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Blood samples from cows and pigs were tested for possible cross-reactivity with a monoclonal antibody-based enzyme-linked immunosorbent assay (ELISA) kit designed for detection of human malaria sporozoites in mosquitoes. The results revealed that 4 of 16 cows (25%) reacted positively with both Plasmodium falciparum (2A10) and P. vivax (NSV3) monoclonal antibodies and 8 (50%) were positive with NSV3 only. One of 12 pigs (8.33%) was positive with both antibodies, and 2 (16.6%) were positive with NSV3 only. The positivity was associated with plasma, but not with the blood cell fraction. Antigenic extracts of Sarcocystis, Toxoplasma gondii and Trypanosoma evansi gave negative ELISA results, suggesting that these were not the factors in animal blood which gave positive results. Laboratory Anopheles dirus A fed on blood of a positive cow by membrane feeding also gave a positive ELISA result. Furthermore, some blood-fed culicine mosquitoes collected directly from a positive cow were ELISA-positive. The cross-reactive factor(s) in plasma has (have) not yet been identified. These false positive ELISA results could complicate the assessment of sporozoite rate in mosquito populations if the study were carried out by ELISA only, especially in areas where cattle and swine are present.

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