Loiasis (African Eye Worm): Prevention, Symptoms and UK Travel Advice

Loiasis is a parasitic tropical disease caused by the Loa loa filarial nematode, often referred to as the African eye worm. While the name may sound alarming, the infection is primarily known for causing temporary swellings and the occasionally visible movement of the adult worm across the surface of the eye. It is endemic to the rainforests and swampy regions of West and Central Africa.

For UK residents travelling to countries such as Cameroon, Gabon, Nigeria, or the Democratic Republic of Congo, understanding the risk is important. The infection is transmitted through the repeated bites of day-biting deerflies (genus Chrysops). Because the worms can live in the human body for over 15 years, symptoms can sometimes emerge long after a traveller has returned to British soil.

Symptoms of loiasis do not usually appear immediately after a bite. It can take months for the larvae to mature into adult worms. When they do, British travellers may notice two distinct clinical features:

• Calabar Swellings: These are localised, itchy, and non-pitting swellings that occur most frequently on the wrists, face, or ankles. They are an immune reaction to the worm and typically disappear within a few days, only to reappear elsewhere.
• Subconjunctival Migration: This is the most famous symptom where a thin, thread-like worm is seen moving beneath the clear membrane (conjunctiva) of the eye. While this can cause significant distress and irritation, it rarely causes permanent vision damage.

Other symptoms include generalised itching (pruritus), hives (urticaria), and muscle pain. In some cases, patients may feel a 'crawling' sensation under the skin as the worms migrate through the subcutaneous tissues.

The Loa loa parasite relies on a specific biological cycle. The deerflies that carry the parasite live in high-canopy forests and are attracted to movement and wood smoke. When an infected fly bites a human, it deposits larvae into the skin. These larvae grow into adult worms, which can reach up to 7cm in length.

These adult worms produce offspring called microfilariae, which circulate in the bloodstream during the day. This timing is strategic, as it coincides with when deerflies are most active and likely to bite, thus continuing the cycle. For the average UK tourist, the risk is generally lower than for long-term expatriates or aid workers, as the infection often requires repeated exposure to fly bites over several weeks or months.

Currently, there is no vaccine to prevent loiasis. If you are planning a trip to Central or West Africa, the best strategy is to avoid deerfly bites. NICE and NHS travel guidelines suggest the following precautions:

• Insect Repellent: Use a repellent containing at least 50% DEET on all exposed skin.
• Protective Clothing: Wear long-sleeved shirts and trousers. Deerflies can bite through thin fabrics, so treating clothes with permethrin is highly recommended.
• Avoid Shady Areas: Deerflies are most active during the day and are often found in shaded, humid areas near water or muddy banks.
• Screening: Ensure your accommodation has fine-mesh insect screens on windows and doors.

For those spending extended periods (over a month) in high-risk areas, some specialists may suggest chemoprophylaxis with diethylcarbamazine (DEC), but this must be discussed with a tropical medicine expert before departure.

If you have recently returned from West or Central Africa and notice unusual skin swellings or a crawling sensation in your eye, you should seek medical advice promptly. You can speak to a GP online to discuss your travel history and symptoms. While loiasis is rare in the UK, an online GP can help differentiate these symptoms from other travel-related issues like malaria, allergic reactions, or other parasitic infections.

A GP will likely refer you to a specialist centre, such as the Hospital for Tropical Diseases in London, for definitive testing. Diagnosis usually involves a daytime blood film (taken between 10 am and 2 pm) to look for the parasite under a microscope. Providing a detailed travel itinerary is crucial for accurate diagnosis.

Treatment for loiasis is effective but must be managed carefully by specialists. The primary medication used is diethylcarbamazine (DEC), which kills both the microfilariae and the adult worms. However, if a patient has a very high 'worm burden' (a large number of parasites in the blood), rapid treatment can lead to serious side effects, including inflammation of the brain (encephalitis).

In cases where the worm is visible in the eye, a doctor can sometimes remove it surgically using a local anaesthetic. This is a quick procedure that provides immediate relief and helps confirm the diagnosis. Long-term management involves monitoring the blood to ensure the parasite has been fully cleared from the system.