Thelaziasis
Encyclopedia
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s of the genus Thelazia
Thelazia
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"...
. The adults of all Thelazia species discovered so far inhabit the eyes and associated tissues (such as eyelids, tear ducts, etc.) of various mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
hosts, including humans. Thelazia
Thelazia
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"...
nematodes are often referred to as "eyeworms."
General life cycle
In the uterus of the adult female, the embryos develop into first-stage larvae (L1), which remain in the eggshell (sheath). The female deposits these sheathed larvae in the tears of the mammal or bird definitive hostParasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
, and the larvae are ingested by tear-feeding flies. In the fly, the larvae "hatch" (exsheath), penetrate the gut wall, and migrate to either the fat body, testes or egg follicles (depending on the species). There they develop into third-stage larvae (L3), which migrate to the head of the fly. The infective L3 larvae wiggle out of the straw-like feeding apparatus of the fly when it feeds on the tears of another mammal or bird host. The L3 larvae develop into adults in the eye or surrounding tissues of the host, where they may live for over one year.
In the definitive host
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
, Thelazia have been found in various tissues of the orbit (or socket) of the eye, including within the eyelids, in the tear glands
Lacrimal gland
The lacrimal glands are paired almond-shaped glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper, outer portion of each orbit, in the lacrimal fossa of the orbit formed by the frontal bone. Inflammation of the lacrimal glands is called...
, tear ducts
Lacrimal canaliculi
The lacrimal canaliculi, also known as the lacrimal canals or lacrimal ducts, are the small channels in each eyelid that commence at minute orifices, termed puncta lacrimalia, on the summits of the papillae lacrimales, seen on the margins of the lids at the lateral extremity of the lacus...
, or the so-called "third eyelid" (nictitating membrane
Nictitating membrane
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...
) or in the eyeball itself.
Hosts and geographic range of thelaziasis
While a few dozen species of Thelazia have been described in the literature, only 2 have been reported to infest humans, and only 7 are commonly reported in veterinary contexts. The remaining species are occasionally found in birds or wild mammals.In humans, dogs and cats, thelaziasis cases due to Thelazia callipaeda
Thelazia callipaeda
Thelazia callipaeda is a parasitic nematode, and the most common cause of "thelaziasis" in humans, dogs and cats. It was first discovered in the eyes of a dog in China in 1910. By 2000, over 250 human cases had been reported in the medical literature.-Hosts:In addition to humans, cats and dogs,...
(Asia, Europe), and occasionally T. californiensis (western North America), have been reported.
Horses are infested by T. lacrymalis (worldwide) and, less frequently, by T. rhodesii (Africa, Asia, Europe).
In cattle, T. gulosa (Asia, Europe, North America), T. rhodesii (Africa, Asia, Europe) and T. skrjabini (Europe, North America) are the primary species of concern.
In camels, T. leesei infestations have been reported from the Post-Soviet states
Post-Soviet states
The post-Soviet states, also commonly known as the Former Soviet Union or former Soviet republics, are the 15 independent states that split off from the Union of Soviet Socialist Republics in its dissolution in December 1991...
and India.
The intermediate hosts
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
of several Thelazia species are known, and in each case they are tear-feeding flies of the genera Musca
Musca
Musca is one of the minor southern constellations. The constellation was one of twelve constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman and it first appeared on a 35-cm diameter celestial globe published in 1597 in Amsterdam by...
(family Muscidae
Muscidae
Muscidae are a family of flies found in the superfamily Muscoidea. The apical segment of the antennae of Muscidae are plumose, and the basal portion is smooth....
), Phortica (family Drosophilidae
Drosophilidae
Drosophilidae is a diverse, cosmopolitan family of flies, which includes fruit flies. Another family of flies called Tephritidae also includes fruit flies. The best known species of Drosophilidae is Drosophila melanogaster, within the genus Drosophila, and this species Is used extensively for...
), or Fannia
Fannia
Fannia is notable as the granddaughter of the well-known Arria Major.Fannia is recorded in the writings of Pliny the Younger as a woman of fortitude and respectability.As with her grandmother, Fannia is described as a political rebel in her own right...
(family Fanniidae
Fanniidae
The Fanniidae are a small group of true flies largely confined to the Holarctic and temperate Neotropical regions.There are 11 Afrotropical species, 29 Oriental, and 14 Australasian. See Biogeographic Regions....
).
Symptoms and diagnosis
In animal and human hosts, infestation by ThelaziaThelazia
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"...
may be asymptomatic, though it frequently causes watery eyes (epiphora), conjunctivitis
Conjunctivitis
Conjunctivitis refers to inflammation of the conjunctiva...
, corneal opacity, or corneal ulcer
Corneal ulcer
A corneal ulcer, or ulcerative keratitis, is an inflammatory condition of the cornea involving loss of its outer layer. It is very common in dogs and is sometimes seen in cats...
s (ulcerative keratitis). Infested humans have also reported "foreign body sensation" the feeling that something is in the eye.
Diagnosis involves simply examining the eyes and nearby tissues for the worms. Adult Thelazia are very active, one author described T. californiensis as a "short lively piece of nylon fishing line about 10 mm long."
Prevalence
By one author's count, 257 human cases of Thelazia callipaedaThelazia callipaeda
Thelazia callipaeda is a parasitic nematode, and the most common cause of "thelaziasis" in humans, dogs and cats. It was first discovered in the eyes of a dog in China in 1910. By 2000, over 250 human cases had been reported in the medical literature.-Hosts:In addition to humans, cats and dogs,...
had been reported worldwide by the year 2000, though thelaziasis is still considered to be a rare disease
Rare disease
A rare disease, also referred to as an orphan disease, is any disease that affects a small percentage of the population.Most rare diseases are genetic, and thus are present throughout the person's entire life, even if symptoms do not immediately appear...
.
Various livestock and wildlife surveys suggest that thelaziasis is quite common among animals.
- A slaughterhouse survey in Canada found that about one-third (32%) of cattle over an 8-month period were infested with eyeworms.
- A survey of horses in Kentucky revealed a 42% rate of infestation with Thelazia lacrymalis.
- In Wyoming and Utah, a survey of hunter-harvested mule deerMule DeerThe mule deer is a deer indigenous to western North America. The Mule Deer gets its name from its large mule-like ears. There are believed to be several subspecies, including the black-tailed deer...
found 15% to be infested by Thelazia californiensis.
- A survey of various sites in Italy found 23-60% of dogs, 5% of foxes and 4 out of 4 cats to be infested with Thelazia callipaeda.
Treatment and control
Because they live so close to the outside of the body, Thelazia is one of the few nematodeNematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
infections which can be treated topical
Topical
In medicine, a topical medication is applied to body surfaces such as the skin or mucous membranes such as the vagina, anus, throat, eyes and ears.Many topical medications are epicutaneous, meaning that they are applied directly to the skin...
ly.
Topical treatment of livestock, dogs and cats with organophosphates (such as ecothiopate iodide or isofluorophate) and systemic treatment with anthelmintics (such as ivermectin, levamisole
Levamisole
Levamisole is an anthelminthic and immunomodulator belonging to a class of synthetic imidazothiazole derivatives. It was discovered at Janssen Pharmaceutica in 1966...
, and doramectin
Doramectin
Doramectin is a veterinary drug approved by the Food and Drug Administration for the treatment of parasites such as gastrointestinal roundworms, lungworms, eyeworms, grubs, sucking lice and mange mites in cattle....
) are recommended by the Merck Veterinary Manual
Merck Veterinary Manual
The Merck Veterinary Manual is a reference manual of animal health care. It is published by Merck & Co., Inc. and Merial Limited. The Merck Veterinary Manual is available as a reference manual or as an online resource for veterinarians, veterinary students, and others involved in animal health...
. Other sources have reported positive results treating dogs with moxidectin
Moxidectin
Moxidectin is a parasiticide used for the prevention and control of heartworm and intestinal worms, and can be found in treatments prescribed for animals such as dogs, cats, horses, cattle and sheep. It is also used for the prevention of red mite in caged birds...
, imidacloprid
Imidacloprid
Imidacloprid is a nicotine-based, systemic insecticide, which acts as a neurotoxin and belongs to a class of chemicals called the neonicotinoids. Although it is now off patent, the primary manufacturer of this chemical is Bayer CropScience,...
, or milbemycin oxime
Milbemycin oxime
Milbemycin oxime is a veterinary drug from the group of milbemycins, used as a broad spectrum antiparasitic. It is active against worms , insects and mites ....
.
For the treatment of human cases, removal of the worm is suggested. Topical treatment with cocaine
Cocaine
Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic...
or thiabendazole
Thiabendazole
Tiabendazole is a fungicide and parasiticide.-Fungicide:...
have also been reported to kill the worms in human cases.
Because most, if not all, species of Thelazia are spread by flies, sanitary practices which reduce the presence of flies will also reduce the spread of thelaziasis.