High resistance to Ichthyophthirius multifiliis in Percocypris pingi, an endemic fish in upper Yangtze River

Ichthyophthirius multifiliis is a ciliated ectoparasite that infects almost all freshwater fishes with low host specificity. However, an endemic fish in the upper Yangtze River, Percocypris pingi, was thought by aqua-culturists to be completely resistant to I. multifiliis. We therefore aimed to determine P. pingi resistance to I. multifiliis. We compared the survival, feed intake, infection intensity, relative number of I. multifiliis tomont and tomont diameter, and the short- (2 months) and long-term (7 months) effects of I. multifiliis on growth between P. pingi, southern catfish (Silurus meridionalis), grass carp (Ctenopharyngdon idella), Schizothorax prenanti and Culter alburnus post I. multifiliis infection at 20,000 theronts/fish under controlled laboratory conditions. We also tested serum and skin mucus immobilization activity to I. multifiliis in uninfected individuals of the five fish species. The results detected mild I. multifiliis infection in P. pingi reflected by significantly lower parasite density and no expression of parasite I-antigen gene IAG52A comparable to the four other species. The low total relative number of tomonts collected from P. pingi during almost the entire parasitic stage (from the third to sixth day post infection) and small tomont size during the later parasitic stage (the fifth and sixth day post infection) suggest that P. pingi was able to limit trophont settlement and/or development. When infected by I. multifiliis, P. pingi displayed almost no mortality compared to high mortality in the other four fish species (14 days post infection) and surviving individuals recovered from the initial appetite loss within seven days post infection and represented normal growth over the following two and seven months. Furthermore, P. pingi mucus and serum have strong immobilization activity to I. multifiliis. It appears that P. pingi is highly resistant to I. multifiliis and may be a good model to understand the underlying immune mechanism of fish to I. multifiliis.