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The Democratic Republic of São Tomé and Príncipe (DRSTP) is situated 220 km off the Africa continent, west of Gabon in the Gulf of Guinea, with a total land area of 1001 km2 and a population of 150,000 inhabitants. Malaria has been endemic, with 
71% of the human population infected with malaria parasites in some districts, and malaria accounted for 32% of national morbidity in 2000 and 47.8% in 2002. All 4 species of human malaria are present at different levels.1 In 1979 hematological surveys showed that 74% of malaria infections were from Plasmodium falciparum, 15.8% from Plasmodium vivax, 0.6% from Plasmodium malariae, and 9.6% from P. vivax + P. falciparum.2 The only vector is Anopheles gambiae s.s., the forest cytoform, and it is largely confined to coastal areas, being virtually absent at altitudes > 200 m.3 In the early 1980s, the malaria control campaign with IRS using DDT successfully reduced malaria prevalence from 19.2% to 0.6%1,4; however, because of financial and administrative problems, the IRS campaign was suspended, and malaria outbreaks occurred in 1985 and 1986, causing high malaria prevalence and mortality.1,4
There is no ideal solution to the problem of malaria control; DDT house spraying has its limitations, and there is a continuing need for operational research to improve the cost-effectiveness of this approach.5 To institute more active ways of vector control to fight malaria in São Tomé and Principe, authorities from DRSTP and Taiwan agreed to initiate a pilot study. The study was carried out in 2003 with 3 methods of intervention, that is, indoor residual spraying (IRS) with alphacypermethrin at a dosage with 50 mg/m2, nocturnal fogging of pirimiphosmethyl at 100 g/hectare, and larval control with an insect growth regulator (IGR), pyriproxyfen, at 2 g/m3. Unpublished data of the pilot study demonstrate that IRS is an effective method for malaria control in DRSTP. Thus, it was then determined to carry out a nationwide IRS campaign with alphacypermethrin once per year, commencing from 2004.
Although IRS with DDT has been criticized because of supposed environmental damage, it is indeed an effective method to eliminate malaria in many countries, such as the United States, the Soviet Union, Europe, and Taiwan, and it also reduced the malaria incidence rate significantly in Southeast and South Asia, southern Africa, and South America.6 At present, IRS is still considered as one of the most effective and economical measures of malaria control in the world.6 In Pakistan, a single round of IRS with alphacypermethrin at a dosage of 25 mg active ingredient per square meter of surface area was demonstrated to be effective for the entire transmission season.7
We report herein the achievements so far made by means of IRS with alphacypermethrin for effective control of malaria in DRSTP. Yearly cycles of nationwide IRS campaigns with the pyrethroid insecticide, alphacypermethrin (10% suspension concentrate), at 50 mg/m2, were initiated in DRSTP. São Tomé Island is administratively divided into 6 districts, and Príncipe Island is an autonomous region (Figure 1
). For IRS, 48 workers were recruited and trained for the operation in Príncipe, and 42 workers, including foremen, were recruited and trained for the operation in São Tomé. The first yearly cycle of spraying campaign was initiated on October 25 and was completed on November 11, 2004, in Príncipe, and various districts of São Tomé were treated between December 2004 and October 2005 as shown in Table 1. The acceptance rate of IRS for dwellings and outhouses varied from 81.9% to 94.7% (av. 87.24%) with population coverage of 93.8%.
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The results of parasitological assessment of the children < 9 years of age regardless of presence of any malaria symptoms before the IRS campaign and 12 months after the first spraying showed that the overall prevalence of malaria parasitemia for all districts was reduced from 20.1% to 2.8% at 12 months after the first IRS, which was reduced further to 0.7% at 8 months after the second IRS (Table 1
).
Bioassays showed the residual efficacy of alphacypermethrin to last for at least 424 days when the pyrethroid was applied to plywood, plastic-coated plywood, and wrapping paper at dosages of 20, 30, and 50 mg of active ingredient per square meter of surface area.8 However, the insecticidal effect of alphacypermethrin sprayed on cement walls persisted for only a few weeks. In DRSTP, most houses are built of wooden materials, especially in the rural areas where malaria is highly endemic.
The IRS campaign with alphacypermethrin has already been shown to have effectively lowered the prevalence of malaria parasitemia and achieved a rapid reduction of malaria infection in DRSTP. IRS with alphacypermethrin for malaria control was highly acceptable to the people in São Tomé and Príncipe due to the following facts: 1) no visible residue remained on the sprayed walls, 2) no domestic livestock died, even if they ingested moribund household pests, such as cockroaches etc., and 3) no cats died.9 Thus it has shown that IRS with alphacypermethrin is an effective measure for malaria control in São Tomé and Príncipe. In Zanzibar, similar recent success was demonstrated with the pyrethroid lamdacyhalothrin.10 Records from hospitals and health centers showed that malaria declined immediately following a 54-day IRS effort.10 Successful use of pyriproxyfen against malaria was demonstrated in Sri Lanka.11 This chemical persists much longer in water than does Bacillus thuringiensis israelensis (Bti). With the aim of preventing development of insecticide resistance by vector mosquitoes, we are planning to commence larval control, using IGR or Bti as an alternative to the IRS campaign, after completion of 3 yearly cycles of IRS with alphacypermethrin.
Received April 10, 2007. Accepted for publication September 28, 2007.
Acknowledgments: We are grateful to the Ministry of Foreign Affairs of Taiwan for financial support, to the International Cooperation and Development Fund of Taiwan (ICDF) for administrative support, and to the Center for Disease Control of Taiwan for technical assistance. Many thanks also go to the staff of CNE for their close cooperation in field operations, to the short-term assistants from the CDC of Taiwan for their technical assistance in the laboratory, and to Chang Chi Lin, the Department of Microbiology and Immunology, National Defense Medical Center, for his PCR tests to determine the vector mosquito identity and insecticide resistance.
* Address correspondence to Jih Ching Lien, Institute of Preventive Medicine, National Defense Medical Center, P.O. Box 90048-700 Sanhsia, Taipei County, Taiwan 237. E-mail: jihchinglien{at}gmail.com 
Authors’ addresses: Lien Fen Tseng, Wen Chun Chang, Cheng Hua Wu, and Jih Ching Lien, The Anti-Malaria Team of Taiwan Medical Mission to São Tomé and Príncipe, C.P. 839, São Tomé e Prncipe. í Maria Conceição Ferreira and Herodes Sacramento Rampão, Centro Nacional de Endemias (CNE), República Democrática de São Tom é e Príncipe. Jih Ching Lien, Institute of Preventive Medicine, National Defense Medical Center, Sanhsia, Taipei County, Taiwan 237, Telephone: +886-2-25944651, Fax: +886-2-25947988, E-mail: jihchinglien{at}gmail.com.
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