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Thelazia is a genus of parasitic nematodes known as "eyeworms", a reference to their primary habitat in their definitive host (i.e., the host in which they reproduce): the host's orbital (eye) cavities and associated tissues. Documented definitive hosts for most Thelazia species are mammals (including various carnivores, ruminants, horses, and humans, depending on the species). At least one species (T. anolabiata) occurs in bird eyes (careful molecular genetic investigations may reveal additional species) (De Oliveira 1992; Elias et al. 2008). The known intermediate hosts for Thelazia eyeworms (i.e., the hosts in which the parasite spends a part of the non-reproductive phase of its life cycle) are various non-biting flies.

The basic Thelazia life cycle is unusual in the mode of transmission between the intermediate host (fly) and definitive host (vertebrate). First stage larvae are released directly (no eggs are laid) in the lachrymal (tear gland) secretions of the hosts by adult female eyeworms after mating with adult males. These larvae then infect flies feeding on lachrymal secretions around the eyes of the vertebrate host. Within the fly, larvae develop from first stage to infective third stage larvae while remaining encapsulated in different parts of the fly's body, yielding infective larvae in around 18 to 25 days. Third stage eyeworm larvae emerge from the labella of infected flies and are transmitted to the eyes of the definitive vertebrate hosts while infected flies are feeding on eye secretions and these third stage larvae develop into adults within the host's ocular cavity. Adult eyeworms are found mainly under the eyelids and nictitating membrane, in nasolachrymal ducts, in conjunctival sacs, or in the excretory ducts of lachrymal gland. The first stage larva is very short-lived in the lachrymal secretions, surviving just a few hours, and transmission thus depends on the continuous presence of appropriate flies to infect. For this reason, thelaziasis (symptomatic infection in the vertebrate host) has a seasonal occurrence that depends on the seasonality of the intermediate hosts.

Six Thelazia species are considered to be of veterinary significance: T. californiensis, T. callipaeda,T. gulosa, T. lacrymalis, T. rhodesi, and T. skrjabini. Two of these, T. callipaeda and T. californiensis, can infect humans.

Thelazia eyeworms are milky-white, with males and females reaching up to around 12 mm and 20 mm, respectively, and a diameter of 250 to 800 μm. Discrimination of Thelazia species by morphology can be extremely difficult, but molecular genetic markers (e.g., Otranto et al. 2001; Otranto and Traversa 2004) promise to provide both valuable diagnostic tools and opportunities to advance our understanding of the biology and true diversity of these worms.

Naem (2011) and and Otranto and Traversa (2005) provide valuable introductions to diverse aspects of Thelazia biology.

(Otranto and Traversa 2005 and references therein; Otranto et al. 2009; Naem 2011 and references therein; Otranto and Eberhard 2011)

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