Re-emergence of Plasmodium malariae in Trinidad, West Indies

A focus of Plasmodium malariae infection has recently occurred on the island of Trinidad, some 30 years after a successful eradication programme. Examination of bloodsmears revealed 22 cases of P. malariae in the Nariva-Mayaro area of Trinidad between August 1994 and September 1995. Most (77%) of the cases were male and, as seven were aged < 25 years of age, it appeared that transmission had been renewed, probably by the vector Anopheles bellator. However, none of the 3000 mosquitoes tested by ELISA for the circumsporozoite protein of P. malariae proved positive. Use of IFAT to check blood samples for P. malariae appeared more sensitive than direct examination of bloodsmears, indicating that 42 (13%) of the 325 samples tested were seropositive (at titres of 1:256 or greater). The levels of transmission of the parasite may therefore be even higher than indicated by examination of bloodsmears. The surveillance measures adopted to understand the epidemiology of this outbreak of P. malariae in Trinidad are described. The need to maintain malaria surveillance in all the countries where P. malariae parasites once existed (prior to eradication) is emphasised.