Johnson, Sterling K.

English

Texas A&amp;M Repository

L-1210ICH DISEASE OF FISHESs. K. Johnson*The ParasiteIchthyopthirius multitilis, the cause of Ich orwhitespot disease, is a common protozoan parasitein most parts of the world. In the southern U.S.,it is found throughout natural waters and is espe-cially a nui ance in cultured fishes.Ich, along with some other protozoans, needsa fish host to complete the life cycle. Consequently,these parasites are passed from fish to fish and donot exist as part of a natural body of water thathas no fish. In contrast, many other fish diseasesare caused by organisms that are present naturally,but affect fish only when they become weakened.Ich is considered contagious where fish arecultured. A contagious parasite not only causesdisease, but spreads readily from one fish to an-other. While many protozoans merely split toproduce a new generation, a single Ich divides intohundreds. at each change in generation. This abilityto produce increased numbers aids the parasite inthe successful completion of the life cycle by increas-ing the chance to find a host fish.The parasite reproduces while separated fromthe host fish. Newly produced parasites must finda fish before further development can take place.The general cycle of Ich is presented in figure I.Fig. 1. Life history of Ich·Extension fish disease specialist, The Texas A&M UniversitySystem.Texas Agricultural Extension Service • The Texas A&M· University System • John E. Hutchison, Director • College Station, TexasThe amount of reproduction that occurs in theparasite popUlation is geared to populations offishes in wild waters. Man's influence in fishculture presents a crowded situation. As a res'ult,infestation exceeds the natural rate, thereby enhanc-ing the contagious potential of the parasite.The parasites appear as whitish to brownishspots on the surface of the fish. The parasites arelodged just beneath the surface of the skin andthe spots that are seen are often a combination ofthe parasite beneath the skin and excess mucussecreted at that place, figure 2. The parasiteobserved at these spots is the large stage, figure 3.A rolling motion and horseshoe shaped nucleus arevisible through a microscope. In the smaller infec-tive stage, the parasite will move about the skinsurface as well, but this stage cannot be seen bythe unaided eye. More parasites of both types alsowill be found on the gills of infected fish.The length of time that Ich spends as a parasiteon a fish varies with temperature of the water. Atoptimum temperature (75-80 degrees F.) the periodis as low as 2 days. (Other temperatures: 70-75degrees F., 4 days; 60 degrees F., 14 days; 50 degreesF. and less, 35 days or more; 80-85 degrees F.,development time extends until 85-90 degrees F.,reproduction ceases). After a parasite leaves thefish, the time lapse for infective stages may last aslong as 5 days in cooler temperatures. After hatch-ing at optimum temperatures, some infective stagesremain alive and free-swimming for 4 days.Since Ich does not reproduce well at high tem-peratures, occurrence is seasonal. August andSeptember are months when Ich is normally absentfrom fish culture systems in southern states.Although strains of Ich may exist with varyingdegrees of virulence, those examined so far haveproved quite strong.The Infected HostIch seems to be able to infest most kinds offresh-water fishes. The natural resistance or suit-ability of a certain kind of fish as a host influencesthe development of the disease. However, thechanging defense mechanisms of a fish often allowfor wrong ideas about the weakness of a particularfish species to the disease.Immunity is one defense mechanism that resultsfrom Ich infestation. Fishes exposed to the diseaseand then relieved of the infestation have tem.porary2Fig. 2. Channel catfish fingerling infected with IchFig. 3. Large or feeding stage of Ichimmunity. However, the immunity usually is notcomplete and the fishes act as undiseased carriersof the parasite. The consequence of release of suchfishes into another fish population usually resultsin a serious problem for the establish~ stock.The actions of stressors influence immunity.Various stressors, such as oxygen shortages andrapid environmental changes, lessen the ability offishes to produce an effective immunity. Carriers,for example, will become diseased if the influenceof stressors lessens their immunity.Soon after an infestation begins to spreadthrough the culture system, fishes begin to showsigns of appetite loss. This usually occurs beforethey begin to die. If a sample of fish is obtained,an early diagnosis of the problem may be made.Other causes of loss of appetite are weather changes,feed changes and other fish diseases. This behavior,therefore, is not only a symptom of leh.Infected fish secrete unusual amounts of mucusand the cells that comprise their outer skin becomeswollen. Sloughing of skin is common, and areaswhere the skin is lost feel rough to the touch.DISEASE MANAGEMENTAvoidance of IntroductionAssuming that a fish system is free of the para-site, the only concern is to avoid parasite intro-duction. Possible sources for introduction arediagrammed in figure 4.Transport of Ich by wind and land animalsinto a culture system is a constant threat. For mostfish farm situations, however, the threat is con-sidered minor.Wild fish act as carriers of the parasite, butthey may be excluded by building spillways thatprevent fish passage. Wild fish populations inthe culture system prior to stocking should beeliminated.Bait fish frequently carry Ich into fishing lakes.Operators who do not allow the use of fish as baitor require use only of their own leh-free minnowswill avoid this threat.If streams or lakes are used for a water sourcefor a fish pond, there is the threat of getting Ichfrom fish or the water itself. Filters will preventpassage of wild fish, but the free-swimming, infec-tive stage of the parasite can pass through com-monly used filters. The best preventative for this3means of introduction of leh is to change to groundwater as the water source.Equipment is a major threat for transfer of theparasite where multiple culture units are used.Dipping of equipment in disinfectants between useswill prevent spread of Ich from one unit to another.The chance of purchase of infected fish shouldbe of utmost concern to those who buy fish eitheras fingerlings or adults. "Eyeballing" the fish forspots will serve to detect leh in advanced stagessuch as shown in figure 2, but a microscopicexamination is needed for effective screening.Examination of gill filaments and scrapings fromthe skin of several fish will usually reveal thepresence of leh in a shipment.Purchase of "treated" fish does not mean thatthe fish will be free of infestation. By being presentbeneath the skin of the fish, leh is sheltered fromthe usual vat treatments that are administered priorto shipment.Quarantine measures are usually helpful indetecting the presence of Ich. Fish may be heldfor several days (depending on the temperature ofthe water) and subsequently examined. Althoughthis method may be effective in detecting Jeh, theproblem of cleaning up the quarantine system stillexists if the disease is found.Corrective MeasuresSelective toxicity. Adding a chemical to theculture system water, in dilutions too weak toseverely damage the fish but strong enough to killthe parasites, is useful. Because Ich lives underthe skin of the host fish and is unexposed to thewater during part of its life cycle, several chemicaltreatments are necessary to kill all parasites whilethey are free-swimming. Temperature determinesthe length of time that the parasite will feed withinthe skin of the fish. Treatment should be extendedfor a week at 70-80 degrees F., 1Y2 weeks at 60-70degrees F. and longer for cooler temperatures.Malachite green (dye), at a concentration of one-tenth part chemical per million parts water andcopper sulfate, at a concentration of one part chem-ical per million parts water, have given goodresults. The latter chemical is ineffective in veryhard waters and may be toxic to the fish at lowconcentrations in soft waters. Malachite greenshould not be used on sunfish (bass, bream) andtetras (tropical fishes) because it is too toxic.When chemicals are used in an aquarium, thetemperature may be adjusted to 80 degrees F. andWINDSURFACE WATERSEQUIPMENTNEW FISHWILD FISHFig. 4. Source of introductionheld during treatment. This promotes quick cyclingof the parasites and, as a result, a better effect bythe chemical treatment.The legal use of treatment chemicals on fishthat are destined as food for humans or animalsis regulated by state and federal agencies. For yourprotection and that of potential customers, checkthe current status of regulations pertaining to aparticular treatment before using it.Temperature change. Since Jch ceases develop-ment at high temperatures, culture systems usedduring hot summers do not have Jch infestations.This should be considered in late summer when"self cure" as a result of 'rising water temperaturescan be anticipated.In an aquarium, gradually raising the water to90 degrees F. and holding it there for 1 or 2 daysusually will result in an effective cure.4Fish removal. If fishes are removed from theculture system, parasites in the fish-free water shouldcycle out in a period of time. The exact time limitfor this to occur is not known, but 2 weeks shouldwork at 70-80 degrees F. Success would, of course,depend on removal of all fish, but in some culturesystems this is not easy to do.Advanced cases. Sometimes a disease is notnoticed until a majority of the fish population isheavily infested. Attempts to relieve the problemwith chemical treatments result in a large numberof the fish dying before cure is attained. In suchcases, it is better to remove the food fish for mar-keting if they are large enough to harvest. Thisdecision should be made prior to treatment, becauseone would not want to market fish that have justbeen treated. Market quality of lch-infested fishis good as the parasites provide no ill-effects tohumans and are removed when the fish are dressed.Management in Different Types of CasesGrow-out ponds. The goal of this type of cul-ture is to raise the fish in a static body of waterfrom fingerlings to harvestable size. By followingthe suggestions for avoidance, lch may be prevented.Since use of the system is on a term basis, the pondmay be dried between crops as an additionalprecaution.Fingerling ponds. This system raises fish fromeggs to stocking size. The commercial fingerlingproducer is not only concerned with Jch-free statusof his fish from the survival standpoint, but alsoin establishing a reputation for producing healthyfingerlings. Frequent transfer of fish from pond topond is common and, consequently, the disinfectionof nets and equipment is an important precaution-ary measure. The keeping of records on the trans-fer of fish is also very important, because it allowsrapid retracing of an outbreak of disease to itssource.Typically, the spread of Jch through fingerlingponds is rapid. The young fish usually have gainedno immunity, and the smaller sizes are less able toproduce an immune response.Fish-out operations. Jch infestation is of para-mount importance in fish-out systems. Since fishare constantly introduced, carriers of lch may bemixed with unexposed stock, resulting in wide-spre,!-d disease. A further complication is that fishwill not bite when heavily infested. Chemicaltreatment must be restrained because the fish areused for food. Purchase of Jch-free fish is essential,5and microscopic surveillance is the most practicalmeans for detection.Closed circuit systems. In recirculating units,~ater is usually. passed through a biofilter to rendersuitable water quality. This is accomplished bymic'roorganisms in the filter that utilize wasteproducts and convert them to harmless fonus.Entry of Jch into such a system is a problem, sincefree-swimming stages of Jch will pass into and backfrom the filter. One must consider that the useof chemical treatment may damage the biofilterby killing the useful microorganisms.Aquaria. Since most aquarists frequently pur-chase new fishes, there is a threat of Ich introduc-tion with each new purchase. Since microscopicalexamination is impractical, quarantine may beused. Should Jch break out anyway, elevation oftemperature may be tried or, perhaps better, chem-ical treatment using commercially prepared reme-dies containing malachite green.Raceways. Raceway systems use flowing waterthat tends to wash away free-swimming infectivestages prior to their contact with fish. Racewayshave great num'bers of fish stocked per unit of waterand this often offsets the disadvantage placed onthe free-swimming Jch by water flow. This isespecially true where long raceways are used orwhere water flow is sluggish.Chemical treatment in raceways is difficultbecause Jch treatments must be maintained in con-tact with the fish for an extended period of time.Shutting off the water flow to hold the treatmentchemical causes strain on the crowded fishes, where-as feeding the chemical into the flowing streamusually requires chemicals in amounts that maketheir use uneconomical.Cages. By being stationary, cages are quite vul-nerable to Jch infestation because both hosts andparasites are localized. Chemical treatment is bestaccomplished by treating the entire culture systemas would be the case for a grow-out pond.Minnow vats. The. same problems are experi-enced in this system as in the fish-out operation.The disease is not as critical, in this case, since theminnows will be marketed in a short time and thevalue of tha fish is much less. Retention of thedisease over a long period of time will cause eco-nomic loss, however. Tank cleaning and separationof old fish, when new minnows are introduced, willeliminate the chance of spread from one to theother. Elevation of temperature is not suggestedfor minnows as ·a method of. Jch removal.Educational programs conducted by the Texas Agricultural Extension Service serve people of all ages regardless of socio-economiclevels, race, color, sex, religion or national origin.Cooperative Extension Work in Agriculture and Home Economics, The Texas A&M University System and the United StatesDepartment of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8, 1914, as amended, andJune 30. 1914.3M-I-74 WM 2

Ich Disease of Fishes.

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Ich Disease of Fishes.

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Ich Disease of Fishes.

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