Human occult loiasis: field evaluation of a nested polymerase chain reaction assay for the detection of occult infection

A nested polymerase chain reaction (nested PCR) assay, targeted on the repeat 3 region (15r3) of the gene coding for a Loaloa 15 kD polyprotein, was developed to detect L. loa infection. The assay has a sensitivity of 95% and is 100% specific with regard to sympatric filarial parasites: Mansonella perstans, Onchocerca volvulus and Wuchereria bancrofti. In this field study in a mixed filarial (L. loa and M. perstans) endemic region of Gabon, 157 L. loa amicrofilaraemic blood samples (AMF; diagnosed by leucoconcentration followed by standard microscopic examination) from the residents from four villages were screened by the 15r3‐nested PCR assay. The assay detected 106 occult infected subjects among the 157 AMF individuals (68%), including 59 of 87 adults (68%) and 47 of 70 children (67%). In each village the prevalence of occult infection was, respectively, 38%, 52%, 79% and 80% for Moyabi, Djoutou, N'djokaye and Okoumbi. The annual transmission potential (ATP) of loiasis has been estimated to be 250 infective larvae (L 3) per man per year for Moyabi and Djoutou, 1800 for N'djokaye and 43000 L3/man/year for Okoumbi. This implies a correlation between occult infection of loiasis and the intensity of transmission. By contrast, the prevalence of L. loa microfilariae was 21% for Okoumbi, 22% for N'djokaye and 19% for Djoutou and Moyabi. These results show that the prevalence of loiasis in this region of Gabon is higher than previously described by standard microscopic examination and that the application of this assay will be significant in the development of control strategies for loiasis.