Interaction between feline leukaemia virus subgroups in the pathogenesis of erythroid hypoplasia

The interaction of feline leukaemia virus (FeLV) of subgroups A and C in the pathogenesis of erythroid hypoplasia in cats was studied. Weanling kittens infected with FeLV-A became permanently viraemic but remained haematologically normal over a period of 36 weeks. Similar kittens inoculated with FeLV-C, which produces erythroid hypoplasia when administered to newborn kittens, neither became viraemic nor developed the disease. However, weanling kittens inoculated with a mixture of FeLV-A and C became viraemic, first with FeLV-A and then additionally with FeLV-C, and the emergence of FeLV-C into the blood coincided with the advent of erythroid hypoplasia. When FeLV-C was inoculated into five older cats which had been viraemic with FeLV-A for several months previously, it appeared in the plasma of three of the cats and erythroid hypoplasia was diagnosed in two of these, 16-20 weeks after infection with FeLV-C. These results show that FeLV-A enhances the growth of FeLV-C in cats and overcomes their age-related resistance to FeLV-C. Also, the appearance of FeLV-C in the plasma of cats viraemic with FeLV-A indicates that erythroid hypoplasia will subsequently occur rapidly. These findings are relevant to the origin of FeLV-C isolates and their occurrence in nature.