Diagnosis of FIP
Presumptive diagnosis of FIP is based on history, clinical signs, and results of laboratory tests. Diagnosis of effusive FIP is based largely on analysis of the characteristic exudate. The fluid typically is sterile, viscous or ropey, and yellow to tan; it may contain fibrin strands, has a high specific gravity (1.017-1.047) and high protein content (5-12 g/dL), and is composed of variable amounts of mixed inflammatory cells (1,600-25,000/µL). Mixtures of neutrophils, lymphocytes, macrophages, and fewer mesothelial cells in a granular, eosinophilic, proteinaceous background are seen on Wright’s-stained smears. Protein determinations and electrophoresis of the exudate may be useful in diagnosis. Exudates with total protein >3.5 g/dL (of which >50% is gamma globulin) and cytology consistent with FIP have a positive predictive value of >90%. In addition, the albumin: globulin ratio of the effusion is usually <0.8; a ratio <0.45 is usually predictive of effusive FIP. Documentation of a coronavirus infection by either a positive titer or coronavirus RNA by reverse-transcriptase PCR support a diagnosis of effusive FIP.
About 50% of cats with effusive FIP (and up to 70% of cats with noneffusive FIP) have an increased total plasma protein (>7.8 g/dL), often with hyperglobulinemia (>4.6 g/dL) and a hypergammaglobulinemia. Serum protein electrophoresis may show increases in α2-globulins and polyclonal (occasionally monoclonal) increases in gamma globulin. Hematologic changes in both effusive and noneffusive FIP, although variable, most consistently show a neutrophilic leukocytosis (>19,000 cells/µL) and a relative lymphopenia (>1,500 cells/µL). Forty to 50% of cats develop a progressive normochromic, normocytic anemia (PCV <24%) that may be severe and nonregenerative if FIP is accompanied by FeLV or infection with Haemobartonella felis . Serologic tests (ELISA, immunofluorescent antibody) that detect antibodies against coronaviral proteins are not specific for FIPV and also detect antibodies against feline enteric coronavirus. The coronavirus titer in cats with FIP is usually increased (1:100 to 1:3,200); some cats with clinical FIP have negative or very low titers. PCR does not differentiate between feline coronaviruses and may be interpreted the same as a positive antibody titer.
Noneffusive FIP is a greater diagnostic challenge. Serum protein abnormalities in debilitated cats with nonresponsive fever, weight loss, multisystemic signs (including ocular and CNS signs), and increased coronavirus titers are suggestive of noneffusive FIP. Blood changes in the noneffusive form must be viewed collectively. Cats that have clinical signs of FIP, lymphopenia, and hyperglobulinemia of >5.1 g/dL have an almost 90% probability of having FIP. In cats that do not meet all 3 criteria, there is a 99% probability that FIP is not the diagnosis. A thorough ophthalmic examination is indicated because nearly 40% of noneffusive cases have ocular lesions. Other ancillary laboratory tests may be helpful; clinical chemistries may indicate organ dysfunction in liver, kidney, or pancreas. In cases of neurologic disease with diffuse meningeal involvement, CSF analysis may show increased protein content (90-2,000 mg/dL) and increased numbers of cells (90-9,250 cells/µL), predominantly neutrophils. The most definitive antemortem diagnostic technique is laparotomy and organ punch biopsy of lesions with subsequent demonstration of typical histopathologic changes.
FIP should be considered in the differential diagnosis of any condition that causes peritoneal or thoracic fluid accumulation and in any chronic wasting disease of cats. Effusive FIP with peritoneal involvement should be differentiated from ascites due to congestive heart failure or hypoproteinemia (renal and liver disease, glomerulonephritis, malabsorption, parasitism), neoplasia, bacterial peritonitis, pansteatitis, toxoplasmosis, tuberculosis, pregnancy, and trauma. Differential diagnoses of effusive FIP with pleural effusion include cardiac insufficiency, neoplasia (lymphoma), pyothorax, chylothorax, cryptococcosis, lung lobe torsion, diaphragmatic hernia, and trauma (hemothorax). Differential diagnosis of noneffusive FIP includes neoplasia and other systemic infectious diseases such as toxoplasmosis, nocardiosis, actinomycosis, tuberculosis, and deep mycotic disease (cryptococcosis, coccidioidomycosis, histoplasmosis, blastomycosis).
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