Pseudocapillaria tomentosa (Nematoda: Capillariidae) in fish and its significance in comprehending host-parasite relationships: A review.

Helminth parasites, including capillariids, pose a significant health risk to animals, including fishes, birds, and mammals. Pseudocapillaria tomentosa is a widespread, but poorly studied parasite primarily infecting freshwater cyprinid fishes in the northern hemisphere. However, despite controlled laboratory procedures, the parasite is also prevalent in many zebrafish (Danio rerio) research facilities due to inadequate measures to ensure biological security and the ability of the nematode to endure chlorine treatment. Pseudocapillaria tomentosa typically causes chronic disease in fish, leading to morbidity, mortality, and economic challenges. Clinical signs include emaciation, ulcers, anemia, and hemorrhage, as well as reduced growth and reproductive ability, and prominent humpback lesions in severe infections. Diagnosing P. tomentosa infections involve observation of eggs in wet mount preparations or worms in histological sections of the intestine. However, definitive species identification requires detailed morphological examination and molecular analyses. Pseudocapillaria tomentosa can be effectively managed through anthelmintic treatments such as emamectin, fenbendazole, albendazole, and mebendazole as well as preventive measures including maintaining optimal water quality, reducing fish density, and implementing strict quarantine protocols. This review discusses the use of the zebrafish to study host-parasite interactions, particularly with the parasitic nematode P. tomentosa. The zebrafish is a suitable model for studying infectious diseases, including parasites, due to its well-characterized immune system, reporter lines and cell lineages. This model organism exhibits immune responses to helminth antigens, including eosinophilia and the upregulation of inflammatory cytokines like Tnf-α and Ifn-γ. The gut microbiota plays a crucial role in susceptibility to parasitic infections in zebrafish and an imbalanced or dysbiotic gut microbiome can predispose fish to parasitic infections, while a healthy, balanced microbiome can enhance resistance. Furthermore, probiotic-based interventions are being explored as a way to boost mucosal immunity and modify the gut microbiome to prevent parasitic infections.