Koi herpesvirus infection is an acute viral disease that can cause high morbidity and mortality in carp and koi. Clinical
signs of the viral infection are nonspecific, but severe gill lesions with red and white patches (edema and necrosis), pale
raised patches of skin, and sunken eyes are common.9 Affected fish may also exhibit neurologic signs such as erratic swimming and disorientation. Clinical disease commonly appears
in a collection of koi and carp during the spring or summer within two to four weeks after introducing new fish to the system
or returning from a koi show. The mortality related to koi herpesvirus infection typically occurs in water temperatures between
64 and 81 F (17.8 and 27.2 C). A definitive diagnosis is based on positive results on culture or polymerase chain reaction
testing (diagnostic laboratory at the University of Georgia). There is no treatment for koi herpesvirus infection, and surviving
fish should be considered carriers. Therefore, depopulation followed by disinfection of the habitat is recommended to control
Parasitic infestations are common in fish. They are diagnosed by wet mount examination of the mucus (from skin), fins, or
gills as mentioned above.
Figure 8: Chilodonella species (40 to 60 µm long) are flattened, oval-to heart-shaped ciliate protozoa that move in a slow circular fashion when
viewed in a wet mount (1,000X).
Ciliate protozoa have a direct life cycle, and most are commensal. However, a few are notoriously pathogenic. Ichthyophthirius multifiliis (known as Ich) in freshwater fish and Cryptocaryon irritans in marine fish are highly pathogenic ectoparasites that feed on host cells. Trichodina species in freshwater and marine fish is another pathogenic ciliate protozoan that can damage the gills and skin when large
numbers of the protozoan are present. Chilodonella piscicola and Chilodonella hexasticha in freshwater fish and Brooklynella hostilis in marine fish are ciliate protozoa that cause excessive mucus production and hemorrhage in the gills (Figure 8). Tetrahymena species in freshwater fish and Uronema species in marine fish are normally free-living commensals that become secondary pathogens that are highly invasive and can
be found in internal organs (Figure 9). Epistylis species in freshwater fish are ectocommensal ciliate protozoa (Figure 10) that create white tufts on hard surfaces of the fish, such as the fin rays and scales.
Figure 9: Tetrahymena species (50 to 100 µm long) are pear-shaped ciliate protozoa that are actively motile in a wet mount (100X).