Innate immune response against an oomycete pathogen Aphanomyces invadans in common carp (Cyprinus carpio), a fish resistant to epizootic ulcerative syndrome

• Cyprinus carpio is known to be naturally resistant to infection with Aphanomyces invadans.
• Experimental infection with A. invadans resulted in an intense inflammatory reaction in C. carpio.
• Respiratory burst, ACH50 and antiproteases activities were significantly higher in infected fish at initial stages.
• Lysozyme, myeloperoxidase and α-2 macroglobulin activities of infected fish were not significantly altered.
• Increase in some innate immunity parameters appears to be effective in resisting A. invadans infection.

Infection with Aphanomyces invadans, also known as epizootic ulcerative syndrome, is a destructive disease of freshwater and brackishwater fishes. Although more than 130 species of fish have been confirmed to be susceptible to this disease, some of the commercially important fish species like common carp, milk fish and tilapia are reported to be resistant. Species that are naturally resistant to a particular disease, provide a potential model to study the mechanisms of resistance against that disease. In the present study, following experimental infection with A. invadans in common carp Cyprinus carpio, sequential changes in various innate immune parameters and histopathological alterations were monitored. Some of the studied innate immunity parameters viz. respiratory burst, alternative complement and total antiproteases activities of the infected common carp were higher compared to control fish, particularly at early stages of infection. On the other hand, some parameters such as myeloperoxidase, lysozyme and alpha-2 macroglobulin activities were not altered. Histopathological examination of the muscle at the site of injection revealed well developed granulomas at 12 days post infection, with subsequent regeneration of muscle fibers. From the results, it could be inferred that innate defense mechanisms of common carp are able to neutralize the virulence factors secreted by A. invadans, thereby, preventing its invasive spread and containing the infection. The results obtained here will help to better understand the mechanisms underlying resistance against A. invadans infection.