Cryptocaryon irritans(Ciliophora): acquired protective immunity in the thick-lipped mullet,Chelon labrosus

Acquired protection toCryptocaryon irritanshas been demonstrated for the first time, using the grey mullet,Chelon labrosus, as an experimental host. Fish, immunized by controlled infections, established immunity against challenge infections withC. irritans, the degree of protection correlating with both intensity and exposure levels, with relatively few fish developing full protection. Protection lasted for six months in the absence of re-exposure to the parasite. There was no evidence of cross-protection in mullet against infection with the closely related fish parasite,Ichthyophthirius multifiliis.