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

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Abstract
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.