ABSTRACT. A syncytlum-inducing viral agent was initially isolated from the ovarian fluld of a domestic white sturgeon Acipenser transrnontanus female using the sturgeon spleen cell line WSS-2. Examination of infected cells by transmission electron microscopy revealed ultrastructural characteristics typical of the Herpesviridae. On the basis of differences shown by comparative in vitro growth studies and serological assays, the new adult isolate was tentatively designated WSHV-2 to distinguish it from a previously reported herpesvirus from juvenile white sturgeon. WSHV (WSHV-1). A water-borne transmission trial at 3 virus concentrations was conducted to determine the susceptibility of juvenile sturgeon to the newly isolated virus. At virus doses of lo5 and 104 TCID,, ml-' cumulative mortality reached 80% within 6 wk at a water temperature of 17°C. Virus was reisolated from all mortallties with concentrations approaching 10' TCIDSo g-I in cutaneous eplthelial tissues where severe hyperplastic lesions were observed. Challenge with 103 TCIDSo m l -' of WSHV-2 falled to induce clinical signs or mortality, although chronic subclinical infections were detected in a majority of survivors 70 d postchallenge. During our investigations of WSHV-2 from adult sturgeon, a second virus isolation was obtained from ulcerative skin lesions of naturally infected subadult white sturgeon. This virus induced identical cytopathic effects in WSS-2 cells to that caused by the adult isolate from ovarian fluid. Serum anti-WSHV-2 neutralizing activity was detected In subadults with skin lesions and among fish recovering from these lesions. Serum from naturally-infected juveniles and serum from adults hyperimmunized with the WSHV-2 from the ovarian fluid were used to compare the adult and subadult viruses. The 2 agents were neutralized with these 2 sera which conversely failed to neutralize WSHV-1 at the lowest dilution tested Our studies demonstrate that at least 2 pathogenic herpesviruses are found among white sturgeon The newly recognized WSHV-2 can cause both acute and chronic infections and possibly a latent carrier state in white sturgeon. WSHV-2 is the fourth virus to be observed as a cause of juvenile mortality among intensively reared white sturgeon populations.
Characteristics of the In vitro propagation of the white sturgeon iridovirus (WSIV) were examined in 6 sturgeon cell lines. One new cell line originating from gonadal tissues (WSGO) produced up to 12-fold more WSIV 1-22 TCID,, (50% tissue culture infective dose) cell-L] than that of a previously established reference spleen cell line (WSS-2). Infected WSGO cell cultures were examined using phase microscopy, viral infectivity assay and transmission electron microscopy (TEM). At 15OC, both mature virions and infectious virus were first detected after 7 d post-infection. Caps~ds acquired envelopes in the cytoplasm and virions remained pnmanly cell-associated during the 35 d replication cycle. Cellular changes including hyper-refractility and cytoplasmic swelling with dense cytoplasmic inclusions correlated to extenslve proliferation of cytoplasmic vesicles and viral assembly sites. These cytological characteristics corresponded to changes in target cells of WSIV-infected juvenile white sturgeon following bath challenge. Microscopic changes in stained tissue sections of the host epithelium were detected 4 d post-challenge, approximately 8 d prior to the onset of clinical signs. Hypertrophied Malpighian cells surrounded by a prominent pericellular cisternum characterized epithelia1 lesions in the skin. Similar changes to epithehal cells of the barbels, olfactory organs and esophagus were also observed. Destruction of the sensory epithelium is suggested as a cause for cessation of feeding which occurs early in the Infection of white sturgeon juveniles with WSIV.
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