Twenty-six, 9 weeks-old Specific Pathogen-Free (SPF) male kittens were obtained from Liberty Research, Inc. (Waverly, NY, USA). Animals were kept under barrier conditions and under optimal ethological and hygienic conditions in one group of 5 cats (uninfected control group) and three groups of 7 cats each (group 1K, 10K, and 100K, based on viral infectious dose). Prior to the beginning of the experiment the cats were tested by PCR, RT-PCR and serology and shown to be negative for FeLV, FIV, Herpes-, Corona-, Calici-, Parvovirus and feline hemotropic mycoplasmas.