''Thelazia'' is a genus of

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

worms which parasitize the eyes and associated tissues of various

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

and

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

hosts, including humans. They are often called "eyeworms", and infestation with ''Thelazia'' species is referred to as " thelaziasis" (occasionally spelled "thelaziosis"). Adults are usually found in the eyelids, tear glands,

tear ducts The nasolacrimal duct (also called the tear duct) carries tears from the lacrimal sac of the eye into the nasal cavity. The duct begins in the eye socket between the maxillary and lacrimal bones, from where it passes downwards and backwards. The o ...

, or the so-called "third eyelid" (

nictitating membrane The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. Most ...

). Occasionally, they are found in the eyeball itself, either under the

conjunctiva In the anatomy of the eye, the conjunctiva (: conjunctivae) 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 gobl ...

(the membrane that covers the white part of the eye) or in the vitreous cavity of the eyeball. All species of ''Thelazia'' for which the life cycle has been studied are transmitted by species of

Diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advance ...

(flies) which do not bite, but which feed on tears.

Representative species

*'' Thelazia anolabiata'' ( Molin, 1860) ** Definitive hosts:

Andean cock-of-the-rock The Andean cock-of-the-rock (''Rupicola peruvianus''), also known as ''tunki'' (Quechua), is a large passerine bird of the cotinga family native to Andean cloud forests in South America. It is the List of national birds, national bird of Peru. I ...

(''Rupicola peruviana'') and many other Brazilian birds ** Intermediate hosts: Not known ** Distribution: South America *'' Thelazia bubalis'' Ramanujachari and Alwar, 1952 ** Definitive hosts:

Water buffalo The water buffalo (''Bubalus bubalis''), also called domestic water buffalo, Asian water buffalo and Asiatic water buffalo, is a large bovid originating in the Indian subcontinent and Southeast Asia. Today, it is also kept in Italy, the Balkans ...

** Intermediate hosts: Not known ** Distribution: India *''

Thelazia californiensis ''Thelazia californiensis'' is a nematode that originates in the genus ''Thelazia'', which comes from phylum Nematoda. This worm has been known to cause Thelaziasis in hosts. Morphology As with most ''Thelazia'' worms, they are small parasites ...

'' Price, 1930 ** Definitive hosts:

Dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the gray wolf. Also called the domestic dog, it was selectively bred from a population of wolves during the Late Pleistocene by hunter-gatherers. ...

(''Canis familiaris''),

cat The cat (''Felis catus''), also referred to as the domestic cat or house cat, is a small domesticated carnivorous mammal. It is the only domesticated species of the family Felidae. Advances in archaeology and genetics have shown that the ...

(''Felis catus''), occasionally

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

(''Homo sapiens''),

domestic sheep Sheep (: sheep) or domestic sheep (''Ovis aries'') are a domesticated, ruminant mammal typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to ...

(''Ovis aries''),

mule deer The mule deer (''Odocoileus hemionus'') is a deer indigenous to western North America; it is named for its ears, which are large like those of the mule. Two subspecies of mule deer are grouped into the black-tailed deer. Unlike the related whit ...

(''Odocoileus hemionus''),

coyote The coyote (''Canis latrans''), also known as the American jackal, prairie wolf, or brush wolf, is a species of canis, canine native to North America. It is smaller than its close relative, the Wolf, gray wolf, and slightly smaller than the c ...

(''Canis latrans'') and

American black bear The American black bear (''Ursus americanus''), or simply black bear, is a species of medium-sized bear which is Endemism, endemic to North America. It is the continent's smallest and most widely distributed bear species. It is an omnivore, with ...

(''Ursus americanus''). ** Intermediate hosts:

Lesser house fly The lesser house fly (''Fannia canicularis'') , commonly known as little house fly, is a species of fly. It is somewhat smaller () than the common housefly and is best known for its habit of entering buildings and flying in jagged patterns in th ...

(''Fannia canicularis'') and ''Fannia benjamini'' ** Distribution: Western North America *'' Thelazia callipaeda'' Railliet & Henry, 1910 (sometimes called "Oriental eyeworm") ** Definitive hosts: Typically reported from

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the gray wolf. Also called the domestic dog, it was selectively bred from a population of wolves during the Late Pleistocene by hunter-gatherers. ...

(''Canis familiaris''),

(''Felis catus''), and occasionally reported from

grey wolf The wolf (''Canis lupus''; : wolves), also known as the grey wolf or gray wolf, is a canine native to Eurasia and North America. More than thirty subspecies of ''Canis lupus'' have been recognized, including the dog and dingo, though gr ...

(''Canis lupus''),

raccoon dog ''Nyctereutes'' (Greek: ''nyx, nykt-'' "night" + ''ereutēs'' "wanderer") is a genus of canid which includes only two extant species, both known as raccoon dogs: the common raccoon dog (''Nyctereutes procyonoides'') and the Japanese raccoon do ...

(''Nyctereutes procyonoides''),

red fox The red fox (''Vulpes vulpes'') is the largest of the true foxes and one of the most widely distributed members of the order Carnivora, being present across the entire Northern Hemisphere including most of North America, Europe and Asia, plus ...

(''Vulpes vulpes''),

European rabbit The European rabbit (''Oryctolagus cuniculus'') or coney is a species of rabbit native to the Iberian Peninsula (Spain, Portugal and Andorra) and southwestern France. It is the only extant species in the genus ''Oryctolagus''. The European rab ...

(''Oryctolagus cuniculus''). Over 250 cases of ''T. callipaeda'' infestation in humans have been reported thus far. ** Intermediate hosts: Fruit flies (''Amiota (Phortica) variegata'' in Europe, and ''Phortica okadai'' in China) ** Distribution: Asia and Europe *'' Thelazia erschowi'' Oserskaja, 1931 ** Definitive hosts:

Pig The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...

(''Sus domesticus'') ** Intermediate host: Not known ** Distribution:

Post-Soviet states The post-Soviet states, also referred to as the former Soviet Union or the former Soviet republics, are the independent sovereign states that emerged/re-emerged from the dissolution of the Soviet Union in 1991. Prior to their independence, they ...

*'' Thelazia gulosa'' (Railliet & Henry, 1910) ** Definitive hosts:

Yak The yak (''Bos grunniens''), also known as the Tartary ox, grunting ox, hairy cattle, or domestic yak, is a species of long-haired domesticated cattle found throughout the Himalayan region, the Tibetan Plateau, Tajikistan, the Pamir Mountains ...

(''Bos grunniens'') and other

cattle Cattle (''Bos taurus'') are large, domesticated, bovid ungulates widely kept as livestock. They are prominent modern members of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Mature female cattle are calle ...

(''Bos taurus''), and rarely

(''Homo sapiens'') ** Intermediate hosts: Face fly ('' Musca autumnalis'') in Europe and North America, ''Musca larvipara'' in Ukraine, ''Musca vitripennis'' in

Crimea Crimea ( ) is a peninsula in Eastern Europe, on the northern coast of the Black Sea, almost entirely surrounded by the Black Sea and the smaller Sea of Azov. The Isthmus of Perekop connects the peninsula to Kherson Oblast in mainland Ukrain ...

, and ''Musca amica'' in the Far East ** Distribution: Asia, Europe, and North America *'' Thelazia lacrymalis'' (Gurlt, 1831) ** Definitive hosts:

Horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 mi ...

(''Equus caballus'') and

(''Bos taurus'') ** Intermediate hosts: Face fly ('' Musca autumnalis'') and '' Musca osiris'' ** Distribution: Asia, Europe, Middle East, North America and South America *'' Thelazia leesei'' Railliet & Henry, 1910 ** Definitive hosts:

Dromedary The dromedary (''Camelus dromedarius''), also known as the dromedary camel, Arabian camel and one-humped camel, is a large camel of the genus '' Camelus'' with one hump on its back. It is the tallest of the three camel species; adult males sta ...

(''Camelus dromedarius'') and

Bactrian camel The Bactrian camel (''Camelus bactrianus''), also known as the Mongolian camel, domestic Bactrian camel or two-humped camel, is a camel native to the steppes of Central Asia. It has two humps on its back, in contrast to the single-humped drome ...

(''Camelus bactrianus'') ** Intermediate hosts: Flies (''Musca lucidulus'') ** Distribution:

and India *'' Thelazia rhodesii'' (Desmarest, 1828) ** Definitive hosts: mainly

bovid The Bovidae comprise the family (biology), biological family of cloven-hoofed, ruminant mammals that includes Bos, cattle, bison, Bubalina, buffalo, antelopes (including Caprinae, goat-antelopes), Ovis, sheep and Capra (genus), goats. A member o ...

(''Bos taurus''),

water buffalo The water buffalo (''Bubalus bubalis''), also called domestic water buffalo, Asian water buffalo and Asiatic water buffalo, is a large bovid originating in the Indian subcontinent and Southeast Asia. Today, it is also kept in Italy, the Balkans ...

(''Bubalus bubalis''),

zebu The zebu (; ''Bos indicus''), also known as indicine cattle and humped cattle, is a species or subspecies of Bos taurus, domestic cattle originating in South Asia. Zebu, like many Sanga cattle breeds, differs from taurine cattle by a fatty hump ...

(''Bos indicus''),

bison A bison (: bison) is a large bovine in the genus ''Bison'' (from Greek, meaning 'wild ox') within the tribe Bovini. Two extant taxon, extant and numerous extinction, extinct species are recognised. Of the two surviving species, the American ...

(''Bison bonasus''), and sometimes

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 mi ...

(''Equus caballus''),

(''Ovis aries''),

dromedary The dromedary (''Camelus dromedarius''), also known as the dromedary camel, Arabian camel and one-humped camel, is a large camel of the genus '' Camelus'' with one hump on its back. It is the tallest of the three camel species; adult males sta ...

(''Camelus dromedarius''), and

goat The goat or domestic goat (''Capra hircus'') is a species of Caprinae, goat-antelope that is mostly kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the ...

(''Capra hircus'') ** Intermediate hosts: Face fly ('' Musca autumnalis'', ''Musca larvipara'', and ''Musca sorbens'') ** Distribution: Africa, Asia, and Europe *'' Thelazia skrjabini'' Erschow, 1928 ** Definitive hosts:

Cattle Cattle (''Bos taurus'') are large, domesticated, bovid ungulates widely kept as livestock. They are prominent modern members of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Mature female cattle are calle ...

(''Bos taurus'') and

yak The yak (''Bos grunniens''), also known as the Tartary ox, grunting ox, hairy cattle, or domestic yak, is a species of long-haired domesticated cattle found throughout the Himalayan region, the Tibetan Plateau, Tajikistan, the Pamir Mountains ...

(''Bos grunniens'') ** Intermediate hosts: Face fly ('' Musca autumnalis''), ''Musca vitripennis'', and ''Musca amica'' ** Distribution: Europe and North America

Human infections/cases

'' Thelazia anolabiata'' Thelazia anolabiata is a spirurid nematode living in the orbits of birds which can cause lacrimation, keratitis, conjunctivitis, and corneal ulcers. This species is reported for the first time from an Andean Cock of the Rock, ''Rupicola peruviana'' (Passeriformes: Cotingidae), from a zoo in Lima, Peru. Clinical signs of keratoconjunctivitis were resolved with the treatment of ivermectin, ciprofloxacin, and an epithelium regenerator, and the host is presently in good health. The nematodes were identified using the primary characteristics used to differentiate the species of this genus as lengths of spicules and other morphologic characteristics. ''

'' The eyeworm, ''Thelazia californiensis'' is a spiruroid nematode. It was first described by Price in 1930, as a parasite of dogs in the western United States. ''T. Californiensis'' can infest dog, cat, sheep, deer and other mammals and causes a disease known as thelaziasis. This is not a common disease. This eyeworm commonly parasitizes in the tear ducts and conjunctival sac of its hosts, and appears as a creamy, thin-white thread measuring slightly over a half an inch in length. The adult worms may be found in the conjunctival sac or tear ducts. The eggs or larvae can be seen when the tears or secretions are examined under the microscope. The eyeworms are most common on the Pacific Coast. Human can become infested, but is extremely rare and thought to be accidental. There are only few reports of ''Thelazia californiensis'' infestation in humans. '' Thelazia callipaeda'' ''Thelazia callipaeda'' eye worm is a nematode transmitted by drosophilid flies not only primarily to carnivores and lagomorphs but also to humans. Only a few cases have been reported in Europe (Italy, France, and Portugal). Here, we report the first eye infection in a German patient. '' Thelazia gulosa'' ''Thelazia gulosa'', the cattle eyeworm, was found in the left eye of a 26-year-old woman from Oregon in 2018. This case was the first-ever reported human infection from this genus. The ten previous cases reported from the United States were caused by ''Thelazia californiensis.'' Human infections caused by ''Thelazia callipaeda'' have also been reported in Japan, China, India, Taiwan, Burma, Thailand, Korea, Indonesia, Italy, Russia, and France. The 26-year-old woman had been practicing horsemanship in Gold Beach, Oregon, which is a region where cattle farming is common. After a few weeks, her left eye started to feel irritated followed by a sensation of a foreign object. On the eighth day of her symptoms, she found and removed a small, translucent worm from her eye. She went to a local physician, who found and removed two more worms. These worms were sent to Northwest Pathology for analysis and identification. It was then forwarded to the Center for Disease Control and Prevention (CDC) Parasitic Diseases Reference Laboratory for identification. The next day, the woman went to an optometrist where three more worms were removed. She was sent to see an infectious disease specialist, who was able to remove a partial worm. The patient was diagnosed with parasitic infiltration of the left periocular tissues and a secondary bilateral papillary reaction of the upper and lower palpebral conjunctive. Even after multiple washouts by ophthalmologists, no more worms were seen, but the patient continued to remove additional worms from her eye. Over 20 days, a total of 14 worms were removed. Since then, the patient no longer had any symptoms and no more worms were detected. '' Thelazia lacrymalis'' Equine thelaziosis is a neglected vector-borne parasitic disease in modern veterinary medicine, lacking recent reports. It is transmitted by ''Musca autumnalis'', and potentially other Muscidae species, by ingesting the lachrymal secretions of its equine host. The distribution of both ''Thelazia lacrymalis'' and its intermediate hosts remains largely unknown throughout Europe, with most studies dating back 20 years. The aim of this study was to assess the presence, prevalence and distribution of ''T. lacrymalis'' in horses from Romania. '' Thelazia skrjabini'' Examination of 122 eyes and 162 nasolachrymal ducts of cattle over 8 years old in Denmark revealed Thelazia skrjabini in 2 cows. This is the first record of Thelazia in Denmark.

References

{{Taxonbar, from=Q3656383 Spirurida Rhabditida genera Parasitic nematodes of vertebrates Veterinary helminthology Taxa described in 1819

Representative species

Human infections/cases

See also

References