Columnaris disease affects a variety of freshwater fish, including all commercially important species worldwide. In the southeastern USA, it is the most reported disease in catfish farming, according to case summary reports by the Aquatic Research and Diagnostic Laboratory at Mississippi State University. This disease is caused by a group of Gram-negative, yellow-pigmented bacteria known as columnaris-causing bacteria (CCB). CCB includes four distinct species with host associations, namely F. columnare, F. covae, F. davisii, and F. oreochromis. Historically, F. columnare, F. covae, and F. davisii have been isolated from columnaris disease cases in the southeastern U.S.
Severe columnaris disease outbreaks recently impacted commercial catfish farms and sport fishing ponds in south Texas. During these outbreaks, hybrid catfish (Ictalurus furcatus x I. punctatus), hybrid tilapia (Oreochromis mossambicus x O. niloticus), red drum (Sciaenops ocellatus), and other sport fish species such as largemouth bass (Micropterus salmoides), bluegill (Lepomis macrochirus), and crappie (Pomoxis spp.) were affected. However, there is limited data on the prevalence of specific CCB species in Texas aquaculture and recreational fish. The study aimed to identify the predominant species of CCB associated with columnaris disease outbreaks and evaluate the isolates’ antimicrobial susceptibility profiles to guide treatment strategies.
Putative CCB isolates (n=40) were recovered following the standard microbiological procedures from the different fish species submitted to the Fish Health and Disease Laboratory at Texas A&M University during columnaris outbreaks throughout 2023-2024. After DNA extraction, a multiplex polymerase chain reaction (PCR) was conducted using four reference CCB strains provided by the USDA-ARS Aquatic Animal Health Research Unit (Auburn, AL) as control to identify the most prevalent species among the samples. 60% of the collected isolates (n=40) were identified as F. covae (n = 24), followed by F. davisii (n = 16). Notably, no isolates were categorized as F. columnare or F. oreochromis. Antibiotic susceptibility tests were performed on representative isolates (n = 13) following established protocols using FDA-approved antibiotics for food fish. F. covae isolates showed an average zone of inhibition of 36.09 ± 2.67 mm against oxytetracycline (30 μg), while F. davisii exhibited an average zone of inhibition of 32.83 ± 0.73 mm. The average inhibition zones for florfenicol (30 μg) were 45.03 ± 0.18 mm and 42.72 ± 0.09 mm for F. covae and F. davisii, respectively. In contrast, F. covae isolates displayed an average zone of inhibition of only 5.75 ± 0.58 mm around Romet-30 discs, and all F. davisii isolates showed no zone of inhibition. This research emphasizes the need to develop alternative therapeutic targeting the most prevalent CCB species to prevent future columnaris disease outbreaks in Texas effectively.