Loa loa filariasis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Pathophysiology

The vector for Loa loa filariasis are flies from two species of the genus Chrysops, C. silacea and C. dimidiata. During a blood meal, an infected fly (genus Chrysops, day-biting flies) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound (1). The larvae develop into adults that commonly reside in subcutaneous tissue (2). The female worms measure 40 to 70 mm in length and 0.5 mm in diameter, while the males measure 30 to 34 mm in length and 0.35 to 0.43 mm in diameter. Adults produce microfilariae measuring 250 to 300 μm by 6 to 8 μm, which are sheathed and have diurnal periodicity. Microfilariae have been recovered from spinal fluids, urine, and sputum. During the day they are found in peripheral blood, but during the noncirculation phase, they are found in the lungs (3). The fly ingests microfilariae during a blood meal (4) . After ingestion, the microfilariae lose their sheaths and migrate from the fly's midgut through the hemocoel to the thoracic muscles of the arthropod (5). There the microfilariae develop into first-stage larvae (6) and subsequently into third-stage infective larvae (7). The third-stage infective larvae migrate to the fly's proboscis (8) and can infect another human when the fly takes a blood meal (1).

Transmission

Loa loa microfilariae are transmitted to humans by the mango (also, mangrove) or deerfly vectors, Chrysops silicea and C. dimidiata. The vectors are blood-sucking and day-biting, and they are found in rainforest-like environments in west and central Africa. Microfilaria mature to adults in the subcutaneous tissues of the human host, after which the adult worms—assuming presence of a male and female worm—mate and produce more microfilaria. The cycle of infection continues when a non-infected mango or deerfly takes a blood meal from a microfilaremic human host, and this stage of the transmission is possible due to the combination of the diurnal periodicity of microfilaria and the day-biting tendencies of the Chrysops spp.[1]

Reservoir

Humans are the primary reservoir for Loa loa. Other minor potential reservoirs have been indicated in various fly biting habit studies: 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 cross-over between the human and simian types of the disease.[2]

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.[3]

Chrysops spp are small (5–20 mm long) with a large head and downward pointing mouthparts.[3] 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.[3] 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.


Shown below is an image Chrysops callidus

References

  1. Padgett JJ, Jacobsen KH (2008). "Loiasis: African eye worm". Trans. R. Soc. Trop. Med. Hyg. 102 (10): 983–9. doi:10.1016/j.trstmh.2008.03.022. PMID 18466939. Unknown parameter |month= ignored (help)
  2. Gouteux JP, Noireau F, Staak C (1989). "The host preferences of Chrysops silacea and C. dimidiata (Diptera: Tabanidae) in an endemic area of Loa loa in the Congo". Ann Trop Med Parasitol. 83 (2): 167–72. PMID 2604456. Unknown parameter |month= ignored (help)
  3. 3.0 3.1 3.2 World Health Organization (WHO). Vector Control – Horseflies and deerflies (tabanids). 1997. Available online at: http://www.who.int/docstore/water_sanitation_health/vectcontrol/ch06.htm#b6-Horseflies%20and%20deerflies%20%28tabanids%29.

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