Ichthyophthirius multifiliis, causing white spot disease, is a serious pathogen in aquaculture as well as for
the ornamental fish industry. In carp, channel catfish and rainbow trout the immune responses against
the parasite have been partly elucidated and these species are able to acquire a high level of immunity
against the disease. Zebrafish are however, known to be more resilient towards the disease than channel
catfish but the pathology and the ability to obtain protection is unknown. In this study a primary
infection in the gills of zebrafish was described and the subsequent acquirement of immunity was
evaluated. The parasites in the gills induced hyperplasia, an increase of mucus cells, adhesion and
shortening of the secondary lamellae, narrowing of water channels and proliferation. The parasite
burden was significantly lower in survivor fish and where all naive fish died within 6 days following
secondary infection (challenge) only one of the survivor fish died. This study demonstrates that zebrafish
are susceptible to I. multifiliis and that gill pathology is similar to the situation observed in rainbow trout.
Furthermore, zebrafish are able to acquire immunity against white spot disease and may therefore be a
suitable model organism to study innate and protective immunology and host/parasite interactions
during I. multifiliis infections.