FIV is a single stranded RNA virus. It is a retrovirus so it requires the enzyme reverse transcriptase to perform reverse transcription of its genome from RNA into DNA so it can insert into host cell DNA. FIV is also a lentivirus, similar to HIV that causes AIDS in humans. FIV is morphologically alike to HIV found in humans but is antigenically distinct (11). Lentiviruses are characterized by long incubation periods, lifelong infection, slowly progressive disease and a high degree of species specificity (8). They have the ability to incorporate a relatively large amount of DNA into the host cell and are capable of infecting neighboring cells in direct contact with the host cells, without having to form extracellular particles (8). The virus in encased in an elongated envelope and ranges in size from 80 to 100 nm in diameter. Surface projections of about 8nm are dispersed evenly over all the surface of the envelope (8). FIV infects and replicates in T lymphocytes (CD4+ and CD8+), B lymphocytes, macrophages and astrocytes (11).
Currently, five subtypes of FIV have been identifies worldwide: A, B, C, D and E. A and B subtypes predominate in the USA with A being more common in the Western USA and B being more common in the Eastern USA. Naturally infected cats can carry multiple subtypes simultaneously and this is referred to as ‘superinfection’ (6).