Migration and death of skin-dwelling Onchocerca volvulus microfilariae after treatment with ivermectin

The effects of a single dose of ivermectin (122-200 micrograms/kg) on Onchocerca volvulus microfilariae (mf) in skin and lymph nodes were studied histologically in a qualitative and quantitative manner over seven days after treatment. Ivermectin caused mf to move from the subepidermal layer into the deeper layers of the dermis, subcutaneous fat and connective tissue, and the regional lymph nodes. No mf were seen migrating upwards through the epidermis. Dead mf were not seen before 24 hr and the numbers found dead in the dermis were far fewer than the total numbers in the skin before treatment. Inflammatory cellular reaction around dead mf in the tissues was usually minimal. The results suggest that most of the mf which disappear from the skin may be destroyed in the lymph nodes and that the histological reaction excited by mf dying under ivermectin is less violent than that after diethylcarbamazine.