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 rou te on transmission of walleye dermal sarcoma, 25-week experimental tra nsmission trials were conducted with cell-free tumor filtrates. Invasi ve dermal sarcomas developed in walleyes Stizostedion vitreum receivin g 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 or al gavage also developed walleye dermal sarcoma. Fish challenged via g avage 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 chall enged by topical application (3%) or gavage (10%). However, several fi sh challenged via gavage developed severe tumors in the oral cavity an d on the head near the oral cavity. Fish challenged via topical applic ation of the inoculum developed tumors that were particularly well dev eloped 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 co nditions than does injection.