Thelaziasis
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Thelaziasis is the term for infestation with
parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...
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 ...
s of the genus '' Thelazia''. The adults of all ''Thelazia'' species discovered so far inhabit the eyes and associated tissues (such as eyelids, tear ducts, etc.) of various
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 ...
and
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 ...
hosts, including humans. ''Thelazia'' nematodes are often referred to as "eyeworms".


Signs and symptoms

In animal and human hosts, infestation by '' Thelazia'' may be asymptomatic, though it frequently causes watery eyes ( epiphora),
conjunctivitis Conjunctivitis, also known as pink eye or Madras eye, is inflammation of the conjunctiva, the thin, clear layer that covers the white surface of the eye and the inner eyelid. It makes the eye appear pink or reddish. Pain, burning, scratchiness ...
,
corneal opacity Corneal opacification is a term used when the human cornea loses its transparency. The term corneal opacity is used particularly for the loss of transparency of cornea due to scarring. Transparency of the cornea is dependent on the uniform diamete ...
, or
corneal ulcer Corneal ulcer, often resulting from keratitis is an inflammatory or, more seriously, infective condition of the cornea involving disruption of its epithelial layer with involvement of the corneal stroma. It is a common condition in humans part ...
s (ulcerative keratitis). Infested humans have also reported "foreign body sensation"the feeling that something is in the eye.


Cause


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 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 ...
, 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 300px, Stained cells of an insect fat body Fat body is a highly dynamic insect tissue composed primarily of storage cells. It is distributed throughout the insect's internal body cavity (the haemocoel), in close proximity to the hemolymph as ...
, 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, ''Thelazia'' have been found in various tissues of the orbit (or socket) of the eye, including within the eyelids, in the 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 ...
) or in the eyeball itself.


Hosts and geographic range

While a few dozen species of ''Thelazia'' have been described in the literature, only three have been reported to infest humans, and only seven 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'' (Asia, Europe), and occasionally ''T. californiensis'' and ''T. gulosa'' (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 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 ...
and India. The
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 ...
of several ''Thelazia'' species are known, and in each case they are tear-feeding flies of the genera ''
Musca is a small constellation in the deep southern sky. It was one of 12 constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman, and it first appeared on a celestial globe in diameter p ...
'' (family
Muscidae Muscidae are a family of flies found in the superfamily Muscoidea. Muscidae, some of which are commonly known as house flies or stable flies due to their synanthropy, are worldwide in distribution and contain almost 4,000 described species i ...
), ''Phortica'' (family
Drosophilidae The Drosophilidae are a diverse, cosmopolitan family of flies, which includes species called fruit flies, although they are more accurately referred to as vinegar or pomace flies. Another distantly related family of flies, Tephritidae, are true f ...
), or '' Fannia'' (family
Fanniidae The Fanniidae are a small (285 species in five genera) group of true flies largely confined to the Holarctic and temperate Neotropical realms; there are 11 Afrotropical species, 29 Oriental, and 14 Australasian. Adults are medium-sized to small ...
).


Diagnosis

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."


Treatment

Because they live so close to the outside of the body, ''Thelazia'' is one of the few
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 ...
infections which can be treated
topical A topical medication is a medication that is applied to a particular place on or in the body. Most often topical medication means application to body surfaces such as the skin or mucous membranes to treat ailments via a large range of classes ...
ly. Topical treatment of livestock, dogs and cats with
organophosphate In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered ...
s (such as ecothiopate iodide or isofluorophate) and systemic treatment with
anthelmintics Anthelmintics or antihelminthics are a group of antiparasitic drugs that expel parasitic worms ( helminths) and other internal parasites from the body by either stunning or killing them without causing significant damage to the host. They may als ...
(such as
ivermectin Ivermectin 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, it is used to treat infestations including head lice ...
,
levamisole Levamisole, sold under the brand name Ergamisol among others, is a medication used to treat parasitic worm infections, specifically ascariasis and hookworm infections. It is taken by mouth. Side effects may include abdominal pain, vomiting, ...
, and
doramectin Doramectin, sold under the brand name Dectomax among others, is a veterinary medication approved by the US Food and Drug Administration (FDA) for the treatment of parasites such as gastrointestinal roundworms, lungworms, eyeworms, Larva, grubs, s ...
) are recommended by the
Merck Veterinary Manual The ''Merck Veterinary Manual'' is a reference manual of animal health care. It was first published by Merck & Co., Inc. in 1955. It contains concise, thorough information on the diagnosis and treatment of disease in a wide variety of species. T ...
. Other sources have reported positive results treating dogs with
moxidectin Moxidectin is an anthelmintic drug used in animals to prevent or control parasitic worms (helminths), such as heartworm and intestinal worms, in dogs, cats, horses, cattle, sheep and wombats. Moxidectin kills some of the most common internal ...
,
imidacloprid Imidacloprid is a systemic insecticide belonging to a class of chemicals called the neonicotinoids which act on the central nervous system of insects. The chemical works by interfering with the transmission of stimuli in the insect nervous system. ...
, or
milbemycin oxime The milbemycins are a group of macrolides chemically related to the avermectins and were first isolated in 1972 from '' Streptomyces hygroscopicus''. They are used in veterinary medicine as antiparasitic agents against worms, ticks and fleas.
. For the treatment of human cases, removal of the worm is suggested. Topical treatment with
cocaine Cocaine is a tropane alkaloid and central nervous system stimulant, derived primarily from the leaves of two South American coca plants, ''Erythroxylum coca'' and ''Erythroxylum novogranatense, E. novogranatense'', which are cultivated a ...
or
thiabendazole Tiabendazole (International Nonproprietary Name, INN, British Approved Name, BAN), also known as thiabendazole (Australian Approved Name, AAN, United States Adopted Name, USAN) or TBZ and the trade names Mintezol, Tresaderm, and Arbotect, is a pr ...
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. In canines, prevention against canine thelaziosis by monthly administrations of a combination of milbemycin oxime and
afoxolaner Afoxolaner (INN) is an insecticide and acaricide that belongs to the isoxazoline chemical compound group. It acts as an antagonist at GABA-receptors (those gated by the neurotransmitter gamma-aminobutyric acid) and other ligand-gated chloride ...
(NexGard Spectra) has been found effective.


Prevalence

By one author's count, 257 human cases of '' Thelazia callipaeda'' had been reported worldwide by the year 2000, though thelaziasis is still considered to be a
rare disease A rare disease is any disease that affects a small percentage of the population. In some parts of the world, the term orphan disease describes a rare disease whose rarity results in little or no funding or research for treatments, without financi ...
. Various livestock and wildlife surveys suggest that thelaziasis is quite common among animals. *A 1978 slaughterhouse survey in Guelph, Ontario, Canada found that about one-third (32%) of cattle over an eight-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 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 ...
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''. *In a study of dogs living in western Spain, 39.9% of the dogs were found to have ''Thelazia callipaeda'' worms living in their eyes.


References


External links

{{Helminthiases Bovine diseases Cat diseases Dog diseases Helminthiases Spirurida Veterinary helminthology