The absence of endemic malaria transmission in Taiwan from 2002 to 2010: The implications of sustained malaria elimination in Taiwan
Introduction
Malaria remains one of the most important public health problems. One hundred and eight countries around the world are endemic for malaria, a disease that infects approximately 350–500 million and causes approximately 1 million deaths worldwide annually.1 The risk of developing malaria may change over time because of global shifts in the epidemiology of malaria, changes in travel habits and patterns of migration, and the development of drug resistance.2, 3 The risk of infection during travel can be reduced by preventing mosquito bites with clothing, insect repellents, and bed nets and by taking an appropriate chemoprophylaxis.4, 5 The extent to which these measures are adopted depends on how well a traveler recognizes and understands the risks.6
The risk of a traveler acquiring malaria is considered the highest in sub-Saharan Africa and Papua New Guinea, intermediate on the Indian subcontinent, and the lowest in Southeast Asia and Latin America.7, 8, 9 The numbers assigned to the relative risk in these regions, however, are quite variable.10, 11, 12 The total number of travelers is often unknown, and most reports are based on national reporting data and therefore lack a denominator. Hence, assessing risk based on such figures is difficult. The country-specific risk for acquiring malaria varied from 714 per 100,000 travelers in Ghana to 2.5 per 100,000 travelers in Thailand.13
The resurgence of malaria in many of the areas from which malaria was previously eliminated during the Global Malaria Eradication Programme serves as a reminder that vigilant systems need to be sustained for as long as the mosquito vectors, a suitable climate and other conditions exist to facilitate disease transmission.14 The risk of resurgence is determined by the prevailing vectorial capacity (receptivity), the malaria importation rate (vulnerability), and the malariogenic potential.15, 16, 17 Therefore, malaria elimination, once achieved, is more likely to be sustained in regions where receptivity is low or decreased by human development and in geographically isolated areas with limited movement across the border and limited importation of parasites.18, 19
Taiwan is located at 23°4′ N and 121°0′ E and has a subtropical climate; temperatures range from cool to hot, and the humidity is relatively high throughout the year. Malaria is documented to have been prevalent throughout Taiwan during the 19th and 20th centuries. The maximum estimated number of cases was 1.2 million in 1952.20, 21 During the late 1960s, a combination of improved housing and socioeconomic conditions, environmental management, an intensive programme of residual spraying with DDT in Taiwan carried out over a period of 5 years, and case management has reduced malaria morbidity to a very low level in Taiwan.22, 23 In November 1965, the World Health Organization (WHO) certified Taiwan as an area where malaria had been eradicated.24 Since then, malaria case surveillance has been maintained to detect locally acquired cases that could indicate the reintroduction of transmission and to monitor patterns of resistance to antimalarial drugs. Imported malaria cases have been diagnosed in this country during the last four decades. The majority of the cases were imported from endemic countries,25 and a few cases were contracted at medical facilities.26 Imported malaria has been an increasing problem in Taiwan and Western countries in the last 2 decades. Two possible reasons for this increase are the increase in the number of travelers to tropical countries and the growing number of immigrants from malaria-endemic countries.6, 27, 28 Although the number of malaria cases has been declining during the past several years in Taiwan,20 the risk for travelers is still evident and should be a concern for physicians who provide pre-travel advice or evaluate a returning traveler with a fever.
To identify trends and risk groups, we analyzed the surveillance data for all malaria cases in Taiwan from 2002 to 2010. We compared the data with information available on the number of travelers and the Anopheles. minimus mosquito distribution in Taiwan to determine whether these data could be useful for improving the existing surveillance system and pre-travel recommendations.
Section snippets
Data sources
Since 1990, the National Notifiable Diseases Surveillance System (NNDSS) has reported malaria cases to the Centers for Disease Control of Taiwan (Taiwan CDC), as previously described in the literature.29 Malaria is a reportable disease in Taiwan. After reports were received, an epidemiologic team (fields epidemiologist, entomologist, public health nurse) was assigned to perform a patient follow-up, verify the diagnosis and complete patient information. Follow-up consisted of in-person
Results
From 2002 to 2010, a total of 195 persons were reported with malaria in Taiwan. Two of these patients were excluded from the study because their infection sources could not be determined. Table 1 shows the socio-demographic characteristics of the patients. The mean age was 39.8 years (SD = 10.2), ranging from 3 to 70 years. Most of the patients (94%) were older than 18 years of age. The male-to-female ratio was 5.2 to 1. The reasons for travel were business (69%), visiting friends or relatives
Discussion
During 2002–2010, 193 cases of imported malaria were reported in Taiwan. Out of the cases with a known place of disease acquisition, 44% were acquired in Africa and 42% were acquired in Asia. P. falciparum (49%) was the dominant imported species; no fatalities were reported. Imported cases were associated with travel to high-risk malaria-endemic areas, such as Africa, primarily for business or to VFR.
The strength of our study is the data, which was coupled with additional data for the number of
Conflict of interest
All authors have confirmed that they have no conflict of interest either financially or via personal relationships as defined by Travel Medicine and Infectious Disease.
Acknowledgments
This study was supported by a grant (DOH100-DC-1018) from the Centers for Disease Control, Department of Health, Taiwan.
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