Loa Loa Filariasis - Vector

Vector

Microfilaria of Loa loa are transmitted by several species of tabanid flies (Order: Diptera; Class: Tabanidae). Although horseflies of the Tabanus genus are often mentioned as Loa vectors, the two prominent vector are from the Chrysops genus of tabanids—C. silicea and C. dimidiata. These species exist only in Africa and are popularly known as deerflies 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, reservoirs, and in rotting vegetation. Female mango and deerflies 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. When fully mature, C. silacea and C. dimidiata assume the day-biting tendencies of all tabanids.

The bite of the mango fly can be very painful, possibly due to the laceration style employed; rather than puncturing the skin like a mosquito does, the mango (and deerfly) make a laceration in the skin and subsequently lap up blood. Female flies require a fair amount of blood for their aforementioned reproductive purposes and thus may take multiple blood meals from the same host if disturbed during the first one.

Interestingly, although Chrysops silacea and C. dimidiata are attracted to canopied rainforests, they do not do their biting there. Instead, they leave the forest and take most blood meals in open areas. The flies are attracted to smoke from wood fires and they use visual cues and sensation of carbon dioxide plumes to find their preferred host, humans.

A study of Chrysops spp biting habits showed that C. silacea and C. dimidiata take human blood meals approximately 90% of the time, with hippopatomus, wild ruminant, rodent, and lizard blood meals making up the other 10%. The fact that no simian (ex: monkeys or apes) blood meals were taken suggests that there is no crossover between the human and simian types of Loa loa. 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.