MFR PAPER 1328

Bacterial Diseases ofFishes and Their Control

S. F. SNIESZKO

In the contemporary approach to­ward understanding communicablediseases, epidemiology or epizootiol­ogy plays an important role in attemptsto explain relationships between thehosts, pathogens, environments, andoutbreaks of diseases (Wedemeyer etal., 1976). Discussing this relationshipseems to be my favorite hobby. It canbe represented in the form of an equa­tion: H + P + 52 =D.

In this equation, H represents thespecies and strain of the host, its age,and inherited susceptibility to any par­ticular disease; P represents the patho­gen causing the disease with all its var­iability; and 5 represents stress of theenvironment. The square of 5 is usedbecause the stress caused by environ­ment increases in geometrical progres­sion when the conditions are approach­ing the limits of tolerance by the host.The D represents the disease which re­sults if the three components listed onthe left side of the equation are in properqualitative and quantitative propor­tions. In discussing bacterial diseasesof aquatic animals, one must alwaysconsider the role of all three factors.

Numerous species of bacteria arecapable of causing diseases in fish andshellfish. Many are well known (Bul­lock et aI., 1971). Unfortunately, whenone tries to identify and name them,one is confronted with problems."Bergey's Manual" (Buchanan andGibbons, 1974) is recognized as themost authoritative text on bacterialtaxonomy; however, taxonomy is be-

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coming a separate branch of bacteriol­ogy that almost exists for its own sake.

In each new edition of "Bergey'sManual," a considerable number ofchanges are made, as would be ex­pected. The new eighth edition listsmany changes of names of bacteria thatcause diseases in fish and shellfish. Al­though such changes are unavoidable,they have resulted in a number of namechanges for several species of fishpathogens during recent years. Con­sequently, it is sometimes difficult toestablish the identity of a fishpathogenic bacterium described onnumerous occasions during the span ofonly a few decades.

The Fish Health Section of theAmerican Fisheries Society hascharged an ad hoc committee with thetask of attempting to clarify thetaxonomic problems. Fish pathologistshave found that the current edition of"Bergey's Manual" does not recog­nize such well-known names asAeromonas Liquejaciens, or the fishpathogenic myxobacteria (exceptChondrococcus columnaris, whosegeneric name has been changed toFlexibacter). Gaffkya homari has beenchanged several times and is nowAerococcus viridans, supposedly iden­tical with the human pathogen causingsubacute endocarditis (Endocarditislenta) (Stewart, 1978). This synonymyis difficult to accept for anyone withexperience with these pathogens whenisolated from lobsters or humans. Alsomissing from the Manual are

S. F. Snieszko is with the NationalFish Health Research Laboratory,National Fisheries Center-Leetown,U.S. Fish and Wildlife Service,Kearneysville, WV 25430.

Hemophilus piscium and Pasteurellapiscicida.

So much for the bacteria; now aboutthe hosts. If one takes into considera­tion the number of species ofpathogenic bacteria and the number ofspecies of fish and shellfish that maybecome infected with these bacteria,the number of bacterial species knownto date is relatively small.

The specific susceptibility of aquaticanimals to various bacteria can be ob­served only in the species that are cul­tured, or at least kept in captivity forawhile. In general, marine fishes aresusceptible to diseases caused by Vib­rio anguillarum, and freshwaterspecies to those caused by Aeromonashydrophila and A. salmonicida. Gaff­kya homari (A erococcus viridans)seems to be an exclusive pathogen oflobsters, unless one accepts the possi­bility that this organism is identicalwith the one causing subacute endocar­ditis in humans.

It seems that the species of the host isnot the only significant factor in deter­mining the species of bacterial patho­gen involved; environment seems to beimportant also. For example, outbreaksof diseases caused by A. salmonicidahave often been reported in nonsaJ­monids that were exposed to infectionat a proper range of temperature. Vib­riosis in marine and estuarine fishes isusually associated with crowding andincreased temperatures. Spring viremiaof carp and bacterial hemorrhagic sep­ticemia (Fijan, 1972), caused by A.hydrophila, are associated with thespring season, temperature, manage­ment practices, and water pollution.Kidney disease of freshwater salmonidsis associated with the nature of thewater supply, and possibly the presenceof external parasites (Warren, 1963).Mycobacteriosis of several species ofPacific salmon was caused by feedinginfected viscera ofraw salmon, some of

Marine Fisheries Review

MFR Paper 1328. From Marine Fisheries Review, Vol. 40, No. 10, October1978. Copies of this paper, in limited numbers, are available from 0822, UserServices Branch, Environmental Science Information Center, NOAA, Rockville,MD 20852. Copies of Marine Fisheries Review are available from the Superin­tendent of Documents, U.S. Government Printing Office, Washington, DC20402 for $1. 10 each.

which contained mycobacteria (Woodand Ordal, 1958). A catastrophic out­break of pasteurellosis of white perch,Marone americana, and striped bass,M. saxatilis, in the Chesapeake Bay in1963 was probably initiated by en­vironmental stress of a hot, dry summer(Snieszko et al., 1964). The samepathogen causes diseases in fish cul­tured in cages in Japan and is alsoknown in the Gulf Coast area.

One could give more examples on therole of environment in the health anddisease of fish and shellfish. This is notnecessary, however, because the su b­ject was recently treated in detail byWedemeyer et al. (1976).

Knowledge of bacterial diseaseswould have little practical value if it didnot result in improved control of thesediseases. In control offish diseases, oneshould avoid ideas based too much onanalogies with diseases of humans anddomestic animals. Fish and shellfish areaquatic and poikilothermic, and theseare important differences.

What means do we have for controlof bacterial diseases of aquatic ani­mals? The first one is avoidance of ex­posure to pathogens. This is possible inrelatively few fish cultural facilities­only those having clean water. One ofthese fortunate areas is in the SnakeRiver Canyon in Idaho, where an al­most limitless supply of fresh waterfrom artesian wells is an effectiv'eshield for protection from infectiousdiseases. But, even in that area, condi­tions are changing for the worse due toincreasing reuse of water and transfer ofinfected fish.

Much pond fish culture and marineculture of fish and shellfish takes placein open waters where fish pathogens arepresent. Under such conditions, avoid­ance is not possible, and use of chemo­therapeutic drugs is expensive andtransitory. Immunization is a recent andgrowing control measure that is provinghighly promising (Anderson, 1974;Fryer et aI., 1(76). Although immuni­zation of humans is the most effectivemeans of disease control, thc immuneresponse in fishes is dependent ontemperature and that in shellfish is still amystery that somehow works very wellin nature.

Immunization of cold-blooded ver­tebrates was started many years ago,but little progress was made because theresponse was negligible at low tempera­tures and the necessity of handling eachanimal made it impractical on a largescale. Oral immunization of salmonidswas introduced by D. C. Duff ofCanada about 40 years ago, but withonly limited success. Only during thelast 10 years has sufficient progressbeen made by G. W. Klontz, J. L.Fryer, D. F. Amend, D. P. Anderson,and others to make it effective, butmainly under laboratory conditions(Fryer et aI., 1976). However, duringthe lastS years, immunization offish byoral administration of antigen and byimmersion of fish, with or without pre­conditioning, is becoming so promisingthat two commercial laboratories in theUnited States are now manufacturingfish vaccines.

A commercial vaccine for immuniza­tion against enteric redmouth disease(ERM) is also available, and a vaccineagainst Vibrio anguillarum has beenlicensed. Other bacterial vaccines areunder development and antiviral vac­cines will most likely be available also.

Outbreaks of bacterial diseases areinfluenced by the susceptibility of thehost, virulence of the pathogens, andquality of the environment (Wede­meyer et aI., 1976). Therefore, controlof diseases of fish and shellfish isprimarily a managerial problem. It isvery important for aquacultural mana­gers to have a thorough understandingof biology, physiology, microbiology,immunology, ecology, and therapy. Itis necessary for managers to be prop-

erly trained and capable of makingproper evaluations of the disease prob­lem. Reduction of losses caused bybacterial and other diseases of fi sheswill require integration and evaluationof all that is known about the nature ofthe disease, and proper and timely ap­plication of control measures.

LITERATURE CITEDAnderson, D. P. 1974. Book 4: Fish immunology.

In S. F. Snieszko and H. R. Axelrod (editors).Diseases of fishes, 239 p. T. F. H. Publica­tions, Neptune, N.J.

Buchanan. R. E.. and N. E. Gibbons (editors).1974. Bergey's manual of determinative bac­teriology. 8th ed. The Williams & WilkinsCo., Baltimore, 1268 p.

Bullock, G. L., D. A. Conroy, and S F.Snieszko. 1971. Book 2A: Bacterial diseasesof fishes. IllS. F. Snieszko and H. R. Axelrod(editors), Diseases of fishes. 151 p. T. F. H.Publications. Neptune, N.J.

Fijan. 1'\. N. 1972. Infectious dropsy in carp-adisease complex. III L. E. Mawdesley-Thomas(editor), Diseases of fish, p. 39-5 I. Symp.Zool. Soc. Lond .. No. 30. Academic Press,Lond.

Fryer, J. L., J. S. Rohovec, G. L. Tebbit, J. S.McMichael, and H. S. Pilcher. 1976. Vaccina­tion for control of infectious diseases in Pacificsalmon. Fish Pathol. 10:155-164.

Snieszko, S. F, G. L. Bullock. E. Hollis. and 1.G. Boone. 1964. Pastellr"l!a sp. from anepizootic of white perch (RvcclIs americanlls)in Chesapeake Bay tidewater areas. J. Bac­teriol. 88:1814-1815.

StewaJ1, 1. E. 1978. Diseases and defensemechanisms of the lobster, Homarllsamericanlls. Mar. Fish. Rev. 40( 10):4-5.

Warren, J. W. 1963. Kidney disease of salmonidfishes and the analysis of hatchery waters.Prog. Fish-Cull 25: 121-131.

Wedemeyer, G. A .. F. P. \1eyer, and L. Smith.1976. Book 5: Environmental stress and fishdiseases. III S. F. Snieszko and H. R. Axelrod(editors), Diseases of fishes, 192 p. T. F. H.Publications, "leptune, N.J.

Wood, J. W., and E. 1. Ordal. 1958. Tuber­culosis in Pacific salmon and steelhead troulFish Comm. Oreg., Contrib. 25, 38 p.

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