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Limited efficacy of an inactivated feline immunodeficiency virus vaccine

FELINE immunodeficiency virus (FIV) is a widespread pathogen of domestic cats associated with a variety of clinical signs, including gingivitis, stomatitis and recurrent infections (Hosie and others 1989). FIV, like human immunodeficiency virus, is a lentivirus of the family Retroviridae. Isolates of FIV are genetically diverse and are classified into subtypes, designated A, B, C, D and E, based on their nucleotide sequence. The prevalence of these subtypes differs throughout the world; for example, subtype A is prevalent in northern Europe, Australia, Canada and California, while subtype B is the major subtype present in eastern and central USA and southern Europe (Steinrigl and Klein 2003, Reggeti and Bienzle 2004). In addition, isolates of FIV differ widely in their biological behaviour. While some isolates achieve high viral loads and produce marked suppression of CD4+ T lymphocytes, others are less virulent, producing relatively low viral loads and having a minimal impact on CD4+ T lymphocytes. There has been considerable effort to develop a prophylactic vaccine against FIV, which ultimately led to the production of a licensed inactivated virus vaccine (Fel-O-Vax FIV; Fort Dodge Animal Health) containing two isolates of FIV, FIV Petaluma (subtype A) and FIV Shizuoka (subtype D). The vaccine is licensed in the USA, Canada and Australia for use in cats over eight weeks of age, and has been shown to provide protection against a number of FIV isolates, including those of subtypes A, B and C (Uhl and others 2002, Pu and others 2005). However, in many cases the challenge viruses used in those studies were poorly characterised or may not have been representative of isolates likely to be encountered in cats in the field. Previous studies by the present authors have used a