IMMUNOLOGICAL PHENOMENA IN THE EFFUSIVE FORM OF FELINE INFECTIOUS PERITONITIS

Abstract

The effusive form of feline infectious peritonitis (FIP) was reproduced by injecting 12-16 wk old kittens i.p. with a cell-free inoculum derived from tissues of infected cats. The kittens used for the study were positive for FIP virus-reacting antibodies [Ab] before inoculation or they were seronegative. Seropositive kittens were obtained from a cattery where the natural infection was enzootic and seronegative kittens were obtained from a specific-pathogen-free cattery. Only about half the kittens that were seronegative before inoculation developed disease or serum Ab to the tissue-derived virus. Seronegative kittens that developed disease showed no signs of illness until 8-10 days after inoculation, and they lived for 7-14 days after clinical signs appeared. Onset of clinical disease coincided with the appearance of serum Ab. All seropositive kittens became ill within 36-48 h after inoculation and died within 5-7 days. If seronegative kittens were treated with immune serum or immunoglobulin (Ig)G, they developed disease with the same frequency, acuteness and severity as seropositive kittens. Foci of hepatitis and serositis in seropositive kittens contained viral antigen, IgG bound to antigen and complement [C]. Serum C activity decreased several days before death in seropositive kittens inoculated with tissue-derived FIP virus. The temporal relationship of clinical disease and the appearance of serum Ab, the more acute and severe nature of the disease produced in seropositive kittens and the presence of Ab and C in the lesions indicated that effusive FIP is immunologically mediated.