Thelazia is a genus of parasitic nematodes known as "eyeworms", a reference to their primary habitat in their definitive host (i.e., the host in which they reproduce): the host's orbital (eye) cavities and associated tissues. Documented definitive hosts for most Thelazia species are mammals (including various carnivores, ruminants, horses, and humans, depending on the species). At least one species (T. anolabiata) occurs in bird eyes (careful molecular genetic investigations may reveal additional species) (De Oliveira 1992; Elias et al. 2008). The known intermediate hosts for Thelazia eyeworms (i.e., the hosts in which the parasite spends a part of the non-reproductive phase of its life cycle) are various non-biting flies.
The basic Thelazia life cycle is unusual in the mode of transmission between the intermediate host (fly) and definitive host (vertebrate). First stage larvae are released directly (no eggs are laid) in the lachrymal (tear gland) secretions of the hosts by adult female eyeworms after mating with adult males. These larvae then infect flies feeding on lachrymal secretions around the eyes of the vertebrate host. Within the fly, larvae develop from first stage to infective third stage larvae while remaining encapsulated in different parts of the fly's body, yielding infective larvae in around 18 to 25 days. Third stage eyeworm larvae emerge from the labella of infected flies and are transmitted to the eyes of the definitive vertebrate hosts while infected flies are feeding on eye secretions and these third stage larvae develop into adults within the host's ocular cavity. Adult eyeworms are found mainly under the eyelids and nictitating membrane, in nasolachrymal ducts, in conjunctival sacs, or in the excretory ducts of lachrymal gland. The first stage larva is very short-lived in the lachrymal secretions, surviving just a few hours, and transmission thus depends on the continuous presence of appropriate flies to infect. For this reason, thelaziasis (symptomatic infection in the vertebrate host) has a seasonal occurrence that depends on the seasonality of the intermediate hosts.
Six Thelazia species are considered to be of veterinary significance: T. californiensis, T. callipaeda,T. gulosa, T. lacrymalis, T. rhodesi, and T. skrjabini. Two of these, T. callipaeda and T. californiensis, can infect humans.
Thelazia eyeworms are milky-white, with males and females reaching up to around 12 mm and 20 mm, respectively, and a diameter of 250 to 800 μm. Discrimination of Thelazia species by morphology can be extremely difficult, but molecular genetic markers (e.g., Otranto et al. 2001; Otranto and Traversa 2004) promise to provide both valuable diagnostic tools and opportunities to advance our understanding of the biology and true diversity of these worms.
Naem (2011) and and Otranto and Traversa (2005) provide valuable introductions to diverse aspects of Thelazia biology.
(Otranto and Traversa 2005 and references therein; Otranto et al. 2009; Naem 2011 and references therein; Otranto and Eberhard 2011)
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:28
Specimens with Barcodes:24
Species With Barcodes:3
Thelazia is a genus of nematode worms which parasitize the eyes and associated tissues of various bird and mammal 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, or the so-called "third eyelid" (nictitating membrane). Occasionally, they are found in the eyeball itself, either under the conjunctiva (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) which do not bite, but which feed on tears.
- Thelazia anolabiata (Molin, 1860)
- Thelazia bubalis Ramanujachari and Alwar, 1952
- Definitive hosts: Water buffalo
- Intermediate hosts: Not known
- Distribution: India
- Thelazia californiensis Price, 1930
- Definitive hosts: Dog (Canis lupus familiaris), Cat (Felis catus), occasionally Human (Homo sapiens), Domestic sheep (Ovis aries), Mule deer (Odocoileus hemionus), Coyote (Canis latrans) and American Black Bear (Ursus americanus).
- Intermediate hosts: Lesser house fly (Fannia canicularis) and Fannia benjamini
- Distribution: Western North America
- Thelazia callipaeda Railliet & Henry, 1910 (sometimes called "Oriental eyeworm")
- Definitive hosts: typically reported from Dog (Canis familiaris), Cat (Felis catus), and Human (Homo sapiens); occasionally reported from Wolf (Canis lupus), Raccoon dog (Nyctereutes procyonoides), Red Fox (Vulpes fulva), European Rabbit (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
- Thelazia gulosa (Railliet & Henry, 1910)
- Thelazia lacrymalis (Gurlt, 1831)
- Thelazia leesei Railliet & Henry, 1910
- Thelazia rhodesii (Desmarest, 1828)
- Definitive hosts: mainly bovids, Cattle (Bos taurus), Buffalo (Bubalus bubalis), Zebu (Bos indicus), Bison (Bison bonasus), and sometimes Horse (Equus caballus), Domestic sheep (Ovis aries), Dromedary (Camelus dromedarius), and Goat (Capra hircus)
- Intermediate hosts: Flies (Face fly (Musca autumnalis, Musca larvipara, and Musca sorbens)
- Distribution: Africa, Asia, and Europe
- Thelazia skrjabini Erschow, 1928
- This image is from the article: Otranto, D. and M. Dutto (2008) "Human thelaziasis, Europe." Emerging Infectious Diseases 14(4):647-649. The journal Emerging Infectious Diseases is published by the Centers for Disease Control, a US government agency.
- Otranto, D. and D. Traversa (2005) "Thelazia eyeworm: An original endo- and ecto-parasitic nematode." Trends in Parasitology 21(1):1-4.
- Xue, C., N. Tian, and Z. Huang (2007) "Thelazia callipaeda in human vitreous." Canadian Journal of Ophthalmology 42(6):884-885.
- de Oliveira Rodrigues, H. (1992) "Thelazia anolabiata (Molin, 1860) Railliet & Henry, 1910 (Nematoda: Thelazioidea), a new host record and systematic considerations." Memórias do Instituto Oswaldo Cruz 87(Suppl 1):217-222.
- Koyama, Y., A. Ohira, T. Kono, T. Yoneyama, and K. Shiwaku (2000) "Five cases of thelaziasis." British Journal of Ophthalmology 84(4):441-442. (Note: This pdf includes pp. 439-440 in addition to the cited article.)
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