F. P. MEYER

MFR PAPER 1295

Incidence of Disease in Warmwater FishFarms in the South-Central United States

ABSTRACT-Case records for the years 1963-68 from the diagnostic laboratoryat the Fish Farming Experimental Station, Stuttgart, Ark., were studied to determineseasonal incidences andlor interrelationships between various disease conditions.Seasonal occurrences were recorded for Trichodina, Ichthyophthirius, Scyphidia,Plistophora, Coslia, Gyrodactylus. Dactylogyrus, Cleidodiscus, and Lernaea.Summer infections of Aeromonas hydrophila showed a relationship to periods ofoxygen depletion or low oxygen stresses. Graphs are presented identifying monthswhen disease problems are most likely to occur and methods of avoiding out­breaks are discussed. Relationships between occurrences ofparticular organismsand water temperature, spawning seasons, and other stress periods are noted.

The greatest incidence of disease was in April which had almost 50 percent morecases than JUly, the next highest month. Outbreaks occurred regularly throughoutthe period from March through July. Seventy-two percent of all cases occurredduring these months. Only Lernaea and myxobacterial infections had their greatestincidence outside this period.

Protozoans constitute the majoretiological agents and were involved in53 percent of the observed cases. Theirprimary effect is greatest on very youngfish; hence, the peak incidence of pro­tozoan epizootics occurs during andimmediately following spawningperiods of both shiners and catfish (Fig.I) .

Danger of parasitism is high fromMarch through July. Young fish arevulnerable to diseases of parasitic ori­gin and are involved in the majority ofcases. Since the danger period em­braces the spawning period of bothshiners and catfish, the importance ofhaving parasite-free broodfish cannotbe overemphasized. Treatments shouldbe made periodically throughout theyear to keep parasite burdens onbroodfish to a minimum. The effective­ness of prophylactic treatments is em­phasized by a lower incidence of parasi­tic diseases on fish farms whereprophylactic measures and sanitationwere practiced.

F. P. Meyer is with the NationalMarine Fishery Research Labora­tory, Fish and Wildlife Service, U. S.Department of the Interior, P. O. Box818, La Crosse, WI 54601.

INCIDENCE OFBACTERIAL INFECTIONS

AND OXYGEN DEPLETIONS

Parasitic diseases cause the greatestnumber of cases but their overalleconomic effect is much less than thatof bacterial disease (Fig. 2). Problemsof parasitic etiology are usually lessacute (although nonetheless serious)and the owner often has an extended

December. Fish which are not mark­eted at harvest are usually placed inholding ponds for winter storage, oftenunder heavily crowded conditions. Ap­parently. parasite burdens do not haveample time to develop to troublesomelevels in storage ponds until some timein January. The jump from three casesin December to 64 in January is be­lieved to be a direct reflection of winterstorage conditions (Meyer, 1970).

INCIDENCE OFPROTOZOAN INFECTIONS

potential danger to fish stocks. Thisperiod embraces the spawning seasonsof golden shiners (Notemigoniscrysoleucas) and catfish (Ictaluruspunctatus) and reflects the problems as­sociated with infections in or on veryyoung fish. Shiners spawn in lateMarch, April, and May. Catfish beginspawning in late May and may continuethrough June into July.

It is relevant that large numbers offish are handled during March andApril, either for marketing, for stock­ing of broodfish, or for stocking fingerl­ings in rearing ponds. This handlingstress is believed to be a factor in initiat­ing some of the problems during thistime of the year.

The low number of disease cases dur­ing December results from several fac­tors. The harvest of fish from rearingponds drastically reduces the number ofstocked ponds during November and

The incidence of disease on fishfarms has been documented by Meyer(1970) and Rogers et al. (1971). Aprilis the most troublesome month of theyear. Nearly one-fifth of all case his­tories recorded over a 5-year period atthe Fish Farming Experimental Station,Stuttgart, Ark., occurred during thisperiod. The number was one-thirdgreater than during July, the next high­est month. The season from I Marchthrough July is a continuous period of

Wannwater fish culture is, to a majorextent, restricted to pond culture.While the incubation of eggs and rear­ing of larval stages of some species maybe done in artificial systems, upwardsof 95 percent of all wannwater fish pro­duced in the United States are reared inpond facilities.

INCIDENCE OF DISEASEON FISH FARMS

38 Marine Fisheries Review

examine stocks of fry and small finger­lings for the presence of parasites. Fail­ure to do so can result in major losses.

Parasite problems encountered inyoung-of-the-year fish can, with fewexceptions, be traced to parasitizedbroodfish. Some farmers, who make noeffort to reduce the number of parasiteson adult fish prior to spawning, sufferlosses of I-hour-old-catfish fry due toTrichodina. Open-pond spawningmethods in which adults and theiroffspring are left in the same pondthroughout the growing season also re­sult in more parasite problems than thepractice of moving egg-laden mats (inthe case of shiners) to rearing ponds orthe use of artificial hatching units forcatfish.

Trichodina is the most commonlyencountered protozoan parasite and isespecially serious to young fish. Newlyhatched channel catfish fry have beenobserved to be so heavily infected withthis parasite that the fish do not survivebeyond I hour. Young golden shinerswhich are heavily infected usually failto feed. Trichodina also causes prob­lems to crowded fish held in winterstorage ponds, as evidenced by thenumber of cases during January.

Farmers who hold fingerlings andbroodfish in the same ponds often en­counter "Ich" disease. The occurrenceof wild fish in rearing ponds is consi­dered to be a primary factor in the intro­duction of Ichthyophthirius. Farmerswho are successful in preventing theentry of wild fish have fewer parasiteproblems.

This low oxygen stress is consideredto be a major factor in outbreaks whichoccur in rearing ponds and gives afurther indication of the relationship ofdisease to water conditions in theponds. Low oxygen levels in pondspass unobserved on many fish farmsand the role of low oxygen in initiatingbacterial infections is often unsus­pected. Periods during which low oxy­gen levels are common correspond tothe periods indicated on the graph foroxygen depletions. The number of oc­currences of sublethal oxygen deple­tions, however, is actually muchhigher.

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SCYPHIOIA

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reflects differences in hosts. Outbreaksin April are usually associated withspawning activity of golden shiners butmay also reflect temperature stresses assuggested by Snieszko (1954). Epizoo­tics in June, July, and August involvecatfish in rearing ponds and coincidewith periods when oxygen levels arelowest. The bacterial infections occurfrom 10 days to 2 weeks following theperiod when the fish have been sub­jected to stresses associated with lowlevels of dissolved oxygen. While totaldepletions cause mortalities due to suf­focation, oxygen levels of 3 ppm or lesswill cause severe stress.

The apparent coincidence of parasi­tic reproduction with fish spawning in­dicates that fish cuiturists should

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TRICHOPHRYA

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Figure I.-The monthly incidence of selected protozoan infections reponed onwarmwater fish farms during a 5-year period.

______ .. ONE CASE HISTORY

period during which diagnosis andtreatment are possible. This is not truein the case of bacterial disease. Thecourse of most bacterial diseases israpid and terminal unless early diag­nosis and prompt treatment are applied(Robinson et aI., 1970).

Next to the danger of oxygen deple­tion, bacterial disease must be consi­dered as a major threat to the successfulproduction of edible-sized fish.

Pseudomonas infections occur in­frequently throughout the year but arenot abundant enough to indicate anyperiodicity.

Aeromonas hydrophila infections aremost prevalent during June, July, andAugust, but a peak has also been notedin April. This two-peak occurrence

March 1978 39

Stresses associated with low oxygenlevels can be combated in a similarway. On well-managed fish farms, dis­solved oxygen levels are usually moni­tored regularly. Anytime the level hasdropped rapidly, the farmer shouldrealize that the fish have been subjectedto a significant stress which will likelylead to bacterial disease. Again, an­tibiotic therapy is indicated.

Losses of broodfish can usually betraced to physiological stresses andphysical abuse during the spawningseason. Feeding adult fish a medicatedration for a 10-day period prior to hand­ling or to the onset of spawning shouldreduce losses significantly. If this is notpossible, an intraperitoneal injection of25 mg of Terramycin t per pound ofbody weight will also combat Aero­monas infection.

While the role of oxygen levels in theincidence of A. hydrophila infectionsappears obvious, the carbon dioxiderole of spawning stresses, tempera­tures, pH changes, high carbondioxide, sulfides, and otherdecomposi­tion products associated with low oxy­gen levels should also be considered aspossible factors.

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GYRODACTYLUS

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DACTYlOGYRUS

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CLEIDOOISCUS

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LERNAEA

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A[ROMONAS LlQU£FACIENS

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MYXOBAC TERIA

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OXYGEN DEPLETIONS

~: ONE CASE HISTORY

Figure 2.-The monthly incidence of bacterial infections, oxygen depletions,Lernaea and monogenetic trematode infestations reponed on warmwater fishfarms during a 5-year period.

Myxobacterial infections are com­mon on golden shiners during theirspawning season and occur on channelcatfish fingerlings after handling inApril. A second and somewhat broaderpeak involves both species of fish dur­ing June, July, and August, again cor­responding closely to periods whenpond conditions are poorest and oxygenlevels are lowest. Although ice cover isuncommon in the fish farming area,outbreaks of myxobacterial infectionsoccur whenever ice persists for longerthan 7 days on storage ponds. De­teriorating pond conditions under theice are believed to be related to theonset of infections due to myxobac­teria.

40

ANTIBIOTICS USEDAS PROPHYLACTICS

Infections associated with stockingor handling can be reduced if handlingis delayed until the fish have received aregimen of antibiotic treatment. Al­though spring stocking may be delayedslightly, the increased survival rateshould more than justify the delay.Stresses due to handling may cause abacterial infection to progress from achronic to an acute stage. Similarly,fish handled in the summer frequentlydevelop infections similar to bovineshipping fever syndrome. Antibioticmedication for 10 days prior to hand­ling serves as an excellent preventivemeasure.

TREATMENT DELIVERYSYSTEMS

Three basic treatment regimens arecurrently used in warmwater fish cul­ture.

Bath Treatment

A bath treatment with selected chem­icals for the removal of external para­sites is the most common treatment reg­imen. These baths may be: I) at lowtreatment rates for extended or in­definite periods; or 2) at high rates forrigidly specified times. Such applica­tions are inefficient and expensive.Technique 1) is usually used for pondapplications. A ton of catfish in a I-acrepond averaging 4 feet deep would con­stitute only 0.018 percent of the mass

I Mention of trade names or commercial productsdoes not imply endorsement by the NationalMarine Fisheries Service. NOAA.

Marine Fisheries Review

by weight. This mean that 99.98 per­cent of the chemical is not utilized. Thecost factor is readily apparent. Intechnique 2), the use pattern is re­stricted to small tanks or raceways.While chemical costs are usually low,labor and time requirements are sig­nificant.

Oral Treatment

Internal diseases of fish can betreated only if a delivery system isavailable to introduce the drugs inter­nally. This means that either the fishmust accept the medication voluntarily(as with their feed) or that the medica­tion must be provided manually (usu­ally by injection). The formertechnique can be used on those specieswhich accept artificial feed only so longas the fish are not too sick to eat. Suc­cess with oral delivery systems isdirectly related to early, accurate diag­nosis of the problem. Cost of oral deliv­ery systems is low and effective. Laborrequirements are also low.

Injection Treatment

Treatment by injection is currentlypracticed in the United States only on

broodstock. The high value of each in­dividual animal more than offsets thelabor and time requirements of this de­livery system. The technique is usedwidely in Europe as a measure to pro­vide protection to young carp frombacterial disease during handling andstocking. It has proven effective andeconomically feasible in Europe be­cause of low labor costs. Injectiontechniques require the handling of eachfish. Even with the most modem injec­tion systems available today, time andmanpower requirements preclude theuse of injection delivery systems onother than broodfish in the UnitedSlates.

Further Delivery SystemsDevelopment Needed

A review of these treatment systemsindicates the following unfilled needs.

I) No delivery system exists for treat­ing systemic bacterial infections inpond-spawned fishes that do not acceptarti ficial feeds.

2) No economical delivery systemexists for mass injection of large num­bers of fish prior to stocking.

3) No efficient treatment system

exists for treating fish in ponds for ex­ternal parasites or bacterial infections.

Considerations should also includedelivery of potential vaccines for theimmunization of fish. Although vac­cines are still in formative stages, it isrelevant that delivery systems be de­veloped. Under the existing state ofknowledge, it appears that the deliveryof antigenic materials faces the samelimitations as chemotherapy. New de­livery systems and alternate culturalpractices are needed if significant gainsare to be achieved in improving fishhealth.

LITERATURE CITEDMeyer, F. P. 1970. Seasonal fluctuations in the

incidence of disease on fish farms. In A sym­posium on diseases of fishes and shellfishes p.21-29. Spec. Pub!. 5. Am. Fish. Soc .• Wash­ington, D.C.

Robinson. J. A., F. P. Meyer. and 1. H.Fribourgh. 1970. Oxytetracycline efficacyagainst bacterial infections in blue and channelcatfishes. U.S. Dep. Inter.• Bur. Sport Fish.Wild!., Tech. Pap. 35. 7 p.

Rogers. W. A.• J. A. Plumb. J. L. Gaines. and R.P. Phelps. 1971. Investigations of parasitesand diseases of cultured fishes. U.S. Dep.Commer.• Natl. Mar. Fish. Serv .• Wash­ington. D.C. Rep. NOAA-72091982, 11 p.

Snieszko. S. F. 1954. Therapy of bacterial dis­eases. Trans. Am. Fish. Soc. 83:313-330.

March /978

MFR Paper 1295. From Marine Fisheries Review, Vol. 40. No.3. March 1978.Copies of this paper, in limited numbers. are available from 0822, User Ser­vices Branch, Environmental Science Information Genter, 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.

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