Outbreak of gill myxoboliosis causing mass mortality in Labeo catla: pathological effects and immune modulation in host

Myxobolus bengalensis is a significant fish parasite responsible for disease and economic losses in aquaculture; however, information on its pathogenicity and impact on fish health remains limited. This study aimed to identify the morphological and molecular features of Myxobolus bengalensis and investigate the immunological changes in infected Labeo catla. Symptomatic fish showed signs of disease, including extreme lethargy, a slender body with a large head (growth retardation), and pale gills with the presence of whitish cysts in the gill lamellae. The Myxozoan parasite was isolated from infected L. catla tissue samples and preliminarily identified as M. bengalensis based on morphology, 18S rRNA PCR sequencing, and phylogenetic analysis. Tissue specimens were collected and underwent examination of immune-related gene expression (in the liver and kidney). The M. bengalensis spores isolated from the gills were ellipsoidal, with ovoid polar capsules. Sequencing and phylogenetic analysis of the 18S rRNA region revealed a ~ 1700 bp fragment, confirming the parasite's identity and placing it within the same clade as other M. bengalensis isolates. The transcription analysis of genes associated with inflammation (TNF-α, IL-1β, iNOS), immune activation (TLR 4, C3, MYD88, NOD 1), and both innate and adaptive immune responses (IFN-γ, Hsp70, Mx, IgM) further highlights that M. bengalensis modulates the expression of genes in the liver and kidney tissue samples of infected L. catla. There is limited data available on the host-pathogen response during M. bengalensis infection; thus, this study provides key insights into the morphology, pathogenicity, and immunological impacts of M. bengalensis in L. catla.