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Pathology of Thelaziasis

Dr Sampurna Roy MD




Thelaziasis (Thelaziosis) is an insect-borne parasitic eye disease.

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More than 30 species of Thelazia were identified in the world and only two species of them, Thelazia callipaeda (T. callipaeda, Railliet and Henry, 1910) and Thelazia californiensis (T. californiensis, Price, 1930), are known to cause human Thelaziasis.

Thelazia callipaeda has been known as the "oriental eye-worm" due to its geographical distribution in the former  Soviet Union and in countries in the Far East, including  China, South Korea, Japan, Indonesia, Thailand, Taiwan, and India.

Recently T. callipaeda has been reported to infect human, dogs, foxes, and cats in Europe (Italy, France, Spain, Portugal, Germany and Balkans).  

Thelazia californiensis has been reported to infect human beings occasionally in the United States.

T. callipaeda infects a range of definitive hosts, such as domestic dogs and cats and wild carnivores (e.g., foxes, wolves, beech martens and wild cats) and humans. This parasite usually lives under the nictitating membrane of the eye, where the adult females release first-stage larvae into the lacrymal secretions.

These 1st stage larvae are subsequently ingested by the intermediate arthropod host (flies) which feed on lacrymal secretions of infected animals and/or humans.

After development within the fly, the 3rd stage larvae is directly transmitted back into the orbit. within which they develop to the infective, third-stage larvae. 

In human thelaziasis  patients present with mild to severe symptoms including lacrimation, epiphora (excessive watering of eye) , conjunctivitis, keratitis, and corneal ulcers.

Human cases are usually associated with poor, rural communities with low health and socio-economic standards, where heavily affected domestic (i.e., dogs and cats) and wild carnivores (e.g., foxes) live in close vicinity with humans.

Children and the elderly seem to be at higher risk.

There are several morphological features that assist in identifying them from other worms that might occur in the orbit, including filaria such as Loa Loa or Dirofilaria.

Thelazia worms are generally removed intact from the eye with foreps.

Prevention of ocular thelaziasis includes avoiding flies in summer that are attracted due to excessive sweating or shedding tears.

The methods to avoid flies include bed nets at night, keeping the face clean during summer, among people living in the rural area as well as in the urban area.

General physicians and ophthalmologists should take human thelaziosis into account in their differential diagnoses of conjunctivitis, ocular lacrimation, and corneal ulcers .

Medical continuing education and awareness of this condition are needed to ensure that the infection does not go undiagnosed and that appropriate treatment for the primary problem and for complications such as allergic reaction or bacterial infection can be prescribed.

Further reading

A Pediatric Case of Thelaziasis in Korea

Two Cases of Human Thelaziasis and Brief Review of Korean cases

Human ocular Thelaziasis in Karnataka

Two cases of human thelaziasis as confirmed by mitochondrial cox1 sequencing in China

Human Thelaziasis, Europe

Zoonotic helminths affecting the human eye

Human ocular thelaziasis in Karnataka

Increase in Eyeworm Infections in Eastern Europe

The spread of zoonotic Thelazia callipaeda in the Balkan area

Thelaziosis in Humans, a Zoonotic Infection, Spain, 2011



Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)






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