ELISA absorbance cut-off method affects malaria sporozoite rate determination in wild afrotropical Anopheles

• Published in: Medical and veterinary entomology Vol. 2; no. 3; pp. 259 - 264
• Main Authors: Beier, J.C, Asiago, C.M, Onyango, F.K, Koros, J.K
• Format: Journal Article
• Language: English
• Published: England 01.07.1988
• Subjects: Animals; Anopheles - parasitology; Anopheles funestus; Anopheles gambiae; Antigens, Protozoan - analysis; disease vectors; Enzyme-Linked Immunosorbent Assay; Female; Insect Vectors - parasitology; malaria; Male; Plasmodium falciparum - growth & development; Predictive Value of Tests; Protozoan Proteins; Kenya
• ISSN: 0269-283X; 1365-2915
• DOI: 10.1111/j.1365-2915.1988.tb00193.x

Malaria sporozoite infection rates in a mixed species group of 244 Anopheles gambiae Giles sensu lato and 115 An.funestus Giles wild female mosquitoes were compared using three methods to determine cut-off absorbance values for positivity of a Plasmodium falciparum Welch enzyme-linked immunosorbent assay (ELISA). Positive controls were based on P.falciparum circumsporozoite protein. As negative controls, four wild male Anopheles were included on each microtitre plate; tests were repeated on four consecutive days for each plate. Infection rates were estimated at 13.1-22.8% using the mean absorbance value of negative controls plus three standard deviations, 11.7-12.8% using double the mean and 12.5-13.6% using the fixed cut-off value of 0.20 (allowing for 20% variation in negative control absorbance values). Observed agreement for positivity or negativity among samples tested four times was 98.6% for the 2 x mean method, 97.2% for the fixed cut-off 0.20 value, but only 82.7% for the mean + 3 SD method. It was concluded that the 2x mean cut-off method is most reliable for field studies. P.falciparum sporozoite rates of 12.2% in An.funestus and 11.9% in An.gambiae s.l. were thus determined on the basis of the 2x mean cut-off method. This comparative evaluation demonstrates that vector infectivity rates can be seriously over-estimated from sporozoite ELISA tests, by as much as 87% in one case considered here, depending on the absorbance cut-off method applied for negative controls.