Attempts to serologically classify feline caliciviruses on a national and an international basis.

A serological classification of five feline calicivirus (FCV) strains of Swiss origin and 13 isolates of Austrian origin was attempted. The antisera used had been prepared in rabbits against the five Swiss strains and in goats against six American strains. Homologous and heterologous neutralization tests were made in tube cultures using sera at dilutions that contained 20 antibody units, in unabsorbed sera at low dilution, and in sera after subjection to three consecutive absorptions with lyophilized feline liver powder. None of these procedures resulted in the delineation of serotypes. A study using 20 antibody units yeilded FCV strains with broad-spectrum antigenicity, which seem promising for vaccination trials and for covering global needs. In addition, readily neutralized strains suitable for epidemiological work on cat sera, and possibly also for measuring humoral response to vaccines, could be recognized. The extent of serum titer variance, after 10-fold variance of virus input, was established as being only two- to threefold. Five Swiss FCV strains have been shown to remain antigenically stable over 10 years of laboratory passage. FCV strains and the procedure using 20 antibody units offered a suitable approach for international comparative work. FCV strains with little cross-neutralization should be subjected to higher antibody concentrations of existing antisera before attempting to create serotypes. Liver powder absorptions, which reduced homologous and heterologous neutralization titers to similar extents, subsequently proved to be unsuitable for use in FCV classification.