Caused by a retrovirus in the type-C oncornavirus subfamily.

Three subtypes of virus (A, B and C ) can be distinguished serologically and by genome analysis. Only FeLV-A is transmissible between cats.

FeLV-B, which is only found in cats also infected with FeLV-A, arises through recombination between type A virus and endogenous FeLV. FeLV-C is the least common subtype encountered and is also only found in cats infected with FeLV. FeLV-C strains appear to be mutants of FeLV-A.

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