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Profile of Morong, Bataan, an area of low malaria endemicity in the Philippines
Acta Tropica 63 (1997) 195 – 207
Profile of Morong, Bataan, an area of low malaria endemicity in the Philippines M.D.G. Bustosa,*, A. Saulb, N.P. Salazarc, M. Gomesd a
Research Institute for Tropical Medicine, Department of Health, Alabang, Muntinlupa, Metro Manila, Philippines b Australian Centre for International and Tropical Health and Nutrition, The Queensland Institute of Medical Research, Brisbane, Australia c SEAMEO TROPMED Central Office, Faculty of Tropical Medicine, Mahidol Uni6ersity, Bangkok, Thailand d Special Programme for Research and Training in Tropical Diseases, World Health Organization, Gene6a, Switzerland Received 22 May 1996; revised 30 September 1996; accepted 7 October 1996
Abstract A malaria study area in the Philippines is described. It consists of the municipality of Morong, Bataan on the Island of Luzon. In January 1992, the population was 19 454 in 106 villages located on a narrow coastal plain, or in valleys of streams running from the mountainous interior. This is an area of low level but persistent seasonal transmission of malaria with approximately one thousand cases reported each year, mainly from February to July. In spite of the low level of malaria, it is apparently quite stable. The study site has been used to investigate parameters leading to stable malaria. Hypotheses tested were that there was substantial under reporting of cases; that there was strain specific immunity stabilising the incidence of malaria and that malaria transmission in this area is highly localised in small regions with a high enough malaria prevalence to account for the year to year stability. The study plan included cross sectional surveys of parasite prevalence and seropositivity, longitudinal surveys, passive case detection, entomological surveys, anthropological surveys to assess knowledge of malaria and documentation of the health-seeking behaviour of the population. © 1997 Elsevier Science B.V. All rights reserved * Corresponding author. Tel.: + 63 2 842 2245/2079/2828/2194; fax: + 63 2 842 2245; e-mail: [email protected] 0001-706X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 0 0 1 - 7 0 6 X ( 9 6 ) 0 0 6 2 1 - 3
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Keywords: Malaria; Field site; Demography; Geography; Philippines
1. Introduction In spite of decades of malaria eradication programs, followed by many years of control, the Philippines, like many other countries in the tropics continues to be endemic for malaria (WHO, 1990). Malaria remains endemic in 72 of the country’s 75 provinces with a current incidence of about 7 cases per 1000 per year. It is one of the 10 leading causes of morbidity (Asinas, 1992). Malaria profiles vary throughout the country, with relatively high endemicity on the island of Palawan to regions of low and sporadic malaria. However, a substantial population lives in areas with relatively low level but persistent malaria. One such area is the municipality of Morong, situated on the Bataan Peninsula to the west of the largest island of Luzon. As detailed below, this area is typical of rural areas associated with forest fringes. Although the level of malaria is quite low, it is remarkably stable with approximately similar numbers of cases reported each year during the main transmission season. Macdonald’s model suggested that the level of transmission in regions such as Morong should be typically ‘unstable’ (Macdonald, 1957). He suggests that such areas will be characterised by fluctuations in incidence which are ‘‘likely to be very marked; at times they would be due to causes so small as to be unapparent except on close study. They might take all forms such as exaggerated seasonal epidemics...’’ (pp. 35 – 36). He further suggests that Anopheles fla6irostris, the major vector in the area, is associated with such unstable malaria (p. 41) and that ‘‘Control falling short of the desired degree would produce little apparent improvement; the ‘wearing out’ of imagicidal control would not be apparent until a late stage but when this stage was reached would produce epidemics which would be both severe and abrupt in their timing’’ (p. 36). Malaria in Morong, like many areas in the Philippines, fails to conform to this pattern. In 1990, the malaria study group of The Research Institute for Tropical Medicine (RITM) commenced an intensive program to investigate reasons for this stability as a prelude to devising more effective control programs. Three hypotheses were proposed to account for the apparent paradox of low level but stable malaria: (1) The level of malaria was much higher than reported and so the paradox was not real. This explanation, if true, raises further questions about why there was such high levels of under reporting: whether most of the population has asymptomatic malaria and if so why, or whether use of the health services was so low that most of the cases of symptomatic malaria went unreported. (2) The malaria transmission potential was high, but the number of cases is limited by strain specific immunity. Experimental infections in patients and volunteers indicate that people exhibit substantial clinical immunity and even sterile immunity when reinfected with the same strain. (McGregor and Wilson, 1988; Powell et al., 1972). High endemic areas are characterised by a very high strain
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diversity (Felger et al., 1994) and this is a factor believed to maintain high levels of parasites in the community (Gupta et al., 1994). However, when this study commenced, little was known about strain diversity in low endemic regions such as Morong. If there is low strain diversity in the study area, and isolate specific immunity is effective, then a model can be readily constructed where new infections only occur when existing immunity wanes. In this case, a negative correlation would be expected between recent history of malaria and risk of a new infection. (3) Malaria transmission is markedly heterogeneous even in a relatively small area such as Morong. The presence of relatively high zones of transmission would maintain malaria in a relatively stable condition. These foci could be due to geographic factors such as living in proximity to larval breeding habitats, be associated with work practices, e.g. workers who frequent forest areas as proposed to account for ‘forest fringe’ malaria from other regions of Southeast Asia (Kondrashin and Rooney, 1992), health-seeking behaviour of minority groups or even human genetic background. This study sought to test these hypotheses by more accurately measuring the prevalence of malaria through active case detection, more intensive passive case detection and serology; malaria incidence and antibody kinetics through a longitudinal survey, entomological factors, the perception of malaria in the population and the propensity to seek treatment; and risk factors associated with location and activities. These are described in a series of seven papers. The background data on malaria prevalence, the demography and geography of the study area are described in this paper. The paper by Torres et al. (1997) describes the entomology. Population beliefs about malaria are described by Espino et al. (1997); the parasitology and serology in the population by Belizario et al. (1997); risk factors associated with malaria by Lansang et al. (1997) and the conclusions of the study by Saul et al. (1997).
2. Study area The study area lies within the municipality of Morong (Fig. 1), with a total land area of 24 968 hectares or 249.68 km2. Morong is 15% of the total land area of Bataan province, lies in the northwestern tip of the Bataan Peninsula, along the west coast of Luzon Island facing the South China Sea. It is 80 km west of Manila, or 175 km by road to the north of Manila Bay and then through the rugged terrain of Bataan Province. This area typifies the topography and ecology of other malaria endemic regions in the country. It has a coastal plain 1–3 km wide and further inland, a mountainous land mass. The study area is dominated to the east by Bataan Peak which rises to over 1000 m only eight km from the coast. This high region is not populated and forms the Subic-Bataan National Park. The lower slopes have been cleared and are now largely covered by low growing secondary vegetation. This consists of woody weeds and coarse grasses. These areas are effectively desolate. In the north, substantial lowland forest still occurs, originally protected by the 17 341 hectare former U.S. Subic Naval Base Reservation, now called the Subic Base Metropolitan Authority.
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The National Park and the former Naval Base are excluded from the study site. There is no permanent human habitation within the National Park. Although there is a considerable population within the Naval Base Reservation, during the study period, movement by land between the study site and the Naval Base was very limited, and few people lived in the Naval Base adjacent to the study site. Thus the study site was effectively isolated on the north and east by the Naval Base and National Park and to the west by Subic Bay and the South China Sea. Contact with the remainder of Bataan only occurs along the narrow coastal strip to the south through an area of very low malaria endemicity. Several rivers and streams, i.e. the Morong and Alilao River in the central part and Mauban and Tuag River southward, form tributaries and creeks cutting across the rugged terrain and provide breeding sites for mosquitoes. Previous studies confirm the presence of the primary vector, Anopheles fla6irostris and 3 other vector species An. maculatus, An. mangyanus and An. litoralis (Salazar, 1989; Salazar and Gomes, 1989). Most houses are located within 1 km from these tributaries as they are a water source for household use and domestic farming. There are also ground springs forming fresh water pools with short tributaries branching in different directions. Small irrigation ditches from these water sources are made by the local
Fig. 1. Map of the Morong study area. Contour lines indicate the height in m. The shaded area indicates the approximate area covered by forest, i.e. areas where there is a continuous forest canopy. For the purposes of the study, the 106 sitios or villages in the area were grouped into sitio clusters to give a population per sitio cluster of at least 100. The position of each cluster is indicated by a dot whose area is proportional to the cluster population. The location of the PRPC and the Naval reserve is shown with cross hatching. The two clinics (RHU and PRPC), which serve the area are shown. The five sitios surveyed in 1990 are indicated: b, Biga; c, Canawan; m, Minanga; n, Nocil; r, Repacpac.
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inhabitants for small crop plantations, which include bananas, yam, sweet potatoes and local vegetables. The major crop on the narrow coastal plain is rice. Small rice paddies have also been developed along stream valleys and irrigation ditches made to direct water into these. Bataan has a tropical climate, with an average annual temperature of 26.1°C, coolest during the months of November to December (23.3°C) and warmest during April (28.6°C). The average annual relative humidity is 78%, ranging from 73% in May to 83% in July. The monthly rainfall varies from 0 (during the dry months of January – May) to 314.6 mm (during the rainy months June–September), with a mean precipitation of 66.7 mm. The rainfall pattern can be variable through the years depending on the onset and end of the monsoon rains. This is reflected in the number of malaria cases monthly, as heavy rains flush out larval habitats and decreases transmission and disease prevalence accordingly. Morong is a municipality, has an elected mayor and local government. The municipality is divided into five regions or barangays: Mabayo, Sabang, Binaritan, Nagbalayong and the Poblacion, the latter being the main town and administrative centre of Morong. Within each barangay, houses are clustered into sitios, which the inhabitants identify as their place of residence. In Morong, there are an average of 35 households per sitio, but these vary from a single household to a small town of 392 houses. About 7 – 9 km from the Poblacion is the Philippine Refugee Processing Center (PRPC), covering about 380 hectares. During the study period, this facility, funded by the United Nations High Commissioner for Refugees, housed 11 000–16 000 Indochinese refugees. They remained in the centre for 6–12 months for processing before they were sent to their countries of destination. There were 2000–2500 staff and dependants staying in the camp. The presence of the camp, established in 1980, provided a source of employment and commerce for the local inhabitants of Morong. This closed in December 1994.
3. Population census In September 1991, an extensive census of the area preparatory to a long-term epidemiological study was completed and updated during a malaria cross-sectional survey from January to April 1992. Sixty volunteer health workers (VHWs) and staff from the Rural Health Unit (RHU) of Morong helped in the enumeration and mapping of the study site. Households were sequentially numbered and individuals given a unique project number identifying the sitio, household and individual. A master list of all inhabitants of Morong was produced to help locate families and individuals and their corresponding household census numbers to be used in the mass blood surveys. A pre-test of the census questionnaires was done before the actual surveys were implemented by the trained VHWs. Six to ten VHWs from each barangay were chosen as the presence of locals in the team helped access, identification and communications, particularly in the remote communities. The census did not include the refugees and staff residing within the PRPC camp.
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Table 1 Population distribution and number per household in Morong Barangay Mabayo Sabang Nagbalayong Binaritan Poblacion Total
Population (%) 2361 3505 4404 4276 4908 19 454
(12.1) (18.0) (22.6) (22.0) (25.2)
Number of households
Average size
463 642 864 768 926
5.1 5.5 5.1 5.6 5.3
3663
5.3
The basic household questionnaire, prepared in the local language, Tagalog, consisted of three parts: (1) administrative information, i.e. name of interviewer, identification of locality and household; (2) demographic information, i.e. age, sex, permanent and secondary addresses; and (3) socio-cultural information, i.e. province of origin and length of stay in Morong. The census proper and data entry with double encoding and analysis of data collected took approximately four months, starting in September 1991 and finishing in time for the January 1992 cross-sectional survey. The census was updated during both the 1992 and 1993 cross sectional surveys.
4. Population distribution and age structure One hundred and six sitios were identified with a population of 19 454 individuals in 3663 households. The population distribution and household size per barangay are shown in Table 1, based on the September 1991 census. The most populous barangay is the Poblacion (25.2% of the population), which is also the centre of activity in Morong. In the rural areas, some of these sitios are quite small. For the purposes of the study, these have been grouped into sitio clusters with nearby sitios to give a population of at least 100. In the town of Poblacion, one or a few streets constitute a ‘sitio’. In some analyses, these have been combined so that the population of the Poblacion was considered as a single entity. As shown in Fig. 1, most of the population is located along the coastal plain. Some sitio clusters are located in the foothills of a denuded forest fringe area with secondary vegetation, within 1–5 km from the heavily forested mountains with continuous canopy. Occasionally, patches of forest clearings show illegal logging activities, which the government has been trying to control. Most families (65%) in the outlying sitios live in a one-room house built of local material such as palm leaf, cogon (a type of coarse grass) and bamboo slats, therefore very vulnerable to mosquito entry. Occupational analyses from a previous survey conducted in 1989 showed most of the population to be farmers or agricultural workers. A small proportion of the population were fishermen living near the coast. Only 48% of the households own land, where they mainly grow rice for home consumption. The
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majority of migrants to the area do not own land and in addition to subsistence farming in garden plots, earn their income from working in the forest areas, collecting wood for charcoal-making activities. There is a widespread belief that in the South East Asian context, forest related activities are risk factors for malaria. Through informal discussions and more structured interviews with health and malaria control staff in the area, we gained the impression that this was also a local impression (Espino et al., 1997). However, there was little pre-existing data on the relationship and this formed a major part of the study (Belizario et al., 1997; Lansang et al., 1997; Saul et al., 1997) reported in this set of papers. Fig. 2 shows the population structure following the 1992 cross-sectional survey, a total of 19 528 inhabitants as of April 1992, with an almost 1:1 male to female ratio in all age groups. Typical of rural areas in developing countries, there is a high birth rate resulting in a high proportion of children. One-third of the population are less than 10 years old. Significant migration into Morong has taken place (Fig. 3). The average length of stay of migrant residents in Morong is 9.06 years. 5412 individuals (27.7%) are not originally from Bataan, while 6.3% come from the adjacent provinces of Region III. In four out of five barangays (Mabayo, Sabang, Nagbalayong and the Poblacion), most of the migrants come from Region VIII (Eastern Visayas), at 3–14% of the population in these barangays. In barangay Binaritan, 9% of the residents come from Region V (the Bicol region). Although malaria is still endemic in these regions, the level of malaria is generally less than that found in Morong. The local perception was that new migrants were more susceptible to malaria than the rest of the community (Espino et al., 1997).
Fig. 2. Structure of the study population based on the total population of 19 545. Number of males in each age group is shown in black, females in white.
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Fig. 3. Length of stay of migrants in Morong, Bataan, 1991 (n = 5412).
5. Health facilities During the study period, there were two health facilities where patients came for free consultation for any illness, the Primary Health Care Clinic or the RHU and the outpatient clinic of the Health Services Group at the PRPC. The RHU is in the town centre. Patients may have to walk as far as 10–15 km for consultation and treatment. Until 1991, there was no microscopist at the RHU. Microscopic confirmation of malarial smears took 1–2 weeks as slides were brought to the Bataan Provincial Hospital or the Malaria Control Service (MCS) Provincial Office in Balanga, approximately 60 km away, examined and then results had to be transmitted back to Morong. Chloroquine and primaquine were given free of charge at the RHU clinic but generally had to be purchased at the PRPC clinic. Sulfadoxine – pyrimethamine was added in cases of suspected chloroquine resistance. Intravenous quinine was given at the PRPC hospital in cases of severe and complicated malaria. Distance, topography and severity of symptoms affect use of the RHU clinic. Past records show that those closest to it use it more often (Gomes and Salazar, 1990), whereas the bulk of malaria cases comes from the outlying sitios. The delivery of health services is described in more detail in Espino et al. (1997).
6. Malaria history Morong is officially classified as a ‘Malaria B Area’, defined as areas ‘‘within forest fringes with a stable population engaged mainly in developed agriculture and with a parasite rate of greater than 2% in children under 10’’ (Malaria Control Service, 1995), although as shown in our study (Belizario et al., 1997), this description is not entirely consistent with the observed pattern. With this classifica-
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tion, a malaria coordinator is assigned to the municipality and performs a multitude of functions such as blood canvasser, vector control supervisor (implementing environmental management and individual protection measures), malaria statistics keeper and several other malaria-related responsibilities. Records kept by the PRPC and RHU are available for the 12 years preceding this study (1981 – 1991, Table 2). Some of the variation in case numbers came from different recording procedures, e.g. the RHU recorded active case detection (ACD) and passive case detection (PCD) cases separately after 1989, and the inevitable changes in staffing levels and quality of service. In spite of these confounding variables, the number of cases shows reasonable consistency from one year to another, with a trend towards more cases as the population grew, especially in the mid 1980’s. Complete records on species are not available, but partial records indicate an average P. falciparum to P. 6i6ax ratio of 5:4 from 1984 to 1991. It primarily includes cases recorded by PCD, i.e. patients sick enough to seek treatment at one of the clinics, although there are some cases found by ACD, i.e. where a malaria coordinator has gone to sitios thought to have an outbreak of malaria and either done random sampling or sampling based on people with symptoms. There is a marked seasonal variation with peak transmission in the dry season. Fig. 4 shows both P. 6i6ax and P. falciparum cases detected by microscopy but only at the PRPC. Besides the general records kept by the RHU and the PRPC, three special surveys were carried out in the area prior to the study reported in this series. Because of concern about the introduction of drug resistant malaria from Indochina, from March to December 1988, refugees were screened. Fourteen of 26 815 tested were found positive within two weeks of arrival, indicating that there was a small potential for introducing new isolates into the area.
Table 2 Cases of malaria at RHU and PRPC clinics 1981 – 1991 Year 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 a
Total populationa
15 842 16 476 16 639 16 751 18 171 18 200 19 454
RHU
PRPC
Total Cases
31 129 140 61 120 251 329 402 519 644 1011
176 133 422 392 1057 1990 956 469 539 732 875
207 262 562 453 1177 2241 1285 871 1058 1376 1986
Population before 1991 supplied from national census data. These are projected values obtained from a 5 yearly census. 1991 data from a census conducted by this project.
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Fig. 4. Seasonal variation in clinical records of malaria over the period Jan 1981 – June 1991. Cases are from the PRPC out-patient clinic. These are predominantly from Filipinos living near the refugee camp, but also include a small number of refugees.
A cross sectional survey of Filipino residents of Morong (excluding the PRPC) was conducted by RITM staff in September and October of 1989. As shown on Fig. 4, this is the period when few malaria cases are reported to the clinics. In all, 11 936 people were screened in an ACD survey. This covered the whole area. Based on the census conducted in late 1991, approximately 85, 65, 60, 60 and 45% of the population was covered in the barangays of Mabayo, Sabang, Nagbalayong, Binaritan and Poblacion, respectively. Twelve were infected with P. 6i6ax and 26 with P. falciparum. A striking feature of the P. falciparum cases were the 22 that were gametocyte positive. In 12 of these, no asexual stages were detected. Most cases were asymptomatic. Five subjects reported fever, two with P. 6i6ax and three who had P. falciparum with both asexual parasites and gametocytes circulating. A limited cross-sectional survey was conducted in 5 sitios in June 1990 (Fig. 1). Three of these (Canawan, Nocil and Repacpac) were adjacent sitios which were analysed as a single sitio cluster in later studies (Belizario et al., 1997). Overall, 10.1% of those sampled were parasite positive (Table 3). In guidelines in force during the study period, Morong was supposed to receive semi-annual residual spraying operations as part of the vector control activities. However, records in the past show this is not always carried out as scheduled because of the delay in release of insecticides and funds to hire local spray men. The PRPC carried out its own regular spraying and fogging operations within the camp and sitios in the vicinity and has sometimes assisted the MCS with personnel and insecticides as needed. The impact of this spraying on the amount of malaria or the year to year stability is not known. On one hand, as detailed by Torres et al. (1997), the principle vector An. fla6irostris tends to bite late at night when most people would be indoors. This may suggest residual house spraying could have an impact. On the other, the vector is predominantly zoophilic and the house construction so open that mosquitoes may have minimal contact with sprayed surfaces.
Number surveyed P. falciparum (gametocyte+) P. 6i6ax
292 7 (5) 10
Minanga, Mabayo
Table 3 Parasite positive cases detected in June 1990
48 5 (1) 3
Canawan, Binaritan
36 3 (3) 1
Nocil, Binaritan
22 2 (2) 3
Repacpac, Sabang
75 10 (4) 4
Biga, Sabang
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7. Summary The survey work and historical records indicated that Morong has the profile expected of much of the malarious area of the Philippines: relatively low levels of highly seasonal malaria; a rural population with low income living by largely subsistence farming; an age structure highly biased towards young children and proximity to remaining forested areas.
Acknowledgements The study described in this series of papers could not have occurred without the cooperation and support of a large number of people. First our thanks go to the people of Morong for their patience and willingness to tolerate the intrusions this study necessarily caused. Facilities were made available and encouragement given by the Malaria Control Service, particularly by the then Director, Engineer C. Asinas; the Philippine Refugee Processing Center Health Services; The Bataan Provincial Health Office and Dr R. Banson; local health staff, Dr B. Dacula, and the nurse and midwives at the Morong RHU. The study was funded by a National Institutes of Health, Tropical Medicine Research Center grant (No. SRC (55) 5 P50 AI030601-02), grants from the World Health Organization/World Bank Special Program for Research and Training in Tropical Diseases, The Australian National Health and Medical Research Council and the Malaria Control Service, Department of Health, Manila, Philippines.
References Asinas, C.Y. (1992) Current status of malaria and control activities in the Philippines. Southeast Asian J. Trop. Med. Public Health 23 (4), 5 – 59. Belizario, V.Y., Saul, A., Bustos, M.D.G. et al. (1997) Field epidemiological studies on malaria in a low endemic community in Bataan, the Philippines. Acta Trop. 63, 241 – 256. Espino, F., Manderson, L., Acuin, C., Domingo, F. and Ventura, E. (1997) Perceptions of malaria in a low endemic area in the Philippines: transmission and prevention of disease. Acta Trop. 63, 221 – 239. Felger, I., Tavul, L., Kabintik, S. et al. (1994) Plasmodium falciparum: extensive polymorphism in merozoite surface antigen 2 alleles in an area with endemic malaria in Papua New Guinea. Exp. Parasitol. 79, 106–116. Gomes, M. and Salazar, N.P. (1990) Chemotherapy: principles in practice — a case study of the Philippines. Soc. Sci. Med. 30 (7), 789 – 796. Gupta, S., Trenholme, K., Anderson, R.M. and Day, K.P. (1994) Antigenic diversity and the transmission dynamics of Plasmodium falciparum. Science 263, 961 – 963. Kondrashin, A.V. and Rooney, W. (1992) Overview: epidemiology of malaria and its control in countries of the WHO South-East Asia Region. Proc. SEAMEO Tropical Medicine Technical Meeting on Advanced Knowledge on Malaria in South East Asia. Southeast Asian J. Trop. Med. Health 23 (4), 13–22. Lansang, M.A., Belizario, V.Y., Bustos, M.D.G., Saul, A. and Aguirre A. (1997) Risk factors for infection with malaria in a low endemic community in Bataan, the Philippines. Acta Trop. 63, 257–265.
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Macdonald, G. (1957) The Epidemiology and Control of Malaria. Oxford University Press, London. Malaria Control Service (1995) Implementation Guidelines on New Directions of the Malaria Control Program. Department of Health, Manila, Philippines. McGregor, I.A. and Wilson, R.J.M. (1988) Specific immunity: acquired in man. In Wernsdorfer, W.H. and McGregor, I. (Eds.), Malaria, Principle and Practice of Malariology, Churchill Livingstone, Edinburgh, pp. 559–619. Powell, R.D., McNamara, J.V. and Rieckmann, K.H. (1972) Clinical aspects of acquisition of immunity to clinical malaria. Proc. Helminthol. Soc. Wash. 39, 51 – 66 (special issue). Salazar, N.P. (1989) The malaria situation in the Philippines: a critique. In Philippine Council for Health Research and Development, State of the Art — Malaria, Technical Report Series No. 7, Manila, Philippines, pp. 1–49. Salazar, N.P. and Gomes, M.F. (1989) Varying malaria epidemiologic patterns in the Philippines. In Proc. 25th Silver Jubilee Annu. Sci. Semin. Malaysian Society of Parasitology and Tropical Medicine, Institute for Medical Research, Kuala Lumpur, Malaysia, 23 – 25 February 1989. Saul, A., Belizario, V.Y., Bustos, M.D.G., Espino, F., Lansang, M.A. and Salazar, N.P. (1997) Stability of malaria in a community in Bataan, the Philippines: prospects for control. Acta Trop. 63, 267 – 273. Torres, E.P., Salazar, N.P., Belizario, V.Y. and Saul, A. (1997) Vector abundance and behaviour in an area of low malaria endemicity in Bataan, the Philippines. Acta Trop. 63, 209 – 220. World Health Organization, (1990) World Malaria situation, 1988. Division of Control of Tropical Diseases. World Health Stat. Q. 43, 68 – 79.
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Profile of Morong, Bataan, an area of low malaria endemicity in the Philippines M.D.G. Bustosa,*, A. Saulb, N.P. Salazarc, M. Gomesd a
Research Institute for Tropical Medicine, Department of Health, Alabang, Muntinlupa, Metro Manila, Philippines b Australian Centre for International and Tropical Health and Nutrition, The Queensland Institute of Medical Research, Brisbane, Australia c SEAMEO TROPMED Central Office, Faculty of Tropical Medicine, Mahidol Uni6ersity, Bangkok, Thailand d Special Programme for Research and Training in Tropical Diseases, World Health Organization, Gene6a, Switzerland Received 22 May 1996; revised 30 September 1996; accepted 7 October 1996
Abstract A malaria study area in the Philippines is described. It consists of the municipality of Morong, Bataan on the Island of Luzon. In January 1992, the population was 19 454 in 106 villages located on a narrow coastal plain, or in valleys of streams running from the mountainous interior. This is an area of low level but persistent seasonal transmission of malaria with approximately one thousand cases reported each year, mainly from February to July. In spite of the low level of malaria, it is apparently quite stable. The study site has been used to investigate parameters leading to stable malaria. Hypotheses tested were that there was substantial under reporting of cases; that there was strain specific immunity stabilising the incidence of malaria and that malaria transmission in this area is highly localised in small regions with a high enough malaria prevalence to account for the year to year stability. The study plan included cross sectional surveys of parasite prevalence and seropositivity, longitudinal surveys, passive case detection, entomological surveys, anthropological surveys to assess knowledge of malaria and documentation of the health-seeking behaviour of the population. © 1997 Elsevier Science B.V. All rights reserved * Corresponding author. Tel.: + 63 2 842 2245/2079/2828/2194; fax: + 63 2 842 2245; e-mail: [email protected] 0001-706X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 0 0 1 - 7 0 6 X ( 9 6 ) 0 0 6 2 1 - 3
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Keywords: Malaria; Field site; Demography; Geography; Philippines
1. Introduction In spite of decades of malaria eradication programs, followed by many years of control, the Philippines, like many other countries in the tropics continues to be endemic for malaria (WHO, 1990). Malaria remains endemic in 72 of the country’s 75 provinces with a current incidence of about 7 cases per 1000 per year. It is one of the 10 leading causes of morbidity (Asinas, 1992). Malaria profiles vary throughout the country, with relatively high endemicity on the island of Palawan to regions of low and sporadic malaria. However, a substantial population lives in areas with relatively low level but persistent malaria. One such area is the municipality of Morong, situated on the Bataan Peninsula to the west of the largest island of Luzon. As detailed below, this area is typical of rural areas associated with forest fringes. Although the level of malaria is quite low, it is remarkably stable with approximately similar numbers of cases reported each year during the main transmission season. Macdonald’s model suggested that the level of transmission in regions such as Morong should be typically ‘unstable’ (Macdonald, 1957). He suggests that such areas will be characterised by fluctuations in incidence which are ‘‘likely to be very marked; at times they would be due to causes so small as to be unapparent except on close study. They might take all forms such as exaggerated seasonal epidemics...’’ (pp. 35 – 36). He further suggests that Anopheles fla6irostris, the major vector in the area, is associated with such unstable malaria (p. 41) and that ‘‘Control falling short of the desired degree would produce little apparent improvement; the ‘wearing out’ of imagicidal control would not be apparent until a late stage but when this stage was reached would produce epidemics which would be both severe and abrupt in their timing’’ (p. 36). Malaria in Morong, like many areas in the Philippines, fails to conform to this pattern. In 1990, the malaria study group of The Research Institute for Tropical Medicine (RITM) commenced an intensive program to investigate reasons for this stability as a prelude to devising more effective control programs. Three hypotheses were proposed to account for the apparent paradox of low level but stable malaria: (1) The level of malaria was much higher than reported and so the paradox was not real. This explanation, if true, raises further questions about why there was such high levels of under reporting: whether most of the population has asymptomatic malaria and if so why, or whether use of the health services was so low that most of the cases of symptomatic malaria went unreported. (2) The malaria transmission potential was high, but the number of cases is limited by strain specific immunity. Experimental infections in patients and volunteers indicate that people exhibit substantial clinical immunity and even sterile immunity when reinfected with the same strain. (McGregor and Wilson, 1988; Powell et al., 1972). High endemic areas are characterised by a very high strain
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diversity (Felger et al., 1994) and this is a factor believed to maintain high levels of parasites in the community (Gupta et al., 1994). However, when this study commenced, little was known about strain diversity in low endemic regions such as Morong. If there is low strain diversity in the study area, and isolate specific immunity is effective, then a model can be readily constructed where new infections only occur when existing immunity wanes. In this case, a negative correlation would be expected between recent history of malaria and risk of a new infection. (3) Malaria transmission is markedly heterogeneous even in a relatively small area such as Morong. The presence of relatively high zones of transmission would maintain malaria in a relatively stable condition. These foci could be due to geographic factors such as living in proximity to larval breeding habitats, be associated with work practices, e.g. workers who frequent forest areas as proposed to account for ‘forest fringe’ malaria from other regions of Southeast Asia (Kondrashin and Rooney, 1992), health-seeking behaviour of minority groups or even human genetic background. This study sought to test these hypotheses by more accurately measuring the prevalence of malaria through active case detection, more intensive passive case detection and serology; malaria incidence and antibody kinetics through a longitudinal survey, entomological factors, the perception of malaria in the population and the propensity to seek treatment; and risk factors associated with location and activities. These are described in a series of seven papers. The background data on malaria prevalence, the demography and geography of the study area are described in this paper. The paper by Torres et al. (1997) describes the entomology. Population beliefs about malaria are described by Espino et al. (1997); the parasitology and serology in the population by Belizario et al. (1997); risk factors associated with malaria by Lansang et al. (1997) and the conclusions of the study by Saul et al. (1997).
2. Study area The study area lies within the municipality of Morong (Fig. 1), with a total land area of 24 968 hectares or 249.68 km2. Morong is 15% of the total land area of Bataan province, lies in the northwestern tip of the Bataan Peninsula, along the west coast of Luzon Island facing the South China Sea. It is 80 km west of Manila, or 175 km by road to the north of Manila Bay and then through the rugged terrain of Bataan Province. This area typifies the topography and ecology of other malaria endemic regions in the country. It has a coastal plain 1–3 km wide and further inland, a mountainous land mass. The study area is dominated to the east by Bataan Peak which rises to over 1000 m only eight km from the coast. This high region is not populated and forms the Subic-Bataan National Park. The lower slopes have been cleared and are now largely covered by low growing secondary vegetation. This consists of woody weeds and coarse grasses. These areas are effectively desolate. In the north, substantial lowland forest still occurs, originally protected by the 17 341 hectare former U.S. Subic Naval Base Reservation, now called the Subic Base Metropolitan Authority.
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The National Park and the former Naval Base are excluded from the study site. There is no permanent human habitation within the National Park. Although there is a considerable population within the Naval Base Reservation, during the study period, movement by land between the study site and the Naval Base was very limited, and few people lived in the Naval Base adjacent to the study site. Thus the study site was effectively isolated on the north and east by the Naval Base and National Park and to the west by Subic Bay and the South China Sea. Contact with the remainder of Bataan only occurs along the narrow coastal strip to the south through an area of very low malaria endemicity. Several rivers and streams, i.e. the Morong and Alilao River in the central part and Mauban and Tuag River southward, form tributaries and creeks cutting across the rugged terrain and provide breeding sites for mosquitoes. Previous studies confirm the presence of the primary vector, Anopheles fla6irostris and 3 other vector species An. maculatus, An. mangyanus and An. litoralis (Salazar, 1989; Salazar and Gomes, 1989). Most houses are located within 1 km from these tributaries as they are a water source for household use and domestic farming. There are also ground springs forming fresh water pools with short tributaries branching in different directions. Small irrigation ditches from these water sources are made by the local
Fig. 1. Map of the Morong study area. Contour lines indicate the height in m. The shaded area indicates the approximate area covered by forest, i.e. areas where there is a continuous forest canopy. For the purposes of the study, the 106 sitios or villages in the area were grouped into sitio clusters to give a population per sitio cluster of at least 100. The position of each cluster is indicated by a dot whose area is proportional to the cluster population. The location of the PRPC and the Naval reserve is shown with cross hatching. The two clinics (RHU and PRPC), which serve the area are shown. The five sitios surveyed in 1990 are indicated: b, Biga; c, Canawan; m, Minanga; n, Nocil; r, Repacpac.
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inhabitants for small crop plantations, which include bananas, yam, sweet potatoes and local vegetables. The major crop on the narrow coastal plain is rice. Small rice paddies have also been developed along stream valleys and irrigation ditches made to direct water into these. Bataan has a tropical climate, with an average annual temperature of 26.1°C, coolest during the months of November to December (23.3°C) and warmest during April (28.6°C). The average annual relative humidity is 78%, ranging from 73% in May to 83% in July. The monthly rainfall varies from 0 (during the dry months of January – May) to 314.6 mm (during the rainy months June–September), with a mean precipitation of 66.7 mm. The rainfall pattern can be variable through the years depending on the onset and end of the monsoon rains. This is reflected in the number of malaria cases monthly, as heavy rains flush out larval habitats and decreases transmission and disease prevalence accordingly. Morong is a municipality, has an elected mayor and local government. The municipality is divided into five regions or barangays: Mabayo, Sabang, Binaritan, Nagbalayong and the Poblacion, the latter being the main town and administrative centre of Morong. Within each barangay, houses are clustered into sitios, which the inhabitants identify as their place of residence. In Morong, there are an average of 35 households per sitio, but these vary from a single household to a small town of 392 houses. About 7 – 9 km from the Poblacion is the Philippine Refugee Processing Center (PRPC), covering about 380 hectares. During the study period, this facility, funded by the United Nations High Commissioner for Refugees, housed 11 000–16 000 Indochinese refugees. They remained in the centre for 6–12 months for processing before they were sent to their countries of destination. There were 2000–2500 staff and dependants staying in the camp. The presence of the camp, established in 1980, provided a source of employment and commerce for the local inhabitants of Morong. This closed in December 1994.
3. Population census In September 1991, an extensive census of the area preparatory to a long-term epidemiological study was completed and updated during a malaria cross-sectional survey from January to April 1992. Sixty volunteer health workers (VHWs) and staff from the Rural Health Unit (RHU) of Morong helped in the enumeration and mapping of the study site. Households were sequentially numbered and individuals given a unique project number identifying the sitio, household and individual. A master list of all inhabitants of Morong was produced to help locate families and individuals and their corresponding household census numbers to be used in the mass blood surveys. A pre-test of the census questionnaires was done before the actual surveys were implemented by the trained VHWs. Six to ten VHWs from each barangay were chosen as the presence of locals in the team helped access, identification and communications, particularly in the remote communities. The census did not include the refugees and staff residing within the PRPC camp.
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Table 1 Population distribution and number per household in Morong Barangay Mabayo Sabang Nagbalayong Binaritan Poblacion Total
Population (%) 2361 3505 4404 4276 4908 19 454
(12.1) (18.0) (22.6) (22.0) (25.2)
Number of households
Average size
463 642 864 768 926
5.1 5.5 5.1 5.6 5.3
3663
5.3
The basic household questionnaire, prepared in the local language, Tagalog, consisted of three parts: (1) administrative information, i.e. name of interviewer, identification of locality and household; (2) demographic information, i.e. age, sex, permanent and secondary addresses; and (3) socio-cultural information, i.e. province of origin and length of stay in Morong. The census proper and data entry with double encoding and analysis of data collected took approximately four months, starting in September 1991 and finishing in time for the January 1992 cross-sectional survey. The census was updated during both the 1992 and 1993 cross sectional surveys.
4. Population distribution and age structure One hundred and six sitios were identified with a population of 19 454 individuals in 3663 households. The population distribution and household size per barangay are shown in Table 1, based on the September 1991 census. The most populous barangay is the Poblacion (25.2% of the population), which is also the centre of activity in Morong. In the rural areas, some of these sitios are quite small. For the purposes of the study, these have been grouped into sitio clusters with nearby sitios to give a population of at least 100. In the town of Poblacion, one or a few streets constitute a ‘sitio’. In some analyses, these have been combined so that the population of the Poblacion was considered as a single entity. As shown in Fig. 1, most of the population is located along the coastal plain. Some sitio clusters are located in the foothills of a denuded forest fringe area with secondary vegetation, within 1–5 km from the heavily forested mountains with continuous canopy. Occasionally, patches of forest clearings show illegal logging activities, which the government has been trying to control. Most families (65%) in the outlying sitios live in a one-room house built of local material such as palm leaf, cogon (a type of coarse grass) and bamboo slats, therefore very vulnerable to mosquito entry. Occupational analyses from a previous survey conducted in 1989 showed most of the population to be farmers or agricultural workers. A small proportion of the population were fishermen living near the coast. Only 48% of the households own land, where they mainly grow rice for home consumption. The
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majority of migrants to the area do not own land and in addition to subsistence farming in garden plots, earn their income from working in the forest areas, collecting wood for charcoal-making activities. There is a widespread belief that in the South East Asian context, forest related activities are risk factors for malaria. Through informal discussions and more structured interviews with health and malaria control staff in the area, we gained the impression that this was also a local impression (Espino et al., 1997). However, there was little pre-existing data on the relationship and this formed a major part of the study (Belizario et al., 1997; Lansang et al., 1997; Saul et al., 1997) reported in this set of papers. Fig. 2 shows the population structure following the 1992 cross-sectional survey, a total of 19 528 inhabitants as of April 1992, with an almost 1:1 male to female ratio in all age groups. Typical of rural areas in developing countries, there is a high birth rate resulting in a high proportion of children. One-third of the population are less than 10 years old. Significant migration into Morong has taken place (Fig. 3). The average length of stay of migrant residents in Morong is 9.06 years. 5412 individuals (27.7%) are not originally from Bataan, while 6.3% come from the adjacent provinces of Region III. In four out of five barangays (Mabayo, Sabang, Nagbalayong and the Poblacion), most of the migrants come from Region VIII (Eastern Visayas), at 3–14% of the population in these barangays. In barangay Binaritan, 9% of the residents come from Region V (the Bicol region). Although malaria is still endemic in these regions, the level of malaria is generally less than that found in Morong. The local perception was that new migrants were more susceptible to malaria than the rest of the community (Espino et al., 1997).
Fig. 2. Structure of the study population based on the total population of 19 545. Number of males in each age group is shown in black, females in white.
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Fig. 3. Length of stay of migrants in Morong, Bataan, 1991 (n = 5412).
5. Health facilities During the study period, there were two health facilities where patients came for free consultation for any illness, the Primary Health Care Clinic or the RHU and the outpatient clinic of the Health Services Group at the PRPC. The RHU is in the town centre. Patients may have to walk as far as 10–15 km for consultation and treatment. Until 1991, there was no microscopist at the RHU. Microscopic confirmation of malarial smears took 1–2 weeks as slides were brought to the Bataan Provincial Hospital or the Malaria Control Service (MCS) Provincial Office in Balanga, approximately 60 km away, examined and then results had to be transmitted back to Morong. Chloroquine and primaquine were given free of charge at the RHU clinic but generally had to be purchased at the PRPC clinic. Sulfadoxine – pyrimethamine was added in cases of suspected chloroquine resistance. Intravenous quinine was given at the PRPC hospital in cases of severe and complicated malaria. Distance, topography and severity of symptoms affect use of the RHU clinic. Past records show that those closest to it use it more often (Gomes and Salazar, 1990), whereas the bulk of malaria cases comes from the outlying sitios. The delivery of health services is described in more detail in Espino et al. (1997).
6. Malaria history Morong is officially classified as a ‘Malaria B Area’, defined as areas ‘‘within forest fringes with a stable population engaged mainly in developed agriculture and with a parasite rate of greater than 2% in children under 10’’ (Malaria Control Service, 1995), although as shown in our study (Belizario et al., 1997), this description is not entirely consistent with the observed pattern. With this classifica-
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tion, a malaria coordinator is assigned to the municipality and performs a multitude of functions such as blood canvasser, vector control supervisor (implementing environmental management and individual protection measures), malaria statistics keeper and several other malaria-related responsibilities. Records kept by the PRPC and RHU are available for the 12 years preceding this study (1981 – 1991, Table 2). Some of the variation in case numbers came from different recording procedures, e.g. the RHU recorded active case detection (ACD) and passive case detection (PCD) cases separately after 1989, and the inevitable changes in staffing levels and quality of service. In spite of these confounding variables, the number of cases shows reasonable consistency from one year to another, with a trend towards more cases as the population grew, especially in the mid 1980’s. Complete records on species are not available, but partial records indicate an average P. falciparum to P. 6i6ax ratio of 5:4 from 1984 to 1991. It primarily includes cases recorded by PCD, i.e. patients sick enough to seek treatment at one of the clinics, although there are some cases found by ACD, i.e. where a malaria coordinator has gone to sitios thought to have an outbreak of malaria and either done random sampling or sampling based on people with symptoms. There is a marked seasonal variation with peak transmission in the dry season. Fig. 4 shows both P. 6i6ax and P. falciparum cases detected by microscopy but only at the PRPC. Besides the general records kept by the RHU and the PRPC, three special surveys were carried out in the area prior to the study reported in this series. Because of concern about the introduction of drug resistant malaria from Indochina, from March to December 1988, refugees were screened. Fourteen of 26 815 tested were found positive within two weeks of arrival, indicating that there was a small potential for introducing new isolates into the area.
Table 2 Cases of malaria at RHU and PRPC clinics 1981 – 1991 Year 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 a
Total populationa
15 842 16 476 16 639 16 751 18 171 18 200 19 454
RHU
PRPC
Total Cases
31 129 140 61 120 251 329 402 519 644 1011
176 133 422 392 1057 1990 956 469 539 732 875
207 262 562 453 1177 2241 1285 871 1058 1376 1986
Population before 1991 supplied from national census data. These are projected values obtained from a 5 yearly census. 1991 data from a census conducted by this project.
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Fig. 4. Seasonal variation in clinical records of malaria over the period Jan 1981 – June 1991. Cases are from the PRPC out-patient clinic. These are predominantly from Filipinos living near the refugee camp, but also include a small number of refugees.
A cross sectional survey of Filipino residents of Morong (excluding the PRPC) was conducted by RITM staff in September and October of 1989. As shown on Fig. 4, this is the period when few malaria cases are reported to the clinics. In all, 11 936 people were screened in an ACD survey. This covered the whole area. Based on the census conducted in late 1991, approximately 85, 65, 60, 60 and 45% of the population was covered in the barangays of Mabayo, Sabang, Nagbalayong, Binaritan and Poblacion, respectively. Twelve were infected with P. 6i6ax and 26 with P. falciparum. A striking feature of the P. falciparum cases were the 22 that were gametocyte positive. In 12 of these, no asexual stages were detected. Most cases were asymptomatic. Five subjects reported fever, two with P. 6i6ax and three who had P. falciparum with both asexual parasites and gametocytes circulating. A limited cross-sectional survey was conducted in 5 sitios in June 1990 (Fig. 1). Three of these (Canawan, Nocil and Repacpac) were adjacent sitios which were analysed as a single sitio cluster in later studies (Belizario et al., 1997). Overall, 10.1% of those sampled were parasite positive (Table 3). In guidelines in force during the study period, Morong was supposed to receive semi-annual residual spraying operations as part of the vector control activities. However, records in the past show this is not always carried out as scheduled because of the delay in release of insecticides and funds to hire local spray men. The PRPC carried out its own regular spraying and fogging operations within the camp and sitios in the vicinity and has sometimes assisted the MCS with personnel and insecticides as needed. The impact of this spraying on the amount of malaria or the year to year stability is not known. On one hand, as detailed by Torres et al. (1997), the principle vector An. fla6irostris tends to bite late at night when most people would be indoors. This may suggest residual house spraying could have an impact. On the other, the vector is predominantly zoophilic and the house construction so open that mosquitoes may have minimal contact with sprayed surfaces.
Number surveyed P. falciparum (gametocyte+) P. 6i6ax
292 7 (5) 10
Minanga, Mabayo
Table 3 Parasite positive cases detected in June 1990
48 5 (1) 3
Canawan, Binaritan
36 3 (3) 1
Nocil, Binaritan
22 2 (2) 3
Repacpac, Sabang
75 10 (4) 4
Biga, Sabang
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7. Summary The survey work and historical records indicated that Morong has the profile expected of much of the malarious area of the Philippines: relatively low levels of highly seasonal malaria; a rural population with low income living by largely subsistence farming; an age structure highly biased towards young children and proximity to remaining forested areas.
Acknowledgements The study described in this series of papers could not have occurred without the cooperation and support of a large number of people. First our thanks go to the people of Morong for their patience and willingness to tolerate the intrusions this study necessarily caused. Facilities were made available and encouragement given by the Malaria Control Service, particularly by the then Director, Engineer C. Asinas; the Philippine Refugee Processing Center Health Services; The Bataan Provincial Health Office and Dr R. Banson; local health staff, Dr B. Dacula, and the nurse and midwives at the Morong RHU. The study was funded by a National Institutes of Health, Tropical Medicine Research Center grant (No. SRC (55) 5 P50 AI030601-02), grants from the World Health Organization/World Bank Special Program for Research and Training in Tropical Diseases, The Australian National Health and Medical Research Council and the Malaria Control Service, Department of Health, Manila, Philippines.
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