What Causes Loiasis

Key points

  • Loiasis is a parasitic infection caused by a microscopic worm.
  • It’s spread by deerflies that breed in African rain forests.
Chrysops silacea or deerfly


Loiasis is an infection caused by the parasitic worm Loa loa. It is spread through the bites of infected deerflies, which breed in the rain forests of Central and West Africa.

How it spreads

When the deerflies bite people with Loa loa, they pick up immature worms, or microfilariae, in that person's blood. The microfilairiae develop into infectious larvae in the fly and can then infect people. When the deerfly bites a person's skin to feed (take a bloodmeal), the larvae enter the wound and begin moving through the person's body.

It takes about five months for the larvae to develop into adult worms. Larvae can only become adults inside the human body. The adult worms live between layers of connective tissue (e.g., ligaments, tendons) under the skin and between the thin layers of tissue that cover muscles (fascia). Their microfilariae will spread into the lymph nodes and eventually the lungs.

People who develop symptoms typically start seeing them about five months after they are infected. This is also when the microfilariae begin to enter the bloodstream. Adult worms can live in the body up to 17 years, reproducing most of this time.