Created By: Elizabeth Dorfman

Human Biology 103

Parasites and Pestilence: Infectious Public Health Challenges

Stanford University


Image from the University of Pittsburgh Medical Center

Introduction: Human infestation by Thelazia spp. is rare. Humans are not believed to be the definitive hosts, and an human infestation, particularly with T. californiensis, is generally agreed to be an accidental event.

Agent: Thelazia callipaeda and Thelazia californiensis
Phylum: Nematoda
Order: Spirusida
Suborder: Spirurata
Superfamily: Spiruroidea

There are two species of the genus Thelazia that have been found in the human eye. Thelazia callipaeda and, more rarely, Thelazia californiensis. Though clinically very similar, the two species are morphologically distinct. This distinction is based on the numbers of pre- and postcloacal papillae in the make and the position of the vulva in the female. (M. Bhaibulaya et. al, 1970) For more on this please see the morphology section.

Synonyms: Conjunctival spirurosis, Oriental Eye Worm

History of Discovery: T. callipaeda was first described by A. Railliet and A. Henry in both man and dog in Thailand in 1910. They only characterized the female of the species, however, and it wasn’t until 1928 that E.C. Faust described the male. T. californiensis was first described by E.W. Price in 1930 as a parasite of dogs in the Western United States. C.A. Kofoid and O.L. Williams first reported human thelaziasis in the United States in 1935.
Clinical Presentation in Humans: The eye worm commonly parasitizes in the tear ducts and conjunctival sacs of its host. Visually it appears as a thin, creamy-white thread. The eggs or larvae can be seen when tears or other eye secretions are examined under a microscope. The most common clinical findings in infected patients include a mild conjunctival infection, foreign body sensation, follicular hypertrophy of the conjunctiva, excessive lacrimation, and hypersensitivity to light. The symptoms can become more serious if a secondary bacterial infection develops.

Image from the University of Toronto ocular teaching website
The anatomy of the nasolacrimal drainage systemImage from the University of Pittsburgh Medical Center

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Clinical presentation of T. californiensis in the human eye

Transmission: Transmission of Thelazia spp. to humans generally occurs via the face fly. As noted, however, humans are believed to be an accidental host to this parasite, and other forms of transmission have been documented and postulated. Please read some interesting Case Studies for more information on various forms of transmission.

Reservoir: T. callipaeda, which is found in China, India, Thailand, Korea, and Japan, has been identified in dogs, cats, cows, badgers, rabbits, foxes, and monkeys. T. californiensis, less well characterized and less frequently observed, has been identified in cats, foxes, coyotes, horses, rabbits, sheep, deer, and black bears.

Thelazia spp. infestation in a dog
Image from the University of Pittsburgh Medical Center

Vector: Several genera of flies have been implicated.
~Amioto okadai, A. magna, and A nagatai Found in Japan, they are vectors of T. callipaeda.
~Musca autumnalis (aka face flies) Found on many continents, the female flies are generally a pest of livestock. They feed on proteins in eye secretions, nasal secretions, and saliva. The flies have rough, spongy mouthparts that are irritating and increase tear production, thus promoting successful transmission.

Musca autumnalis
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~Fannia canicularis (aka little house fly) Another face fly that promotes transmission similarly to M. autumnalis. This species has been implicated specifically in the transmission of T. californiensis.

Image from fliegen.html

Two views of Fannia canicularis

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Incubation Period: After being deposited in the tears of the new host, the larvae take 3-6 weeks to reach maturity.

Life Cycle/Stages:
~The adult female worm lays her eggs in the tears.
~Face flies ingest the embryonated eggs (eggs in the last 2/3 of their incubation period) when they feed on the ocular secretions.
~The embryonated eggs develop into larvae in the fly’s body cavity. This takes about 15-30 days. (This is problematic because the life entire life cycle of several of the vectors is only 14-21 days)
~After 15-30 days the larvae move toward the fly’s mouth. When the fly next feeds the larvae move out of the fly’s mouth and migrate to the conjunctiva of the new host.
~In 3-6 weeks the larvae mature and the adult worms deposit their ova in the conjunctival sac and lacrimal ducts of the new host.

Morphology: Adult worms for both species are creamy-white and measure up to 0.75 X 13.00 mm in males and 0.85 X 17.00 mm in females. The male is identified by ventral curving of the posterior end and the female is identified by the vulva, which opens mid-ventrally.
T. callipaeda can be distinguished morphologically from T. californiensis based on the numbers of pre- and postcloacal papillae in the male and the position of the vulva in the female.
~Male T. callipaeda have 8-10 pairs of precloacal papillae while T. californiensis have 6-7 pairs.

Drawing of the posterior end of a male T. callipaeda

Drawing of the posterior end of a male T. californiensis
Both images from The British Journal of Ophthalmology, 1999

~In female T. callipaeda the vulva is located anterior to the esophago-intestinal junction (referred to as ei in the figure below), and in T. californiensis the vulva is positioned posterior to the esophago-intestinal junction.

Drawing of the anterior end of female T. callipaeda showing the position of the vulva (v) and the esophago-intestinal junction (ei).

Drawing of the anterior end of female T. californiensis showing the position of the vulva (v) and the esophago-intestinal junction (ei).
Both images from The British Journal of Ophthalmology, 1999

Diagnostic Tests
: A diagnosis of Thelaziasis is usually made by the clinician based on visualization of the parasite on the conjunctiva. The eggs or larvae can be seen when tears or other eye secretions are examined under a microscope.

Image from the University of Pittsburgh Medical Center

Management and Therapy: The treatment of human Thelazia spp. infections is relatively simple. Adult worms can be removed with fine forceps, under local anesthesia. The symptoms described above generally resolve immediately after the removal of the worms.
Pharmacological indications include:
~Irrigation with Lugol’s iodine or 2-3% boric acid immediately after worm removal or for parasites that are in the lacrimal ducts where they cannot be removed manually.
~Levamisole, either orally or parenterally, at 5mg/kg (it is shed in secretions produced by the lacrimal glands) or 2ml injected into the conjunctival sac.
~A dose of 1mg/lb of Ivermectin given subcutaneously has been shown to cure similar infestations in Asia and Europe.

Epidemiology: Thelaziasis has been reported in Africa, Asia, Europe, and North America. Nearly 250 T. callpiaeda infestations in humans have been reported worldwide. Cases have been reported in China, Japan, Korea, India, Thailand, Russia, and Indonesia. T californiensis infestation sare much more rare in humans, and have occurred exclusively in the Western United States. The majority of cases have occurred in the Sierra Nevada Mountains of California.

Public Health and Prevention Strategies
: Thelaziasis prevention is extremely low-priority in all countries, and understandably so. Humans are not the definitive hosts of Thelazia spp. and the associated condition, Thelaziasis, is unnoticeable at best, uncomfortable at worst. Still, in some areas, like parts of India where there is a high infestation rate among dogs, there is cause to act. Public health and prevention strategies include fly population management, aversion mechanisms like netting around beds, and treatment of all infected domestic animals. There is no vaccine for Thelaziasis.

Useful Web Links
British Journal of Ophthalmology: letter to the editor on 5 cases of Thelaziasis
University of Pittsburgh School of Medicine Department of Pathology: A case of T. californiensis in human
College of Veterinary Medicine, University of Missouri-Columbia: Veterinary parasitology
West Virginia University Extension Service, Integrated Pest Management: Face Fly Biology and Management

~Bhaibulaya M, Prasertsilpa S, Vajrasthira S. Thelazia callipaeda Railliet and Henry, 1910, in man and dog in Thailand. Am J Tropical Med and Hygiene 1970 476-479
~Doezie AM et. al. Thelazia californiensis Conjunctival Infestation. Ophthalmic Surgery and Lasers 1996 716-719
~Kirschener BI, Dunn JP, Ostler HB. Conjunctivitis caused by Thelazia californiensis. Am J Ophthal 1990 573-574.
~Koyama Y, Ohira A, Kono T, Yoneyama T, Shiwaku K. Five cases of thelaziasis. Brit J Ophthalmology 2000
~Peng Y, Kowalski R, Garcia LS, Pasculle W. Case 279—A case of Thelazia Californiensis conjunctival infestation in human. Department of Pathology, University of Pittsburgh-Columbia 2001
~Zakir R, Zhong-Xia Z, Chiodini P, et al. Intraocular infestation with the worm, Thelazia callipaeda Brit J Ophthalmology 1999 1194-1195