SRAC Publication No. 478

VI September 1998 PR

Aeromonas Bacterial Infections Ñ Motile Aeromonad Septicemia

A.C. Camus1, R.M. Durborow2, W.G. Hemstreet3, R.L. Thune1 and J.P. Hawke1

Bacterial infections, caused by ceptible to infection. Under cer- Factors causing motile members of the genus tain conditions mortalities can disease outbreaks Aeromonas, are among the most approach 100 percent. common and troublesome dis- Aeromonas infections also occur Motile aeromonads are among the eases of fish raised in ponds and in other vertebrates, including most abundant bacteria found in recirculating systems. The wide- frogs, turtles, alligators and, fresh water aquatic environments. spread distribution of these bacte- sometimes, humans. They also occur in brackish ria in the aquatic environment waters, but are found less fre- and the stress induced by inten- Cause quently as salinity increases above sive culture practices predisposes 15 parts per thousand (about half fish to infections. Motile aeromon- Aeromonas hydrophila, A. sobria, the strength of seawater). ad infections have been recog- A. caviae, and possibly other aero- Aeromonads are facultative, nized for many years and have monads, are capable of producing which means they are capable of been referred to by various disease in fish. While all members utilizing nutrients present in names, including motile aeromon- of this group are small, motile, water and surviving for long peri- ad septicemia (MAS), motile gram-negative, rod-shaped bacte- ods in the absence of host fish. aeromonad infection (MAI), hem- ria and all share certain biochemi- They occur in the greatest num- orrhagic septicemia, red pest, and cal characteristics, their scientific bers in organically rich waters, red sore. In this publication, they names are constantly under revi- such as those found in ponds and are referred to simply as sion and subject to change in the other aquaculture systems. These aeromonas infections. Aeromonas future. Numerous strains of these bacteria can also be isolated from bacteria causing these infections bacteria exist, and they vary great- the skin and intestinal tracts of are called aeromonads. ly in their ability to cause disease. healthy fish, from pond mud, Whether acting alone or in mixed In general, strains isolated from aquatic plants and certain proto- infections with other organisms, the environment are less patho- zoan parasites. These factors make the motile aeromonads are genic than those isolated from dis- the elimination of this group of responsible for significant finan- eased fish. The marked genetic bacteria from fish rearing systems cial losses annually. All species of diversity among different aero- impossible. fish, scaled and unscaled, are sus- monad strains has made it diffi- Aeromonads are considered to be cult to develop effective vaccines. opportunistic pathogens, capable A non-motile aeromonad (Aero- of producing disease only in 1School of Veterinary Medicine, Louisiana monas salmonicida), not discussed State University weakened populations of fish or 2 here, produces severe losses in as secondary invaders in fish suf- Cooperative Extension Program, salmon, trout, goldfish and koi. Kentucky State University fering from other diseases. 3Alabama Fish Farming Center Aeromonas infections in large- may be seen in the skin only, as an of depigmentation (paleness) that mouth bass and other scaled fish internal systemic disease (sep- can develop anywhere on the are sometimes associated with ticemia), or as a combination of body surface. The skin overlying infestation by the protozoan para- both. Outbreaks may be chronic these sites is eventually lost, site Epistylis sp. Environmental (long-term) and affect only small exposing the muscle below (Fig. stress factors, particularly those numbers of fish or may produce 2). These open sores or ulcers may associated with poor water quali- acute (intense and short-term) remain superficial or they can be ty conditions, enhance the devel- infections accompanied by rapidly extensive and invade deeply into opment of disease. These factors increasing, high mortality rates. muscle, revealing underlying include high water temperatures, In unscaled fish (e.g., catfish), bone in some cases (Fig. 3). These high ammonia and nitrite levels, there is often fraying and redden- ulcers often have ragged white pH disturbances, and low dis- ing of fins (Fig. 1), accompanied margins bordered by a narrow solved oxygen levels. Heavy par- by irregular, variably sized areas zone of hemorrhage. In scaled fish asite burdens, overcrowding, high organic loads in the water, spawning activity, seining activi- ties, rough handling and trans- port also may lead to outbreaks of disease. Serious episodes of stress, such as oxygen depletion or cases of brown blood disease (caused by nitrite toxicity), often are followed by outbreaks of aeromonas infection within a week. Aeromonas infections are more common in warmwater and tem- perate species than in coldwater fish. Infections can occur in any age fish, but losses are usually most severe in fry and small fin- gerlings. Outbreaks are usually seasonal, with peaks in the spring Figure 1. Channel catfish with severe erosion of the anal and caudal fins caused to early summer and in the fall by Aeromonas sobria. Skin at the fin bases is swollen and bright red from hem- when water temperatures are orrhage. (Photo by Al Camus) between 65 to 85o F. Spring out- breaks may be related to decreased disease resistance in fish that are in poor condition from overwintering or after spawning. Extensive handling and transport of young fish in the fall also may cause outbreaks. Aeromonas infections do not fol- low strict temperature ranges and have been reported during every month of the year. Although rarely found in the winter, aeromonads have been isolated from lesions associated with win- ter kill (winter fungus). Clinical signs or symptoms Signs of disease associated with aeromonas infection are non-spe- cific and may be easily confused Figure 2. Channel catfish with a shallow skin ulcer caused by Aeromonas with other diseases. Infections sobria. Margins of the ulcer are white and ragged. Muscle tissue can be seen vary greatly in appearance and beneath the ulcerated skin. (Photo by John Hawke) Skin lesions caused ease. Losses from aeromonas by aeromonads infections seldom exceed 50 per- often have fungus cent; however, mortality is strong- or columnaris bac- ly influenced by the general teria present. Fish health status of the fish popula- affected only with tion, stress level, and virulence of skin lesions may the particular bacterial strain continue to feed infecting the fish. Mortalities occa- and survive for sionally approach 100 percent in extended periods, fry and small fingerlings. despite the presence of severe ulceration Diagnosis (Fig. 3). Daily mor- talities associated Because aeromonas infection may with this chronic mimic other diseases, sick fish Figure 3. These channel catfish infected with form of disease may should be submitted to a diagnos- Aeromonas hydrophila, which caused complete erosion be low, but can rise tic laboratory for a complete eval- uation that includes bacterial of the caudal peduncle, were captured alive. In this partic- to high levels over time. identification and antibiotic sensi- ular case, the disease ran its course in this pond, mortali- tivity testing to determine which ties stopped, and no treatment of the surviving catfish The internal or sep- antibiotic will best treat the infec- population was needed. (Photo by Bob Durborow) ticemic form of dis- tion. Motile aeromonads grow on ease typically fol- most common culture media in 24 lows a more acute (e.g., largemouth bass), skin hours, but complete identification course with a sudden onset of rel- lesions begin as small hemor- and antibiotic sensitivity testing atively high mortalities. Affected rhages within scale pockets will usually require an additional fish usually do not eat and com- (Fig. 4) that can rapidly expand to 24 to 48 hours. monly will be seen swimming larger areas. Affected scales are lazily near the waterÕs surface or Several fish, showing signs typical eventually lost and ulcers form. in shallow areas of a pond. If dis- of the diseased population as a Aeromonas infection may also turbed, the fish move into deeper whole, should be collected, sealed include any or all of the following water, but typically return to the in plastic bags without adding external signs: exophthalmia surface within a short period of water, and placed in a cooler sur- (popeye), abdominal distention time. The internal organs may be rounded by ice or ice packs (see (swelling of the abdomen), and enlarged, reddened or pale, or SRAC publication number 472, pale gills. Scaled fish often accu- have a mottled pattern of deep Submitting a Sample for Fish Kill mulate edema (fluid) in their scale red hemorrhage interspersed with Investigation). Also, a water sam- pockets. This condition, called pale areas of tissue destruction or ple from the pond or culture sys- lepidorthosis, creates a roughened necrosis (Fig. 4). Organs with sig- tem should be submitted in a or bristled appearance. nificant tissue necrosis become clean container. Whenever possi- weak and are easily ble, moribund fish (alive but near damaged when han- the point of death) should be col- dled. The intestinal lected. Avoid submitting fish that tract typically will be are obviously decomposed or devoid of food, red- have been floating dead. Well-iced dened, and filled fish collected fresh can usually be with cloudy yellow used for diagnostic purposes for or bloody fluid and about 24 to 48 hours. Do not mucus. The freeze samples; freezing seriously abdomen may be hinders diagnostic evaluation. filled with clear, Consult your local diagnostic lab- cloudy or bloody oratory for specific details con- fluid. The gall blad- cerning sample submission. der will be filled Figure 4. Skin hemorrhaging under the scales is especial- with large amounts Prevention and treatment of green bile. Highly ly apparent in the caudal peduncle of this largemouth Whenever aeromonas outbreaks virulent (deadly) bass infected with Aeromonas hydrophila. The pale occur, every attempt should be strains may cause made to identify and eliminate white liver in the front of the body cavity and the mot- sudden mortalities sources of environmental stress. tled kidney at the top rear of the cavity are clinical signs with few external or This alone will often correct the of internal aeromonas infection. (Photo by Bob internal signs of dis- disease problem. Avoid handling Durborow) fish when they are in a weakened body weight per day, while a 1.25 feed. The recommended dosage state or when environmental con- g/lb formulation is fed at 2 to 3 is 10.1 g of the Romet B¨ premix ditions are less than optimal. Fish percent body weight per day. The per 100 pounds of fish per day for should never be handled or trans- FDA has established a 21-day 5 days. The amount of feed to be ported during an aeromonas out- withdrawal period following fed (calculated as a percent of break. Terramycin¨ administration body weight) for various concen- trations is listed in the table Chemical treatments with potas- before fish or salmonids can be marketed for human consump- below. sium permanganate (KMnO4), at a rate of 2 to 4 parts per million tion. The Romet B¨ is first mixed with (milligrams/liter), are sometimes Romet¨ medicated feed, available corn oil or 5 percent gelatin (1 gal- useful in the treatment of infec- as a floating pellet, is fed at a rate lon of oil per 200 pounds of feed), tions limited to the skin. Potas- of 23 mg of active drug per pound then is applied to a floating pellet- sium permanganate treatments of fish per day for 5 days, fol- ed feed to give a uniform coating are of greatest value when fish are lowed by a 3-day withdrawal (a cement mixer works well for feeding poorly or not at all, and period before catfish may be sold this). The coated feed should be medicated feed is not an option. for human consumption air-dried and used immediately or Potassium permanganate is (salmonids have a 42-day with- rebagged and stored for no more presently on deferred status by drawal period). Romet¨ may be than 6 months in a cool, dry envi- the Food and Drug Administra- milled into feed at concentrations ronment. The drug has a long tion; it may be used but is not ranging from 5.6 to 6.6 pounds of shelf life even after addition to officially approved. drug premix per ton of feed. The feed but the nutritional value of the feed will become degraded Systemic infections can only be amount of medicated feed to be with prolonged storage. No feed successfully treated by the use of fed daily will vary with the spe- should ever be used if it has medicated feeds containing antibi- cific formulation, but the dosage become moldy. otics. For medicated feeds to be of 50 mg/kg/day remains the effective, it is essential that an same. Palatability problems are Before any treatment is initiated ¨ early diagnosis is made and that associated with Romet feeds, an evaluation of feeding activity, the fish be fed as soon as possible, particularly at the higher concen- value of the fish, daily mortality, before the disease causes them to trations, and fish may initially and potential future losses should stop eating. Treatment with med- refuse to consume it. This prob- be weighed against the cost of icated feeds will not be effective if lem may be alleviated to some treatment. It should also be noted a large portion of the fish popula- degree by increasing the amount that the improper use of tion has already stopped feeding. of fish meal (for more desirable Terramycin¨ and Romet¨ med- Usually there is not enough time flavor) or by adding the drug to icated feeds has led to the emer- to wait for the results of antibiotic the feed at a lower concentration gence of aeromonad strains resis- sensitivity tests. Sensitivity test- and increasing the amount that is tant to one or both drugs. These ing, however, may indicate that a fed daily. Infected catfish finger- antibiotics should be fed at the different, more effective medi- lings are now commonly fed full therapeutic dose for the full ¨ cated feed should be used. Romet 30 formulated at 11.1 number of days recommended on pounds of premix per ton of feed the label. At lower-than-recom- Oxytetracycline (Terramycin¨) is (the tag on the bag will indicate mended doses and feeding rates, approved for control of motile the formulation). This particular the bacteria are better able to aeromonad infections in catfish. formulation is fed to the fish at 3 adapt to the drugs by mutating Sulfadimethoxine plus ormeto- percent of their body weight each and developing resistance to the prim (Romet¨) is approved for day for 5 days. Romet¨ is a heat- antibioticÕs lethal effects. controlling Edwardsiella ictaluri stable drug, so it can be formulat- (ESC) infections in catfish. ed into a floating pellet. Use of a Terramycin¨ medicated feed, floating pellet is available as a sinking pellet, is advantageous because Feed intake of fish Pounds of Romet B¨ administered at a rate of 25 to 37.5 it allows producers to (% body weight) premix per ton of feed observe feeding milligrams of active drug per 0.5 88.8 pound of fish per day for a period responses in diseased 1 44.4 of 10 days. Actual feeding rates fish populations. 2 22.3 vary according to the strength of Romet B¨ is the form 3 14.8 the medicated feed mixture. A of Romet¨ that can be 4 11.1 2.5 g/lb (grams of oxytetracy- 5 8.9 bought by individuals 6 7.4 cline/pound of feed) formulation to mix into their own is fed at a rate of 1 to 1.5 percent

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.