The Myth Revealed

One of the biggest misconceptions about Koi is that they are a hearty breed.  Koi are not hearty at all.  In fact, because they have been bred to display colors that are not seen in common carp (like red and white) and a standard of body shape that is desirable in the Koi culture, they also inherently display a weak immune system and low tolerance to parasites, injuries, infections, and to great changes in their environment. 

All Koi Owners Should Know About the Following Deadly Virus:

KHV (Koi Herpes Virus) click on here for more information

The Injury & Illness

Parasites

Koi are susceptible to parasites, injury, fungal infections, bacterial infections (both internal and external), and viral infections (also both internal and external).  Any one, or all, of these issues will occur at some point in every Koi owner's fish, and it is very important to understand this.

There are two external parasites that invade Koi that are visible to the naked eye:  Argulus (lice) and Anchor Worm, and these can be easily detected and treated.  There are 5 common external parasites that are only visible through a microscope and most commonly invade the slime coat, skin, and gills of Koi:  (in no particular order) Trichodina; Chilodonella; Ichthyophthirius multifilis (commonly known as 'Ich'); Oodinium; and Flukes.  All of these parasites can be eradicated with the appropriate treatment if they are detected.  Reputable retail Koi dealers will quarantine, heat and 'scope' their incoming fish prior to selling them, but it is possible for some of those parasites to slip by.  This is one of the most important reasons that everyone should quarantine new fish.  It is also important for the serious Koi owner to learn how to anaesthetize and scrape Koi, and then immediately view the slide under a microscope.  Parasites will only live a short time after being removed via scraping (about 5 minutes), and that is the reason that you can not take a specimen to someone with the knowledge to detect various parasites.  Also, the reputable retailers that have the equipment and ability to diagnose the problem do not want any live Koi brought into their establishment - it is too easy to transfer the 'problems' into their facilities.

Injuries

Injuries can occur in many ways.  To acquire your Koi, the fish must be transported from breeder to wholesaler, in many cases from wholesaler to retail dealer, and then finally home to your pond.  Many injuries can happen during this process:  they frequently become bruised and these bruises can develop into an ulcer; sometimes they will jump out of their pond or holding container; sometimes they will become 'spooked' and dart off into a wall or plumbing fixture; worst of all for the person handling the fish as well as the fish itself, sometimes they get dropped; and lastly, they can suffer injury due to chemical burns from Ammonia and Nitrites to water treatments and medications.  All of these injuries can develop into ulcers.  This creates an open area with easy-access for parasites, bacteria and virus to invade.

Infections

Fungal infections can occur under several circumstances.  Two of the most common reasons that fungal infections occur are secondary to an injury and as a secondary infection from bacterial or viral infections.  Most fungal infections can be treated if they are caught early and treated properly.

Bacterial and viral infections are a different matter.  Because they are not easily diagnosed, they are often treated too late to save the life (or lives) of the Koi.  Bacteria and virus can easily invade weak areas and ulcerations on Koi because of their aquatic environment. Water in the pond allows for easy transference of the 'bugs' into the flesh of the skin.  Also, because the Koi are in water, they can not heal easily, because they are battling the hydrophilic nature of bodies - when there is an open wound water and infectious materials are drawn into their bodies.  This is one of the reasons that salt is often used in the pond during an illness or injury - the salt will draw out the non-salty, infectious fluids that are in the fish.  There are specific treatments for infections that include injections of antibiotics and medications added to the water.