EVALUATION OF COFACTOR EFFECT OF FELINE SYNCYTIUM-FORMING VIRUS ON FELINE IMMUNODEFICIENCY VIRUS-INFECTION

Although feline immunodeficiency virus (FIV) and the unrelated retrovi rus feline leukemia virus (FeLV) are associated with acquired immune d eficiency in cats, experimental and field evidence indicates that coin fection with both viruses may lead to more serious disease syndrome. A third feline retrovirus, feline syncytium-forming virus (FeSFV), whic h is far more prevalent than either FIV or FeLV and is considered nonp athogenic in nature, is consistently coisolated from sick Fiv-infected cats. To determine the potential role of FeSFV in enhancement of FIV- mediated disease, persistent FeSFV infection was established in 14 of 24 nine-month-old cats. Four months later, half the FeSFV-infected and half the noninfected cats were inoculated with blood obtained from a cat persistently infected with the Petaluma strain of FIV. At postinoc ulation week 17, 1 male cat infected with only FIV died of bacterial b ronchopneumonia that could have been attributed to FIV-induced acquire d immune deficiency-like syndrome. However, none of the remaining cats had clinical illness, whether infected with either virus alone or coi nfected with both viruses. As early as postinoculation week 6, decreas es were observed in the CD4+ to CD8+ T-lymphocyte ratio of both groups of cats inoculated with FIV. Infection with FeSFV had no effect on th e CD4+ to CD8+ T-cell ratio. Mitogen stimulation assays and total WBC count were unaffected by FeSFV infection, although an increase in numb ers of neutrophils from FeSFV-infected cats was consistent, especially when compared with the decrease observed after FIV infection. Overall , results of the study indicate that coinfection with FeSFV may not en hance progression of the FIV-induced early stages of disease and that the positive correlation between FeSFV and FIV-induced acquired immune deficiency-like syndrome is attributable to a common mode of transmis sion, rather than to a synergistic effect of coinfection.