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Oriental Eyeworm

(Thelazia callipaeda)

Thelazia callipaeda is a spirurid of dogs, which can also be found in cats and wildlife such as foxes and hares. This parasite is transmitted to dogs by Phortica variegata, which is a fruit fly that feeds on lachrymal secretions of mammals. It is zoonotic.

Parasite: Thelazia callipaeda
Common name: The oriental eyeworm
Hosts: Dogs, cats, several wildlife species, and humans
Pre-patent period: 3 weeks
Location of adults: conjunctival sac
Distribution: some parts of Asia and Europe
Transmission route: through secretophagous flies (Phortica variegata)
Zoonotic: Yes

Distribution

It has been reported in several parts of Europe and Asia, including China, India, Bangladesh, Myanmar, Indonesia, Japan, Korea, Taiwan, and Thailand.

Clinical signs

In most cases, T. callipaeda infection in dogs is asymptomatic, but clinical signs may include mild conjunctivitis, blepharitis, epiphora, periocular pruritus and, in severe cases, corneal oedema and keratitis (Fig 1). Blindness may eventually occur in severe cases that are left untreated.

Figure 1. Thelazia callipaeda in the eye of a dog. (Image credit: Dr Filipe Dantas-Torres DOI: 10.1186/s13071-015-0881-7)

Diagnosis

Diagnosis is achieved by visual inspection and retrieval of adult worms in the eye of infected hosts. First-stage larvae of the parasite may also be found in ocular secretions.

Treatment

Mechanical removal of the worms by flushing saline solution in the affected eyes is usually successful. A single application of topical imidacloprid plus moxidectin (2.5 mg/kg) killed worms within 7 days of application. Two oral doses of milbemycin oxime (0.5 mg/kg) administered, one week apart reached 100% efficacy 28 days following treatment. Alternatively a single dose of 200 µg/kg oral ivermectin achieved 100% efficacy 25 days following off-label administration.

Control

Control of T. callipaeda infections in dogs may be achieved by avoiding wooded environments inhabited by Phortica variegata and by treating infected animals.

Public health considerations

Several cases of human thelaziosis have been recorded in Asia and Europe, especially in people living near wooded environments, where the natural life cycle of this parasite takes place. Clinical signs resemble those of dogs listed above.