Feline immunodeficiency virus (FIV) is a Lentivirus that affects cats worldwide, with 2.5% to 4.4%[1][2] of felines being infected. FIV differs taxonomically from two other feline retroviruses, feline leukemia virus (FeLV) and feline foamy virus (FFV), and is more closely related to human immunodeficiency virus (HIV). Within FIV, five subtypes have been identified based on nucleotide sequence differences coding for the viral envelope (env) or polymerase (pol).