Answer: Onchocercoma. Definition: a subcutanteous tumor-like mass caused by the presence of adult filarial worms Onchocerca volvulus. Note the fibrous tissue surrounding the coils of the adult worms, and the microfilariae within the classic 'double uterus' and in the surrounding tissue. All adult female filarial worms that infect humans have a classic double uterus, seen best in cross-section.
The disease caused by Onchocerca volvulus is called onchocerciasis. The symptoms are caused almost entirely by the release of the microfilariae into the surrounding skin where they cause extremely itchy dermatitis. The most severe damage, however, is caused by the migration of the microfilariae through the chambers of the eye, and the retina, resulting in permanent scarring and blindness. Thus, the common name for onchocerciasis is "river blindness". The onchocercoma is typically painless and causes very little symptoms.
In this case, the differential diagnosis includes other adult filarial worms, such as the agents of lymphatic filariasis (i.e. Wuchereria bancrofti and Brugia spp.). These adults are not found in subcutaneous nodules like onchocerciasis, but within lymphatics and lymph nodes. The symptoms of lymphatic filariasis typically include inflammation of the lymphatics (lymphangitis), obstruction of the lymph channels, and eventually scarring, and edema (elephantitis). Unlike onchocerciasis, the disease of lymphatic filariasis is caused primarily by the presence of the adults. The microfilariae are released into the lymphatics, and cause little associated symptoms.