Externally parasitic

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"living on the surface or in the skin of animals," 1832, from epizoon + -ic .

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Epizoic \Ep`i*zo"ic\, a. (Zo["o]l.) Living upon the exterior of another animal; ectozoic; -- said of external parasites. [1913 Webster] ||

WordNet Word definitions in WordNet
adj. living or growing on the exterior surface of an animal usually as a parasite; "an epizoic plant parasite" [ant: entozoic ]

Wiktionary Word definitions in Wiktionary
a. living or growing on the external surface of an animal (not always as a parasite)