Parasitic infections in teleost fish are limited by constitutive innate defence mechanisms that
render the host refractory or reduce the severity of infection. Controlled challenge trials using
naive animals provide indirect evidence of innate immunity as well as identifying the host
range or specificity of a parasite, often when specific details of defence mechanism (s) are
lacking. Examples of parasites for which innate immunity may be inferred from cross-
infectivity studies include Gyrodactylus spp., Lepeophtheirus salmonis, Cryptobia spp …