The parasitic nematode Thelazia callipaeda infects the eyes of several vertebrate hosts including humans. This parasite requires a dipteran (fly) intermediate host, which also serves as a vector, delivering the parasite to the definitive vertebrate host (i.e., the host in a multi-host life cycle in within which reproduction occurs) when the fly lands around the vertebrate's eye and feeds on lachrymal secretions. (Neem 2011)  See EOL Thelazia page for more details.