Experimental Loma morhua (Microsporidia) infections reveal early-onset production implications for Atlantic cod (Gadus morhua) aquaculture

Widespread Loma morhua (Microsporidia) epizootics in cultured Atlantic cod populations have been linked to reduced immune function and growth. Importantly, the lack of appropriate diagnostics and basic knowledge of the L. morhua life-cycle have limited the development of disease management strategies for mitigating infection during aquaculture. As a consequence, L. morhua epizootics are commonplace within hatchery facilities and during net-pen grow-out and limit commercialization of cod culture. Herein, we detail experimental L. morhua infections in naive Atlantic cod to mimic infection phenotypes observed during natural epizootics, to better validate diagnostic procedures and to understand the effect parasitism has on cod growth. An integrative approach for parasite detection incorporating xenoma enumeration (via gross morphological examination) and quantitative PCR (qPCR) analyses reveals that intraperitoneal (IP) delivery of L. morhua spores was dramatically more effective for experimental infection, relative to gastric intubation (GI), with 100% prevalence of infection achieved by 28 weeks post-exposure (WPE). Subclinical infections persisted in 31.8% of IP-injected fish, as detected via qPCR analysis. This supports previous observations that individual cod vary widely in their response to infection and require such integrative diagnostic approaches for parasite detection. Quantification of parasite xenomas reveal static intensities in spleens 16-28 WPE (Kruskal-Wallis, p = 0.3), but a phasic reduction in gill xenoma intensities during this same period (Mann-Whitney-Wilcoxon, W = 113, p = 0.037). In situ examination of gills in live fish also support the significant reduction in gill xenoma intensity observed 22-28 WPE (2-37 fold reduction, Wilcoxon signed rank test, V = 78, p = 0.0004) and offers a useful in vivo monitoring approach. Importantly, experimental infections induce a period of considerable growth rate suppression at 2-4 WPE. Though growth rates recovered, the effect of early growth impairment persisted at 28 WPE, with similar to 19.3% fillet loss occurring in parasitized fish as compared with uninfected controls. The negative consequences of L. morhua infections on cod growth emphasize the translational importance of incorporating eukaryotic pathogens like microsporidians into management, biosecurity, and selective breeding programs for fish species of importance for basic and applied research, or for aquaculture.