Thelazia callipaeda, the Oriental Eye Worm, is one of two Thelazia spirurid nematodes (roundworms) that have been implicated in human infection (the other being T. californiensis). Dogs and other canids, cattle, and horses are the usual definitive hosts (the hosts in which the parasite matures and reproduces) for Thelazia nematodes (roundsworms), although other mammals, including cats, lagomorphs, cervids and humans, can also become infected. Adults reside in the conjunctival sac of the definitive host where they shed first-stage larvae. These larvae are sheathed. The first-stage larvae are ingested by the intermediate host (usually flies, including drosophilid flies in the genus Amioto and muscid flies in the genera Musca and Fannia) when they feed on tears and other lacrimal secretions. In the digestive tract of the intermediate host, the larvae shed the sheath and invade various host tissues, including the hemocoel, fat body, testis, and egg follicles, where they develop in capsules. The encapsulated larvae become infective L3 larvae after two molts. Afterwards, the L3 larvae break out of the capsules and migrate to the fly’s mouthparts, where they remain until the fly feeds on the tears of the definitive host. The larvae invade the conjunctival sac of the definitive host and become adults after about a month and two additional molts. Humans may also serve as a final host after infected flies feed on tears or other lacrimal secretions. The geographic distribution of these parasites is presumed to be worldwide. Human infections have been recorded from the United States, China, Russia, India, Japan, and Thailand.
The parasitic nematode Thelazia callipaeda is prevalent in dogs, cats, and humans in the former Soviet Union and in eastern Asia, including China, Korea, Burma, Japan, Indonesia, Thailand, Taiwan, and India (Neem 2011).
Life History and Behavior
The parasitic nematode Thelazia callipaeda infects the eyes of several vertebrate hosts including humans. This parasite requires a dipteran (fly) intermediate host, which also serves as a vector, delivering the parasite to the definitive vertebrate host (i.e., the host in a multi-host life cycle in within which reproduction occurs) when the fly lands around the vertebrate's eye and feeds on lachrymal secretions. (Neem 2011) See EOL Thelazia page for more details.
Molecular Biology and Genetics
Barcode data: Thelazia callipaeda
Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.
See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
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Download FASTA File
Statistics of barcoding coverage: Thelazia callipaeda
Public Records: 10
Specimens with Barcodes: 10
Species With Barcodes: 1
Relevance to Humans and Ecosystems
The parasitic nematode Thelazia callipaeda is one of the two Thelazia species known to infect humans, the other (typically infecting canids and several othe mammal hosts but occasionally infecting humans) being T. californiensis (Neem 2011).
Thelazia callipaeda is a parasitic nematode, and the most common cause of "thelaziasis" (or "eyeworm" infestation) 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.
In addition to humans, cats and dogs, definitive hosts of T. callipaeda include the Wolf (Canis lupus), Raccoon dog (Nyctereutes procyonoides), Red Fox (Vulpes fulva), and European Rabbit (Oryctolagus cuniculus). This species has been found in China, France, Germany, India, Indonesia, Italy, Japan, Korea, the Netherlands, Russia, Switzerland, Taiwan and Thailand.
Two intermediate hosts have been identified so far: Amiota (Phortica) variegata (Diptera: Drosophilidae) in Europe and Phortica okadai in China, which feed on tears of humans and carnivores. Some data suggests that only the males of A. (P.) variegata carry Thelazia callipaeda larvae. This is noteworthy because in all other known cases of blood-feeding flies which transmit parasites, the parasites are carried by the females.
The eggs of Thelazia callipaeda develop into first stage larvae (L1), in utero while the female is in the tissues in and around the eye of the definitive host. The female deposits these larvae, which are still enclosed in the egg membranes, in the tears (lacrymal secretions) of the host. When a tear-feeding fly (intermediate host) feeds, it ingests the T. callipaeda larvae. Once inside the fly, the L1 larvae "hatch" from the egg membrane and penetrate the gut wall. They remain in the hemocoel (the fly's circulatory system) for 2 days, and then invade either the fat body or testes of the flies. In these tissues, the larvae develop into third stage larvae (L3). The L3 migrates to the head of the fly, and is released in or near the eye of a new host mammal when the fly feeds again. Once in the eye, eyelid, tear glands, or tear ducts of the mammalian host, the L3 larvae develop through the L4 larval stage and into adults in about 1 month. The seasonal timing of L1 and L4 larvae in the lacrymal (tear) secretions of naturally infested dogs in Italy was found to coincide with the activity of the fly vectors.
Symptoms, diagnosis and treatment
Symptoms of T. callipaeda infestation include conjunctivitis, excessive watering (lacrimation), visual impairment, and ulcers or scarring of the cornea. In some cases, the only symptom is the worm obscuring the host's vision as a "floater".
Diagnosis is made by finding the adult worms in the eye or surrounding tissues. Human cases are treated by simply removing the worms. In canines, topical imidacloprid with moxidectin, or milbemycin oxime (Interceptor) have been recommended.
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