Insights into mucosal and systemic immune responses of <scp>African</scp> catfish, <i>Clarias gariepinus</i>, to <scp>chilodonellosis</scp>: A natural infection study

A natural parasitic infection with the external ciliate protozoan, Chilodonella hexasticha, was recorded 3 days post-transportation (PT) in the gills and skin of African catfish (Clarias gariepinus). Infected fish displayed behavioural changes and typical signs of infection. Mortalities started on day 7 PT and stopped on day 13 PT, where cumulative mortality reached 62.2%. The expression of immune-relevant genes, interleukin-1β (IL-1β), major histocompatibility complex class II (MHC-II) and interleukin-10 (IL-10) in the gills, head kidneys and spleen of the Chilodonella-infected and control uninfected fish were investigated using quantitative real-time polymerase chain reaction (PCR) on days 7-, 14- and 28 PT. Expression levels of IL-1β in the gills showed a significant upregulation on day 7 PT but were not significantly different from those of the control fish in the head kidneys and spleen at all time points investigated. Expression levels of MHC-II were significantly elevated in the gills (days 7- and 14 PT), head kidneys (days 7-, 14- and 28 PT) and spleen (days 7- and 14 PT), whereas IL-10 showed a significant upregulation only in the gills on day 14 PT, with no significant changes in all other tissues examined. Additionally, histological studies were conducted to investigate the alterations in the head kidney and spleen tissue structures associated with the immune response to infection and the changes in the expression profiles of the immune-related genes in these organs. Depletion of the renal and splenic tissues, simultaneously with prominent melanomacrophage infiltration, was observed. Also, the histopathologic changes caused by C. hexasticha in the gills of infected fish involved aggravated tissue damage characterized by hyperplasia, and necrosis of the gill lamellae was recorded.