ORIGINAL ARTICLES

Epidemiology of Malaria in Thailand Krongthong Thimasarn, Siriporn Jatapadma, Saowanit Vijaykadga,Jeerapat Sirichaisinthop, and Chansuda Wongsrichanalai Background: In spite of significant achievements in malaria control in the past two decades, about 150,000 malaria cases still occur in Thailand each year. Although most short-term visitors to Thailand stay in malaria-free areas, an increasing number of more adventurous travelers are exposed to the disease. Method; Since 1987, the Malaria Division of the Thai Ministry of Public Health has maintained a computerized database that includes all malaria cases recorded at malaria clinics, government health institutions, and private hospitals nationwide. In this article, we analyze the 1992 data. R e s u k T h e provinces of Trad, Tak, and Kanchanaburi had the highest incidence of locally transmitted cases. Trad Province was also responsible for the highest number of imported cases. The highest incidence rate was found to be 426.5 per 1000 persons per year in a group of villages in Maesod District, Tak Province. Districts and provinces with 2 20 cases per 1000 persons per year are listed in this report. Peak transmission seasons and species prevalence of different endemic areas are described. Analysis of case investigation, a part of this database, indirectly supported the presence of mefloquine resistant Plasmodium falciparum strains on the Thai-Cambodian border. Conclusions: This paper describes the characteristics of malaria in different parts of Thailand and pinpoints areas with significant transmission. However, in accordance with the present policy of the Thai national malaria control program, we do not recommend chemoprophylaxis, but we do strongly encourage personal protection, early diagnosis, and prompt treatment. ( J Travel Med2:59-65, 1995)

Methods

Currently, 12.7 million (24.1%)Thai people live in malaria “control” areas. The rest (40.3 million) live in “eradicated” or malaria-free areas.’ Control areas, where efforts directed toward control of transmission are in progress, include mostly the forested and mountainous parts of the country, along the borders with Myanmar and Cambodia. Malaria in Thailand is known for its severity because of the prevalent multidrug resistant strains of Plasmodium fak@arum in certain parts of the country. By knowing whether a traveler intends to visit places located in endemic areas, appropriate prevention can be planned and undertaken.

Since 1987, the Malaria Division, Department of Communicable Disease Contro1,Thai Ministry of Public Health has conducted malaria surveillance countrywide. Diagnostic, treatment, and case investigation data are collected monthly from all malaria sectors (No. = 302), government health institutions, and private hospitals throughout the country.These data are entered into a computerized database.Eighty percent of the malaria cases were identified by malaria sectors through malaria clinic services.In addition to mosquito control, the three major tasks of a malaria sector include the following: malaria clinic services; examination of malaria smears collected by village health volunteers; and active case finding (e.g., a periodic screening for malaria regardless of symptoms). Each malaria sector is responsible for a defined control area and operates one or more malaria clinics. Patients, especially those who are local residents of the villages primarily served by the malaria sector, are interviewed concerning possible places of recent exposure to malaria, antimalarial drug use, etc, and their previous health records are checked. The computerized database provides the source of malaria statistics forThailand, which are reported annually by the Malaria Division. Detailed analyses of these data, however, have not yet been reported.

Krongthong Thimasarn, MD: Director, Center for Malaria Region 5, Nonthaburi; Siriporn Jatapadma, MSc, and Saowanit Vijaykadga, BSc: Malaria Division, Department of Communicable Disease Control, Ministry of Public Health, Nonthaburi; Jeerapat Sirichaisinthop, MD: Center for Malaria Region 1 , Pra Buddhabaht, Saraburi; Chansuda Wongsrichanalai, MD: Department of Immunology, Armed Forces Research Institute of Medical Sciences (AFRIMS), Bangkok. Reprint requests: Dr. Chansuda Wongsrichanalai, Department of Immunology, AFRIMS, 315/6 Rajvithi Road, Bangkok 10400, Thailand

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Journal of Travel Medicine, Volume 2, N u m b e r 2

Table 1 Number and Percentage of Fakiparum Malaria Cases by Province

Province

Trat Tak Kanchanaburi Chanthaburi Mae Hong Son Prajuab Kirikhan Prajeenburi Ratchaburi Suratthani Ranong Chumphon Chiangmai Sisaket Phetchaburi Ubon Ratchathani

29,623 25,260 13,016 11,173 8,242 5,600 4,939 4,376 3,834 3,301 3,186 3,102 2,626 2,370 2,319 122,967

19,911 16,453 7,449 6,731 4,882 2,857 3,021 2,323 1,331 1,603 1,171 1,526 1,575 1,162 1,630

67.2 65.1 57.2 60.2 59.2 51 .O 61.2 53.1 34.7 48.6 36.8 49.2 60.0 49.0 70.3

*Data are from the 15 provinces with the highest number of malaria cases,Thailand, 1992

Results Malaria-Free Areas

Currently, 14 of the 76 Thailand provinces have been designated malaria &ee.Theyare as follows: Bangkok, Nonthaburi, Pathumthani, Ang T h o n g , Ayuthaya, Singburi, Nakhonpathom, Samut Prakan, Samut Sakhon, Samut Songkhram, Chainat, Pichit, Maha Sarakham, and Phuket.All of these provinces, except Maha Sarakham and Phuket, are situated in the central plain (Fig. 1).In addition to these 14 provinces, all provincial municipal areas of the country are free of malaria transmission. Figure 1 Map of Thailand showing the 73 provinces in 1992. In 1993, three districts, namely, "Srakaew" of Prajeenburi Province, "Nong Bua Lamphu" of Udon Thani Province and "Arnnat Charoen" of Ubon Ratchathani Province, were elevated to province status, thereby resulting in a total of 76 provinces.

AU original DBase IV, data files for 1992 were converted and loaded into aVAX4000 computer system located at the Armed Forces Research Institute of Medical Sciences (AFRIMS).The files were merged, and the data analysed using SPSS-X statistical packages.* An area-specific malaria incidence rate was calculated by dividing the number of local (resident) cases reported by a particular malaria sector with the midyear population of the corresponding control area and multiplied by 1000. The area-specific populations were derived from registers ofArea Stratification for Malaria Control, 1992 (available at the Malaria Division).

Species and Seasonal Peaks

In 1992, 152,240 cases of malaria were reported. Fifteen provinces with the highest number of cases reported a combined total of 122,967 o r 80.8% of the total (Table 1). Malaria transmission in Thailand occurs year round, with one or two seasonal peaks. Pfalciparum accounted for 87,126, or 57.2% ofthe total number of cases.Almost all other cases were due to Plasmodium uivax (64,274 cases or 42.2%) and mixed (Pfalciparum and I? vivax) infection (648 cases or 0.4%).AU number and percentages of P j d ciparum cases presented elsewhere in this paper included mixed infection. T h e Plasmodium malariae species is uncommon, and Plasmodium ovule is only exceptionally found in Thailand. The number of cases by province shown in Table 1 does not necessarily reflect the degree of malaria transmission in that particular province. One-third of patients (40,939 out of 120,047 or 34.1%) contracted malaria

Thimasarn et al, Malaria in Thailand

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Table 2 Number and Percentage of Imported Malaria Cases by Province of the Fifteen Provinces

Province

New Cases Investigated (No.)

Central region, eastern part 25,067 Trat Chanthaburi 8,841 Prajeenburi 3.621 Northern region Tak 16,459 Mae Hong Son 6,803 Chiangmai 2,457 Central region, western part 9,973 Kanchanaburi Ratchaburi 3,862 Phetchaburi 1,821 Prajuab Kirikhan 3,659 Southern region Chuniphon 2,667 Ranong 2,869 Suratthani 3,745 Northeastern region Srisaket 2,370 Ubon Ratchathani 2,134

Imported Cases (No.)

PA)

18,847 3,414 2.235

75.2 38.6 61.7

1,310 1,987 519

8.0 29.2 21.1

1,867 670 19 653

18.7 17.3 1.o 17.8

512 1,687 175

19.2 58.8 4.7

727 522

30.7 24.5

outside Thailand. Table 2 shows the percentages of imported cases in the 15 provinces. Results of detailed analyses of these 15 provinces (see Tables 1 and 2), plus those which reported more than 1000 cases in 1992,are presented in detail. Central Region, Eastern Part

Trat Province had the highest number of malaria cases (seeTable l).Trat Province is located on the eastern border with Cambodia. Two seasonal peaks occur, one during the dry season (3802 cases in January), and the other at the beginning of the rainy season (2806 cases in June).The number of cases dropped to its lowest level in September (1282 cases).The majority of these was reported from Borai District (67.4%; 19,718 of 29,273). Borai District is the center of ruby trading and the gateway to gem mining in Western Cambodia. Pfakiparum was the predominant species. Trat Province also had the highest number of imported cases (75.2%, seeTable 2).Almost all of these cases were contracted in Cambodia. Only 5101 cases (20.4%) were reported to be due to local transmission within the province. For Borai District, local transmission accounted for only 16.7% (2864117,104). In addition to some remote areas in the forested hills along the border with Cambodia where malaria control is difficult to achieve, a place with significant local transmission withinTrat is Koh Chang 1sland.The island is a popular destination for both backpackers from western countries and for local tourists. A total of 377 cases were reported in Koh Chang in 1992 and 90.2% (340 of 377)

were due to transmission within the island. The overall malaria incidence rate was 95.1 per 1000 persons per year, but for certain villages on the fiinge of the thick rain forest covering the core part of the island, the incidence is thought to be much higher. Since Chanthaburi Province is adjacent toTrat, and itselfborders Cambodia, the number of malaria cases was also affected by ruby mining in Cambodia.Two seasonal peaks similar to that found inTrat occur (1383 cases in January and 1034 cases in July). Other provinces in the region that need to be mentioned are Rayong and Chonburi. Rayong reported 1674 cases of malaria, with 51.7% (865 of 1674) due to Pfakiparurn and 21.4% (243 of 1136) imported. Chonburi reported 1025 cases, 47.2% (484 of 1025) ofwhich were Pfakiparum and only 13.7% (94 of 687) imported. Prajeenburi Province possesses a few crossing points to Cambodia that have, until recently, been rather busy.This partly explains the relatively high percentage of imported cases in this province (61.7%, see Table 2). Northern Region

Tak Province is located on the western border with Myanmar. Malaria transmission usually presents in two peaks, similar to Trat.The 1992 peaks were observed in January (2213 cases) and in July (2879). Case numbers were lowest in April (1047); Maesod District accounted for the majority of cases. In contrast to Trat, only 8.0% of the local resident cases were contracted outside the country. Similar to Trat, the percentage of Pfakipanrm was high (65.1%, see Table 1). Gem miners routinely travel between Maesod and Borai District, the “gem trade route.” Mae Hong Son is a mountainous province north of Tak; it also borders Myanmar. Both Mae Hong Son and Chiangmai Provinces had a single transmission peak in the rainy season, which occurs from June to August. Chiangrai and Kamphaengphet Provinces also had a single midyear transmission peak. One thousand six hundred and forty one cases were reported in Chiangrai (57.8% Pfakiparum and 54.9% imported), and 1141 cases were reported in Kamphaengphet (56.1% Pjilipanrm and 36.6% imported). Western Region, Central Part

The Kanchanaburi provincial centre is only 128 km northwest of Bangkok, but the province contains a number of malaria-endemic remote villages along the ThaiMyanmar border. Peak transmission occurred in June and July, 2023 and 2034 cases, respectively. In contrast to Trat, only 18.7% of the cases reviewed were imported. Cases were concentrated in the border districts, namely, Sangklaburi,Thong Phaphum, and SaiYoke. Ratchaburi Province is just south of Kanchanaburi. Major endemic areas are in the border district of Suan Phueng. Cases in Phetchaburi Province were almost all

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Table 3 Districts with Malaria Incidence Rates of >I00 Cases per 1000 Persons per Year

Table 4 Districts with Malaria Incidence Rates of 40-100 Cases per 1000 Persons per Year

Province

Province

District (Subdistrict)

Central region, eastern part Trat Borai (except Nong Bon District) KhongYai Koh Kut Northern region Tak Thasongyang (Maetan and Maela subdistricts) Maeramat (Maeramat, Maetuen and Kanejue subdistricts) Maesod Umphang (Nongluang and Maechan subdistricts) Central region, western part Kanchanaburi Saiyok Srisawat Ratchabur i Suan Phueng Phetchaburi NongYa Plong Prajuab Kirikhan Hua Hin (Nongphlap and Huay SatYai subdistricts only) Kuiburi Muang

due to local transmission. Ratchaburi, Phetchaburi, and Prajuab Kirikhan followed Kanchanaburi in terms of exhibiting a single midyear seasonal peak. Southern Region

Malaria seasons in southern Thailand vary from province to province. In 1992, Ranong, Krabi, and Suratthani exhibited a minor peak in January and a major one around June-August. In Chumphon, Nakhon SriThammarat, and Songkhla, only one peak (from May to August) was observed. Border posts in Ranong were known for handling the busy trade trafic with Myanmar, and this partly explained the higher percentage of imported cases (58.8%) in Ranong than elsewhere in the South. In other provinces, transmission was largely local, and the percentages of P!fakipamm cases were relatively low: Krabi (1495 cases, 0.7% imported, 38.3% P$lcipamm), Suratthani (4.7% imported 34.7% P$lcipamm; seeTable 2). Nakorn SriThammarat (1427 cases, 7.6% imported, 31.3% Pj.1cipamm),Songkhla (1018 cases, 2.4% imported, 39.6% I? falcipamm). In the far south,Yala, Pattani, and Narathiwat experienced a minor transmission peak in February-March and a major one in June-August. InYala, 1699 cases were noted (34.6% I?!faukcipamm),in Pattani, 1169 cases, (37.9% P$kipamm),and 1485 cases (45.3% P$lcipumm)

District (Subdistrict)

Central region, eastern part Trat Borai (Nong Bon subdistrict) Laem Ngop (including Koh Chang island) Muang Chanthaburi Pong Namron Makham Soidao Kaeng Hang Maew Khlung Chonburi Bo Thong Prajeenburi Nadee Prajantakharn Khlong Hat Watthana Nakhorn Muang Nakhon Nayok Pakphli Banna Muang Northern region Tak Urn Phang (Mokro, Mae Lamung, and Urn Phang subdistricts) Phobphra Maeramat (Sam Muen, Maejarao subdistricts) Mae Hong Son Sop Moey Muang Chiangmai Fang Mae-ai Chaiprakan Central region, western part Kanchanaburi Sangkhlaburi Thong Phaphum Dan Makhamtia Ratchaburi Phaktho Chombueng Phetchaburi Thayang Kaeng Krachan Prajuab Kirikhan Pranburi Bangsaphan Northeastern region Srisaket Phusing Khun Han Kan Thoralak Khu Khan

in Narathiwat. Imported cases are not common in provinces along the Thai-Malaysian border. Northeastern Region

Ubon Ratchathani, Srisaket,and Surin Provinces also accounted for some imported cases from Cambodia. Similar to Trat, high percentages of the infections were

Thimasarn et al, Malaria in Thailand

Table 5 Districts with Malaria Incidence Rates of 20-39.9 Cases per 1000 Persons per Year Province

District (Subdistrict)

Central region, eastern part Trat Khao Saming Chanthaburi Thamai Rayong Namyen Ban Khai Pluak Dang Prajeenburi Sa Kaew Cha Choeng Sao Tah Takiap (Klong Takrao subdistrict) Central region, western part Prajuab Kirikhan Bang Saphannoi Northern region Tak Bantak Sam Ngao Muang Mae Hong Son KhunYuam Mae Lanoi Pai Pang Mapha Mae Sarieng Chiangmai Mae Chaem Hod Jomthong Doitao Chiangrai Chiangkho Wiang Kaen Nan Mae Charim Santisuk Muang Southern region Suratthani Phra Saeng Chaiburi Muang Pounpin Koh Pa-ngan Island Kiancha Phanom Ban Takhoun Chumphon Thasae Langsuan Lamae Ranong Kraburi La-oun Songkhla Sadao Yala Yaha Kabang Narathiwat Ruesoh Srisakhorn R a Ngae Chanae

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due to Pfulciparurn (see Table 1). Peak transmission occurred during the dry season from December to February. Surin Province had a total of 1613 cases (48.9% Pfakiparurn) and 45.8% (548 of 1196) imported. In the upper part of the region along theThai-Laotian border, malaria is generally minimal compared to incidences in the Thai-Myanmar and Thai-Cambodian borders. This region along the Thai-Laotian border is flat, a plain on the bank of Mekong River, with little dense forest. Area-Specific Malaria Incidence Rates

Results of data analyses by district are the most useful as indicators of malaria risk in Thailand. Tables 3-5 list districts by level of malaria incidence in 1992. A group of villages in the Maesod District ofTak Province was found to have the highest incidence rate (426.5 of 1000 persons per year). Districts with less than 30 cases per year or incidence rates less than 20 of 1000 persons per year are not listed. For more specific risk areas within a particular district, interested readers may check with the Malaria Division or the corresponding author. Areas with Data Suggesting Mefloquine Resistance

Since 1985, under the national control program, mefloquine, in combination with sulfadoxine and pyrimethamine (MSP) has been used as the first line drug for the treatment offalcipartrrn malaria in Thailand. In our case investigation system, “new” cases and cases

Table 6 Number and Percentage of Repeat Cases “not cured”, bv Province of the Fifteen Provinces Cases Investigated (No.)

Province ~

Repeat Cases “Not Cured” (No.)

(%)

33,667 10,357 4,446

4,597 938 423

13.7 9.1 9.5

18,004 6,959 2,523

816 70 45

4.5 1.0 1.8

10,321 4,076 1,906 3,902

117 52 17 74

1.1 1.3 0.9 1.9

2,870 3,000 3,822

63 34 24

2.2 1.1 0.6

2,846 2,211

219 26

7.7 1.2

~

Central region, eastern part Trat Chanthaburi Prajeenburi Northern region Tak Mae Hong Son Chiangmai Central region, western part Kanchanaburi Ratchaburi Phetchaburi Prajuab IOrikhan Southern region Chumphon Ranong Suratthani Northeastern region Srisaket Ubon Ratchathani

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returned within 3 months (“repeat cases”) are grouped separately. Each repeat case is classified as either a reinfection or a recrudescence and/or relapse (“not cured”) based on an interview and review of treatment records. A comparative analysis of the proportion of cases “not cured” by province showed the results to be compatible with the occurrence of mefloquine resistance along the Thai-Cambodian border (7.7-13.7%, in Srisaket, Prajeenburi, Chanthaburi, and Trat) and in Tak Province (4.5%) (Table 6).

Discussion The data reviewed in this article encompasses all malaria sectors and health institutions in Thailand.Theoretically it is almost complete. However, it is not possible to include malaria cases that are self-treated because a prescription is not required for the purchase of antimalarial drugs in Thailand. The case investigation component of the system is complete, except when a patient attends a malaria clinic that belongs to a different malaria sector from the one responsible for hidher home village.This may be associated with a small degree of underestimation of cases. O u r data analyses include all malaria cases reported from January to December 1992. An apparent discrepancy between our total number of cases and what has been previously reported by the Malaria Division is noted.The Malaria Division’s report represents a fiscal year period (i.e., the 1992 statistics cover the period from October 1991 to September 1992). In addition, some 40,000 cases of malaria among migrants from neighboring countries (Myanmar, Cambodia and Laos) are reported each year.These are not included in our report. Malaria cases in Thailand concentrate along the Thai-Cambodian and Thai-Myanmar borders. Infection due to transmission outside Thailand is common. In 1990, hundreds of gem miners Gom all over Thailand and neighboring countries traveled to western Cambodia each day through crossing points in Borai District, Trat Province. Lack of adequate medical services in Cambodia usually caused those who became ill to return to Borai for treatment. Gem mining in Cambodia declined in 1991-1992 and has substantially dropped since 1993. In early 1994, an average of 500 malaria cases per month were reported in Borai.The malaria situation described in this paper, particularly for the Thai-Cambodian border areas, may be quite different when this article appears in print. The overall risk of malaria within Thailand is relatively small.Travelers who join an organized tour, follow tour schedules, such as daytime excursions to rural villages, and who stay in air conditioned hotels in urban areas are essentially at no risk. Malaria-endemic areas, especially those listed in Tables 3-5, generally offer few places of

J o u r n a l o f T r a v e l M e d i c i n e , V o l u m e 2, N u m b e r 2

attraction to most foreign tourists. Also, for each at-risk district listed, the risk is not uniform, and parts of listed districts may be malaria free. For example, Hua Hin is a popular beach resort, for both local and foreign visitors. The seaside resort located in Hua Hin District, Prajuab Kirikhan Province, is totally malaria-bee. Malaria-endemic areas are confined to Nong Plub and Huay SatYai subdistricts (see Table 3 ) ,far from popular tourist tracks. Adventurous tourists, however, who prefer to visit remote areas and pursue outdoor activities such as trekking, rafting, elephant riding, and camping in the jungles, are at risk of malaria. Popular destinations for such activities are in the mountainous areas along the ThaiMyanmar border in Kanchanaburi, Mae Hong Son, Chiengmai, and Chiengrai. The three levels of malaria incidence rates for 1992 presented (see Tables 3-5) are indicative of different degrees of risk. Malaria seasons inThailand vary hpm place to place, and sometimes, from year to year, depending on the monsoons.Variation in rainfall and geographic characteristics has a differential effect upon the population dynamics of the two major mosquito vectors: Anopheles minimus and Anopheles dims. Usually two transmission peaks forTrat andTak provinces occur, a single rainy season peak for the west-central region, either one peak or one major and one minor for the South, and only a single dry season peak for the Northeast. However, all levels of endemicity should be considered risky regardless of season. It should also be noted that the incidence rates shown in this report were underestimated due to the unknown number of self-treated cases and incomplete case investigation. The rates reflect risks among semiimmune villagers, and not nonimmune visitors, in whom the risks are likely to be several times higher. Resistance of I?fak@artrm to mefloquine in some localized areas in Thailand has been documented both in vivo and in vitro.”s Resistance was first observed on the Thai-Cambodian border, but is now believed to have spread to the Thai-Myanmar border in the direction of the gem trade route from Borai to Maesod District. Treatment failure of mefloquine alone, as high as 67% has been recorded in Trat Province.6 The data on areas suggestive of mefloquine resistance (see Table 6) indirectly provide supportive evidence and is meaningful only in a comparative sense, (e.g.,9.1% of the cases in Chanthaburi returned within 3 months as “not cured”versus 1.l% in Ranong) .These percentages do not by themselves indicate the extent of mefloquine resistance, which is likely to be underestimated by these statistics for two obvious reasons.The first reason is that in our previous crosssectional surveys of gem miners in Borai, asymptomatic low levels of parasitemia and selftreatment for parasitemia with mild symptoms were common. These patients tended not to return to the malaria clinics, unless clin-

T h i m a s a r n e t al, M a l a r i a i n T h a i l a n d

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ically worse.The other reason is that those who showed up as treatment failures at a different malaria clinic may not have been traced. Alternatively, the number of cases “not cured” (the numerator used in this analysis) was inclusive of both recrudescent cases of Pfakcipamm and relapse cases of I? vivax (chloroquine-resistant I? vivax has not yet been documented inThailand) and therefore may have contributed to some degree of overestimation. Malaria incidence in Thailand has significantly decreased during the past two decades. Several more provinces, especially those in the central plain and the northeastern plateau are to be declared malaria-free in the near future.The situation along the borders, however, is unpredictable, in spite of the rigorous malaria control program within the Thai territory. Success depends largely on population movement.This is subject to political and economic stability of our neighboring countries. Eradication is unlikely without at least an equal effort being made across the borders. Problems of multidrug resistantfakiparum malaria in certain areas ofThailand mean that, at present, the Thai Ministry of Public Health does not recommend chemoprophylaxis.The intention is rather to encourage rigorous use of personal protective measures,to diminish a false sense of protection by reliance on prophylactic regimens, and to urge the need for prompt medical attention when fever develops. Malaria clinics are located in all endemic areas of the country and provide services regardless of a patient’s nationality. Microscopic diagno-

sis is usually made within half an hour, and those with positive blood films receive antimalarial drugs at n o cost.

Editorial Commentary

of malaria, even visitors to areas that border Cambodia and Burma (Myanmar) rarely require chemoprophylaxis, and the Thai Ministry of Public Health does not recommend it. Travelers with a febrile illness can readily receive medical attention from Thai physicians or malaria clinics located in all endemic areas of the country. Microscopic diagnosis is usually made rapidly, and appropriate treatment is available at no cost. For the same reason, antimalarial drugs should not be prescribed for self treatment of a febrile illness.

The information in this article “Epidemiology of Malaria in Thailand” enables practitioners of travel medicine to counsel travelers to Thailand about the malaria situation in that country and to advise them not to use malaria chemoprophylaxis.The article reports the incidence of malaria in the regions and provinces ofThailand. It is important to note two limitations to these data. First, the incidence data in this review may not represent the local risk of infection for the Thai population since in some provinces, a high proportion of cases is imported from outsideThailand. Second, it is difficult to infer the risk of malaria for foreign visitors to Thailand from the incidence of malaria among the Thai population.The risk of malaria for U.S. and U.K. visitors toThailand is estimated to range between 1-10 cases per 100,000 visitors per year. Although U.S. Peace Corps volunteers often live in remote areas ofThailand and do not use chemoprophylaxis, malaria cases have not been identified among these volunteers. Because of the very low risk

Acknowledgments We are grateful to Dr. Henry Wilde, Dr. L. Molineaux, and Dr. D. Gray Heppner for reviewing the manuscript and to Dr. Apiluck Tumtavitikul for her kind assistance on the phonetic transcription of the district names. References 1. Ministry of Public Health, Malaria Division. Annual report for the fiscal year of 2534. Bangkok: Ministry of Public Health, Department of Communicable Disease Control, 1993:lO. 2. SPSS-X user’s guide. 3rd Ed. Chicago: SPSS Inc, 1988. 3. Boudreau EF, Webster HK, Pavanand K,Thosingha L. Type II mefloquine resistance in Thailand. Lancet 1982; 2: 1335. 4. Nosten F, Imvithaya S,Vincenti M, et al. Malaria o n the Thai-Burmese border: treatment of 5,192 patients with mefloquine-sulfadoxine-pyrimethaminecombination. Bull World Health Organ 1987; 655391-896. 5 . Wongsrichanalai C,Webster HK,WimonwattrawateeT,et al. Emergence of multidrug resistant Plasmodium falcipamm in Thailand: in vitro tracking. Am J Trop Med Hyg 1992; 47: 112-1 16. 6 . Ketrangsee S,Vijaykadga S,Yamokgul P, et al. Comparative trial on the response of PIasmodiumfaIcipanrm to halofantrine and mefloquine inTrat Province, easternThailand. Southeast Asian J Trop Med Public Health 1992; 23:55-58.

Hans 0.Lobel, MD Division of Parasitic Diseases, National Centerfor Infectious Diseases, Centersfor Disease Control, Atlanta, GA

The views expressed in this editorial are those of the author and do not necessarily reflect the official policy or position of the Public Health Service or the US. Department of Health and Human Services.