California Sea Grant College Program

Since 1998, episodes of mass mortality have occurred in populations of common carp Cyprinus carpio carpio and koi Cyprinus carpio koi. A herpesvirus, termed koi herpesvirus (KHV) isolated from infected fish has been shown to be the cause of the disease which has now been detected in North America, Europe, Israel and Asia.

KHV has 31 virion polypeptides. Both virion polypeptide and restriction fragment length polymorphism (RfLP) analyses of genomic DNA showed that the first two isolates of KHV isolated from koi in Israel and the U.S. were identical; furthermore, KHV was clearly distinguished from Herpes... (CHV) the only other herpesvirus isolated from Cyprinus carpio.

A polymerase chain reaction (PCR) assay to detect the virus in koi tissues was developed viral sequences obtained from a restriction fragment of KHV genomic DNA. The PCR detected KHV DNA in koi from naturally occurring outbreaks and from koi following experimental infections with KHV induced by intraperitoneal injection or bath challenges.

A comparison of the virion polypeptides and genomic RfLP of 7 geographically diverse isolates of KHV indicated they represent a homogeneous group with the exception of a single isolate from koi in Israel.

Optimal KHV growth occurred in the koi fin (KF-1) cell line at temperatures from 15–25°C. Experimental infections of koi by bath exposures to KHV resulted in the greatest cumulative mortality (95.2%) at a water temperature of 23°C. Significant mortality also occurred among virus-exposed koi at water temperatures of 28 and l8°C. No mortality was observed at water temperature of l3°C but, when virus-exposed fish originally held at 13°C were shifted to water at 23°C, a rapid mortality ensued. Survival analyses indicated a signiticant difference in risk of mortality between the virus-exposed tish at the different temperatures.

A real-time TaqMan PCR assay was developed to detect and quantify KHV DNA for diagnostic and research purposes. Virus concentrations in tissues of experimentally infected koi increased over time at all water temperatures tested ( 13, 18, 23, and 28°C). High concentrations of the virus at early time points in the mucus suggest that the skin may be an initial site of virus replication. At subsequent time points the virus was found in gill, kidney, spleen, liver, gut and brain. The principal target tissues during active infection are the gill, kidney and spleen. In addition, low copy numbers of KHV DNA were detected in fish that survived KHV at 62 - 64 d post initial virus exposure. There was little evidence for anti-KHV neutralization activity in the serum of koi previously exposed to the virus, thus detection of KHV DNA may be the most reliable indicator of prior exposures to the virus.