Feline infectious peritonitis (FIP) results from mutations in the viral genome 
during a common feline enteric coronavirus (FECV) infection. Since many 
virological and immunological data on FECV infections are lacking, the present 
study investigated these missing links during experimental infection of three 
SPF cats with FECV strain UCD. Two cats showed mild clinical signs, faecal 
shedding of infectious virus from 4 dpi, a cell-associated viraemia at 
inconsistent time points from 5 dpi, a highly neutralising antibody response 
from 9 dpi, and no major abnormalities in leukocyte numbers. Faecal shedding 
lasted for 28-56 days, but virus shed during this stage was less infectious in 
enterocyte cultures and affected by mutations. Remarkably, in the other cat 
neither clinical signs nor acute shedding were seen, but virus was detected in 
blood cells from 3 dpi, and shedding of non-enterotropic, mutated viruses 
suddenly occurred from 14 dpi onwards. Neutralising antibodies arose from 21 
dpi. Leukocyte numbers were not different compared to the other cats, except for 
the CD8(+) regulatory T cells. These data indicate that FECV can infect immune 
cells even in the absence of intestinal replication and raise the hypothesis 
that the gradual adaptation to these cells can allow non-enterotropic mutants to 
arise.