Epidemiology of malaria in the Papua New Guinean highlands.

OBJECTIVES: To conduct an in-depth investigation of the epidemiology of malaria in the Papua New Guinea (PNG) highlands and provide a basis for evidence-based planning and monitoring of intensified malaria control activities. METHODS: Between December 2000 and July 2005, 153 household-based, rapid malaria population surveys were conducted in 112 villages throughout the central PNG highlands. The presence of malaria infections was determined by light microscopy and risk factors assessed using a structured questionnaire.The combined dataset from all individually published surveys was reanalysed. RESULTS: The prevalence of malaria infections in the different surveys ranged from 0.0% to 41.8%(median 4.3%) in non-epidemic surveys and 6.6% to 63.2% (median 21.2%, P < 0.001) during epidemics. Plasmodium falciparum was the predominant infection below 1400 m and during epidemics, Plasmodium vivax at altitudes >1600 m. Outside epidemics, prevalence decreased significantly with altitude, was reduced in people using bed nets [odds ratio (OR) = 0.8, P < 0.001] but increased in those sleeping in garden houses (OR = 1.34, P < 0.001) and travelling to highly endemic lowlands (OR = 1.80, P < 0.001). Below 1400 m, malaria was a significant source of febrile illness. At higher altitudes, malaria was only a significant source of febrile illness during epidemic outbreaks, but asymptomatic malaria infections were common in non-epidemic times. CONCLUSIONS: Malaria is once again endemic throughout the PNG highlands in areas below 1400­1500 m of altitude with a significant risk of seasonal malaria outbreaks in most area between 1400­1650 m. Ongoing control efforts are likely to result in a substantial reduction in malaria transmission and may even result in local elimination of malaria in higher lying areas.

The epidemiology of malaria in the Papua New Guinea highlands: 6. Simbai and Bundi, Madang Province.

Although predominantly a lowland province, Madang also includes highland areas such as Simbai and Bundi along the northern highland fringe. While the malaria situation in the coastal lowlands has been studied in great detail, the current malaria situation in the highland fringe communities has not been studied in depth since the 1960s. A series of recent malariological surveys found that the malaria situation has changed little over the last 40 years in both Simbai and Bundi. In the Simbai area there is little malaria transmission in villages above 1400 m, with a prevalence rate (PR) of 2.5-4.2%. Below 1400 m, however, there is moderate to high transmission (PR 8.6-24.7%) with surprisingly little difference in prevalence rates between survey villages, despite large differences in altitude. Prevalence rates of malaria infection were low in all Bundi villages (2.5-8.5%) with most infections occurring in adolescents and adults, which indicates limited acquisition of effective immunity to malaria and the possibility that many infections are acquired when travelling to the highly malarious lowlands area. Based on spleen rates the lower Simbai area would be regarded as mesoendemic, and the upper Simbai and Bundi areas as hypoendemic. Only in the lower Simbai area is malaria a major cause of febrile illness. However, in all areas village mean haemoglobin (Hb) levels were highly correlated with the prevalence of malaria infections, while concurrent parasitaemia reduced individual Hb levels by 1.3 g/dl (CI95 [1.0-1.5], p < 0.001) and significantly increased the risk for moderate-to-severe anaemia (Hb < 8 g/dl) (adjusted odds ratio 5.6, CI95 [3.6-8.6], p < 0.001). Based on the survey results, areas of different malaria epidemiology are delineated and options for control in each area are discussed.

Epidemic malaria in the highlands of Papua New Guinea.

As part of a larger study into the epidemiology of malaria in the highlands of Papua New Guinea, outbreak investigations were carried out at the end of the 2002 rainy season in 11 villages situated between 1,400 and 1,700 meters above sea level that had reported epidemics. Locations and timing of these epidemics corresponded largely to those reported in the pre-control era of the 1960s and 1970s. On average, 28.8% (range = 10.3-63.2%) of people in each of the 11 villages were found to be infected with malaria. Plasmodium falciparum accounted for 59% of all identified infections and P. vivax for 34%. The majority (53%) of infections were symptomatic. Although symptomatic infections were most common in children 2-9 years of age (36%), even in adults a prevalence of 20% was observed. A comparison with earlier non-epidemic data in three of the villages without easy access to health care showed markedly increased levels of morbidity, with 6-10-fold increases in parasite prevalence, a 3-fold increase in both measured and reported fevers, and a 12-fold increase in enlarged spleens. The average hemoglobin levels were reduced by 2.3-3.5 g/dL, with a concurrent increase in moderate to severe anemia (hemoglobin level < 7.5 g/dL) from 0.0-3.3% to 3.8-18.4%. These massive increases in morbidity have devastating impact on the affected communities and highlight that malaria epidemics are a serious and increasing public health problem in the highlands of Papua New Guinea.

The epidemiology of malaria in the Papua New Guinea highlands: 3. Simbu Province.

Two very distinct malaria zones can be found within Simbu Province. The north of the province is characterized by the absence or very low level of local malaria transmission, but there is a considerable risk of epidemics prevalent in the lower-lying parts. During non-epidemic periods, parasite prevalence was usually under 5%, with similar frequencies for Plasmodium falciparum and P. vivax (47% each), and malaria was an only minor source of febrile illness. During epidemics, however, 13-36% of people were infected, predominantly with P. falciparum (64%), and high levels of severe morbidity were present. In south Simbu malaria is clearly endemic with an overall prevalence of 35%, combined with a strong age-dependence of infections, low haemoglobin levels, high rates of enlarged spleen and moderate to severe anaemia (haemoglobin level < 7.5 g/dl) in children. The malaria epidemiology in south Simbu is thus more similar to the lowlands than to other highlands areas. Epidemic prevention, surveillance and response in the north, and bednet distribution and strengthening of curative services in the south, are therefore the priorities for malaria control in Simbu Province.

The epidemiology of malaria in the Papua New Guinea highlands: 2. Eastern Highlands Province.

The epidemiology of malaria in Eastern Highlands Province (EHP) is characterized by generally very low-level or no local malaria transmission but a considerable risk of epidemics. In non-epidemic situations, parasite prevalence was under 5% in all but one area, Lufa Valley, where P. vivax was the most common parasite (prevalence rate of 58%). During epidemics, however, 23% (range 8-63%) of people were infected, with P. falciparum the predominant parasite. Mixed infections were very common (12%) but P. malariae and P. ovale were rare. Outside epidemics, malaria was only a minor source of febrile illness and enlarged spleens and anaemia (haemoglobin level < 7.5 g/dl) were virtually absent. However, epidemics were associated with a high incidence of relatively severe morbidity, both in terms of fevers and moderate to severe anaemia. Epidemic prevention, surveillance and response therefore are priorities in malaria control in EHP.

Complex patterns of malaria epidemiology in the highlands region of Papua New Guinea.

A cross-sectional malaria survey of the Pabrabuk area in the Western Highlands Province found that all 4 human malaria species were present in a single village, with an overall parasite prevalence rate of 27%. Plasmodium falciparum was the most frequently detected infection (14%) followed by P. vivax (11%), P. malariae (5%) and P. ovale (3%). 10 of the 51 infections were mixed. Anopheles punctulatus was the most frequent vector species in the area, but both An. farauti no. 6 and An. karwari were also present in low numbers. This diversity in both parasite and vector populations indicates that complex malaria patterns are found in Papua New Guinea even at the moderate transmission levels found in low-lying inter-montane valleys.