Human thelaziosis – a neglected parasitic disease of the eye

The oriental eyeworm, Thelazia callipaeda (Spirurida, Thelaziidae), infects a range of definitive hosts, such as dogs, cats, foxes, rabbits, and humans. This parasite usually lives under the nictitating membrane of the eye, where the adult females release first-stage larvae into the lachrymal secretions; these larvae are subsequently ingested by the intermediate arthropod host within which they develop to the infective, third-stage larvae. The latter larvae are then deposited into the eyes of the definitive host. Recently, T. callipaeda has been reported to infect dogs, foxes, and/or cats in Europe (Italy, France, and Germany). Human thelaziosis (HT) is considered to be an underestimated parasitic disease, whose prevalence appears to have increased in poor socioeconomic settings in many Asian countries, including China. In humans, the disease can be subclinical or symptomatic, exhibiting epiphora, conjunctivitis, keratitis, excessive lachrymation, corneal opacity, and/or ulcers. Knowledge about HT is presently fragmentary and mainly limited to clinical case reports. This article provides a background on the parasite and its life cycle, reviews cases of human thelaziosis, summarizes key aspects regarding the diagnosis of thelaziosis, and proposes future research and methods of control of the disease in humans, particularly in Asia.