World Aquaculture - September 2023

WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2023 35 industry from mortality, missed feeding days, increased treatment expenses and required changes to production practices. Aeromonas hydrophila is a Gram-negative, motile, rod-shaped bacterium that occurs naturally in most freshwaters worldwide (Cipriano et al. 1984). Typically, A. hydrophila is regarded as an opportunistic pathogen in freshwater aquaculture systems, causing secondary infections; however, vAh has been acting as a primary pathogen (Peatman et al. 2018). Since the turn of the century, catfish producers in the southeastern United States have been fighting against this atypical bacterium. At present, there are no approved vaccines for prevention of MAS mortality and oxytetracycline dihydrate Type A1 is the only antimicrobial approved by the U.S. Food and Drug Administration (FDA) for treatment. Risk Factors Associated with MAS Outbreaks in Catfish Ponds The epizootiology of MAS is intricate and not wellunderstood. Despite the extensive economic losses that MAS inflicts on the aquaculture industry, risk factors for outbreaks and ecological attributes of A. hydrophila in catfish ponds remain a mystery. Various potential predictor variables, including algal blooms, nitrogenous waste accumulation and water temperatures have been considered; however, high variability in water quality and phytoplankton communities, as well as the densities of earthen aquaculture ponds, pose challenges to predicting MAS outbreaks reliably. To date, no study has successfully predicted MAS outbreaks within catfish ponds. Commercial aquaculture is a global, multibillion-dollar industry that fights diseases and pathogens constantly. Catfish farming dominates the aquaculture industry in the US in terms of pounds produced and total value. According to recent reports by The United States Department of Agriculture, the value of catfish sales reached $421 million in 2021 (USDA 2022). The majority of catfish are grown in earthen ponds in the Southeastern US. The top four catfishproducing states are Mississippi, Alabama, Arkansas, and Texas. Farmers raise catfish at high stocking densities to achieve economies of scale, but this practice may result in greater susceptibility to diseases. A newly emergent highly virulent strain of Aeromonas hydrophila (vAh) has been responsible for extensive losses in farm-raised catfish for over a decade, especially in the West Alabama and East Mississippi regions. This pathogen is a “jack-of-all-trades” and has ravaged the US catfish farming industry since its discovery in 2009. This bacterium is one of the mesophilic (moderate temperature) Aeromonads causing a disease known as motile Aeromonas septicemia (MAS) in fish. Motile Aeromonas septicemia results in the loss of millions of pounds of catfish production every year. Outbreaks of disease caused by vAh are characterized by rapid onset and high mortality rates ranging from 10 percent to 100 percent in a short period, usually less than one week (Baumgartner et al. 2017). The classical external clinical signs of the disease include hemorrhagic dermatitis with diffuse haemorrhages on the fish’s body, red fins, and haemorrhagic eyes (Figure 1). Internally, hemorrhagic visceral organs and bloody fluid in the abdomen are common findings. The disease impacts primarily food-sized, marketable catfish, resulting in significant economic losses to the Can High Salinity Affect the Catfish Pathogen Aeromonas hydrophila? Haitham H. Mohammed, Wenlong Cai and Timothy J. Bruce FIGURE 1. Channel catfish infected with virulent A. hydrophila (vAh) showing typical signs of motile Aeromonas septicemia (MAS) including diffuse haemorrhages, red fins, and bloody eyes. (CONTINUED ON PAGE 36) FIGURE 2. Gram-stained smears of A. hydrophila cultures at different salt levels. Notice many normal-looking short bacterial cells (coccobacilli) at 5 g/L salt (left), fewer but still normal cells at 15 g/L salt (middle), and very few elongated cells at 45 g/L salt (right). Light microscopy, 40 X magnification.