The diagnosis of feline infectious peritonitis (FIP) is hampered by the characteristic etiopathogenesis of the disease. 9 The coronavirus responsible for the disease (FIPV) originated by a mutation in the widespread feline enteric coronavirus (FECV). 9,14 These coronaviruses are both phenotypically and genotypically identical, as demonstrated by polymerase chain reaction (PCR). 14 Both feline coronaviruses (FCoVs) are able to trigger the production of antibodies. High antiFCoV titers are also detectable in healthy cats in FECVendemic catteries, 9,10 and the formation of immune complexes could results in negative serology in symptomatic cats. 8 Furthermore, false-positive results occur from cross-reactivity with coronaviruses of other species, such as canine coronavirus (CCV) or transimissible gastroenteritis virus (TGEV), or with extraneous noncoronavirus proteins that are copurified with the FCoV and included in commercially available diagnostic enzyme-linked immunosorbent assay kits. 1 Hematology, serum chemistry, and serum protein electrophoresis give only a strongly suggestive but nonpathognomonic pattern of FIP. 9,12,13 In the effusive forms, diagnosis could be improved by cytology 3 and protein analysis of the fluid. 9,11-13 Although FCoV RNA has been demonstrated in the plasma of healthy cats in FECV-endemic catteries, 6 FCoVs detected from areas other than the intestinal lumen should be interpreted as FIPV, particularly if the viruses are detected in lesions 15 or effusions. 2 In this study, the results obtained using 3 different diagnostic methods (anti-FCoV direct immunofluorescence test, cytology, and protein analysis) for 110 cats with effusions over a 5-year period were compared to determine the utility of these methods for diagnosing FIP from analysis of the effusion alone. Cats with clinically detectable effusion were examined (Table 1), although in some cases clinical and laboratory data (serum chemistry, ultrasonography, macroscopic appearance of the effusion) suggested diseases other than FIP.Approximately 2 ml of effusion was withdrawn from each cat before death (72 cats) or within 2 hours after death (38 cats). The sampled fluid was put in a tube containing ethylenediamine tetraacetic acid. The protein content of the effusions of 51 cats was evaluated in a discrete autoanalyzer a by the biuret method. b Fifty to 100 l of fluid was cytocentrifuged c within 15 hours at 130 ϫ g for 10 minutes. Two Received for publication August 25, 1997.slides were obtained from each fluid sample; 1 slide was routinely stained with May Grünwald-Giemsa and the other was used immediately or after storage at Ϫ20 C for no more than 1 week for direct immunofluorescence (DIF) tests as previously described. 2 The slides were fixed and dehydrated in acetone : methanol (3:1) for 20 minutes and incubated for 30 minutes at 37 C in a moist chamber with 100 l of a feline polyclonal fluorescein-conjugated antiserum d detecting FCoV biotypes I and II and cross-reacting with TGEV and CCV. After washing 4 times for 10 minutes e...

Twenty‐one cases of feline infectious peritonitis (FIP) were diagnosed using a direct immunofluorescence test on cytocentrifuged pleural and peritoneal effusions from cats sampled in vivo (11 cases) and at necropsy (10 cases). A commercial fluorescent polyclonal antiserum of feline origin reacting with FIPV and cross reacting with transmissible gastroenteritis virus and canine coronavirus was used. Eleven cats with ascites of a different origin were used as negative controls. The direct immunofluorescence test was 97 per cent reliable (31 cases of 32) and can be used in routine diagnosis.

Biopsy of liver and kidney can correctly identify FIP lesions. However, false-negative results or inadequate samples occur with moderate frequency, especially for immunochemical analysis. Diagnostic sensitivity may be increased when both TCB and FNAB specimens from the same organ are examined.

Blood was collected from 55 cats with feline infectious peritonitis (FIP) and from 50 control cats in order to define whether differences in pathological findings and in distribution of feline coronaviruses (FCoV) can be associated with changes in haemograms, serum protein electrophoresis, and antibody titres. Compared to controls, the whole group of FIP-affected cats had blood changes consistent with FIP. Based on the pathological findings or on the immunohistochemical distribution of viral antigen, FIP-affected cats were divided in the following groups: subacute against acute lesions; low against strong intensity of positivity; intracellular against extracellular positivities; positive against negative lymph nodes. Lymphopenia was more evident in cats with acute forms, strong intensity of positivity, extracellular antigen and negative lymph nodes. Cats with positive lymph nodes had the most evident changes in the protein estimations. These results suggest that differences in pathological findings might depend on different reactive patterns to the FCoVs.

Summary In focal lesions of feline infectious peritonitis (FIP), the cells involved in the delayed‐type hypersensitivity were identified in formalin‐fixed paraffin‐embedded and frozen samples taken from 35 affected cats. The clinical diagnosis of FIP was confirmed by necropsy, histology and direct immunofluorescence against the coronaviruses on cryostatic sections. The immune cells were detected immunohistochemically by the Avidin‐Biotin‐Complex (ABC) method using either polyclonal antibodies against lymphoid antigens (CD3) or monoclonal antibodies against lymphoid (PAN‐T, CD4, CD8) and myeloid antigens (MAC387). Better identification of T cells and macrophages was found on formalin‐fixed paraffin‐embedded sections than on cryostatic ones, while T lymphocyte subpopulations could be differentiated only in cryostatic sections. Type IV hypersensitivity was detected in focal feline infectious peritonitis virus (FIPV)‐induced lesions from progressive activation of T lymphocytes, mainly CD4+, and the presence of granulocytes and macrophages. The FIPV‐induced lesions could be studied as examples of granulomas caused by unconventional antigens, such as viruses or immune complexes.