Asymptomatic malaria parasitaemia and seroreactivities to Plasmodium falciparum antigens in blood donors from Ibadan, south-western Nigeria.

Malaria parasite rates, parasite densities and seroreactivities to two Plasmodium falciparum antigens (Pf155/RESA and circumsporozoite protein) were investigated in a random sample of 416 blood donors attending the Blood Transfusion Unit of the University College Hospital in Ibadan, south-western Nigeria: 224 in October-November 1991 and 192 in March 1992. The incidence of malaria parasitaeia observed in 1991 was significantly higher than that seen in 1992 (41% v. 19%; P < 0.001). In contrast, the geometric mean parasite density in 1992 was significantly higher than in 1991 (440 v. 191) parasites/microliters blood; P < 0.001). Although parasite rates were highest in the group aged 25-31 years in both surveys, there was no apparent correlation between age of donor and parasite density in either survey. Parasite density was significantly higher in AA- than in AS-haemoglobin individuals only in the 1992 survey (P = 0.050). All the blood donors were seropositive for antibodies to crude parasite antigens, indicating heavy exposure to malaria infection. Seroreactivity to Pf155/RESA was similar in the two surveys but that to circumsporozoite protein (CSP) was significantly higher in 1991 than in 1992 (P < 0.001). The seropositivity rates were generally similar to malaria-positive and -negative blood donors. In 1992, however, all the blood donors with high reactivities to Pf155/RESA, as detected by erythrocyte membrane immunofluorscence, were negative for malaria parasites, indicating that this group was relatively protected against malaria parasitaemia. It is recommended that blood samples from prospective blood donors be examined for malaria parasites and that recipients of malaria-infected blood samples be given a curative regimen of antimalarials.