External parasites include chewing lice of the genus "Saemundssonia", feather lice and fleas such as "Ceratophyllus borealis". Lice are often host specific, and the closely related common and Arctic terns carry quite different species. Internal parasites include the crustacean "Reighardia sternae", and tapeworms such as "Ligula intestinalis" and members of the genera "Diphyllobothrium" and "Schistocephalus". Terns are normally free of blood parasites, unlike gulls that often carry "Haemoproteus" species. An exception is the brown noddy, which sometimes harbours protozoa of that genus. In 1961 the common tern was the first wild bird species identified as being infected with avian influenza, the H5N3 variant being found in an outbreak involving South African birds. Several species of terns have been implicated as carriers of West Nile virus.