Mastering Freshwater Aquarium Ecosystems
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Fish Health (Cont.)

KHV infection typically does not show symptoms below 72° F (22° C) or above 86° F (30° C).  It takes up to two weeks for koi to show symptoms of KHV in temperatures between 72° and 78°F (22° and 25.5°C). Once koi are infected with KHV it is believed that they will become carriers of the virus for life. In California, some koi hatcheries have been shutdown by the State, and made to destroy all fish, resulting in severe economic hardship for farmers. KHV has been known to still be active in a system for up to three days after the fish have been removed.

Lab tests have been developed to verify KHV infection. A KHV vaccine has been developed that has shown positive results in adult koi, but has caused mortality in some juveniles.

Goldfish Herpesvirus

Goldfish herpesvirus (GHV, cyprinid herpesvirus-2 or CyHV-2) that causes the same symptoms as KHV, but only in goldfish.

Dwarf Gourami Iridovirus

Dwarf gourami iridovirus (DGIV) was believed to infect only the dwarf gourami (Colisa lalia, also known as Trichogaster lalius), but later studies have shown that it may be the same virus (or very closely related virus) that infects many other species, both freshwater and saltwater. It is believed that the virus originated in with imported dwarf gouramis from Singapore. The iridovirus is spread through the water. Other fish that are believed to be susceptible to the virus are swordtails (Xiphophorus hellerii), mollies (Poecilia sp.), guppies (Poecilia reticulata), cleaner wrasse (Labroides dimidatus), and banggai cardinalfish (Pterapogon kauderni).


Iridovirus under a electron microscope. Photo by Dr. Thomas B. Waltzek.

DGIV has often been misdiagnosed as a bacterial infection. DGIV causes lesions on the fish that look similar to fish TB (Mycobacterium marinum). It causes 90% or more mortality in fish exposed to the iridovirus.


Ich (Ichthyophthirius multifiliis)

Ich (Ichthyophthirius multifiliis) is the most common parasite hobbyist will have to face. Ich is a microscopic parasite that you will see evidence of on the fish when it is near the end of its lifecycle. Small white dots will appear on the fish when ich is present. The ich parasite not only attacks the fins and body of the fish, but it also the gills. When the ich reaches significant numbers, it will make it difficult for the fish to respire. If not treated early, the parasite can kill all the fish in the aquarium. When observing fish in the aquarium store, be aware that not all infected fish will show spots on the body or fins, they can also be in the gills where it can go undetected.

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