Title: Thelaziasis

{{Infobox medical condition (new)
| name          = Thelaziasis
| synonyms      = Thelaziosis
| image         = Thelazia callipaeda in dog.jpg
| caption       = ''Thelazia callipaeda'' infestation in a dog&lt;ref name=pmid18394285/&gt;
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| field         = [[Infectious disease (medical speciality)|Infectious disease]]
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'''Thelaziasis''' is the term for infestation with [[parasite|parasitic]] [[nematode]]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]] and [[bird]] hosts, including humans.&lt;ref&gt;{{cite journal |pmid=15639731 |year=2005 |last1=Otranto |first1=D |last2=Traversa |first2=D |title=Thelazia eyeworm: an original endo- and ecto-parasitic nematode. |volume=21 |issue=1 |pages=1–4 |doi=10.1016/j.pt.2004.10.008 |journal=[[Trends in Parasitology]]}}&lt;/ref&gt; ''Thelazia'' nematodes are often referred to as &quot;eyeworms&quot;.

==Signs and symptoms==
In animal and human hosts, infestation by ''[[Thelazia]]'' may be asymptomatic, though it frequently causes watery eyes ([[Epiphora (medicine)|epiphora]]), [[conjunctivitis]], [[corneal opacity]], or [[corneal ulcer]]s (ulcerative keratitis).&lt;ref name=pmid18394285&gt;{{cite journal |pmid=18394285 |pmc=2570937 |url=https://www.cdc.gov/eid/content/14/4/pdfs/647.pdf |year=2008 |last1=Otranto |first1=D |last2=Dutto |first2=M |title=Human thelaziasis, Europe. |volume=14 |issue=4 |pages=647–9 |journal=Emerging Infectious Diseases |doi=10.3201/eid1404.071205}}&lt;/ref&gt;  Infested humans have also reported &quot;foreign body sensation&quot;{{spaced ndash}}the feeling that something is in the eye.&lt;ref&gt;{{cite journal |title=Human Thelaziasis: Emerging Ocular Pathogen in Nepal |journal=  Open Forum Infectious Diseases|date=September 15, 2018 |doi=10.1093/ofid/ofy237 |pmc=6189630 |last1=Sah |first1=Ranjit |last2=Khadka |first2=Shusila |last3=Adhikari |first3=Mahesh |last4=Niraula |first4=Reema |last5=Shah |first5=Apoorva |last6=Khatri |first6=Anadi |last7=Donovan |first7=Suzanne |volume=5 |issue=10 |article-number=ofy237 |pmid=30349847 }}&lt;/ref&gt;&lt;ref&gt;{{cite journal |title=A Second Case of Human Conjunctival Infestation With Thelazia gulosa and a Review of T. gulosa in North America |website=academic.oup.com |date=22 October 2019 |url=https://academic.oup.com/cid/article/70/3/518/5602294}}&lt;/ref&gt;

==Cause==
===Life cycle===
In the uterus of the adult female, the embryos develop into first-stage larvae (L1), which remain in the eggshell (sheath).&lt;ref&gt;{{cite book |last1=Anderson |first1=R. C. |year=2000 |chapter=''Thelazia'' |title=Nematode Parasites of Vertebrates: Their Development and Transmission |url=https://archive.org/details/nematodeparasite00ande |url-access=limited |edition=2nd |publisher=CABI Publishing |location=Wallingford, England |isbn=0-85199-421-0 |pages=[https://archive.org/details/nematodeparasite00ande/page/n424 404]–407}}&lt;/ref&gt; The female deposits these sheathed larvae in the tears of the mammal or bird [[parasitic life cycles|definitive host]], and the larvae are ingested by tear-feeding flies.  In the fly, the larvae &quot;hatch&quot; (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, ''Thelazia'' have been found in various tissues of the orbit (or socket) of the eye, including within the eyelids, in the [[lacrimal gland|tear glands]], [[lacrimal canaliculi|tear ducts]], or the so-called &quot;third eyelid&quot; ([[nictitating membrane]]) or in the eyeball itself.&lt;ref&gt;{{cite journal |pmid=16995411 |year=2006 |last1=Shen |first1=J |last2=Gasser |first2=RB |last3=Chu |first3=D |last4=Wang |first4=Z |last5=Yuan |first5=X |last6=Cantacessi |first6=C |last7=Otranto |first7=D |title=Human thelaziosis--a neglected parasitic disease of the eye. |volume=92 |issue=4 |pages=872–5 |journal=The Journal of Parasitology |doi=10.1645/GE-823R.1|s2cid=22281032 }}&lt;/ref&gt;

===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.&lt;ref&gt;{{cite journal|last1=Bradbury |display-authors=etal |title=Case Report: Conjunctival Infestation with Thelazia gulosa: A Novel Agent of Human Thelaziasis in the United States|journal=The American Journal of Tropical Medicine and Hygiene|year=2018 |volume=98 |issue=4 |pages=1171–1174 |doi=10.4269/ajtmh.17-0870|pmid=29436343 |pmc=5928835}}&lt;/ref&gt;

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]] and India.

The [[parasitic life cycles|intermediate hosts]] of several ''Thelazia'' species are known, and in each case they are tear-feeding flies of the genera ''[[Musca (fly)|Musca]]'' (family [[Muscidae]]), [[Phortica variegata|''Phortica'']] (family [[Drosophilidae]]),  or ''[[Fannia (fly)|Fannia]]'' (family [[Fanniidae]]).

==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 &quot;short lively piece of nylon fishing line about 10 mm long.&quot;&lt;ref name=pmc1130199&gt;{{cite journal |pmc=1130199 | pmid=4546457 |year=1974 |last1=Chiapella |first1=KJ |title=Letter: Verminous ophthalmia. |volume=120 |issue=6 |page=500 |journal=The Western Journal of Medicine}}&lt;/ref&gt;

==Treatment==
Because they live so close to the outside of the body, ''Thelazia'' is one of the few [[nematode]] infections which can be treated [[topical]]ly.

Topical treatment of livestock,&lt;ref&gt;{{Cite web|url=http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/30401.htm|title=Merck Veterinary Manual}}&lt;/ref&gt; dogs and cats&lt;ref&gt;{{Cite web|url=http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/30402.htm|title=Merck Veterinary Manual}}&lt;/ref&gt; with [[organophosphate]]s (such as [[ecothiopate iodide]] or [[Diisopropyl fluorophosphate|isofluorophate]]) and systemic treatment with [[anthelmintics]] (such as [[ivermectin]], [[levamisole]], and [[doramectin]]) are recommended by the [[Merck Veterinary Manual]]. Other sources have reported positive results treating dogs with [[moxidectin]], [[imidacloprid]], or [[milbemycin oxime]].

For the treatment of human cases, removal of the worm is suggested.  Topical treatment with [[cocaine]] or [[thiabendazole]] have also been reported to kill the worms in human cases.&lt;ref name=pmc1130199/&gt; 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]] (NexGard Spectra) has been found effective.&lt;ref name=&quot;LebonGuillot2019&quot;&gt;{{cite journal|last1=Lebon|first1=Wilfried|last2=Guillot|first2=Jacques|last3=Álvarez|first3=Maria-Jesús|last4=Antonio Bazaga|first4=José|last5=Cortes-Dubly|first5=Marie-Laure|last6=Dumont|first6=Pascal|last7=Eberhardt|first7=Marianne|last8=Gómez|first8=Héctor|last9=Pennant|first9=Olivier|last10=Siméon|first10=Noémie|last11=Beugnet|first11=Frederic|last12=Halos|first12=Lénaïg|title=Prevention of canine ocular thelaziosis (''Thelazia callipaeda'') with a combination of milbemycin oxime and afoxolaner (Nexgard Spectra®) in endemic areas in France and Spain|journal=Parasite|volume=26|year=2019|page=1|issn=1776-1042|doi=10.1051/parasite/2019001|pmid=30644355|pmc=6333103|url=}} {{open access}}&lt;/ref&gt;

==Prevalence==
By one author's count, 257 human cases of ''[[Thelazia callipaeda]]'' had been reported worldwide by the year 2000,&lt;ref&gt;{{cite journal |pmid=10777285 |pmc=1723424 |year=2000 |last1=Koyama |first1=Y |last2=Ohira |first2=A |last3=Kono |first3=T |last4=Yoneyama |first4=T |last5=Shiwaku |first5=K |title=Five cases of thelaziasis |volume=84 |issue=4 |page=441 |journal=The British Journal of Ophthalmology |doi=10.1136/bjo.84.4.439c}}&lt;/ref&gt; though thelaziasis is still considered to be a [[rare disease]].

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.&lt;ref&gt;{{cite journal |pmid=7189135 |year=1980 |last1=Moolenbeek |first1=WJ |last2=Surgeoner |first2=GA |title=Southern Ontario survey of eyeworms, Thelazia gulosa and Thelazia lacrymalis in cattle and larvae of Thelazia spp. in the face fly, Musca autumnalis |volume=21 |issue=2 |pages=50–2 |pmc=1789681 |journal=The Canadian Veterinary Journal}}&lt;/ref&gt;
*A survey of horses in Kentucky revealed a 42% rate of infestation with ''Thelazia lacrymalis''.&lt;ref&gt;{{cite journal |pmid=10936545 |year=2000 |last1=Lyons |first1=ET |last2=Swerczek |first2=TW |last3=Tolliver |first3=SC |last4=Bair |first4=HD |last5=Drudge |first5=JH |last6=Ennis |first6=LE |title=Prevalence of selected species of internal parasites in equids at necropsy in central Kentucky (1995-1999) |volume=92 |issue=1 |pages=51–62 |journal=Veterinary Parasitology |doi=10.1016/S0304-4017(00)00266-1}}&lt;/ref&gt;
*In Wyoming and Utah, a survey of hunter-harvested [[mule deer]] found 15% to be infested by ''Thelazia californiensis''.&lt;ref&gt;{{cite journal |pmid=11085445 |url=http://www.jwildlifedis.org/cgi/content/abstract/36/4/783 |journal=Journal of Wildlife Diseases |title=Bacteria and nematodes in the conjunctiva of mule deer from Wyoming and Utah |volume=36 |issue=4 |pages=783–7 |author=Dubay, SA |last2=W |first2=E |last3=M |first3=K |last4=B |first4=A |year=2000 |doi=10.7589/0090-3558-36.4.783|s2cid=38137704 }}&lt;/ref&gt;
*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''.&lt;ref&gt;{{cite journal |pmid=14580802 |year=2003 |last1=Otranto |first1=D |last2=Ferroglio |first2=E |last3=Lia |first3=RP |last4=Traversa |first4=D |last5=Rossi |first5=L |title=Current status and epidemiological observation of Thelazia callipaeda (Spirurida, Thelaziidae) in dogs, cats and foxes in Italy: a &quot;coincidence&quot; or a parasitic disease of the Old Continent? |volume=116 |issue=4 |pages=315–25 |journal=Veterinary Parasitology |doi=10.1016/j.vetpar.2003.07.022}}&lt;/ref&gt;
*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.&lt;ref&gt;{{cite journal |last1=Miro |first1=Guadalupe |last2=Montoya |first2=Ana |last3=Hernandez |first3=Leticia |last4=Dado |first4=Diana |last5=Victoria Vazquez |first5=Maria |last6=Benito Manuel Villagrasa |first6=Marta |last7=Brianti |first7=Emanuelle |last8=Otranto |first8=Domenico |title=Thelazia callipaeda: infection in dogs: a new parasite for Spain |journal=Parasites &amp; Vectors |date=27 July 2011 |volume=4 |issue=148 |page=1756-3305 |doi=10.1186/1756-3305-4-148 |pmid=21791108 |pmc=3158752 |doi-access=free }}&lt;/ref&gt;

== References ==
{{Reflist}}

== External links ==
{{Medical resources
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{{Helminthiases}}

[[Category:Bovine diseases]]
[[Category:Cat diseases]]
[[Category:Dog diseases]]
[[Category:Helminthiases]]
[[Category:Spirurida]]
[[Category:Veterinary helminthology]]