Effect of ivermectin on the vertical distribution of Onchocerca volvulus microfilariae in the skin

A new technique for assessing vital and degenerative microfilariae (mf) in different skin layers of onchocerciasis patients is described. Vertical sections at least 5 mm deep were collected from the surgical nodulectomy wounds before and 2, 4 and 28 days after treatment with ivermectin and fixed in ethanol. Skin biopsies were punched horizontally with trephines and digested enzymatically with collagenase. In untreated biopsies 80% of the mf were located in the uppermost layer of 1 mm and only 1.2% were degenerated. After treatment with ivermectin the total number of mf was significantly reduced, and the distribution of living and remnant mf in the different skin layers had changed. The proportion of degenerated mf had increased markedly, but did not exceed 6% of the total pre-treatment mf level, i.e., the vast majority of the mf had actually disappeared from the skin. It is suggested that ivermectin reduces microfilarial motility slightly, and that mf are then attacked by adhering immunocompetent cells of the host and cleared by the lymphatic drainage.