Perennial Malaria Transmission Research Articles

Immune responses to band 3 neoantigens on Plasmodium falciparum-infected erythrocytes in subjects living in an area of intense malaria transmission are associated with low parasite density and high hematocrit value.

During the intracellular development of the human malarial parasite, Plasmodium falciparum, cryptic regions of the erythrocyte band 3 protein are exposed. Antibodies against these band 3-related neoantigens block cytoadherence, and peptides based on amino acid sequences of putative exofacial loops of band 3 protein block the in vitro and in vivo adherence of P. falciparum-infected erythrocytes. At present, it is not known whether reactivity to these antigens is related to exposure to the malaria parasite or is correlated with protective immunity. The reactivities of plasma to peptides containing amino acid sequences of putative exofacial loops 3 and 7 of human band 3 protein were determined for children and adults living in an area of perennial malaria transmission (Liberia) and for donors who had never been exposed to malaria (Denmark). Plasma samples from children and adults living in an area of intense malaria transmission showed a much higher reactivity with the band 3 peptides than did those from nonimmune individuals. High reactivity to the loop 3 peptide (amino acids 546 to 555) was correlated with lower mean parasite density in children in the 5- to 9-year-old age group. The presence of antibodies against loop 3 and 7 peptides was not associated with a low packed erythrocyte volume (hematocrit); in fact, higher-than-average reactivities to both peptides were positively correlated with high hematocrit values, indicating that antibodies which specifically recognize the band 3-related neoantigens are not involved in hemolysis (autoimmunity).

Plasmodium falciparum sporozoite and entomological inoculation rates at the Ahero rice irrigation scheme and the Miwani sugar-belt in western Kenya.

Anopheles arabiensis and An. funestus were collected by pyrethrum spray sheet collections in houses and by human-bait catches at a village in western Kenya adjacent to the Ahero rice irrigation scheme; and using the same methods, An. gambiae s.l. and An. funestus were collected at Miwani, a village in the sugar-cane belt. Plasmodium falciparum sporozoite rates were determined by ELISA. At Ahero the mean sporozoite rates were 1.1% and 4.3% in An. arabiensis and An. funestus, respectively, while at Miwani the rates were 6.0% in An. gambiae s.l. and 4.3% in An. funestus. Entomolgoical inoculation rates (EIR) were derived from both human-bait collections (IR-HBC) and by the proportion of human blood-fed females caught resting indoors (IR-HBF). The IR-HBF appeared to be a more realistic index of EIR. At Ahero and Miwani people were exposed to an average of 416 and 91 infective bites/person/year, respectively. The main vectors were An. funestus at Ahero and An. gambiae s.l. at Miwani. In view of the intense and perennial malaria transmission at Ahero, vector control by insecticides should be considered, while at Miwani, where transmission is seasonal, permethrin-impregnated bed nets could be an alternative to indoor spraying. These measures must be augmented with availability of effective antimalarials.

In vitro lymphoproliferative responses to malaria antigens: a prospective study of residents of a holoendemic area with perennial malaria transmission

A longitudinal, prospective study to examine the relationship between the outcome of infection with Plasmodium falciparum parasites and in vitro T-cell proliferative responses to a P. falciparum schizont extract (PfSE) was conducted in a village in south-eastern Gabon, an area where malaria is holoendemic and transmission is intense and perennial. The donor's age was found to have a strong independent influence on all malariometric indices. At the community level, the in vitro lymphoproliferative response to PfSE was bimodal with 30% of the villagers studied showing persistently low responses. The frequency of low or high responders within the study population did not show any consistent relationship with the community parasite rates or the number of either patent parasitaemic episodes or clinical malarial attacks per individual. At the individual donor level, the response was negatively correlated with P. falciparum parasite density in those donors who were parasitaemic at the time of sampling. High in vitro lymphoproliferative responses to PfSE were predictive of resistance to clinical malaria. The PfSE-induced in vitro lymphoproliferative response was dependent on antigen presenting cells, CD4+ T-cells and UCHL-1+ cells.