Cryptobia iubilans Infections in Discus Fish in Trinidad and Tobago

Pathological and Tissue-Based Molecular Investigation of Granulomas in Cichlids Reared as Ornamental Fish

Simple Summary

The global ornamental fish trade has an estimated value of USD 15–30 billion per year and more than a 10% average annual growth. Despite their economic importance, the management of ornamental fish is challenged by a paucity of information, including data on the fish health status. Pathological and microbiological investigations were conducted on ornamental cichlids sampled during routine management activities held at an aquarium commercial facility, in order to evaluate the presence of granuloma in the organs. Cutibacterium acnes and Mycobacterium spp. were detected by molecular methods and immunohistochemistry. These bacteria represent potential zoonotic agents, and the advancement of their knowledge could significantly improve the management of ornamental fish and reduce the risk of exposure for people, such as hobbyists, fish handlers, aquarists, and dedicated personnel.

Abstract

Cichlids include hundreds of species with a high economic value for aquaculture. These fish are subjected to intensive trade and farming that expose them to the risk of infectious diseases. This work focuses on ornamental cichlids held in an aquarium commercial facility presenting emaciation, in order to evaluate the presence of lesions in fish skin and organs. The fish were sampled during routine management activities and subjected to pathological and molecular investigations. The presence of lymphocystis disease virus, typically associated with cutaneous nodular disease, was ruled out. Histologically, they presented granulomas in the spleen, sometimes extending to the other visceral organs. Bacterial heat-shock protein 65 PCR products were detected in tissues associated, in the majority of cases, with granulomas; molecular investigation identified Mycobacterium spp. in two cases and Cutibacterium acnes in seven cases. Immunoreactivity to anti-Mycobacterium and anti-C. acnes antibodies was detected within granulomas. The presence of C. acnes within granuloma is elucidated for the first time in fish; however, similarly to what is found in humans, this bacterium could be harmless in normal conditions, whereas other contributing factors would be required to trigger a granulomatogenous response. Further confirmation by bacterial culture, as well as using large-scale studies in more controlled situations, is needed.

Piscine mycobacteriosis in the ornamental fish trade in Trinidad and Tobago

The freshwater ornamental fish trade represents a major contributor to the livelihoods of many producers in Trinidad and Tobago, with stocks destined for local, regional and international markets. A review of clinical cases presented to the Aquatic Animal Health Unit at the University of the West Indies, School of Veterinary Medicine for the period September 2010 to December 2012 suggested that piscine mycobacteriosis may be widespread throughout the local ornamental fish industry. Thus, to determine the prevalence of mycobacteriosis in ornamental fish sold in pet stores, a total of 122 specimens were sourced from 24 retail suppliers across Trinidad. Fish were killed and internal organs were examined for lesions suggestive of granulomas. All wet-mount slides were acid-fast stained, regardless of the presence or absence of observed granuloma-like lesions. Histological analysis was performed on one randomly selected whole specimen from each facility. Mycobacterium sp. was identified using real-time PCR detecting the 16S rRNA gene in tissue samples. Associations between parasitism, facility biosecurity and presence of positive animals were determined. The prevalence of Mycobacterium sp. infection was 61 ± 7% (74/122), with positive specimens being acquired from 54.2% (13/24) of facilities examined. Further, 100% of facilities did not employ optimum biosecurity measures.