Administration of dehydrated oxytetracycline effectively reduces francisellosis mortality in Nile tilapia

Abstract

Francisellosis is a disease responsible for severe economic losses in aquaculture. Currently, this disease is controlled via biosecurity measures and treatment of affected batches. In this study, tilapia juveniles were challenged with Francisella orientalis via immersion and fed with oxytetracycline‐medicated feed, at 100 mg kg fish−1 and 200 mg kg fish−1, under prophylactic and therapeutic strategies for 16 and 15 days respectively. All doses and treatment strategies reduced the fish mortality. The detection of the pathogen from the spleen was lower in the treated groups. Histopathology analysis showed that challenged fish that received the highest dosage presented fewer granuloma in their spleens and less inflammatory infiltrate in their livers than untreated fish. Fish that received the prophylactic treatment, regardless of the dose, presented fewer granuloma in their livers than untreated fish. Fish treated with 100 mg kg fish−1 had better scores for hepatocyte accumulations than fish treated with the higher dosage. Furthermore, 20% of the unchallenged fish that received the higher dosage presented mild inflammatory infiltrate in their livers. No differences were observed in the serum hepatocyte lesion enzyme quantifications. The results indicate that oxytetracycline‐medicated feed is safe, even at the higher dose, and effective at controlling francisellosis in tilapia.

Therapeutic efficacy of enrofloxacin in treatment of Francisella orientalis infections in juvenile Nile tilapia (Oreochromis niloticus L.)

“…Further evidence for these studies is the fact that FLO, which is commonly used to treat francisellosis in the field, is not effective in controlling the disease in Nile tilapia (O niloticus) farms because the pathogen has a propensity for latency (Soto et al, 2010), and there are descriptions of treatment failures with this drug (unpublished data). This study is not only the first report on the therapeutic efficacy of enrofloxacin in the treatment of Nile tilapia (O. niloticus) fingerlings affected by francisellosis but also confirms the data in the literature on the efficacy of OTC in controlling mortality in cases of acute F. orientalis infection (Favero et al, 2021). OTC and ENR differ in their antibacterial efficacy.…”

“…This enables the pathogen to remain in the fish population, resulting in disease recurrence. The occurrence of a carrier state after treatment with OTC was also confirmed against F. orientalis by Favero et al (2021) and other fish pathogens, including Flavobacterium psychrophilum (Bruun et al, 2003) and Streptococcus agalactiae (Darwish et al, 2002;Faria et al, 2013). The presence of disease carriers complicates any attempt to control the disease at the farm level.…”

“…The therapeutic efficacy of florfenicol (FLO) and oxytetracycline (OTC) in treating francisellosis in Nile tilapia ( O niloticus ) has been demonstrated. FLO used at doses of 15 and 20 mg/kg LW for 15 days (Soto et al, 2013), and OTC at doses of 100 and 200 mg/kg LW for the same period (Favero et al, 2021), reduced the mortality rate caused by F. orientalis infection. However, none of these antimicrobials proved to be efficient in eliminating pathogens from the host (Favero et al, 2021; Soto et al, 2013).…”

“…Some producers have obtained positive results with doses up to three times higher than those recommended by the manufacturer (100 mg/kg of LW; TM700, Phibro) while other farmers reported that the use of OTC has been ineffective. Recently, Favero et al (2021) reported the only study describing OTC as effective in controlling the mortality of Nile tilapia ( O. niloticus ) infected with F. orientalis under experimental conditions at doses of 100 and 200 mg/kg body weight. However, high carrier rates were also observed in animals treated with the drug.…”

Investigating the Effect of an Oxytetracycline Treatment on the Gut Microbiome and Antimicrobial Resistance Gene Dynamics in Nile Tilapia (Oreochromis niloticus)

“…Oxytetracycline is a natural broad-spectrum antibiotic extensively used in global aquaculture due to its limited side effects in the host and broad-spectrum activity against both Gram-negative and Gram-positive bacteria, through the inhibition of protein syn-thesis [11][12][13]. In tilapia, OTC has been shown to successfully treat a range of bacterial diseases including Francisellosis, motile Aeromonas septicemia, and Streptococcosis, which are significant disease challenges for this sector [14][15][16]. Across the global aquaculture industry, OTC is often administered orally in the diet during disease outbreaks, where doses range 50 to 250 mg/kg/day for typically between four and eight days, up to 21 days, depending on the fish species and farming country [17,18].…”