Human
filariasis due to Loa loa differs from other
filariasis in that the majority of infected subjects are without circulating microfilariae (occult
loiasis). In search for alternative diagnostic methods, which do not depend on circulating microfilariae or the (rather infrequent) eye-passage of adult worms, it was shown earlier that
IgG4 antibodies directed against Loa loa adult worm
antigen are apparently a good marker of occult
loiasis and specific with regard to the sympatrically occurring Mansonella perstans. In this study we evaluated an
IgG4 antibody-based ELISA using
crude extract of Loa loa microfilariae (which is easier to obtain than adult worm) to estimate the prevalence of
loiasis in 3 villages in South-East Gabon. Of 222 examined individuals (80 children < 16 years, 142 adults) 44 (20%) carried Loa loa microfilariae and 170 (77%) M. perstans. Using the mean OD-value + 1 standard deviation of 9 sera from patients solely infected with M. perstans (from the Gambia, where Loa loa is not endemic) as a cut-off, 35 of the 44 microfilaraemic Loa loa patients and 2 of the 9 Gambian controls were positive. This shows that our method had a sensitivity of 80% and a specificity of 78%. Among the remaining 178 subjects who had no microfilariae of Loa loa, as many as 97 (55%) had significant levels of specific
IgG4 antibodies against Loa loa, suggesting that they carried occult
loiasis. The mean
IgG4 level in these putatively occult
loiasis patients was slightly but significantly lower than in microfilaraemic subjects (P < 0.03). In conclusion, despite the limited sensitivity and specificity of our method, IgG4- ELISA at present is a very useful tool in estimating the real prevalence of
loiasis in epidemiological surveys and at the individual level can confirm the diagnosis of L. loa amicrofilaraemic subjects with clinical signs suggesting
loiasis.