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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.
o N 0
J'MAMJJASONO
SCYPHIOIA
~JF'MAMJJASONO
CHILOOONELLA
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
", ..... JJ.80NO
TRICHOPHRYA
,JF ........ "'JASONDTRICHOOINA
-------J"MAMJJASO~D
Ii[NNEGUYA
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.
JFMAMJ,JASOND
GYRODACTYLUS
JFMAMJJASOND
DACTYlOGYRUS
JfMAMJ,JASOND
CLEIDOOISCUS
JFJolAMJJA
LERNAEA
•o • 0
JFMAMJJASONO
A[ROMONAS LlQU£FACIENS
JFM.MJJASONO
MYXOBAC TERIA
JfNAMJJAS
PSEUDOMONAS
•_. .
~.l~JFMAMJJASOHD
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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