Reviews
Lymphatic Filariasis in the Philippines M. Kron, E. Walker, L. Hernandez, E. Torres and B. Libranda-Ramirez Lymphatic filariasis caused by Wuchereria bancrofti and Brugia malayi is endemic throughout most of the southern half of the Philippine archipelago. Economic and manpower shortages prior to 1996 made it difficult to acquire new prevalence data and vector control data concurrently from all provinces. Nevertheless, analysis of cumulative prevalence data on filariasis indicates the persistence of filariasis in each of the three major island groups – Luzon, Visayas and Mindanao – including 45 out of 77 provinces. Here, Michael Kron and colleagues summarize the prevalence data, and review host, parasite and vector characteristics relevant to the design and implementation of disease control initiatives in the Philippines planned for the year 2000. The Philippine Island archipelago contains 7100 islands covering 298 000 km2, and has a population of approximately 73 million. Filariasis was first described in the Philippines more than 90 years ago, and over the past 50 years has been reported from most provinces south of central Luzon1 (Fig. 1). In 1959, the first survey of microfilaremia prevalence (Mf prevalence) included 18 provinces and revealed a mean prevalence of 8.8%. In 1960, 43 of the 63 existing provinces were shown to be endemic for the disease, leading to the creation of the National Filariasis Control Program (NFCP) in 1963. In 1987, governmental reorganization placed the NFCP under the Communicable Disease Control Service, Department of Health, sharing a general service budget with other communicable disease control programs; in 1996, however, a separate budget was finally allotted to the NFCP. Case-finding activities and accomplishment reports for the NFCP originate in its implementing arms – the provincial Filariasis Control Units (FCUs). Although filariasis is a reportable disease in the Philippines, funds for active surveillance are limited and FCU prevalence surveys are not always obtained in the same villages. Thus, the true incidence of infection and disease is still unknown in most areas. Prevalence of infection and clinical disease In 1984, it was estimated that approximately 20 million people were at risk of lymphatic filariasis (LF) in the Philippines. Analysis of disease surveillance data from 1960–1996 (Table 1) concluded that filariasis is now endemic in 45 out of 77 provinces2,3. Twelve new provinces have been created since 1960. Two provinces, Marinduque (Region 4) and Sarangani (Region 11) Michael Kron is at the Department of Medicine, Division of Infectious Diseases, B323 Life Sciences Building, East Lansing, Michigan State University, MI, USA. Edward Walker is at the Department of Entomology, Michigan State University, USA. Leda Hernandez is Program Coordinator for the National Filariasis Control Program, Department of Health, Manila, The Philippines. Elizabeth Torres is at the Department of Parasitology and Medical Entomology, Research Institute for Tropical Medicine, Department of Health, Manila, Philippines. Bernadette Libranda-Ramirez is at the Department of Biochemistry and Molecular Biology, University of The Philippines, Manila, Philippines. Tel: +1 517 353 3747, Fax: +1 517 353 1922, e-mail:
[email protected]. Parasitology Today, vol. 16, no. 8, 2000
became endemic after 1963. By WHO criteria4, 38 provinces are defined as having a low prevalence of the disease (10%). In Marinduque5, a previously nonendemic area, 22.4% of people aged 40–49 years and 13.6% of all ages (n 5 1349) were microfilaremic. Other provincial Mf prevalence rates range from 0.02% (Cebu) to 10.08% (Sulu). These percentages are believed to be inaccurate and potentially gross underestimates for several reasons. First, the data include prevalence studies from more than five years ago. Second, FCUs do not always have the resources to collect night-time blood in areas where nocturnally periodic Wuchereria bancrofti exists. Third, FCUs still use thick blood films to identify Mf, and thus define a case of filariasis based only on positive blood films. A study of 143 subjects in 1994 from a moderate prevalence village by three diagnostic regimens – Giemsa-stained thick blood smears, nucleopore filtration of anticoagulated blood (the gold standard) and acridine orange-stained blood* – indicated that the use of thick blood smears systematically misses at least 10% of positive samples. Prior to 1990, health statistics only included persons with Mf in blood, ignoring the existence of acutely or chronically infected amicrofilaremic subjects. Two cross-sectional data collections began in 1994 to attempt to quantitate filarial disease using both Mf prevalence and physical or historical evidence of chronic disease. Studies in rural Mapaso and Añog Sorsogon (n 5 518) revealed that common acute clinical manifestations of filariasis are fever and local lymphatic inflammation (lymphadenitis or lymphangitis). Men develop chronic hydrocoele with or without chyluria6–10. In the 518 individuals surveyed, 80% of cases of Mf were in people aged 15–69 years, and there was a distinct male to female predominance of almost 3:1 based on blood smears; 22% had physical evidence of chronic filarial disease in the absence of Mf, and another 17.5% demonstrated active Mf10. Therefore, the true prevalence of acute and chronic filariasis may be double the current estimates. Parasite and vector species Two species of filaria are known to exist in the Philippines. Nocturnally periodic W. bancrofti is the predominant organism. Nocturnally subperiodic Brugia malayi has been reported from seven provinces. Four provinces – Davao Oriental, Palawan, Eastern Samar and Surigao del Sur – have both species. Brugia malayi was first confirmed in 1964 from Palawan in a survey of 314 natives from nine villages; 33.1% were positive for Mf and eight cases were mixed infections with W. bancrofti. *Ramirez, B. et al. Acridine orange staining of microfilaria: a rapid and highly sensitive technique for the detection of microfilariae cases in a filariasis endemic community. 5th Scientific Exhibit on Health and Related Technologies, Manila, the Philippines, 16–22 July 1994.
0169-4758/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0169-4758(00)01705-1
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Reviews have been one of the most highly endemic areas for bancroftian filariasis. Villages (barangays) of average 2 population size 700–1000 can be found throughout the region, and Luzon Sea terrain varies considerably, from low mountains to volcanic lakes and Philippine Sea large rivers. Rain falls throughout the year, with particularly heavy 1 rains during the monsoon months CAR of November to January. One of the primary agricultural products of this region is the abaca plant (Musa NCR textilis), which is cultivated for proLuzon duction of Manila hemp. Larvae of Ae. poicilius utilize the water-filled 5 3 leaf axils of abaca as breeding sites th 14 parallel (Fig. 2). Aedes poicilius larvae also dwell in leaf axils of other plants, Bicol peninsula such as banana (Saba variety: Musa 8 sapientum), taro (Colocasia) and screwpine (Pandanus). However, the extensive plantings of abaca underVisayas Samar island neath a tall canopy of coconut palms create the conditions for large populations of Ae. poicilius to occur. 4 Several biological characteristics Palawan of Ae. poicilius contribute to its caisland pacity to facilitate transmission of W. bancrofti16. Aedes poicilius has a high human blood index (
