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FIELD
OBSERVATIONS OF A HERPES VIRAL DISEASE OF KOI CARP (CYPRINUS
CARPIO) IN ISRAEL S.
Tinman and I. Bejerano Central
Fish Health Laboratory, 19150 Nir David, Israel During
the first week of May 1998 and following uncontrolled introduction of Koi carp
from Europe, massive mortality of Cyprinus carpio species was observed
along the Coastal plain. This outbreak, first reported during the early spring
months, was followed by 2 further outbreaks during the fall of 1998, and the
early spring of 1999. Economic losses included more than 600 metric tons of
common carp and 4 million dollars of high quality Koi intended for export. Although
this disease process was observed as being highly contagious and extremely
virulent, morbidity and mortality were restricted to Koi and common carp
populations. Several other species (including other cyprinids such as Goldfish)
within the same ponds remained
completely asymptomatic to the disease. During the three-week period of the
outbreak, every Cyprinus carpio population within the region was
affected. Mortality rates were consistently 80% in every pond. The disease process was triggered by specific temperatures, as indicated by its reappearance during the transient range of spring and fall (18-270C). Temperature seems to be the most dominant environmental factor affecting the clinical presence of disease. Gross external clinical signs included severe necrosis of gill tissue, superficial hemorrhages, and increased mucus production. Internally fish showed petechial hemorrhages on the liver, and other inconsistent pathological changes. Behavioral symptomology included fatigue and exhibition of gasping movements in areas of shallow water. Affected C. carpio populations were characterized as highly susceptible to various non-specific secondary infection of parasitic, bacterial, and fungal origin. Biochemical asays of C. carpio blood samples revealed severe osmoregulatory dysfunction, a general hypoproteinemia, and hepatic dysfunction. Additional studies exhibited a general state of immunosupression in infected fish, compared to normal populations. All tests yielded negative results for all known viral diseases of carp. Histological sections of hematopoeitic kidney tissue and gill epithelium of infected fish revealed the presence of intranuclear inclusion bodies. These particles, viewed with electron microscopy, were found to be characteristic of herpesvirus-like forms. Lately, the agent was cultured and identified by Hedrick et al. (USA) as Koi herpesvirus (KHV). Recently similar results were found in Israeli laboratories. In the attempt to characterize the epidemiological character of the virus several cohabitation trials were conducted early in the diagnostic process. The majority of Koi and common carp, which were regarded as survivors of clinical outbreaks, were found to be resistant to consecutive disease outbreaks and/or challenge. Our studies show that this resistance could not be transferred congenitally to the next generation. Current studies are focused on development of rapid diagnostic methods (molecular, cell lines and blood analysis). In addition, much effort is focused on development of resistant carp populations (resistant strains of carp and artificial immune resistance).
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