Development of virus non-producer lymphosarcomas in pet cats exposed to FeLv

Naturally occurring oncoviruses of several species are transmitted contagiously and cause lymphosarcoma (LSA) or leukaemia in their hosts. All naturally occurring oncoviruses replicate in vivo in the tumours they induce or, as with bovine leukaemia virus, can be isolated from tumour cells grown in short-term cell culture. However, we have shown that feline leukemia virus (FeLV) is not present in a significant minority of pet cats that develop LSA. Unlike experimentally induced virus-negative leukaemias and sarcomas of other species, LSA cells from FeLV-negative LSA cats lack any FeLV proteins, including p15 or p12, and complete functional copies of FeLV provirus and thus do not produce FeLV when grown in cell culture. Thus, except for FeLV, the naturally occurring animal leukaemogenic oncoviruses seem to induce only virus-producing lymphoid tumours. Our earlier findings prompted a study to determine the frequency of occurrence of FeLV non-producer (NP) LSA in pet cats and whether NP LSAs develop in cats exposed to FeLV. We report here epidemiological data which indicate that development of NP LSAs in pet cats is associated with exposure to FeLV and suggest that FeLV may be the aetiological agent for FeLV NP feline LSAs. Thus, feline NP LSAs may be suitable for studying the potential viral aetiology and mechanism of leukaemogenesis of human lymphoid tumours in which no oncoviruses have, as yet, been proved to cause the disease.