If you are considering Koi keeping, know that most illnesses and pathogens affecting these fish are preventable or treatable. However, three types of viral infections exist for which insufficient treatments are available. Of these, two are fatal. Understanding these viral infections can help you make an early decision about how to handle an outbreak.
SVC
Spring Viremia of Carp (SVC) is a highly contagious viral infection. It primarily attacks in the spring, though it can occur in other seasons. It originated in Europe in the 1960s, and though it’s prone to regions with colder weather, it has spread to most of the world since the late 1990s.
The virus attacks any member of the carp species, including ornamental Koi and goldfish. It is fast-spreading, as infected fish’s urine, feces, and mucus contaminate the water and expose the entire pond population to the virus. Each virus strain is typically fatal to most Koi varieties that contact it; however, some evidence suggests that the SVCV Id strain is most lethal to Beni Kikokuryu and Sanke Koi.
If your pond experiences an SVC outbreak, you will often notice behavioral changes before physical ones. Typically, active fish will become more sedentary, breathe slowly, and appear off balance when swimming. These are common signs of illness in Koi, but the fish won’t respond to your typical first-line treatments. Eventually, the skin and gills will darken, and the vent may expel bloody mucus. Eye bulging and bloating are common indicators of SVC infection as well.
SVC can enter a pond through contaminated equipment, parasites, predatory birds, or new fish that are introduced to a pond without quarantine. Even fish that survive this incurable disease will continue to be carriers. Therefore, an SVC outbreak will most likely require you to eliminate everything living in the pond.
KHV
With a mortality rate of 80% to 100%, fish infected with Koi herpes virus (KHV) will either die or become asymptomatic carriers. KHV shares symptoms with other common, treatable Koi illnesses, so the only way to definitively diagnose KHV is through laboratory testing. Afflicted Koi will be unable to eat, have difficulty breathing, and shed large amounts of skin and mucus. Fish typically succumb to KHV infection within 48 to 72 hours of the onset of symptoms.
Even with precautions taken to reduce the likelihood of an outbreak, this pathogen has successfully mutated into several new strains and continues to enter pond ecosystems. In addition, KHV viruses can lay dormant in fish for quite some time, attacking the host once pond conditions fall into their optimal parameters, between 60 and 80 degrees.
Signs of KHV infection include lethargy, increased gill movement, gasping at the surface, the appearance of ulcers, fraying fins, whitening gills or mucus, decreased appetite, and sudden death. Some enthusiasts report that using potassium permanganate or chloramine T helps treat the infection, but this information is inaccurate. Any survival is due to natural immunity.
The most effective way to test for KHV is by PCR testing DNA. If the test is positive, you can force the virus to run its course more quickly by raising the water temperature to 87°F for a few weeks. Remember that surviving fish can transmit the disease, so you can never add new fish to your pond or transfer the existing fish anywhere else.
Carp Pox
Carp pox is another common herpes virus in Koi. However, unlike KHV, it is not usually fatal. This strain causes abnormal cell development that produces growths similar to warts on the Koi’s skin. These lesions often resemble candle wax but are distinct growths and not mucus. Other symptoms include thickening of the fins and growths in and around the mouth.
Symptomatic outbreaks will become more prevalent at colder water temperatures, and Koi will shed lesions when the water temperature reaches 70°F. The absence of lesions in an infected Koi only means the virus is dormant–it is not gone. As with SVC and KHV, once the carp pox virus enters a pond, you can assume all your fish are exposed and are carriers. The only way to test for carp pox is to swab an active lesion.
The biggest risk carp pox poses is secondary infections. The virus weakens the Koi’s immune system, increasing the risk that it will catch other contagions and face greater difficulty fighting them. Therefore, it is essential that you are diligent about water and food quality for carp pox-positive fish and that you check for bacterial and parasitic infections regularly. In the event of a secondary infection, early intervention is crucial.
There is no current treatment for a carp pox outbreak. If you want to show a Koi, you may alleviate the symptoms of carp pox by raising your water temperature for a few weeks. This may trigger a shed and improve the fish’s aesthetic. However, the lesions will return when the water cools again. Never surgically remove skin lesions. Doing so will increase the Koi’s susceptibility to secondary infections, and the lesions will return relatively quickly.
Deep Cleaning a Pond
In order to disinfect a pond, you must dispose of every fish, plant, and substrate. The next step is to run bleach solution at 1 cup per 1,000 gallons through the mechanical filtration for 24 hours. Afterward, drain the pond, pressure wash it, and spray it with bleach solution. Let the bleach sit for 24 hours and then rinse thoroughly, as bleach remnants in the pond will harm new fish.
Leave the pond dry for several days so that any lingering virus will die. Refill the pond with water and treat it with Cloram X or another sodium thiosulfate product to neutralize chlorine and make the system safe for fish again. Follow the manufacturer’s recommendations for dosage. Also, disinfect any pond equipment, such as nets, buckets, etc., and leave them to dry for several days.
Restocking a Pond
The best time to restock your pond is during spring or fall. The milder temperatures in these seasons produce higher oxygen levels, making it easier for new fish to acclimate. Don’t immediately introduce the same number of fish as in your prior population. Start with a small number and gradually add over time until you reach your desired density. This will allow you to observe the fish in your new ecosystem and run water quality checks.
Preventing Viruses
As with all Koi illnesses, isolating new fish before introducing them to the pond is vitally important. Although quarantine will not eliminate the risk of SVC, KHV, or carp pox infections, it will greatly reduce it. Quarantine should last between two and four weeks. If you are introducing multiple fish, the quarantine period begins as a group after the last fish is placed into the quarantine tank.
It is also important to maintain excellent water quality and to change between 10% and 20% (excluding evaporation) of the pond water every one or two weeks. The goal is to keep the new water as similar as possible to the old–only cleaner. Temperature testing is a good idea during this process. Fish are more stressed and susceptible to disease when rapid temperature shifts occur.
Robust biological filtration can also support the health of your Koi. Cover 40% to 60% of your pond with a mixture of floating, shallow, and submerged plants. If that is not possible, add beneficial bacteria to the pond with your water changes. Beneficial bacteria break down waste products into less harmful substances by converting ammonia and nitrites into nitrates.
Diet is also important in illness prevention, and high-quality Koi food is worth the investment. We recommend a fish meal-based formula that is high in protein, like Blue Ridge Platinum Pro. Food that is fortified with vitamins and minerals Koi need strengthens their immune systems and protects them from illnesses related to nutrient deficiencies.
It is never an easy decision to disinfect your pond, but sometimes it is the right choice. Next Day Koi is here to help you recover with excellent deals on high-quality Koi and Goldfish year-round.