The dynamics of neutrophils in zebrafish (Danio rerio) during infection with the parasite Ichthyophthirius multifiliis

• Zebrafish were infected with the parasite Ichthyophthirius multifiliis.
• Neutrophils were attracted to the infection in the caudal fin of the fish.
• Neutrophils were quantified in the caudal fin during infection.
• Following 24 h post infection the neutrophil number decreased.
• The parasites may have actively evaded the innate immune system.

Ichthyophthirius multifiliis is a ciliated protozoan parasite infecting the skin and gills of freshwater fish. Neutrophils are attracted to the infection sites, as a part of the innate immune response. In this study a transgenic line of zebrafish (Tg(MPO:GFP)i114) with GFP-tagged neutrophils was infected with I. multifiliis and the neutrophil influx in the caudal fin was quantified. Twenty-four hours post infection (pi) the neutrophil count had gone up with an average of 3.4 fold. Forty-eight h pi the neutrophil count had dropped 12% and 72 h pi it had dropped to 21% compared to 24 h pi. At 72 h pi the neutrophil count was 2.7 times higher than prior to infection. A few dead parasites were observed, which were disintegrated and covered internally and externally with neutrophils. Live parasites, both surrounded by neutrophils and with no neutrophils in the near vicinity, were found during the infection. Neutrophils interacted directly with the parasites with pseudopod formation projecting towards the pathogen. These results indicate a strong innate immune response immediately following infection and/or a subsequent immune evasion by the parasite.