What are parasites?

A parasite is an organism that lives in or on another organism, which is called the host. Through this interaction, the parasite receives benefits, such as nutrients, at the expense of the host.

There are three types of parasites:

  • Protozoa. These are single-celled organisms that are able to grow and multiply within the host. Examples include Plasmodium species and Giardia species, which can cause malaria and giardiasis, respectively.
  • Helminths. Helminths are larger wormlike parasites. Examples include roundworms and flatworms.
  • Ectoparasites. Ectoparasites include organisms such as lice, ticks, and mites, which can attach to and live on the body of a host.

Some parasites can infect humans, causing a parasitic infection. They typically enter the body through the skin or the mouth. Once inside the body, these parasites can travel to other organs, including the eyes.

Read on to learn more about eye parasites, including how to tell if you have one and what to do next if you do.

Parasitic eye infections don’t always cause symptoms, which can make them hard to recognize.

When symptoms do occur, they can include:

  • eye pain
  • redness or inflammation in the eye
  • excessive tear production
  • blurry vision
  • the presence of floaters (small spots or lines) in your field of vision
  • sensitivity to light
  • crusting around the eyelids and eyelashes
  • redness and itching around the eye
  • retinal scarring
  • loss of vision and blindness


Acanthamoebiasis is caused by a protozoan parasite. Acanthamoeba is a very common organism within freshwater and marine environments worldwide. While it usually doesn’t cause an infection, when it does, it can potentially damage your vision.

Acanthamoeba is transmitted through direct contact with the parasite and the cornea of your eye. Poor contact lens care is a major risk factor for developing acanthamoebiasis.


Toxoplasmosis is also caused by a protozoan parasite. It’s prevalent in the environment and can be found in animal waste, especially that of domestic cats.

The parasite can enter your body when you ingest it. It can also be passed from mother to child during pregnancy.

Most people who get toxoplasmosis won’t develop any sort of eye disease. But when this does happen, it’s referred to as ocular toxoplasmosis. People with weakened immune systems and newborns who’ve acquired the infection from their mother are more likely to develop ocular toxoplasmosis.

If left untreated, ocular toxoplasmosis can cause scarring in the eye and lead to vision loss.


Loiasis is caused by a helminth parasite that’s found in Africa.

You can acquire the infection through the bite of an infected fly. Once inside the body, the parasite continues to develop and can migrate to various tissues. It also produces larvae, called microfilariae.

Both the adult worm and its larvae can cause eye pain, impaired eye movement, and vision problems, including sensitivity to light.


Gnathostomiasis is caused by a helminth parasite that’s mostly found in Asia, particularly parts of Southeast Asia, Thailand, and Japan. It can also be found in parts of Africa, South America, and Central America.

You can acquire the parasite through eating raw or undercooked meat or fish. The parasite exits your gastrointestinal tract. From there, it can move to other parts of your body, including your eyes. If this happens, it can result in partial or full blindness.

River blindness (onchocerciasis)

River blindness, also called onchocerciasis, is caused by a helminth parasite. The parasite can be found in parts of Africa, the Middle East, South America, and Central America.

You can get river blindness if you’re bitten by an infected blackfly.

The larvae of the parasite burrow through your skin, where they can develop into adult worms. These worms then produce more larvae, which can move into different tissues. If they reach your eye, they can cause blindness.


A helminth parasite causes toxocariasis. It can be found globally and is most often found in domestic dogs and cats.

You can acquire the parasite by ingesting its eggs, which are often found in soil that’s been contaminated with animal feces. The eggs hatch in your intestines, and the larvae can then migrate to other parts of your body.

Toxocariasis rarely affects the eye, but when it does, it can cause loss of vision.

Crab lice

Crab lice, also called pubic lice, are found worldwide. They’re small insects that typically colonize the hair of the genital region. But they can also be found in other hair areas, including eyelashes.

They’re usually spread through sexual contact, but contaminated personal items, like clothing or towels, can also spread them.

Demodex folliculorum

D. folliculorum are mites that are found in hair follicles of humans around the world. This includes the hair follicles of your eyelashes.

Occasionally, these mites can cause a condition called demodicosis. Demodicosis can cause irritation around the eyelashes and lead to loss of eyelashes, conjunctivitis, and decreased vision.

Treating a parasitic infection depends on the type of parasite that’s causing the infection. But many types are treated with oral or topical medications, such as pyrimethamine, ivermectin, and diethylcarbamazine.

In some cases, adult worms will need to be removed from your eye. This is a common part of the treatment of loiasis, gnathostomiasis, and river blindness.

While it’s hard to completely avoid parasites, there are several things you can do to reduce your risk of developing a parasitic infection in your eye.

Practice good hygiene

Wash your hands frequently, particularly before eating, after using the bathroom, and after picking up animal waste. Avoid sharing personal items like clothes, towels, and bed sheets.

Cook food properly

If you’re traveling in an area where parasitic infections are common, avoid eating raw or undercooked food. Be sure that all food is cooked through to the proper internal temperature. If you’re handling raw food, wear gloves and wash your hands afterward.

Prevent insect bites

If you’re going to go outside during times of the day when insects could bite you, apply an insecticide to exposed skin or wear protective clothing.

Properly care for contact lenses

If you wear contact lenses, don’t clean or store them with tap water. Use only sterile products approved for cleaning contacts. When storing your contacts, replace the contact solution in the case every time.

Make sure to wash your hands before handling or applying contact lenses. You should also try to avoid wearing your contact lenses while sleeping, especially after swimming.

There are many parasites throughout the world that can infect humans. Some of these parasites can infect your eyes. A parasitic infection in your eye won’t always cause symptoms. But if you notice any unusual eye pain, inflammation, or vision changes, make an appointment with a doctor. Left untreated. some parasitic infections can cause permanent vision loss.