Effects of Fish Age and Challenge Route in Experimental Transmission of Walleye Dermal Sarcoma in Walleyes by Cell-Free Tumor Filtrates

To determine the effects of fish age at challenge and of challenge route on transmission of walleye dermal sarcoma, 25-week experimental transmission trials were conducted with cell-free tumor filtrates. Invasive dermal sarcomas developed in walleyes Stizostedion vitreum receiving intramuscular injections of tumor filtrate at 6 or 8 weeks posthatch. Invasive tumors were observed in few (5%) 12-week-old fish and in no 1-year-old fish injected with the tumor filtrates. Fish challenged at 12 weeks after hatching via topical application of inoculum or via oral gavage also developed walleye dermal sarcoma. Fish challenged via gavage developed tumors at a variety of locations on the body and fins. Again. invasive tumors were seen in few of the 12-week-old fish challenged by topical application (3%) or gavage (10%). However, several fish challenged via gavage developed severe tumors in the oral cavity and on the head near the oral cavity. Fish challenged via topical application of the inoculum developed tumors that were particularly well developed at the site of application. The topical contact and oral routes of experimental transmission of walleye dermal sarcoma are likely to more closely proximate routes of infection that occur under natural conditions than does injection.