Loiasis (African Eye Worm) Clinical Presentation

Updated: Jun 03, 2020
  • Author: Darvin Scott Smith, MD, MSc, DTM&H; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Patients with L loa infection must have been exposed to the competent vector within the endemic areas in Africa. The diagnosis should be considered with such exposure in the setting of eosinophilia, urticaria, subcutaneous swellings, and joint pain. Less commonly, some patients report a history of visualizing the migrating adult worm across the conjunctiva. This transient event, which may last between 30 and 60 minutes, [8] is notable for some eye irritation described as sand in the eye with movement.

Symptoms of loiasis reflect the developmental stage of the parasite and the degree of immune response by the host. Clinical manifestations are highly variable between temporary residents of endemic areas and people native to these areas. Visitors and expatriates had low levels of microfilariae yet exhibited marked immunological hyperresponsiveness (symptomaticity). In contrast, people living in endemic regions were often asymptomatic despite high levels of circulating microfilariae. [19]

Symptoms of loiasis do not appear until after the L4 larvae mature into adult worms. Clinical symptoms may appear as soon as 5 months but up to 13 years (average, 17 months) after the initial infecting bite. The interval between the acquisition of the parasite and onset of symptoms is sometimes referred to as the prepatent phase. [8]



L loa is transmitted by the bite of infected Chrysops flies. The fly bites during daylight hours. Chrysops flies can be found in the tropical rainforests of West and Central Africa, are attracted to movement and smoke from wood fires, and are most abundant during the rainy season. [1, 7]

Most L loa infections are asymptomatic, and many symptoms result from adult worm migration or the host immune response to adult worms or the microfilariae they produce. [1, 8]


Physical Examination

Physical examination may reveal skin findings such as urticaria accompanied by pruritus. The next most common finding in patients with chronic or longer-term infection is joint swelling with pain (calabar swelling), which may be seen in the wrists and elbows or lower-extremity joints. These swellings are related to angioedema (associated with immune reactions) and are nonpitting and nontender and often associated with localized pruritus. They may disappear spontaneously, typically after a few days, and then reappear on any part of the body at irregular intervals. [8]

Adult female worms are 40-70 mm in length and 0.5 mm in diameter; males are 30-34 mm in length and 0.35-0.43 mm in diameter. [7] ​ The migration of an adult worm across the conjunctiva may be felt or observed by the patient, accompanied by itching and photophobia. The worm may take between 30 to 60 minutes or up to 1 day to transit across the eye in the conjunctival space. [8]

Live adult Loa loa worm in the anterior chamber of Live adult Loa loa worm in the anterior chamber of the eye. Courtesy of PLoS Neglected Tropical Diseases.


Although L loa infections are typically described as benign and the vast majority of cases are asymptomatic, a 2017 retrospective review of 28 Cameroonian villages found heightened mortality rates (4.1%) in individuals with over 30,000 microfilariae per mL of blood, as well as significantly earlier deaths among individuals older than 25 years with over 30,000 microfilariae per mL of blood, compared to amicrofilaremic individuals, suggesting an increased mortality rate associated with high-grade L loa infection. [15]

Beyond these findings, many complications due to L loa infection have been described, albeit in a minority of cases. Worm migration through the subconjunctiva may progress to invasion of the eye itself by adult worms, causing pain, intraocular inflammation, and even blindness. [16]

In addition, encephalitis is known to occur in individuals with L loa infection, often due to administration of diethylcarbamazine (DEC) or ivermectin, drugs used in the treatment of other nematode infections, such as onchocerciasis (river blindness). This pathology is believed to result from the high microfilaricidal effects of these drugs and the immune system's response to dead microfilariae in blood. Most such cases of encephalitis have been found in individuals with over 30,000 microfilariae per mL of blood, although neurologic symptoms have been observed in patients with lower levels of microfilariae in the bloodstream. [21, 22] This encephalitis may lead to events such as coma and may be fatal. [17]

Endomyocardial fibrosis has been observed in cases of chronic loiasis and may progress to cardiomyopathy and, sometimes, death. [18, 19] ​ These cardiac manifestations of infection are associated with eosinophilia and eosinophilic infiltration of the heart and its tissue, [23] which typically results from the immune response to L loa infection. [24]

Renal involvement has also been reported in individuals with loiasis. The mechanisms that mediate this involvement are unclear, but the immune response is believed to play some role. Kidney failure due to loiasis-related nephropathy is uncommon but has been documented. [19]