Reliability of detection of microfilariae in skin snips in the diagnosis of onchocerciasis

To quantify the sensitivity of skin snips for the diagnosis of onchocerciasis, data collected over a 2 year period from 261 Liberians, 12-60 years of age, were examined. The overall sensitivity of 6 snips per person was estimated to be 91.6%. When the microfilariae density was ≥3.5 microfilariae/mg skin, there were no false negatives. At lower microfilaria densities, the sensitivity declined markedly. Furthermore, it was found that smaller and lighter snips were more likely to be classified as being falsely negative. The load of infection at the community level may be reduced after intervention strategies, such as vector control or community-based chemotherapy. Interpretation of the impact of these programs on the prevalence of infection should take into account the lower sensitivity of the skin snips at lower levels of infection.