Detection of Onchocerca volvulus infection in Simulium ochraceum sensu lato: comparison of a PCR assay and fly dissection in a Mexican hypoendemic community

Detection of Onchocerca ca volvulus larvae in vector populations is of prim e importance in the assessment of the effectiveness of onchocerciasis contr ol programmes. Traditionally, detection of larvae is attained by the dissec tion of flies, but this time-consuming method cannot easily discriminate be tween species of Onchocerca ca. The genome of all Onchocerca species has a unique 150 bp repeat, which can be amplified by PCR, and O. volvulus-specif ic DNA probes can detect these products by Southern blot (SB). This study o ptimizes a PCR/SB assay, and compares it with fly dissection to estimate th e prevalence (p) and intensity of infection (m) in the local vector populat ion of a Mexican community that has become hypoendemic as a result of 7 yea rs of treatment with ivermectin and nodulectomy. The PCR detected 1 infecte d fly in a pool of 99 uninfected flies, but the optimal pool size was 50 fl ies. At the community level, 1 out of 10550 flies was positive (p = 00095%, 95 % confidence intervals CI = 0.00024-0.05280 %,; m = 0.00027 larvae/paro us fly, CI= - 0.00026-0.00081) by PCR, and 4 out of 10772 flies (p=0.0371%, CI=0.01012-0.09505 %; m=0.00107 larvae/parous fly, 95 %, CI=0.00002-0.0021 2) by dissection (observed m=0.0005). Both methods produce statistically si milar estimates of the prevalence and intensity, indicating that pool scree ning is a viable alternative for entomological surveillance in areas where the intensity of transmission is becoming extremely low as a result of cont rol interventions.