Parasites In Humans
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Loa Loa - Subcutaneous Filariasis

Loa loa is a thread-like worm that lives under the skin in the subcutaneous fat causing loiasis. Because it is often spotted migrating in the eye, it is known as the eye worm. Loiasis is endemic to 11 countries and 12 million Africans are infected. The disease is also known as subcutaneous filariasis, Calabar swellings, African eye worm infection, Loa loa filariasis and Fugitive swelling.

Loiasis is carried by day-biting deer and mango flies (Chrysops silacea and Chrysops dimidiata). They both live in rain forests and warm swamps in central and western Africa. Deer fly's bite is very painful because it has to rip skin apart in order to drink blood. It does not have a narrow sophisticated feeding tube like that of a mosquito. A deer fly is about 5–20 mm in length.

The life cycle of Loa loa starts, when a Loa loa female gives birth to living microfilariae (prelarval eggs) inside the skin of an infected human. The microfilariae travel in peripheral blood during daytime, but during the night (noncirculation phase) they reside in the lungs. When a noninfected deer fly takes a blood meal from an infected human, it ingests microfilariae. The microfilariae lose their sheaths, migrate from the fly's midgut to the hemocoel and eventually to the thoracic muscles. There they develop into first stage and eventually into third stage (infective) filarial larvae. The development inside the fly takes up to two weeks. They migrate to the fly's proboscis (the snout) and invade another human during the next blood meal. The third stage larvae are transferred from the deer fly's mouth parts to the skin. They burrow into the bite wound and enter the subcutaneous layer where they mature into adults in one year. The cycle is completed, when male and female mate and release microfilariae into the bloodstream. Loa loa adults live up to 17 years.

Loa loa life cycle

Adult Loa loa male is about 30–34 mm long and 0.35–0.42 mm thick; Loa loa female about 40–70 mm long and 0.5 mm thick. Loa loa migrates in the subcutaneous layer. When it stops moving, the surrounding tissue develops Calabar swelling. This often occurs in joints, which are its usual stopping points. If an adult Loa loa is killed, its body becomes waste, which causes abscesses and pus. The waste products need to be eliminated to make sure that the surrounding tissue and cells are not disturbed. If the body cannot get rid of the waste, it restricts it by fibrosis.

Loiasis can be asymptomatic. Native people are more likely to have symptoms than tourists. Adult worms (both living and dead) and their metabolic byproducts (waste) can cause localized subcutaneous itching and allergic reactions. The inflammation may cause red skin eruptions and swelling in the deeper layers of the skin which can last many weeks. Migrating adults can cause instant pain, when moving in sensitive areas, for example, across the surface of the eye. Loiasis is often the cause of eosinophilia (large quantities of eosinophil granulocytes, white cells, that use antigens and special proteins to fight against foreign organisms such as parasites.). Other symptoms that may occur:

  • arthritis (joint pain)
  • colonic lesion (damaged large intestine)
  • inflammation, swelling and accumulation of fluid in testicles
  • lymphadenitis (infection of the lymph glands)
  • membranous glomerulonephritis (kidney disease)
  • peripheral neuropathy (damaged peripheral nervous system)
  • retinopathy (damaged retina (thin layer on the back wall of the eye)).

Loa loa microfilaria is about 0.25 mm long and 6–8 µm (micrometer = 0.001 mm) thick. It is sheathed, its tail is tapered and its nuclei extend to the tip of the tail. It can only develop into a larva inside the intermediate host (deer fly).

Diagnosis can be made by analyzing blood under a microscope to find Loa loa microfilariae. Microfilariae have also been found in urine, sputum and spinal fluids. The blood sample needs to be taken during the day, when the microfilariae are travelling in the bloodstream (peripheral blood circulation). According to some estimates 30 % of loiasis patients do not have microfilariae in their bloodstream or have so small quantities that they are rarely seen in microscopic examinations. An immunoassay for detecting circulating antigens excreted by adult Loa loa can be used (depending on your health care provider). Identification of an adult worm is possible from a tissue sample collected during a subcutaneous biopsy. Sometimes adult worms are seen migrating across the eye, but the short time (often only 15 minutes) for the worm’s passage through the conjunctiva makes this observation less used. Calabar swellings are also of diagnostic value.

Loiasis is usually treated with diethylcarbamazine (DEC) or sometimes with ivermectin. Both drugs have severe side effects which include death. DEC is effective against microfilariae and less effective against adult worms. Treatment with DEC is not advised for patients with high quantities of microfilariae because those patiens can develop encephalopathy (brain disease). When side effects need to be minimized, albendazole can be used instead. Sometimes eye worms can be removed surgically, although the time frame during which the removal must be done is very short. After the surgery, DEC therapy should follow to manage other adult worms and microfilariae.

Also check out the Loa loa pictures and videos.

Loa loa Quiz

To reveal the answer you need to click the correct option.

How long does it take for a third stage larva to develop into an adult and where does it take place?

+ A) 1 day in the subcutaneous layer
+ B) 1 day in the bloodstream
+ C) 1 month in the subcutaneous layer
+ D) 1 month in the bloodstream
+ E) 1 year in the subcutaneous layer
+ F) 1 year in the bloodstream

How long can Loa loa live?

+ A) 1 year
+ B) 10 years
+ C) 17 years

Why is it commonly known as the eye worm?

+ A) Because it is the only parasitic worm that has eyes
+ B) Because it is sometimes caught crawling on the eyeball thus making the diagnosis easy
+ C) Because it lives in the human eye

How is loiasis treated?

+ A) By surgical removal
+ B) With DEC

What steps of Loa loa life cycle take place inside a fly?

+ A) Microfilaria --> L1 larva --> L3 larva
+ B) L1 larva --> L3 larva
+ C) Microfilaria --> L1 larva