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Enhancement of feline leukemia virus‐induced leukemogenesis in cats exposed to methylnitrosourea

The effect of methylnitrosourea (MNU), a potent resorptive carcinogen, was evaluated for its influence on the susceptibility of adult cats to infection and induction of oncornavirus disease by feline leukemia virus (FeLV). Young adult cats at an age previously demonstrated to be highly resistant to FeLV, were injected intravenously with moderately toxic doses (15–20 mg/kg) of MNU alone or with infectious FeLV (Rickard strain). Following exposure to virus and chemical, cats were monitored for antibody to the feline oncornavirus‐associated cell membrane antigen (FOCMA), viremia by direct infectivity and the presence of gsa in peripheral blood leukocytes, and for toxic effects of MNU by hemogram analyses on peripheral blood. Of 8 cats injected with MNU+FeLV, 6 developed persistent viremia, 5 of which became debilitated from thymic lymphoma. Only 1 of 6 non‐MNU‐treated and infected cats of the same age became transiently viremic. FOCMA antibody development was markedly depressed in MNU+FeLV inoculated cats compared with cats inoculated with FeLV alone. Results show that MNU was apparently responsible for the obliteration of age‐related susceptibility in cats to FeLV infection and induction of FeLV‐related disease, and suggest that in nature exposure to toxic chemical carcinogens may act as factors which determine susceptibility to feline oncornaviruses in the cat.