Health and Medicine

You Can See And Feel This Parasitic Worm As It Wiggles In Your Eyes


Francesca Benson

Junior Copy Editor and Staff Writer

clockFeb 10 2021, 14:04 UTC
adult loa loa

When inside a person, they can live up to 17 years. Image Credit: NIAID (CC BY 2.0)

Do you hate the feeling of getting an eyelash stuck in your eye? Well, there is something way worse that can get into your peepers. Meet Loa loa (literally meaning “worm worm” in Latin), also known as the eye worm.


This parasitic nematode worm lives in Central and West Africa, mainly around rainforests. Infection with Loa loa causes a disease called loiasis or Loa loa filariasis. An estimated 14.4 million people live in areas with high rates of infection, where over 40 percent of people report having come down with a case of eye worm. A further 15.2 million live in areas with a slightly lower reported rate of 20 to 40 percent.

Many people with loiasis do not develop symptoms. The most common symptoms that do occur are itching and Calabar swellings on arms, legs, and near joints. However, there is one nasty symptom that makes infection very obvious and gives the worm its nickname. Loa loa can migrate into the subconjunctival tissues of the eye, causing pain, itching, and light sensitivity. The worms are very visible when in this area, which they can be in for hours or even up to a week. Luckily, despite sometimes being quite painful, this does not usually cause much damage to the eyes. On top of wriggling around in the eyes, some sources have reported the worms being seen in the penis and nipples.

Unfortunately, humans are the definitive host of Loa loa, meaning they can only reach their mature form in our bodies. When inside a person, making themselves at home between layers of connective tissue, they can live up to 17 years. Their intermediate hosts are flies of the genus Chrysops, also known as deer flies or mango flies. These flies normally bite during the day and are attracted by human movement and wood smoke.

  1. C. silacea and C. dimidiate are the two main culprits of transmitting the eye worm to humans, biting humans to feed on our blood. When a fly carrying Loa loa bites, larvae enter the bite wound. These larvae take about 5 months to mature into their adult form. Females can reach up to 7 centimeters (2.75 inches) in length and 0.5 millimeters (0.02 inches) in diameter, and males can measure up to 3.4 cm (1.33 inches) in length and 0.43 mm (0.017 inches) in diameter. 

  2. Females can produce thousands of microfilariae – worms very early in development – every day. The tiny immature worms are usually found in the lungs, but have also been found in urine and spinal fluid. They enter the blood between 10 am and 2 pm, which means that blood drawn at that time can be used to diagnose infection. When a fly bites an infected human, the microfilariae are ingested, continuing the grim circle of life that enables them to be passed on. The microfilariae migrate to the fly’s thoracic muscles through their midgut, maturing into larvae ready to infect their next victim within 12 days. At this point, the larvae migrate to the proboscis, ready to hop into an unfortunate human. 

Surgically removing the worms can help treat local symptoms, which may be a relief when you have one of these parasites living in your eye, but does not cure the disease. Luckily, treatment for loiasis is available in the form of antiparasitic drugs. Killing both microfilariae and adult Loa loa, diethylcarbamazine is the drug of choice in this scenario.



Health and Medicine