Some parasites can infect human eyes. Without treatment, these infections can lead to eye pain, vision problems, and in some cases, blindness.

Parasites are organisms that live in, or on, another organism. The host organism provides the parasites with ideal living conditions and a source of nutrients. Most parasites damage their hosts in some way.

The Centers for Disease Control and Prevention (CDC) identifies three types of parasites that cause diseases in humans. These are:

  • protozoa, or single-celled organisms, which can multiply inside of the body, which can lead to serious infections
  • helminths, which include flatworms, flukes, and roundworms
  • ectoparasites, or blood-sucking invertebrates, include mosquitoes, lice, ticks, and mites

Most parasites secrete toxic substances, damaging their hosts. These substances can destroy the tissue or cause inflammation.

This article looks at some parasitic infections of the eyes. It also looks at how doctors treat parasitic infections and how people can prevent them.

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Acanthamoeba keratitis is a rare but serious eye infection that occurs due to a single-celled organism called an amoeba. These are present in bodies of water, the soil, and the air.

Acanthamoeba parasites infect the cornea, the transparent covering of the eye. Without treatment, Acanthamoeba keratitis can lead to severe pain, and in some cases, loss of vision.

Although anyone can develop the infection, in the United States, approximately 85% of infections develop in those who wear contact lenses.


Symptoms can include:

  • blurred vision
  • eye pain
  • eye redness
  • light sensitivity
  • excessive tearing
  • the feeling that something is in the eye

Toxoplasma gondii is another common protozoan parasite. The CDC estimates that as many as 11% of people 6 years old and older have the infection in the US.

If a person develops the infection during, or just before, pregnancy, toxoplasmosis can damage the fetus’ eyes.

Most people contract the parasite as a result of eating undercooked meat or shellfish. Pets, including cats and dogs, can pass on the infection through their feces.


The CDC notes that most people will not experience symptoms. However, others may feel as if they have the flu. They will develop swollen lymph glands and aching muscles for a month or more.

If it affects the eyes, a person may experience:

  • reduced vision
  • pain, often due to bright lights
  • eye redness
  • tearing

A parasitic worm called the Onchocerca volvulus causes onchocerciasis. A person can contract the parasite due to repeated bites from infected blackflies. The flies carry microscopic worm larvae that penetrate a person’s skin.

These worms can cause lesions in the eyes, which can lead to blindness. If the worms infect the optic nerve, cornea, or retina, the resulting inflammation can also lead to vision loss.

People are most likely to contract the infection in tropical areas. The World Health Organization (WHO) notes that over 99% of people with the infection live in thirty-one countries in sub-Saharan Africa. Those most at risk of contracting the infection live or work near fast-flowing water, such as rivers and streams.


People may not develop any symptoms. However, if symptoms do occur, a person may experience:

  • an itchy skin rash
  • vision changes
  • nodules under the skin

Severe symptoms include visual impairment and vision loss.

Toxocara are parasitic roundworms that often infect cats and dogs. The worm’s eggs pass through the infected animal’s intestines and contaminate their feces.

People contract the infection if they accidentally swallow contaminated dirt.


Many people who contract the infection do not present with symptoms and do not become ill. However, if the larvae travel to the host’s eyes, they can cause swelling and scarring on the retina, which can lead to vision loss.

Ocular toxocariasis usually only occurs in one eye.

Tiny mites, called Demodex folliculorum, live in the hair follicles on human skin.

They are usually harmless, but when their numbers increase on the face, they can cause eye problems.


People with large numbers of these mites in their eyelashes may develop:

Loa loa is a parasitic worm from West and Central Africa. It causes loiasis, or African eye worm.

The CDC states that people are most at risk of infection if they live in, or visit, rain forests of the area, and are repeatedly bitten by deerflies.

People typically contract the infection after being bitten by deerflies for many months. However, a person can contract the infection after less than 30 days.


Symptoms do not typically develop, but if they do, they can take months to show up. People may develop itching and swelling, most commonly in the joints.

If the worms travel to a person’s eyes, it can make them itchy, painful, and sensitive to light. People may see a worm crawl across the surface of the eye.

Gnathostomiasis is another infection that occurs due to a parasitic worm.

Although these worms are present throughout the world, most diagnoses occur in Southeast Asia, Thailand, and Japan.

People can pick up the parasites if they eat raw or undercooked freshwater fish carrying the larvae.


Symptoms develop as the parasite moves through the body.

According to the CDC, as the parasite moves through the wall of the intestine, stomach, or liver, a person may experience:

This can last for 2–3 weeks.

As the parasite moves under the skin, people experience itchy swellings under their skin. This occurs 3–4 weeks after ingestion, but it can last for up to 10 years.

Although it is rare, it can enter other parts of the body, including the eyes. This can lead to vision loss or blindness.

Depending on the type of infection, doctors may prescribe antiparasitic drugs. Doctors use different drugs for each group of parasites.

For example, doctors treating worm infestations may use vermicides or vermifuges. Vermicides kill the worms, while vermifuges help expel the worms from a person’s body.

Treatments may include eye drops, oral medicines, or in some cases, surgery to remove the parasites from the eye.

Not all parasitic infestations are preventable, but there are several ways people can reduce their risk of contracting an infection.

People should make sure they only eat properly cooked food, and that they drink water from a clean source, especially if they are traveling to an area where infections are high.

Many biting insects transmit infections, and these can be active at any time of day. A person can reduce the risk of insect bites by keeping all of their skin covered and using an insecticide on their skin.

People who wear contact lenses need to practice good hygiene before handling their lenses and use sterile solutions to store them. The American Optometric Association (AOA) cautions against using tap water, even when rinsing lens cases.

Gardeners, pet owners, and people who work outside regularly should wash their hands thoroughly after handling any dirt or feces.

Many people do not experience any symptoms of parasites, but the American Academy of Ophthalmology (AAO) recommends speaking to an ophthalmologist if a person has any unusual vision symptoms. These include:

  • loss of vision
  • blurred vision
  • pain in, or around, the eye
  • excessive tearing
  • redness or inflammation around the eye
  • sensitivity to light
  • floaters, or unusual shapes that cross a person’s field of vision
  • itchy or crusty eyelids

People may also experience the sensation that something is in their eye.

Parasites can infect the eyes, and in some cases, lead to blindness.

Although many infections do not cause symptoms, anyone experiencing pain or discomfort in, or around, the eye should speak with a doctor.

Doctors can treat many infections with antiparasitic medicines before any lasting damage occurs.