Citrobacter freundii: A Threat To Aquaculture Development

Aquaculture is the fastest-growing agricultural activity in the world, and fish represent the most produced animal protein on the planet. According to the Food and Agriculture Organization (FAO), this production tends to increase, as aquaculture is expected to exceed production of catch fish by 2020.

However, this intensification has led to increased mortality due to infectious diseases, among which those of bacterial origin are the most prevalent. The bacterial and parasitic diseases generate direct and indirect economic losses that were estimated in US$ 84 million per year in Brazilian fish farms.

Citrobacter freundii is a Gram-negative bacillus and facultative anaerobic opportunistic pathogen of the family Enterobacteriaceae. It is an emerging bacterium that has been associated with infections in freshwater fish, leading to high mortality and serious economic losses to fish producers. There are reports of this bacterium affecting catfish (fish of the order Siluriformes).

The silver catfish (Rhamdia quelen) is the most produced native fish in the South Region of Brazil, and the presence of a highly-virulent (biofilm producer) C. freundii strain causing mortality in this fish species was reported. Worsening the situation, this strain is multidrug-resistant, making treatment of infected animals a challenge. Only three antimicrobials are approved by the Food and Drug Administration (FDA) for use in aquaculture, which are oxytetracycline, florfenicol, and sulfadimethoxine/ormetoprim. Therefore, there are few drug options for treatment, decreasing the possibility of therapeutic success.

The infected silver catfish presented several clinical signs, such as lethargy, skin darkening with depigmentation areas, cutaneous ulcers, and the presence of a yellow mucus. Internally, these animals presented splenomegaly, pale and friable liver, and gallbladder distension. Figure 1 exemplifies some of these external findings.

Figure 1: (a) Healthy silver catfish; (b) Silver catfish infected with Citrobacter freundii showing skin darkening with depigmentation areas and cutaneous ulcers. Photo: Guerino Bandeira Junior.

Regarding the hematological effects of this bacteriosis, it was verified that chronic infection leads to arregenerative anemia and leucopenia. In relation to the histology, liver degeneration, decrease in the amount of renal hematopoietic tissue, and the presence of melanomacrophage centers (MMCs) in the spleen and cephalic kidney of infected fish were reported. The cephalic kidney is the most important fish hematopoietic organ. Therefore, it can be concluded that C. freundii infection in silver catfish can affect organs essentially involved in hematopoiesis and the immune system, causing histological and hematological alterations that contribute to the pathophysiology of the disease and the high mortality of the affected animals.

Stressful events, such as transportation and excessive or inappropriate management, lead to the release of hormones such as catecholamines and cortisol, which are immunosuppressive, predisposing fish to infections by opportunistic bacteria. Therefore, we emphasize the importance of proper management of these animals, avoiding unnecessary stress, in order to prevent infections by opportunistic bacteria. As future prospects, we can highlight the importance of conducting research on the discovery of new antimicrobial drugs effective in combating this bacteriosis.