Eye Worm

What is an eye worm nematode and which species are known to infect humans?

The term “eye worm” specifically refers to the nematode species Loa loa, a parasite found in central and western Africa. It is transmitted to humans through the bite of a deerfly (Chrysops), which acts as an intermediate host. Once the larvae enter the human host, they migrate through the bloodstream and often settle in tissues under the skin or within the eye, where the adult worm may be visible beneath the conjunctiva.¹²

While Loa loa is the primary species associated with the name “eye worm,” other filarial nematodes can also affect the eyes:

• Onchocerca volvulus: This parasite causes onchocerciasis, or river blindness. It is transmitted by Simulium flies and can invade the eyes, leading to vision loss.²³
• Brugia malayi and Wuchereria bancrofti: While these are primarily associated with filariasis and lymphatic damage, they belong to the same superfamily of threadlike nematodes that infect human tissues.²

Treatment for these infections typically involves surgical removal of the worms in the case of Loa loa or the use of specific chemotherapeutic agents to manage the parasites.²³

How do eye worm nematodes enter the human body and migrate to the eyes?

The eye worm (Loa loa) enters the human body through the bite of an infected deerfly (Chrysops), which serves as the intermediate host. When the fly lands on a human to feed, the infective larvae are deposited onto the skin and burrow into the host.¹²

Once inside, the larvae migrate through the bloodstream to reach various tissues, including those just beneath the skin and the eyes. Adult worms often settle in the eye, where they may become visible beneath the conjunctiva, or in subcutaneous tissues, where their movement can cause itching and localized swellings known as Calabar swellings. The worms reach maturity within the human host in approximately one year, at which point the adult females produce microscopic embryos called microfilariae that circulate in the blood, allowing the cycle to continue when another deerfly bites the host.¹²

Are there any animals that commonly carry eye worm nematodes?

The Loa loa nematode, commonly known as the eye worm, is a parasite that primarily infects humans and other primates in central and western Africa.¹

The life cycle of this parasite relies on the deerfly (Chrysops) as an intermediate host. The fly facilitates the transmission of the parasite by feeding on the blood of infected primates, including humans. Once the fly ingests the microfilariae from an infected host, the larvae develop within the insect for approximately two weeks before they are ready to be transferred to a new host during a subsequent blood meal.¹

What is river blindness and how does it differ from loiasis?

River blindness, also known as onchocerciasis, is a parasitic disease caused by the nematode Onchocerca volvulus. While both river blindness and loiasis are forms of filariasis, they differ significantly in their transmission, symptoms, and potential for long-term damage.⁵

Key Differences

Feature

River Blindness

Loiasis

Causative Agent

Onchocerca volvulus

Loa loa

Vector

Black flies

(

Simulium

)

Deerflies

(

Chrysops

)

Primary Symptoms

Subcutaneous nodules, skin lesions, and potential blindness

Calabar swellings, visible worms in the eye

Severity

Can cause permanent blindness

Generally causes irritation but rarely permanent damage

River blindness is transmitted by flies that breed near fast-moving streams, leading to the formation of nodules under the skin and, in about 5 percent of cases, severe ocular damage. In contrast, loiasis is transmitted by deerflies and is characterized by transient allergic reactions known as Calabar swellings and the occasional migration of adult worms across the surface of the eye. While loiasis is often uncomfortable, it seldom results in the permanent vision loss associated with onchocerciasis.⁵