| Seasonal trends in the prevalence of virally induced skin tumors are first reviewed for a variety of wild fish with emphasis on neoplasms in the walleye Stizostedion vitreum. Walleye dermal sarcoma (WDS) is a seasonally limited neoplasm caused by a retrovirus. In our initial fieldwork, we followed the prevalence of visible neoplasms among walleyes age 3–8+ years from 1995 to 1999 in Oneida Lake, New York. The percent of fish developing tumors increased from age 3 to age 6 but decreased thereafter. These findings give further support to the hypothesis that walleyes from Oneida Lake contract the disease during the spawning run and develop visible neoplasms in the fall and winter of that same year but return in the spring with regressing tumors that disappear by summer and do not return. Our data indicate that over time, almost all walleye of a year-class may contract the disease.; Next, a temperature-associated model for tumor regression was established using the virally induced WDS. The regression of these tumors was associated with increased temperature over the course of a 10-month study. The incidence of tumor regression was significantly higher in walleyes held at 15°C or 20°C than in those held at 10°C. A second trial (which more closely replicated the temperatures of Oneida Lake, New York, during the early spring) demonstrated somewhat lower rates of regression than those observed in the first trial did.; To study the possibility of resistance, walleyes infected with WDS virus (WDSV) were rechallenged in two trials. The rate of tumor redevelopment was compared with naive fish that received the same inoculum. The incidence of tumor redevelopment was significantly lower in walleyes that previously had been challenged with WDSV. This resistance to WDS recurrence also helps explain the epidemiology of WDS seen in wild fish.; Finally, we have determined that there are approximately 1010 viral particles in 100 μl of WDS-tumor inoculum. The minimum dose of WDSV that could induce tumors by the topical challenge method was the 1,000-fold dilution of this 1010 inoculum (i.e. 10 7 viral particles). |
