Thelazia callipaeda

Thelazia callipaeda is a species of parasitic worm in the Nematoda phylum. It is one of the most common types of worm to infect the eyes of cats, dogs, and humans. This infection is known as thelaziasis or “eye worm” infestation. This species was discovered in 1910, in the eyes of a dog in China. In 2000, it was reported that 250 cases of eye worm infection had been documented in humans. Other hosts of this species include wolves, red foxes, European rabbits, and raccoon dogs. It has a large range that includes India, China, France, Germany, Japan, Korea, Indonesia, Russia, Italy, the Netherlands, Taiwan, Thailand, and Switzerland.

There have been two recognized species that serve as intermediate hosts to Thelazia callipaeda. These are fruit flies known as Phortica okadai in China and Amiota (Phortica) variegata (Diptera: Drosophilidae) in Europe. It is thought that these flies drink the salty tears of humans and other animals, producing a means of transport for Thelazia callipaeda. Unlike other species of flies that transfer worms, the males of A. (P.) variegate are the ones who transmit the worms instead of females.

The first stage of life for Thelazia callipaeda begins when the eggs develop into larvae. These eggs are placed into the tears of the definitive host while the female resides within the tissues in and around the eye. Tear-feeding flies consume the eggs with the tears of the definitive host, effectively becoming intermediate hosts. The eggs will hatch within the flies and penetrate the lining of the stomach. In this stage two form, the larvae are able to move around the circulatory systems of the flies. The larvae will then move into the fat or testes of the fly, developing into their third stage forms. In the third stage, the larvae will move into the head of the fly, after which they will reach the eye of a definitive host when the fly feeds. After about one month within the tear ducts, tear glands, eyelids, or eyes, the larvae mature into stage four adults and are able to breed and lay more eggs.

The host of Thelazia callipaeda will display many symptoms of infection including visual weakening, conjunctivitis, extreme watering of the eyes known as lacrimation, and scarring or ulcers on the eye. However, some human hosts have reported only one symptom, known as a “floater” that obscures the host’s vision. It is relatively easy to diagnose and treat infections of this species in humans, because the worms must only be found in a small area and then physically removed. In animals, like cats and dogs, topical crèmes including moxidectin, imidacloprid, or Interceptor (milbemycin oxime) are typically used to remove Thelazia callipaeda from the eyes.

Image Caption: Female Thelazia callipaeda. Credit: Wikipedia