centre for sight africa Loa Loa_ The _African_ eye worm By Dr Enaholo Ehimare

Loa Loa: The “African” eye worm By Dr Enaholo Ehimare. S



Numerous diseases result from invasion of the body by disease-causing organisms. Such causative organisms include: bacteria, viruses, fungi and parasites. However, for the purpose of this article, the parasites are of particular interest to us.

A parasite is, simply put, an organism which feeds from and depends on its host to grow and reproduce while also having a negative impact on the overall wellbeing of its host. The term ‘host’  used here simply refers to the organism a particular parasite depends upon.

‘Loiasis,  commonly referred to as the “African eye worm”, is the condition that arises when the host system is invaded by parasitic ‘loa loa’ roundworms. Loa loa parasites do not invade the host in their adult form but only attain maturity within the body of its host after several months. Adult worms can live up to seventeen years while constantly reproducing within the host. The adult loa loa migrate frequently underneath the skin as well as other superficial areas of the body surface till they eventually reach the eye, giving the appearance of a transparent worm dwelling underneath the conjunctiva (superficial part of the ‘white’ of the eye).  Apart from the obvious feelings of discomfort and irritation which result from the movement of an adult worm underneath the conjunctiva; the sight of a sizeable worm moving in front of one’s eye is also very undesirable and bound to raise panic.  

What is the cause of this condition?

Mangrove flies (including the deer fly or the mango fly), which breed in certain rain forests of West and Central Africa, act as vectors (carriers) of the parasite. Hence, loiasis is transmitted to humans via repeated bite incidents by infected mangrove flies. This occurs when infected flies obtain a blood meal, in turn, parasites are released into the blood stream from the mouth organ (proboscis) of the deer fly. Therefore, only infected flies can transmit the loa loa parasites to their host. However, an uninfected fly becomes an ‘active carrier’ of the loa loa parasite after ingesting a blood meal from an individual affected by loiasis.

Who are those facing the greatest risk of infection?

Around 14 million people live in areas with high rates of loiasis infection while about 15 million individuals live in other areas with past history of loiasis.

Most people who are at risk of loiasis live in certain rain forests of West and Central Africa. The deerflies which transmit the parasite to humans usually biteduring the day and are more common during rainy seasons. They are attracted by the movement of people and smoke from wood fires. Rubber plantations are areas where more deerflies may be found. These flies typically do not enter homes, but they might be attracted to well lit homes.

What are the signs of loiasis?

The condition does not usually involve any symptoms. Vision is unaffected and patients are not at risk of blindness directly from this disease. Occasionally, itchy and swollen areas known as ‘Calabar swellings’  are found along the arms, legs and near joints.  Adult worms may also be spotted when they migrate to the eye’s conjunctiva. At this point, there may also be some accompanying itchiness and pain sensation from the eyes. However, this ‘eye worm’ sign usually lasts for only a few hours to less than a week.

An adult filarial worm pictured underneath the conjunctiva of an individual infected with loiasis

Prevention measures and treatment options

 – Individuals travelling to high risk areas may be placed on preventive treatment by qualified health professionals.

– Frequently wearing long sleeved shirts and trousers, particularly during the day, may also limit the chances of deer fly bites.

– Use of insect repellants containing a compound known as DEET.

If you, or someone you know suddenly feels a sensation of movement within the eye or even notices a transparent worm coursing the front surface of the eye; such an individual should not attempt to remove this worm forcibly. On the other hand, the case should be reported to a specialist eye clinic, where the offending worm can be removed safely; and such patients placed on adequate therapy.

In the absence of the ‘eye worm’ sign, proper diagnosis of loiasis is aided by  laboratory assessments of blood samples and staining techniques to differentiate other types of parasitic worms from the organism of concern.

Timely and proper treatment of loiasis is essential, as greater multiplication of parasite populations within a human host greatly increases the risk of severe health complications arising.

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