The definitive host for Loa loa is usually a human. Loa loa infection has been found in some African primates. However the worm infecting primates appears to be a species distinct from the one infecting humans because in primates microfilariae circulate with a nocternal periodicity (Rakita et al., 1993). Adult Loa loa worms live in the subcutaneous and deep connective tissues. Female Loa loa are typically 6 cm long and .5 mm wide while adult males are typically 3 cm long and .4 mm wide. Cross sections of male worms contain 1 reproductive tube while cross sections of female worms contain between 3 and 5 reproductive tubes. Adult worms continuously migrate through tissue at a rate of about 1 cm per minute. After adult male and female worms mate, the female begins to excrete microfilariae which penetrate capillaries and enter the host's bloodstream.
Loa loa microfilariae (see above image) are sheathed and about 300 micrometers long. Microfilariae of Loa loa have body nuclei that are continuous from tip to tail. This distinguishes them from microfilariae of Wuchereria bancrofti and Brugia malayi which have extended sheaths on both ends. Loa loa microfilariae circulate in the bloodstream with a diurnal periodicity. This periodicity coincides with the feeding pattern of the insect vector, which typically bites humans between 10 am and 4 pm. When microfilariae are not in the blood they can typically be found in the lungs.
The intermediate host and vector for Loa loa is the mango fly of
the Chrysops species. The mango fly (see above image) ingests
microfilariae when taking a blood meal from an infected host. Following
ingestion, microfilariae loose their sheaths and bore through the gut
wall. The larvae then migrate to the fat body where they undergo two
molts. The larvae become infective after approximately 10 to 12 days.
When the mango fly bites a person, the larvae are released from the
proboscis and enter the bite wound. Larvae in the subcutaneous tissue
take between 6 months and 4 years to molt into adults and the entire
cycle begins anew. Adult worms can survive for up to 17 years. For a
summary of the Loa loa lifecycle see the diagram below.