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The filariae are parasitic nematodes (roundworms) that cause significant human morbidity in tropical regions worldwide. The macroscopic adults live in the human host and release microscopic offspring (microfilariae) into the blood or skin. The microfilariae of Wuchereria bancrofti, Brugia malayi, B timori, Loa loa, Mansonella perstans, and M ozzardi are found in the blood, while the microfilariae of Onchocerca volvulus and M streptocerca are found in the skin. If microfilariae are taken up by a biting insect vector (mosquitos, blackflies, midges, and deer flies), they undergo further development in the insect and can then be transmitted to other humans.
W bancrofti and the Brugia species cause a serious condition called lymphatic filariasis. The adults live in the lymphatics and cause inflammation and scarring of the lymph vessels. Over time, the lymphatic channels are obstructed and fluid cannot drain back to the heart, resulting in massive lymphedema (elephantiasis) of the affected limb or groin. W bancrofti is found in the tropics worldwide, while Brugia species are found in parts of Asia and Southeast Asia.
Loa loa causes migratory subcutaneous angioedema referred to as "calabar swellings" as the adult worm migrates throughout the body. The adult occasionally migrates across the surface of the eye, giving it the moniker "the African eye worm." Loa loa is only found in Africa.
Finally, M perstans and M ozzardi cause a relatively mild form of filariasis. Patients are often asymptomatic. When present, symptoms include fever, angioedema, headache, myalgias, arthralgias, pruritus, and neurologic manifestations. M perstans is found in parts of Africa and South America, while M ozzardi is only found in Mexico and Central and South America.
The microfilariae of these filarial worms can be seen on conventional thick and thin blood films, which allows for their definitive identification. However, microfilariae may be in low numbers and, therefore, use of concentration methods such as the Knott's technique improves the detection sensitivity. Some microfilariae are released into the blood at certain times of the day; W bancrofti and Brugia species are usually released between 10 p.m. and 2 a.m. (nocturnal periodicity), while L loa is released mostly from 10 a.m. and 2 p.m. (diurnal periodicity). It is therefore important to collect blood during these time periods for optimal detection sensitivity. Mansonella species microfilariae do not exhibit any periodicity and, therefore, a random blood draw is acceptable. Since the levels of parasitemia may fluctuate, multiple smears may be needed to detect the filarial worms. Blood should be obtained and examined every 8 to 12 hours for 2 to 3 days before excluding infection.
Detection of microfilariae in peripheral blood
Positive results are provided with the genus and species of the microfilariae, if identifiable.
This exam will not detect the microfilariae of Onchocerca volvulus and Mansonella streptocerca since they are found primarily in the skin. The "skin-snip" examination is the preferred method for detecting the microfilariae of these worms.
Microfilariae may be seen in peripheral blood on routine thick and thin blood films, but concentration techniques such as the Knott's concentration and Nucleopore membrane filtration technique offer increased detection sensitivity.
Multiple smears may be needed to detect microfilariae in blood. Repeat specimens can be collected every 8 to 12 hours over a period of 2 to 3 days.
The blood collection should be timed to correspond with the release of microfilariae in the peripheral circulation. For the agents of lymphatic filariasis, blood should be collected between 10 p.m. and 2 a.m., whereas for detection of Loa loa, blood should be collected between 10 a.m. and 2 p.m.
If positive, organism is identified.
Centers for Disease Control and Prevention, Division of Parasitic Diseases and Malaria. DPDx, Diagnostic Procedures. 2013. Available at http://www.cdc.gov/dpdx/diagnosticProcedures/blood/specimencoll.html