Can parasites be present on healthy fish? The discussion will surprise you – simple steps for excellent fish biosecurity.

Some people come from a view point of keeping a balance in systems of pathogens, host and environment as occurs in nature. I do not disagree with that, however, we ARE keeping animals in less than natural environments. Additionally, from a medical stand-point, we would prefer to keep pathogens out.

Many of the parasites I deal with are obligate pathogens (e.g. skin flukes [Gyrodactylus], gill flukes [Dactylogyrus], white spot disease [Ichthyophthirius multifilis], etc.) and do not exist as commensals, nor are they found free-living in the environment (although they may be temporarily recovered from the water, environment and objects, associated with infected fish).

Some apparently healthy looking fish can be carrying pathogens, however, you cannot see evidence of the parasites (they are microscopic) or they may not be showing signs of disease… yet. Over time, the pathogens may cause stress (even in low numbers), picking off the most susceptible fishes first, as they slowly build in numbers. Then suddenly, their population explodes and you have to deal with whole tanks of sick fish.

How do we prevent this?
Well, you’d have to practice good biosecurity and you’ll have no issues. I’ve attached a section from my book on what sorts of treatments you’d need to apply to new fish before introducing them to your tank/pond. This is ideal for high through-put systems such as ornamental fish retailers/wholesalers and re-sellers. This should remove the bulk of pathogens. For those who have time on their side (e.g. home pond owners), or those dealing with highly valuable fish (e.g. koi and discus), there is an additional step.

What’s this additional step?
Sentinels! After following the prophylactic dips, the new fish are held in quarantine in a separate pond/tank for 4-6 weeks. At this time, 2-3 of your resident fish are placed in the pond/tank to cohabit with the new fish. The reason for doing this is because the different populations of fish have not been exposed to each other’s mix of microflora (and possibly viruses). This will give you time to react in a closely monitored system, without exposing your beloved collection to disease.

And to make it even more fool-proof, there is an additional “pre-step”!
A pre-purchase exam. Just as they do for farm and domestic pets, we can also apply this to fish. The new fish can be examined for health by microscopic exam, blood tests and their environment checked. Preventive medicines can be given and advise of low stress methods in transporting fish can be given. So, before you make that next big purchase, make sure you book TheFishVet to check everything out.

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Yours sincerely,

Dr Richmond Loh DipProjMgt, BSc, BVMS, MPhil (Pathology), MANZCVS (Aquatics & Pathobiology), CertAqV, CMAVA, NATA Signatory.
THE FISH VET, Perth, Western Australia, AUSTRALIA.
Mobile Aquatic Veterinary Medical & Diagnostic Services.
Ph: +61 (0)421 822 383
Skype: thefishvet

President WAVMA 2014


Adjunct Lecturer Murdoch University | Secretary Aquatic Animal Health Chapter – ANZCVS.


2 thoughts on “Can parasites be present on healthy fish? The discussion will surprise you – simple steps for excellent fish biosecurity.

  1. Does the recent study showing increased mortality from columnaris disease in fish pretreated with potassium permanganate change your new fish quarantine protocol?


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