of potential targets and mechanisms for immunity have been identified for P. vivax,1315 little is known about acquisition of immunity to P. malariae and P. ovale. A better understanding of the incidence of infection and disease caused by different Plasmodium species in areas coendemic for all species is needed to properly assess differences in the acquisition of clinical immunity to different species. Because mixed infections are common in malariaendemic areas, but often remain undetected by light microscopy,10,16 polymerase chain reaction (PCR)based diagnostic methods are needed for quantifying risk of infection and morbidity reliably. To determine epidemiologic patterns of infections and disease with P. falciparum and P. vivax and investigate possible mechanisms of immune protection, we conducted a longitudinal treatment re-infection study of 206 Papua New Guinean elementary school children that combines repeated blood sampling and molecular detection of parasitemia with a large array of classic and functional immune assays. We describe the general study design and report patterns of incidence of infection and disease with all four human malaria parasite species. Detailed investigations of immunity to P. falciparum and P. vivax malaria will be the topic of future reports. MATERIALS AND METHODS Field study. This study was conducted between June and December 2004 at the Mugil and Megiar elementary schools situated on the northern coast of Papua New Guinea, 50 km north of Madang. The catchment area of both schools is serviced by a single health center at Mugil (Figure 1) run by the Catholic Health Services. Although the Mugil school is within easy walking distance of the health center, the Megiar schools are 4 km away along a sealed road but with frequent transport available. Bed net use in the study area is limited, with retreatment of bed nets virtually absent. This study was reviewed and approved by institutional review boards of the Papua New Guinea Medical Research Advisory Council, the Walter and Eliza Hall Institute, and the Veterans Affairs Medical Center (Cleveland, OH). After obtaining community support and written parental consent, children from all three grades in Mugil and grades 1 and 2 in Megiar were enrolled. Demographic information was collected from all participating children; the location of each childs home was recorded using a hand-held global positioning system (GPS) receiver (GPS 315; Magellan, Santa Clara, CA). Before starting treatment, each child was clinically examined: axillary temperature was measured using digital thermometers, the spleen was palpated, and a standard questionnaire of common signs and symptoms of malarial illness was administered. Hemoglobin (Hb) levels were measured using a portable device (HemoCue, ngholm, Sweden). A 10-mL venous blood sample was collected using EDTA-Vacutainer tubes (Becton Dickinson, Franklin Lakes, NJ) and two blood slides (thick and thin films) were made for determination of malarial infection. All children were subsequently treated with a seven-day course of artesunate monotherapy according to Papua New Guinea National treatment guidelines (i.e., 4 mg/kg on day 1 and 2 mg on days 27). All children received all seven doses with at least five of them directly observed. After treatment, children were actively followed-up at the schools every two weeks for new infections and febrile illness with the first visit taking place two weeks after receiving the first artesunate treatment. A total of 12 follow-up visits were conducted. Because of national holidays that prevented field work, the eighth follow-up period had to be extended to three weeks, which resulted in a total length of active follow-up of 25 week. Follow-up visits were conducted on a class-by-class basis with one class checked every day. Children that did not attend school on the day of scheduled follow-up were checked the next day o (...truncated)