Best Treatment Practices of Warmwater Fish Pathogens Using Feed Medicated with Aquaflor

Introduction

Aquaflor (florfenicol) is a fast-acting, palatable, broad-spectrum antibiotic provided as a feed premix. It is proven to be an effective and safe antibiotic for use in fish. To maintain the efficacy of this valuable antibiotic and obtain maximal benefit from feed medicated with Aquaflor, producers and diagnosticians must understand the rationale behind the label indications and they must work cooperatively.

Aquaflor is approved for use in finfish in over 20 countries. Indications differ from country to country but include control of mortality due to diseases associated with the warmwater bacterial pathogens Edwardsiella ictaluri, Streptococcus iniae, Streptococcus agalactiae, Flavobacterium columnare, Francisella asiatica and Aeromonas hydrophila.

In the US, the approval of Aquaflor for use in fish involved many experimental efficacy and safety studies required by the United States Food and Drug Administration, a US regulatory body. In the US, Aquaflor is currently approved for control of mortality due to enteric septicemia of catfish, furunculosis and coldwater disease in salmonids. Aquaflor-CA1 (florfenicol) is conditionally approved for control of mortality due to columnaris disease in catfish.

The objective of this paper is to explain the best practices for incorporating Aquaflor into overall disease management programs for warmwater fish production.

Efficacy Studies

In vivo and in vitro efficacy studies with Aquaflor have been conducted.

In vitro efficacy studies: The sensitivity of bacterial pathogens to florfenicol (FFC) has been assessed by determining the minimal inhibitory concentrations (MIC) of FFC for bacteria from both experimental studies and from diagnostic specimens obtained from disease outbreaks on farms. This laboratory assay quickly determines the susceptibility of bacteria to drug.

MIC values for warmwater fish pathogens of interest are presented in Table 1. The values indicate that a wide variety of warmwater bacterial pathogens are susceptible to FFC. MIC values correlated with data from pharmacokinetic studies can be used to predict and validate the clinical performance of Aquaflor on farms.

In vivo studies: Experimental Aquaflor efficacy studies in warmwater fish were conducted at a dose rate of 10 mg FFC/kg bodyweight against the pathogens E. ictaluri and F. columnare in catfish (Ictalurus punctatus) fingerlings. Aquaflor was highly efficacious (p < 0.001) when compared to controls (Figures 1 and 2).

In 2009, high mortality in market-size catfish (~2 lbs or 0.907 g) associated with A. hydrophila occurred on several farms in Alabama, USA. Aquaflor was used to contain the outbreak. There were no untreated controls in these studies for both animal welfare and economic reasons. Nonetheless, comparison of mortality before and after treatment with feed medicated with Aquaflor showed a dramatic decrease and, in some cases, complete cessation of mortality (Figure 3).

Pharmacokinetic and Residue Depletion Studies

Pharmacokinetic studies were conducted to determine the disposition of FFC in warmwater fish (reared in freshwater) and to confirm the dose rates determined by efficacy studies. After oral dosing of catfish at 10 mg/kg (Figure 4) and tilapia at 15 mg/kg, a high concentration of FFC was absorbed quickly from the intestine, rapidly disseminated throughout the body and maintained at a steady concentration during the 10-day dosing period.

Tissue concentrations of FFC in catfish and tilapia during the 10-day treatment under the specified experimental conditions were greater than the MIC values of five pathogenic bacteria studied (Table 1). This tissue concentration was considered sufficient to effectively combat the pathogenic bacteria.

In a single oral-dose study conducted with catfish, the mean concentration of Aquaflor in plasma declined below 1 ?g/mL by 36 hours; this concentration is below the highest MIC efficacy value for most catfish pathogens (Table 1). Therefore, it is critical to continue daily dosing (every 24 hours) for 10 days to achieve and maintain an efficacious dose at the rate necessary to combat bacterial infections.

Residue depletion studies were conducted to determine the calculated withdrawal time, which is the time between the last dose of FFC and the time when the drug residue levels fall below the 1 ?g/g tolerance. The results support a withdrawal time of 12 days for warmwater fish following treatment with Aquaflor administered according to label directions.

Best Treatment Practices

Feed medicated with Aquaflor should be used in conjunction with proper husbandry, available vaccines and genetically improved fish. Good fish husbandry includes environmental management, attention to stocking density, biosecurity and good recordkeeping.

The ideal aquaculture environment is maintained at optimal water temperatures, dissolved oxygen and chemistries for each warmwater species. Although emphasis is placed on increased stocking rates to maximize production, fish should be stocked near, but not in excess of, the maximal capacity to avoid poor water quality, suboptimal dissolved oxygen concentrations and traumatic injury. Any of these factors could stress and predispose fish to disease.

Biosecurity measures such as footbaths and cleaning, disinfecting and rinsing equipment will help prevent the spread of infectious agents on the farm. Diseases can be transmitted by humans, predators and scavengers, equipment and water. Although predators and scavengers are difficult to deal with, the control of other vectors is manageable. Newly introduced fish should be quarantined (and treated if necessary) for 3 to 6 weeks before introduction into the growout facilities on farms that share common water systems.

Proper farm management requires managers to maintain fish health records that help enable recognition of diseases and their proactive management. Important farm records include the seasonality of outbreaks, fish eating patterns and behavior. For example, if fish become anorexic during summer when water temperature ranges from 24 C to 28 C (75 F to 82 F), the problem pathogen is more likely to be S. agalactiae rather than S. iniae.

Producers should submit live, sick fish to a diagnostic laboratory for examination of lesions and culture of the suspected pathogenic bacterium. Dead fish often have been contaminated with additional bacteria that are not the source of a disease outbreak, which is why live, sick fish should be examined and lesions cultured; the results must be carefully interpreted to avoid an incorrect diagnosis.

It is advantageous for producers to stock robust fish strains bred for fast growout and high survival rates. Genetically improved stock fed optimal diets yield high quality alevins resulting in larger, more disease-resistant juveniles that perform well in the field.

Prevention of disease is increasingly becoming a reality due to the availability of warmwater fish vaccines, which vary from country to country. Vaccines against E. ictaluri and F. columnare (AquaVac-ESC and AquaVac-COL) in catfish are commercially available in the US; AquaVac Strep Sa is the first-ever intraperitoneal vaccine for tilapia and protects against S. agalactiae Biotype II, an important cause of streptococcosis. However, vaccinated fish can still succumb to pathogens if they are stressed and if husbandry is less than optimal.

Use Aquaflor at the First Sign of Disease, According to Label Directions

Feed medicated with Aquaflor should be administered as soon as bacterial disease is recognized in fish. A delay of just a few days after the onset of disease signs can mean the difference between successful therapy and treatment failure.

Important bacterial pathogens of freshwater tilapia include S. iniae, S. agalactiae, A. hydrophila, F. asiatica, F. noatunensis and F. columnare. Fish infected with the first four of these pathogens will quickly lose their appetites, which decreases medicated feed intake.

Experimental studies of F. columnareinfected catfish demonstrate that the bacterial infection does not cause anorexia in catfish.8 However, anecdotal reports indicate that appetite is affected in columnaris disease under field conditions when the mouth becomes extremely necrotic. As stated previously, FFC tissue concentration must exceed its MIC value for the bacteria during the treatment period. Fish that are not eating or only eating small quantities will be inadequately medicated.

Lethargic fish will likely have lesions that are more indicative of a given disease than are clinical signs. Streptococcosis in tilapia is most frequently caused by S. agalactiae and S. iniae. External lesions include exophthalmia bulging eyes with corneal opacity (Figure 5), darkening of the skin, necrosis of gills and hemorrhage of the skin, opercula, vent and muscles. Internal lesions include fluid in the coelomic cavity, hemorrhage and enlargement of internal organs as well as inflammation in joints and the heart.

A. hydrophila in tilapia is characterized by external lesions of hemorrhage, ulceration and necrosis of the skin, base of the fins and occasionally the muscle. Exophthalmia is occasionally reported with this disease. Internal lesions include bloody fluid in the coelomic cavity and hemorrhage in the organs.

Francisellosis in tilapia is caused by F. asiatica and F. noatunensis. Typical lesions of Fransicella spp. infection include exophthalmia, whitish nodules (which represent areas of necrosis) in the gills, skin ulcerations (from secondary infections) and enlarged internal organs, especially of the spleen and kidney, with whitish necrotic nodules (Figure 6).

Columnaris disease caused by F. columnare in tilapia is characterized by external lesions of ulceration and necrosis of the gills, mouth, skin, muscle and fins, giving a ragged appearance of the fin rays caused by sloughing of the epithelium.

Because Aquaflor requires a prescription in many markets, such as the US and Brazil, a veterinarian or other fish health specialist should examine affected fish and determine that they are sick based on the signs of disease, lesions or the results of diagnostic tests such as bacterial culture.18 If the fish have a bacterial disease that is treatable with Aquaflor, the veterinarian will issue a medicatedfeed order. Dosing should be followed according to label directions, using the effective approved dose rate for 10 days duration. A dose rate of 10 mg/kg bodyweight in catfish is efficacious against E. ictaluri and F. columnare; in tilapia alevins and juveniles, this dosage has been shown to be efficacious against F. columnare, A. hydrophila and S. agalactiae.

Medicated feed should be purchased from a feed mill that follows Good Manufacturing Practices to ensure quality, purity and the indicated label concentration. For practical reasons, the Aquaflor feeding rate (calculated as a percentage of bodyweight) for medicated feed may be predetermined by commercial feed mills on the basis of local feeding practices. Custom blends can be produced by farm-mixing where permitted.

Photo courtesy of Dr. Juan A. Morales, Escuela de Medicina Veterinaria, Universidad Nacional de Costa Rica; and Dr. Esteban Soto, Ross University School of Veterinary Medicine, St. Kitts

It is imperative that producers feed the full regimen of feed medicated with Aquaflor and that feed medicated with Aquaflor is fed as the sole ration during the treatment period. If the farmer is uncertain that the medication will work, bacterial isolation and susceptibility testing must be performed at a diagnostic laboratory.

Skipping treatment days or administering feed medicated with Aquaflor for fewer days than recommended will lead to wasted resources. In addition, lower tissue concentrations of the drug in fish can lead to the selection of resistant bacteria, and resistance to one medicated feed can lead to resistance against other medicated feeds, eventually leaving the farmer with fewer options for bacterial treatment.

Alternatively, producers may be tempted to administer medicated feed excessively because Aquaflor is highly palatable, but this will only lead to higher tissue concentrations than necessary, which wastes resources. Producers should adhere to recommended dosages and required withdrawal times to prevent prohibitive residues.

Conclusion

Aquaflor is effective against aquatic warmwater bacterial infections such as F. columnare, S. iniae, S. agalactiae, A. hydrophila and E. ictaluri. Fish showing early signs of disease associated with these pathogens should be examined for lesions and cultured at a diagnostic laboratory to identify the bacteria.

It is imperative that producers use feed medicated with Aquaflor according to label directions to ensure maximal efficacy, preserve bacterial susceptibility and prevent prohibited tissue residues. When used in conjunction with good husbandry, vaccines and improved genetic stock, judicious use of Aquaflor is a valuable tool for controlling warmwater fish mortality from bacterial infections.

September 2012