The interaction between Plasmodium falciparumand P. vivaxin children on Espiritu Santo island, Vanuatu

Studies of the prevalence and incidence of malaria were conducted in children <10 years old living in 10 rural villages on the island of Espiritu Santo, Vanuatu, south-west Pacific. Malaria prevalence remained stable at 30% throughout the year but the relative contributions of the 2 major species were highly dependent on season. Plasmodium falciparumpredominated in the long wet season (November–May) and P. vivaxin the dry season (June–October). Case definitions for malaria, derived using a multiple logistic regression method, showed that parasite densities associated with clinical disease were low; case definitions for P. falciparum(>1000 parasites/µL in children > 1 year old and >500/µL in infants) and P. vivax(>500 parasites/µL at all ages) were both associated with a specificity and sensitivity of >90%. Like prevalence data, malaria morbidity was highly seasonal; 80% of clinical P. falciparuminfections occurred in the wet season and 66% of clinical P. vivaxin the dry season. Mixed infections were rare. Malaria was an important cause of morbidity with children <5 years old experiencing 1.3–2.0 episodes of clinical malaria per year and 23% of fevers being attributable to malaria in this age group. Children aged 5–9 years continued to suffer one episode of clinical malaria per year. The peak incidence of P. vivaxmalaria occurred earlier in life than the peak incidence of P. falciparummalaria. The possible interactions between these 2 parasite species are discussed.