Diagnosis
Microscopic examination of microfilariae is a practical diagnostic procedure to find Loa loa. It is important to time the blood collection with the known periodicity of the microfilariae (between 10:00 a.m. and 2:00 p.m.). The blood sample can be a thick smear, stained with Giemsa or haematoxylin and eosin (see staining). For increased sensitivity, concentration techniques can be used. These include centrifugation of the blood sample lyzed in 2% formalin (Knotts technique), or filtration through a nucleopore membrane.Antigen detection using an immunoassay for circulating filarial antigens constitutes a useful diagnostic approach, because microfilaremia can be low and variable. Though the Institute for Tropical Medicine reports that no serologic diagnostics are available, tests that are highly specific to Loa loa have been developed in recent years. This is despite the fact that many recently developed methods of antibody detection are of limited value because substantial antigenic cross-reactivity exists between filaria and other parasitic worms (helminths), and that a positive serologic test does not necessarily distinguish among infections. The new tests have not reached the point-of-care level yet, but show promise for highlighting high-risk areas and individuals with co-endemic loiasis and onchocerciasis. Specifically, Thomas Nutman and colleagues at the National Institutes of Health have described the a luciferase immunoprecipitation assay (LIPS) and the related QLIPS (quick version). Whereas a previously described LISXP-1 ELISA test had a poor sensitivity (55%), the QLIPS test is practical, as it requires only a 15 minutes incubation, while delivering high sensitivity (97%) and specificity (100%).
Reservoir
Humans are the primary reservoir for Loa loa. Other minor potential reservoirs have been indicated in various fly-biting habit studies, such as hippopotamus, wild ruminants (e.g. buffalo), rodents and lizards. A simian type of loiasis exists in monkeys and apes but it is transmitted by Chrysops langi. There is no crossover between the human and simian types of the disease. A related fly, Chrysops langi, has been isolated as a vector of simian loiasis, but this variant hunts within the forest and has not as yet been associated with human infection. Vector
Loa loa is transmitted by several species of tabanid flies (Order: Diptera; Family: Tabanidae). Although horseflies of the genus Tabanus are often mentioned as vectors, the two most prominent vectors are from the tabanid genus Chrysops—C. silacea and C. dimidiata. These species exist only in Africa and are popularly known as deer flies and mango, or mangrove, flies.Chrysops spp. are small (5–20 mm long) with a large head and downward-pointing mouthparts. Their wings are clear or speckled brown. They are hematophagous and typically live in forested and muddy habitats like swamps, streams and reservoirs, and in rotting vegetation. Female mango and deer flies require a blood meal for production of a second batch of eggs. This batch is deposited near water, where the eggs hatch in 5–7 days. The larvae mature in water or soil, where they feed on organic material such as decaying animal and vegetable products. Fly larvae are 1–6 cm long and take 1–3 years to mature from egg to adult.