Life cycle for Thelazia
Dogs and other canids, cattle, and horses are the usual definitive hosts (the hosts in which the parasite matures and reproduces) for Thelazia nematodes (roundsworms), although other mammals, including cats, lagomorphs, cervids and humans, can also become infected. Adults reside in the conjunctival sac of the definitive host (5) where they shed first-stage larvae (1). These larvae are sheathed. The first-stage larvae are ingested by the intermediate host (usually flies, including drosophilid flies in the genus Amioto and muscid flies in the genera Musca and Fannia) when they feed on tears and other lacrimal secretions (2). In the digestive tract of the intermediate host, the larvae shed the sheath and invade various host tissues, including the hemocoel, fat body, testis, and egg follicles, where they develop in capsules. The encapsulated larvae become infective L3 larvae (3) after two molts. Afterwards, the L3 larvae break out of the capsules and migrate to the fly’s mouthparts, where they remain until the fly feeds on the tears of the definitive host. The larvae invade the conjunctival sac of the definitive host and become adults after about a month and two additional molts (4). Humans may also serve as a final host (6), after infected flies feed on tears or other lacrimal secretions. Two Thelazia species have been implicated in human infection, Thelazia californiensis (the California Eye Worm) and T. callipaeda (the Oriental Eye Worm).
From Centers for Disease Control Parasites and Health website.