Loiasis (African eye worm): symptoms, diagnosis and treatment for travellers and migrants

Loiasis is an infection with the roundworm Loa loa, a filarial worm. The MSD Manual (consumer version) explains that infected African deer flies deposit larvae when they bite, the larvae enter the bite wound and mature into adult worms in the tissues under the skin. Adult worms travel under the skin and under the clear membrane covering the eye, and produce immature larvae, called microfilariae, that circulate in the blood during the day and are found in the lungs at night.

The infection is spread when a fly bites an infected person during the day, when microfilariae are in the bloodstream, and then bites someone else. The MSD Manual's professional version adds that humans are the only known natural reservoir of the parasite.

Because loiasis is confined to a particular region, it is encountered in the UK in people who were born or have lived in affected countries, and in travellers who stay for long periods. The BMJ case report from Rotherham says that, due to international travel and migration, various tropical diseases are being reported across Europe and North America.

Most people with loiasis have no symptoms, according to the MSD Manual. The symptoms that can occur are as follows.

• Calabar swellings: itchy swellings that develop mainly on the arms and legs, though they can occur anywhere. They are thought to be an allergic reaction to substances released by travelling adult worms and usually last 1 to 3 days in residents of endemic areas, but are more frequent and severe in travellers.

• A worm moving across the eye (subconjunctival migration): the MSD Manual says this may make the eyes itch or feel irritated, and although unsettling it usually does not cause permanent damage. The Rotherham case describes an acute, painful, red eye of 3 days with a live thread-like worm seen in the temporal subconjunctival space.

• Occasional involvement of the heart, kidneys or brain: the MSD Manual says symptoms are usually mild. Kidney involvement may show as extra protein, and a little blood, in the urine.

• A raised eosinophil count in blood tests, described in the professional version as common but non-specific.

See a GP, or a travel or tropical medicine clinic, if you have lived in or travelled through western or central Africa and have recurrent itchy swellings, or if you notice a worm or an unexplained red, painful or irritated eye. The BMJ case report advises considering these infections in patients with an acute red eye who have recently lived, worked or travelled in affected regions. A red painful eye should be seen promptly in any event; NHS 111 or an emergency eye service can advise out of hours.

Call 999 or go to A&E if there is sudden loss of vision, severe headache with confusion or drowsiness, a seizure or severe difficulty breathing. These are general emergency signs rather than features specific to loiasis; the MSD Manual notes brain involvement is occasional and usually mild.

The MSD Manual says doctors suspect loiasis in people with eye worms or swellings who have travelled to or emigrated from western and central Africa. Diagnosis is made by identifying microfilariae in a blood sample examined under a microscope, with the sample taken between 10 AM and 2 PM when the most microfilariae are in the blood. Occasionally doctors diagnose loiasis by seeing worms under the conjunctiva or by identifying worms removed from the eye or skin. The professional version also lists quantitative PCR as a way of counting microfilariae.

In the Rotherham case, the worm was removed live under topical anaesthesia, the daytime blood sample showed microfilariae at a low load and a night-time sample did not, and the patient was screened for other parasites and for onchocerciasis before treatment. This order of events reflects a common practical point: the count of microfilariae matters, because it determines how treatment is done.

The MSD Manual says the only medicine that kills both microfilariae and adult worms is diethylcarbamazine, taken by mouth for 21 days and sometimes repeated. Before using it, doctors check for onchocerciasis (another filarial infection), because diethylcarbamazine can cause serious side effects when both are present, and measure the microfilariae count. In heavy infection, the consumer version says doctors reduce the number first with albendazole or by a blood-filtering procedure (apheresis), and that diethylcarbamazine can have serious, sometimes fatal side effects especially if infection is severe.

The professional version states that treatment of loiasis is complicated and that clinicians should seek expert advice before starting. It names a threshold of more than 8000 microfilariae per millilitre of blood as a risk for potentially fatal encephalopathy, which is why this is managed in specialist tropical medicine or infectious diseases services, not by general practice alone.

In the Rotherham case, the worm seen in the eye was removed live under topical anaesthesia, and the patient was also investigated for other parasites, with the blood findings guiding the next steps.

The MSD Manual says people who will be staying a long time in an affected area can take diethylcarbamazine to help prevent loiasis. To reduce deer fly bites, it advises using insect repellents, wearing clothing treated with permethrin, and wearing loose-fitting long-sleeved shirts and long trousers. Because the flies bite during the day, it says mosquito netting over beds does not help.

A GP surgery nurse or travel clinic can discuss the specifics of your itinerary and whether any preventive medicine is suitable for you.

The MSD Manual says most infected people have no symptoms and that residual eye damage is uncommon. In the NHS, diagnosis and treatment for imported tropical infections are usually through infectious diseases or tropical medicine departments, with eye clinics involved if a worm is seen in the eye. Private travel and tropical medicine clinics also offer testing, but the treatment for loiasis is specialist and should be supervised by a unit experienced in managing filarial infections.