Which microfilariae are usually not found circulating in the peripheral blood?

Onchocerca volvulus is known as the causative agent of river blindness (onchocerciasis), and its microfilariae do not circulate in the peripheral blood. Instead, they are typically found in the skin, where the adult worms reside. The migration of microfilariae occurs in the tissues rather than in the bloodstream, which is key to identifying the infection.

In contrast, Brugia malayi, Wuchereria bancrofti, and Loa loa are filarial worms whose microfilariae do circulate in the peripheral blood. Wuchereria bancrofti is associated with lymphatic filariasis and is commonly found in the bloodstream, particularly during the night when hosts are most active and capable of transmitting the disease. Brugia malayi is primarily located in Southeast Asia and the South Pacific, and its microfilariae can also be detected in the blood. Loa loa, known as the African eye worm, similarly has microfilariae that are present in the peripheral blood, which can be confirmed through examination, especially during the day.

This distinction regarding microfilariae distribution is crucial for diagnosis and understanding the transmission and epidemiology of these parasitic diseases.