Malaria and its possible control on the island of Príncipe

Abstract

Background: Malaria can be eradicated from islands. To assess the prospects for eradication of malaria from the island of Príncipe in the Gulf of Guinea, we fitted a mathematical model to age-prevalence curves and thus obtained estimates of the vectorial capacity and of the basic reproductive number (R0) for malaria.

Methods: A cross-sectional malariological survey was carried out, in mid-1999, in six communities, comprising circa 17% of the total 6,000 population of the island. All houses in these communities were registered and their mode of construction recorded. Thick and thin blood films were prepared from all consenting individuals. Each individual was asked whether they possessed a mosquito net, whether they had slept under a mosquito net the previous night, whether they were allergic to chloroquine, and whether they had visited the main island of São Tomé since the beginning of the year. Outpatient records from March 1999 until the end of December 2000 were also examined and the age and place of residence of diagnosed cases noted.

Results: 203 (19.8%) of the 1,026 individuals examined were found to be infected with Plasmodium falciparum. By fitting the mathematical model of the Garki project to the age-prevalence curve we estimate that the basic reproductive number, R0, on the island is approximately 1.6. Over a period of one year, a total of 1,792 P. falciparum cases reported to an outpatient facility at the island's hospital. Overall, 54% of the people interviewed slept under mosquito nets and were at reduced risk of infection. Conversely, people living in houses with openings between the top of the wall and the roof had higher risk of infection.

Conclusion: This high incidence suggests that most of the malaria cases on the island attend the hospital and that treatment of these cases is an important factor reducing the effective rate of transmission. Providing that clinical cases are effectively treated, endemic malaria can probably be eliminated from the island mainly by reducing exposure to the vector with simple measures such as insecticide-treated nets and mosquito-proofing of dwellings. In contrast to traditional malaria eradication strategies, this would avoid the risk of malaria epidemics because the reduction in R0 should be sustainable.

Figure 1

Map of Príncipe showing collection sites

Figure 2

Prevalence of microscopy positive P. falciparum by age. Dots: observed prevalence, line: fitted line (using the Garki model, adjusted for seasonality)

Figure 3

Seasonal pattern of fever incidence at the hospital, Santo Antδnio, Príncipe.

Figure 4

Age-sex pattern of incidence of fever with P. falciparum parasitaemia at the hospital, Santo Antδnio, Príncipe.