Diagnosis and Off-Label Treatment of Subcutaneous Putative Dracunculoid Nematodes in Apteronotus albifrons Originating From Wild-Caught Fisheries.

Apteronotus albifrons (Gymnotiformes: Apteronotidae), commonly known as the black ghost knifefish, is a high-value ornamental fish that is susceptible to various parasites. The affected fish were wild-caught fisheries and subsequently introduced into a commercial breeding system. Among parasitic diseases, subcutaneous infections with putative dracunculoid nematodes are rarely documented in ornamental fish, and their management may be particularly challenging in wild-caught specimens introduced into breeding systems. This study aimed to describe an outbreak of subcutaneous helminthiasis in A. albifrons, morphologically characterise the parasites and report the clinical outcome of off-label treatment attempts. Affected fish presented with cutaneous nodules, oral expulsion of worms and reduced reproductive performance. Necropsy and morphological analysis revealed subcutaneous nematodes with features compatible with dracunculoid nematodes; however, genus-level identification was not confirmed because no male specimens were recovered and molecular analyses were not performed. Following off-label oral treatment and post-treatment monitoring, clinical improvement was recorded, and no detectable parasites were observed during the 4-week monitoring period. The findings highlight the importance of accurate parasite identification, cautious interpretation of treatment outcomes and the need for controlled studies to validate safe and effective treatments for subcutaneous nematode infections in ornamental fish.