Can Parasites be present on healthy fish?

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Can Parasites be present on healthy fish?

January 05, 2016 - 08:39

This was the question that was posed to a vet, and you'll find his response below by clicking on the title or picture...

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?
The first answer is to practice good biosecurity, and you’ll have no issues.  The new fish should be held in quarantine in a separate pond/tank for 4-6 weeks.  The temperature in a quarantine tank is critical - you want any disease to manifest itself while the new fish is isolated and easily treatable.  Quarantine tanks for Koi should be held at 70-80 degrees for a minimum of 3 weeks.  If the temperature goes above 80 or below 70, correct the temperature and start timing the 3 weeks over again.  Just as is done for farm and domestic pets, the new fish should be examined for health by performing microscopic exams,and doing an environment check.  There are other tests that can be done if specific disease is suspected.  Chemical dips/treatments may be used if parasites are found.

After the first 3 weeks, and before introducing the new fish to the pond, 2-3 of your resident fish should be placed in the quarantine pond/tank as 'sentinels' to cohabit with the new fish for another 2-3 weeks. 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.

But by far, the most important part of introducing any new fish into a collection is observation time in quarantine.

Posted by Dr. Richmond Loh, The Fish Vet, Perth, Western Australia


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