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: 7. Southern Highlands Province.

As the last part of a program to survey the extent of malaria transmission in the Papua New Guinea highlands, a series of rapid malaria surveys were conducted in 2003-2004 and 2005 in different parts of Southern Highlands Province. Malaria was found to be highly endemic in Lake Kutubu (prevalence rate (PR): 17-33%), moderate to highly endemic in Erave (PR: 10-31%) and moderately endemic in low-lying parts (< 1500 m) of Poroma and Kagua (PR: 12-17%), but was rare or absent elsewhere. A reported malaria epidemic prior to the 2004 surveys could be confirmed for the Poroma (PR: 26%) but not for the lower Kagua area. In Kutubu/Erave Plasmodium falciparum was the most common cause of infection (42%), followed by P. vivax (39%) and P. malariae (16%). In other areas most infections were due to P. vivax (63%). Most infections were of low density (72% < 500/ microl) and not associated with febrile illness. Overall, malaria was only a significant source of febrile illness when prevalence rates rose above 10%, or in epidemics. However, concurrent parasitaemia led to a significant reduction in haemoglobin (Hb) level (1.2 g/dl, CI95: [1.1-1.4.], p < 0.001) and population mean Hb levels were strongly correlated with overall prevalence of malarial infections (r = -0.79, p < 0.001). Based on the survey results, areas of different malaria epidemiology are delineated and options for control in each area are discussed.

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.

The epidemiology of malaria in the Papua New Guinea highlands: 5. Aseki, Menyamya and Wau-Bulolo, Morobe Province.

Although not strictly a highlands province, Morobe encompasses large highlands areas, the most important being Aseki, Menyamya and Wau-Bulolo. A series of rapid malaria surveys conducted in both the wet and dry seasons found malaria to be clearly endemic in areas below 1400 m in Menyamya and Wau-Bulolo, with overall prevalence rates in the wet season (25.5%, range: 9.1%-39.2%) greatly exceeding those in the dry season (8.3%, range: 2.4%-22.8%; p < 0.001). In the wet season surveys Plasmodium falciparum was the clearly predominant species, accounting for 63% of all infections. P. vivax increased in frequency in the dry season (from 27% to 46%, p < 0.001), while P. falciparum and P. malariae decreased. In line with past surveys a low prevalence of malaria was found in the Aseki area. Malaria was found to be the main source of febrile illness in the wet season with at least 60% of measured or reported fever associated with parasitaemia. Other causes of febrile illness dominated in the dry. In villages with parasite prevalence rates < 20% mean haemoglobin levels and prevalence of severe anaemia were strongly correlated with overall parasite prevalence. In addition concurrent malarial infections were associated with a strong reduction of individual haemoglobin levels (-1.2 g/dl) and there was increased risk of moderate-to-severe anaemia with concurrent malaria. Malarial infections are thus the most significant cause of febrile illness and anaemia in the highlands fringe populations in Morobe. As a consequence all villages below 1500-1600 m in Morobe Province should be included in malaria control activities.

The epidemiology of malaria in the Papua New Guinea highlands: 4. Enga Province.

Of all Papua New Guinea provinces, Enga has the largest proportion of people living at altitudes that preclude malaria transmission. However, the first systematic surveys in 1979 showed that malaria was endemic in lower-lying valleys to the north and east of the province. A series of new surveys conducted in both wet and dry seasons showed that these areas remain the main malaria focus in Enga. However, over the last 25 years the risk of malarial infections has increased substantially in areas < 1200 m (from 10% to 37-41%). In these low-lying areas people acquire substantial antimalarial immunity and most infections are asymptomatic. However, people in villages in these areas had significantly lower mean haemoglobin levels (13.0 vs 14.0 g/dl, p < 0.001) than in areas above 1200 m, where overall prevalence rates (0-9%) have not changed much. In areas between 1200 and 1600 m epidemics with parasite prevalence rates in excess of 20% have been found to occur. Malaria was a significant cause of febrile illness only in endemic areas or during outbreaks. Although rarely used, sleeping under a bednet was associated with a significant reduction in risk of malaria infection (adjusted OR = 0.45, p = 0.01). On the other hand, sleeping in garden houses away from the main villages increased the risk of malaria infection (adjusted OR = 1.6, p = 0.03). Malaria control in outlying, malarious areas of Enga province could therefore be based on the distribution of long-lasting impregnated bednets, while at the same time addressing the additional risks posed by the high mobility of many of these populations through targeted health education.

The epidemiology of malaria in the Papua New Guinea highlands: 1. Western Highlands Province.

Despite a resurgence of malaria in many Papua New Guinea highlands and highlands fringe areas after the cessation of control activities in the early 1980s the malaria situation in these areas has received little attention. A series of cross-sectional surveys were therefore carried out to provide accurate and up-to-date information on the prevalence of malaria and the risk of epidemics and to propose adequate malaria control strategies. Studies in 24 villages in Western Highlands Province found the prevalence of malarial infections to be strongly correlated with altitude, ranging from 1.6% at altitudes of 1500-1800 m to over 30% in villages below 900 m. Malaria outbreaks were observed at the end of the rainy season. All four human malaria species were present with P. falciparum infections clearly dominating. The relative importance of P. vivax increased with altitude, while both P. malariae and P. ovale were rare. Many infections were of low density. While malaria is an important source of febrile illness in endemic areas below 1500 m altitude, only few observed or reported fevers are due to malarial infections in higher, nonendemic areas. Rates of enlarged spleens, mean haemoglobin levels and the prevalence of anaemia (Hb <7.5 g/dl) were strongly linked to the level of malaria found in each community and were associated with both altitude and concurrent malarial infection. Based on the survey results, areas of different malaria epidemiology are delineated and options for control in each area are discussed.