Loiasis (Loa loa Infection)

Updated: Jul 31, 2018
  • Author: Darvin Scott Smith, MD, MSc, DTM&H; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Overview

Background

Loiasis is an infectious disease caused by the nematode Loa loa, which is transmitted to humans via the bite from one of two female Chrysops deerfly species: Chrysops silacea and Chrysops dimidiata. The risk of infection is highest in the rainforests of West and Central Africa and during the rainy season, when the deerfly (or mango fly or mangrove fly, as they are commonly known) are most prevalent. It is estimated that 3-13 million people have loiasis. [1]

A fly of the Chrysops genus. Courtesy of Flickr [J A fly of the Chrysops genus. Courtesy of Flickr [Judy Gallagher].

The worm larvae that Chrysops flies deposit in human hosts mature into adults, which commonly reside in subcutaneous tissue. Most L loa infections are asymptomatic, although some progress to loiasis, which is characterized by localized painful subcutaneous joints (calabar swellings) and the migration of adult worms through the subconjuctiva of the eye.

Loiasis is commonly known as "African eye worm," since the adult form of the parasite can sometimes be found migrating across the conjunctiva of the eye. Repeated exposure to the bites of infected flies increases the risk of infection and disease. [4]

Adult Loa loa worm. Courtesy of Flickr [NIAID/NIH] Adult Loa loa worm. Courtesy of Flickr [NIAID/NIH].

L loa is endemic in eleven West and Central African countries, as follows (listed alphabetically):

  • Angola
  • Cameroon
  • Central African Republic
  • Chad
  • Congo
  • Democratic Republic of Congo
  • Equatorial Guinea
  • Ethiopia
  • Gabon
  • Nigeria
  • Sudan

Ten of these countries have foci where the prevalence of infection exceeds 40%. An estimated 14.4 million people live in these regions of high prevalence. [2, 3] This range overlaps significantly with that of onchocerciasis and is of great epidemiological significance, since ivermectin, the major and most effective treatment for onchocerciasis, often leads to severe, and sometimes fatal, complications in individuals who are co-infected with L loa. [4]

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Pathophysiology

L loa microfilariae mature into their infective third-stage larval forms in their Chrysops hosts, a process that typically takes 10-12 days to complete. [6] These larvae make their way into humans when female flies take blood meals and migrate from the proboscis of an infected fly to the bite wound it creates.

Chrysops bites are typically quite painful, since, unlike mosquitoes, which use their probosces to puncture skin and suck blood out of their hosts, these flies use their probosces to create lacerations in the host’s skin and then lap up the blood from the wound that they create. When interrupted during a blood meal, Chrysops flies have been known to make new lacerations to continue to feed from the same host owing to the female's high blood demand for reproductive purposes. [7] In their human hosts, larvae migrate through the lymphatic system and mature into adults, residing in subcutaneous tissue for approximately 19 days.

Giemsa stained blood showing microfilaria of Loa l Giemsa stained blood showing microfilaria of Loa loa with white blood cells present. Courtesy of Wikimedia Commons [Stefan Walkowski].

Adult L loa worms also migrate through subcutaneous host tissue, and the host’s immune response to this migration causes many of the characteristic symptoms of loiasis, such as urticaria, pruritus, and calabar swelling (painful swelling in the joints). Occasionally, adult worms migrate through the subconjunctiva of the eye and are visible therein, giving rise to the disease's common name, "African eye worm."

Once fertilized, female worms release thousands of microfilariae per day (up to 22,000). These microfilariae accumulate in the pulmonary circulation, which serves as a reservoir from which they invade peripheral blood systems. Only a fraction of the microfilariae in the lungs exit into circulation, a process that may be mediated by their age. About 17 months pass between the introduction of L loa larvae into the host and the appearance of microfilariae in the blood. Microfilariae have lifespans of approximately 6-12 months, and the cycle of infection is continued when Chrysops flies feed on the blood of humans with microfilariae in their blood. [5]

L loa has diurnal periodicity, meaning that levels of microfilariae are higher in the blood during the daytime, typically highest between 10:00 am and 3:00 pm. [5] However, microfilariae have also been found in blood samples collected at night. [24]

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Etiology

Loiasis is caused by infection with L loa (tissue nematode) parasites transmitted by Chrysops flies (commonly known as the mango fly or mangrove fly). The adult L loa worm can migrate across the subconjunctiva of the human eye, giving rise to the worm's colloquial name, "African eye worm." Beyond this ocular pathology, loiasis also commonly manifests as skin findings (urticaria and pruritus), swelling (calabar swelling), and joint pain due to the immune reaction against the parasite. [23]

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Epidemiology

Frequency

United States

Loiasis is not acquired in the United States. Some cases have been reported in travellers and immigrants from endemic areas. Months to years of exposure are typically required to establish infection, although some cases have been reported after as little as four days of exposure.

International

L loa is endemic in eleven West and Central African countries, as follows (listed alphabetically):

  • Angola
  • Cameroon
  • Central African Republic
  • Chad
  • Congo
  • Democratic Republic of Congo
  • Equatorial Guinea
  • Ethiopia
  • Gabon
  • Nigeria
  • Sudan

Ten of these countries have foci where the prevalence of infection exceeds 40%. An estimated 14.4 million people live in these regions of high prevalence. [2, 3, 35] In these regions, the vast majority of infections are asymptomatic, so the range and endemicity of loiasis in many regions have not yet been determined. The probability of infection has been found to increase with age and depends on vector abundance, which, in turn, depends on local ecology.

The Chrysops vectors that carry L loa typically bite during the daytime and are most prevalent during the rainy season. They are attracted to signs of human activity, such as movement and smoke from wood fires. [8]

The range of loiasis overlaps significantly with that of onchocerciasis and is of great epidemiological significance, since ivermectin, the major and most effective treatment for onchocerciasis, often leads to severe, and sometimes fatal, complications when administered to individuals infected with L loa. [4]

Geographical distribution of RAPLOA surveyed villa Geographical distribution of RAPLOA surveyed villages in 11 APOC countries. Courtesy of PLoS Neglected Tropical Diseases.

Morbidity/Mortality

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. [11]

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. [12, 13]

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 encephalitis may lead to events such as coma and may be fatal. [16]

Endomyocardial fibrosis has also been observed in cases of chronic loiasis and may progress to cardiomyopathy and, sometimes, death. [17]

Renal involvement has also been reported in loiasis. Kidney failure due to loiasis-related nephropathy is uncommon but has been documented. [20]

Sex

Loiasis does not have a sexual predilection. [22]

Age

The prevalence of loiasis in a population increases with age. [21, 22]

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