Argulus sp., commonly referred to as fish lice, are crustacean ectoparasites of fishes. The hematophagous parasites attach to and feed off the integument of their hosts. Outbreaks of epizootics have been reported worldwide, causing mass mortalities and having serious economic implications for fish farms and culture efforts. Argulus fish lice may also serve as vectors of infectious diseases and as intermediate hosts of other parasites. Two native European species, A. foliaceus and A. coregoni, as well as the invasive Japanese fish louse A. japonicus, have previously been recorded in Slovakia. This study investigated samples collected at fish farms and culture sites of Common carp (Cyprinus carpio L.), Pike-perch (Sander lucioperca L.) and Brook trout (Salvelinus fontinalis M.) in Eastern Slovakia, as well as samples collected from live fish imported to the Slovak Republic. A quantitative description of the of Argulus sp. was recorded from each locality. Samples from Common carp were identified as the invasive A. japonicus, and samples from Pike-perch and Brook trout were identified as A. foliaceus. Evidence of a mixed infection of Pike-perch with both A. foliaceus and A. japonicus was found in samples from Zemplínska Šírava, which was substantiated by electron microscopic examination. Morphometric characteristics were measured and averages and ranges produced for each species and sex.
