THE INTERACTION BETWEEN PLASMODIUM-FALCIPARUM AND PLASMODIUM-VIVAX INCHILDREN ON ESPIRITU-SANTO ISLAND, VANUATU

Studies of the prevalence and incidence of malaria were conducted in c hildren <10 years old living in 10 rural villages on the island of Esp iritu Santo, Vanuatu, south-west Pacific. Malaria prevalence remained stable at 30% throughout the year but the relative contributions of th e 2 major species were highly dependent on season. Plasmodium falcipar um predominated in the long wet season (November-May) and P. vivax in the dry season (June-October). Case definitions for malaria, derived u sing a multiple logistic regression method, showed that parasite densi ties associated with clinical disease were low; case definitions for P . falciparum (>1000 parasites/mu L in children >1 year old and >500/mu L in infants) and P. vivax (>500 parasites/mu L at all ages) were bot h associated with a specificity and sensitivity of >90%. Like prevalen ce data, malaria morbidity was highly seasonal; 80% of clinical P. fal ciparum infections occurred in the wet season and 66% of clinical P. v ivax in the dry season. Mixed infections were rare. Malaria was an imp ortant cause of morbidity with children <5 years old experiencing 1.3- 2.0 episodes of clinical malaria per year and 23% of fevers being attr ibutable to malaria in this age group. Children aged 5-9 years continu ed to suffer one episode of clinical malaria per year. The peak incide nce of P. vivax malaria occurred earlier in life than the peak inciden ce of P. falciparum malaria. The possible interactions between these 2 parasite species are discussed.