Loa loa
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''Loa loa'' is a
filaria ''Filaria'' is a genus of nematodes belonging to the family Filariidae. The genus has cosmopolitan distribution. Species: *'' Filaria acutiuscula'' *'' Filaria bufonis'' *'' Filaria loliginis'' *'' Filaria martis'' *'' Filaria piscium'' ...
l (
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
-borne)
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
(roundworm) that causes ''Loa loa'' filariasis. ''Loa loa'' actually means "worm worm", but is commonly known as the "eye worm", as it localizes to the
conjunctiva The conjunctiva is a thin mucous membrane that lines the inside of the eyelids and covers the sclera (the white of the eye). It is composed of non-keratinized, stratified squamous epithelium with goblet cells, stratified columnar epithelium ...
of the eye. ''Loa loa'' is commonly found in Africa.Schmidt, Gerald et al. "Foundations of Parasitology". 7th ed. McGraw Hill, New York, NY, 2005. It mainly inhabits rain forests in West Africa and has native origins in Ethiopia. The disease caused by ''Loa loa'' is called loiasis and is one of the
neglected tropical diseases Neglected tropical diseases (NTDs) are a diverse group of tropical disease, tropical infections that are common in low-income populations in Developing country, developing regions of Africa, Asia, and the Americas. They are caused by a variety ...
. ''L. loa'' is one of three parasitic filarial nematodes that cause subcutaneous
filariasis Filariasis is a parasitic disease caused by an infection with roundworms of the Filarioidea type. These are spread by blood-feeding insects such as black flies and mosquitoes. They belong to the group of diseases called helminthiases. These ...
in humans. The other two are ''
Mansonella streptocerca ''Mansonella streptocerca'', (formerly ''Diptalonema streptocerca''), is a filarial (arthropod-borne) nematode (roundworm) causing the disease streptocerciasis. It is a common parasite in the skin of humans in the rain forests of Africa, where ...
and
Onchocerca volvulus ''Onchocerca volvulus'' is a filarial (arthropod-borne) nematode (roundworm) that causes onchocerciasis (river blindness), and is the second-leading cause of blindness due to infection worldwide after trachoma. It is one of the 20 neglected trop ...
'' (causes
river blindness Onchocerciasis, also known as river blindness, is a disease caused by infection with the parasitic worm ''Onchocerca volvulus''. Symptoms include severe itching, bumps under the skin, and blindness. It is the second-most common cause of blindne ...
). Maturing
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...
e and adults of the "eye worm" occupy the subcutaneous layer of the skin – the fat layer – of humans, causing disease. The ''L. loa'' adult worm which travels under the skin can survive up to 10–15 years, causing inflammations known as Calabar swellings. The adult worm travels under the skin, where the female deposits the
microfilaria ::''Microfilaria may also refer to an informal "collective group" genus name, proposed by Cobbold in 1882. While a convenient category for newly discovered microfilariae which can not be assigned to a known species because the adults are unknown, ...
e which can develop in the host’s blood within 5 to 6 months and can survive up to 17 years. The young larvae, or microfilariae, develop in horseflies of the
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
''
Chrysops Deer flies (also known in some parts of the mid-Atlantic United States as sheep flies) are bloodsucking insects considered pests to humans and cattle. They are large flies with large brightly-coloured compound eyes, and large clear wings with da ...
'' (deer flies, yellow flies), including the species '' C. dimidiata'' and '' C. silacea'', which infect humans by biting them. After bites from these infected flies, the microfilariae are unique in that they travel in the peripheral blood during the day and migrate into the lungs at night.Turkington, C., & Ashby, B. (2007). Encyclopedia of Infectious Diseases. New York: Facts on File.


Morphology

''L. loa'' worms have a simple structure consisting of a head (which lacks lips), a body, and a blunt tail. The outer body of the worm is composed of a cuticle with 3 main layers made up of collagen and other compounds which aid in protecting the nematodes while they are inside the digestive system of their host. Juveniles have a similar appearance to adult worms, but are significantly smaller. Male adults range from 20 to 34 mm long and 350 to 430 μm wide. Female adults range from 20 to 70 mm long and can be about 425 μm wide. They vary in color.


Lifecycle

The human is the
definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
, in which the parasitic worms attain sexual maturity, mate, and produce
microfilariae ::''Microfilaria may also refer to an informal "collective group" genus name, proposed by Cobbold in 1882. While a convenient category for newly discovered microfilariae which can not be assigned to a known species because the adults are unknown, ...
. The flies serve as
intermediate hosts In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
in which the microfilariae undergo part of their morphological development, and then are borne to the next definitive host. Two species of ''Chrysops'' deerflies, ''C. silacea'' and ''C. dimidiata'', are the main vectors for this filariasis. # A fly bearing third-stage
filarial Filariasis is a parasitic disease caused by an infection with roundworms of the Filarioidea type. These are spread by blood-feeding insects such as Black fly, black flies and mosquitoes. They belong to the group of diseases called helminthiases. ...
larvae in its proboscis infects the human host through the bite wound. # After entering the human host, the larvae mature into adults, commonly in subcutaneous tissue. Adult females measure about 40 to 70 mm in length and 0.5 mm in diameter. Males measure some 30 to 34 mm by 0.35 to 0.43 mm. # The adult female produces large numbers of microfilariae, about 250 to 300 μm in length and 6-8 μm in width. She continues to do so continuously for her lifetime, which typically spans several years. # Microfilariae tend to reside within spinal fluids, urine, and sputum; by day, they also circulate in the bloodstream. Apart from their presence in bodily fluids, however, microfilariae in the noncirculating phase also occur in the lungs. # The vector fly ingests microfilariae while feeding on the host's blood. # Once inside the vector, the microfilaria sheds its sheaths and escapes through the walls of the midgut into the fly’s
haemocoel The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
. # It then migrates through the
hemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...
into the wing muscles in the fly's thorax. # In the thoracic muscles, the microfilaria develops successively into a first-stage larva, second-stage larva, and finally into the infectious third-stage larva. # The third-stage larva migrates to the fly’s proboscis. # Once the larva is established in the proboscis and the fly takes its next human blood meal, the cycle of infection continues.


Disease


Signs and symptoms

Usually, about five months are needed for larvae (transferred from a fly) to mature into adult worms, which they can only do inside the human body. The most common display of infection is the localized allergic inflammations called
Calabar Calabar (also referred to as Callabar, Calabari, Calbari and Kalabar) is the capital city of Cross River State, Nigeria. It was originally named Akwa Akpa, in the Efik language. The city is adjacent to the Calabar and Great Kwa rivers and cre ...
or Cameroon swellings that signify the migration of the adult worm in the tissues away from the injection site by the vector. The migration does not cause significant damage to the host and is referred to as benign. However, these swellings can be painful, as they are mostly found near the joints. Although most infections with ''L. loa'' are asymptomatic, symptoms generally do not appear until years, or even more than a decade, after the bite from an infected fly. However, symptoms can appear as early as four months after a bite. These parasites have a diurnal periodicity in which they circulate in the peripheral blood during the daytime, but migrate to vascular parts of the lungs during the night, where they are considered non circulatory. Therefore, the appearing and disappearing characteristics of this parasite can cause recurrent swelling that can cause painful enlargements of
cyst A cyst is a closed sac, having a distinct envelope and cell division, division compared with the nearby Biological tissue, tissue. Hence, it is a cluster of Cell (biology), cells that have grouped together to form a sac (like the manner in which ...
s in the connective tissue surrounding tendons. Additionally, chronic abscesses can be caused by the dying worms. The most visual sign of an adult worm infections is when the worm crosses the sclera of the eye, which causes significant pain to the host and is usually associated with inflammation and less likely, blindness. Eye worms typically cause little eye damage and last a few hours to a week. Other tissues in which this worm can be found includes: the penis, testes, nipples, bridge of the nose, kidneys, and heart. The worms in these locations are not always externally visible.


Risk factors

People at the highest risk for acquiring
loiasis ''Loa loa'' filariasis is a skin and eye disease caused by the nematode worm ''Loa loa''. Humans contract this disease through the bite of a deer fly or mango fly (''Chrysops'' spp.), the vectors for ''Loa loa''. The adult ''Loa loa'' filarial ...
are those who live in the rainforests of West or Central Africa. Furthermore, the probability of getting bitten by a deerfly increases during the day and during rainy seasons. The flies are also attracted to smoke from wood fires. These flies do not commonly enter houses, but they are attracted to the houses that are well lit, so will congregate outside. Travelers can be infected in less than 30 days after arriving in an affected area, although they are more likely to be infected whilst being bitten by multiple deerflies over the course of many months. Men are more susceptible than women due to their increased exposure to the vectors during activities such as farming, hunting, and fishing.Zierhut, M., Pavesio, C., Ohno, S., Oréfice, F., Rao, N. A. (2014). Intraocular Inflammation. Springer Dordrecht Heidelberg London New York.


Diagnosis

The main methods of diagnosis include the presence of microfilariae in the blood, the presence of a worm in the eye, and the presence of skin swellings. However, in cases where none of those is the case, a blood count can be done. Patients with infections have a higher number of blood cells, namely
eosinophil Eosinophils, sometimes called eosinophiles or, less commonly, acidophils, are a variety of white blood cells (WBCs) and one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. A ...
s, as well as high
IgE Immunoglobulin E (IgE) is a type of antibody (or immunoglobulin (Ig) " isotype") that has been found only in mammals. IgE is synthesised by plasma cells. Monomers of IgE consist of two heavy chains (ε chain) and two light chains, with the ε c ...
levels that indicate an active infection with
helminth Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as schi ...
parasites. Due to the migration of microfilariae during the day, the accuracy of a blood test can be increased when samples are taken between 10 am and 2 pm. A
Giemsa stain Giemsa stain (), named after German chemist and bacteriologist Gustav Giemsa, is a nucleic acid stain used in cytogenetics and for the histopathological diagnosis of malaria and other parasites. Uses It is specific for the phosphate groups of ...
is the most commonly used diagnostic test that uses a thick blood smear to count the microfilariae. Other than blood, microfilariae can also be observed in urine and saliva samples.


Treatment

Adult worms found in the eye can be surgically removed with forceps after being paralyzed with a topical anesthesia. The worm is not paralyzed completely, so if it is not extracted quickly, it can vanish upon attempting extraction.
Ivermectin Ivermectin (, '' EYE-vər-MEK-tin'') is an antiparasitic drug. After its discovery in 1975, its first uses were in veterinary medicine to prevent and treat heartworm and acariasis. Approved for human use in 1987, today it is used to treat i ...
has become the most common antiparasitic agent used worldwide, but can lead to residual microfilarial load when given in the management of loiasis. Treatment with ivermectin has shown to produce severe adverse neurological consequences in some cases. These treatment complications can be increased in individuals co-infected with
onchocerciasis Onchocerciasis, also known as river blindness, is a disease caused by infection with the parasitic worm ''Onchocerca volvulus''. Symptoms include severe itching, bumps under the skin, and blindness. It is the second-most common cause of blindne ...
. Some of these patients experienced cases of coma and resultant
encephalopathy Encephalopathy (; from grc, ἐνκέφαλος "brain" + πάθος "suffering") means any disorder or disease of the brain, especially chronic degenerative conditions. In modern usage, encephalopathy does not refer to a single disease, but r ...
,
parkinsonism Parkinsonism is a clinical syndrome characterized by tremor, bradykinesia (slowed movements), rigidity, and postural instability. These are the four motor symptoms found in Parkinson's disease (PD), after which it is named, dementia with Lewy b ...
, and death. After about 12 hours, the first signs start to appear and include fatigue, pain in joints,
mutism Muteness or mutism () is defined as an absence of speech while conserving or maintaining the ability to hear the speech of others. Mutism is typically understood as a person's inability to speak, and commonly observed by their family members, caregi ...
, and
incontinence Incontinence or Incontinent may refer to: *Fecal incontinence, the involuntary excretion of bowel contents *Urinary incontinence, the involuntary excretion of urine * Lack of moderation or self-control, especially related to sexual desire - see I ...
. Severe disorders of the consciousness start to develop after about a day. Although Ivermectin is a common treatment for loiasis, the
Centers for Disease Control The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
(CDC) recommends treatment with
diethylcarbamazine Diethylcarbamazine is a medication used in the treatment of filariasis including lymphatic filariasis, tropical pulmonary eosinophilia, and loiasis. It may also be used for prevention of loiasis in those at high risk. While it has been used for ...
(DEC). Symptoms may be resolved with as little as 1–2 courses of DEC. DEC is chosen over Ivermectin because evidence supports its ability to kill both the adult worms and the microfilariae, which are the main cause of the severe neurological problems mentioned above. In some cases, albendazole may also be an effective treatment used to reduce the microfilariae prior to treatment with DEC. The body's response to albendazole is slow, so the patient being treated must be monitored closely and frequently to ensure it is effective.


Prevention

Currently, no control programs or vaccines for loiasis are available. However,
diethylcarbamazine Diethylcarbamazine is a medication used in the treatment of filariasis including lymphatic filariasis, tropical pulmonary eosinophilia, and loiasis. It may also be used for prevention of loiasis in those at high risk. While it has been used for ...
treatment is suggested to reduce risk of infection. Avoiding areas where the vectors, deerflies, are found also reduces risk. This includes swamps, bogs, and shaded areas near rivers or near wood fires. Fly bites can be reduced by using insect repellents such as DEET and wearing long sleeves and pants during the daytime.
Permethrin Permethrin is a medication and an insecticide. As a medication, it is used to treat scabies and lice. It is applied to the skin as a cream or lotion. As an insecticide, it can be sprayed onto clothing or mosquito nets to kill the insects th ...
treatment on clothes is an additional repellent that could be used. Also, using malaria nets can reduce the number of fly bites acquired.


Epidemiology

Reports of microfilaremia have been made in Angola, Benin, Cameroon, Central African Republic, Congo, the Democratic Republic of Congo, Equatorial Guinea, Gabon, Nigeria, and Sudan, and possibly rare cases in Chad, Ghana, Guinea, Liberia, Uganda, and Zambia. Of the 10 countries that have high rates of infection, about 40% of the people who live in the area have reported being infected with the worm in the past. The population in high-risk areas is about 14.4 million; in addition, 15.2 million people live in areas where around 20–40% of people admitted to having the worm in the past.Centers for Disease Control and Prevention (CDC). (2015 January 20). Parasites – Loiasis. Global Health – Division of Parasitic Diseases. Retrieved from: https://www.cdc.gov/parasites/loiasis/ Epidemiological studies have been emphasized in the western part of Africa. In this area, the disease is considered endemic. A study conducted by the Research Foundation in Tropical Diseases and Environment in 2002 had a sample of 1458 persons, spanning 16 different villages, and found'' Loa loa'' presence in these villages ranging from 2.22 to 19.23% of the population. The disease was found to be slightly more prevalent in men. In a different country in western Africa, a cross-sectional community survey was conducted in Gabon, Africa. The study was performed by the department of Tsamba-Magotsi from August 2008 to February 2009. The study of 1,180 subjects evaluated the presence of microfilaria using microscopy. The carriage rate of ''L. loa'' in the subjects tested was 5%. This rate falls within the range of the study listed above. In the western part of Africa, an increase in prevalence has been associated with the distribution of ivermectin, which is used to prevent the infection of onchocerciasis, which is also very prevalent in the same region. Patients with ''L. loa'' who are treated with ivermectin have extreme adverse effects, including death. Therefore, a prevalence mapping system was created called REMO. REMO is used to determine which areas to distribute the ivermectin based on lower levels of ''L. loa'' prevalence. The area discovered to be the most overlapping was where Cameroon and the Democratic Republic of Congo overlap. A study performed to review reported cases of ''L. loa'' in nonendemic countries in the past 25 years reviewed 102 cases of imported loiasis, 61 of them from Europe and 31 from the US. Three-quarters of the infestations were acquired in three countries considered endemic: Cameroon, Nigeria, and Gabon. In the subjects viewed, peripheral blood microfilariae were detected 61.4% of the time, eye worm migration 53.5% of the time, and Calabar swellings 41.6% of the time. A trend appeared in the symptoms of the patients where Calabar swellings and eosinophilia were more common among travelers. African immigrants tended to have microfilaremia. Eye worm migration was observed in a similar proportion between the two groups. Only 35 of the patients underwent clinical follow-up. The researchers concluded that ''L. loa'' would end up migrating to Europe and the United States, due to increased travel to already endemic regions.


See also

*
List of short species names Living organisms are known by scientific names. These binomial names can vary greatly in length, and some can be very short; genus or species names composed of only one letter are not allowed by any of the nomenclature codes, but any combination of ...


References


Taxonomy Browser: Loa Loa.
National Center for Biotechnology Information (NCBI). {{DEFAULTSORT:Loa Loa Parasitic diseases Parasitic nematodes of humans Nematodes described in 1864 Spirurida Taxa named by Thomas Spencer Cobbold