Sunday, February 13, 2011

Loa loa: A Parasite Worth Seeing

The Loa loa is a nematode, found only in the rainforests and swamps of Central and West Africa [1], and causes causes a disease known as loiasis. The filarial adult worm migrates to the lymphatics [2] and subcutaneous tissues (the third layer of the skin); it may also migrate to the eye under the conjunctiva [3]. Humans become infected when bitten by Chrysops silacea and Chrysops dimidiate (two species of Mango fly) [4].

Adult Males are 20-34 mm long by 350-430 um wide, while females are 20-70 mm long and about 425 um wide [3]. The female vulva is about 2.5 mm from the anterior end, and the tail is about 265-300 um long. Both genders of the L. loa live anywhere from 4 to 17 years [1]. The worm has a simple head, a characteristic of nematodes, with no lips, eight cephalic papillae (sensory organs around the mouth), a long, slender body; and a blunt tail [3].

The definitive host of the worm is humans, whom receive the parasite via its vectors Chrysops silacea and C. dimidiate. Infected mango flies pass on their infected larvae to humans. In turn, a healthy mango fly becomes infected from an infected human [4]. A more detailed description of the process of human infection, and Loa loa life cycle is given below.

Life Cycle

When the Mango Fly bites a human to take a meal Mango Fly larvae are left on the skin and enter through bite.Once inside the body the infective larvae wonder through the subcutaneous tissue while they develop into a mature adults. Adults produce microfilariae measuring 250 to 300 μm by 6 to 8 μm, which are sheathed and have diurnal periodicity (circadian rhythms). Microfilariae have been recovered from spinal fluids, urine, and sputum. During the day they are found in peripheral blood, but during the night, they are found in the lungs.

When another fly ingests microfilariae during a blood meal, the microfilariae lose their sheaths and migrate from the fly's midgut through the hemocoel to the thoracic muscles. It is here that the microfilariae develop into infective larvae. The infective larvae migrate to the fly's proboscis where they can infect another human when the fly takes in a meal.

The Painful Interaction
Worms often go unnoticed as they travel through subcutaneous tissues but can be painful as they pass over the eyeball or bridge of the nose. Swelling of the conjunctiva and eye lid are other characteristics of infection.Patients may describe literaly seeing something crawl across their eye [4]. While in the subcutaneous tissues, the worms trigger immune responses that result in swelling of specific areas often a hand or foot. When they remain in one spot for a short time, localized Calabar swellings, especially in the wrist and ankles, appear, which disappear when the worm moves on. There may also be intense itching, joint pain and fatigue [3]. Other symptoms may include: lymphadenitis (infection of the lymph glands),kidney disease, peripheral neuropathy (damaged peripheral nervous), and retina damage [5]

Class-Like Example
The Loa loa worm is a prime example of a parasite that is increasing its "encounter" filter, enabling it to infect a new host. Emergence rhythms allow for maximizations of encountering a host [6].Because humans are greatly dispersed and always on the move, the worms must develop strategies to make the most of their energy. By only coming to the surface of the skin during the day when the flies are feeding, the worms increase their protection by retreating back within the body where they are less likely to be removed.

Once the Loa loa worm has made its way into the eye, it can be surgically removed. Want to check it out, or is this parasite really not worth ever seeing? You decide, but the video is at the end of this article:

6.Combes, C.(2005).The Art of Being A Parasite. University of Chicago.


  1. Can the loa loa parasite be transmitted out of the host of humans other then being bitten? Can the blood be sucked up from an open wound or from infected blood?

  2. It's interesting that the microfilariae migrate to the lungs at night. In your research of the parasite, did you find out why that is? Or is the actual migration from the lungs to the blood during the day? What is their task or reason for traveling to each location? Also, I'm curious about whether or not their presence in the lungs has any negative effects on the lungs (breathing or chest pain, etc.)

  3. You mentioned that the parasite is generally spread through the mango flies. Generally, when is this parasite spread? It was said that the larvae are found in the peripheral blood vessels during the day, would it be safe to say that the mango flies bite and carry the parasite during the day? And to further Josh's question, what would be the significance of the parasites being found in the lungs at night? Could this parasite be spread through a person coughing at night?