Infectious Leukemias in Domestic Animals

Viruses have been regarded as substantial candidates for the aetiology of Cancer in man since the demonstration that the prime factors in the pathogenesis of naturally occurring leukemias in certain domestic animals were viruses of what is now known as the retrovirus group. Early in this century leukosis in domestic poultry was shown experimentally to be infectious and enzootic bovine leukosis was long considered by many veterinarians in continental Europe to be transmissible on epidemiological grounds. Later the discovery of feline leukemia virus added further substance to the suggestion that leukemia in most, if not all, animal species might be caused by viruses. Since then viruses have been isolated from spontaneous leukemia in captive gibbons but no further leukemogenic viruses have been isolated from any other domestic animal and the evidence for viral aetiology of leukemia in man is still equivocal. Obviously it is naive to consider that the natural history of leukemia in all species might be the Same: That virus might be isolated from each case of neoplasia as easily as it is found in leukemia in chickens or that the thread of transmission might be followed as easily as in cats. There is, however, a great deal to be learnt from the epidemiological studies which have been carried out in these species which is relevant to studies of leukemia in man. In this brief review I will discuss some of the evidence from which we derive our interpretation of the natural occurrence of virus-induced leukemias in animals and will outline similarities and differences between the disease in each species. Where appropriate the relevance to human leukemia will be discussed.