HEMATOLOGY AND SERUM BIOCHEMISTRY EVALUATION

Hematology results often are abnormal in cats with FIP, but the changes are not pathognomonic. White blood cell counts can be decreased or increased. Lymphopenia commonly is present, mainly caused by apoptosis of uninfected T cells, primarily CD8+ T cells, as a result of high tumor necrosis factor-α (TNFα) concentrations produced by virus-infected macrophages.²⁰ However, lymphopenia in combination with neutrophilia generally is common in cats as a stress leukogram, is not typical for FIP, and can occur in many severe diseases in cats. A mild to moderate nonregenerative anemia also is a common finding in cats with FIP, but is another nonspecific laboratory abnormality that may occur in almost any chronic disease in cats.

The most common laboratory abnormality in cats with FIP is an increase in total serum protein concentration caused by increased globulins, mainly γ-globulins.^21,22 In one study, hyperproteinemia was present in about 50 per cent of cats with effusion and in 70 per cent of cats without effusion.²³ After experimental induction of FIP, an early increase of α₂-globulins is present, while γ-globulins and antibody titers increase only just before appearance of clinical signs.^24,25 The characteristically high levels of γ-globulins and the increased antibody titers invite the conclusion that hypergammaglobulinemia is due to a specific anti-FCoV immune response. Antibody titers and hypergammaglobulinemia show a linear correlation, but the wide variation in anti-FCoV titers at a given concentration of γ-globulins indicates that additional (autoimmune) reactions play a major role in the pathogenesis of FIP. It has been discussed that stimulation of B cells by interleukin-6, which is produced as part of the disease process, contributes to the increase in γ-globulins.²⁶ Thus, total protein in cats with FIP can reach very high concentrations of up to 120 g/L (12 g/dL) and higher. Albumin-to-globulin ratio, however, has a significantly higher diagnostic value to differentiate FIP from other diseases than total serum protein or γ-globulin concentrations, because a decrease in serum albumin also may occur through a decrease in production in association with liver failure or protein loss.^27,28 Protein loss in cats with FIP is caused by glomerulopathy secondary to immune complex deposition, by loss of protein due to exudative enteropathy in the case of granulomatous changes in the intestines, or by extravasation of protein-rich fluid during vasculitis.²⁸ An optimal cut-off value of 0.8 was determined for the albumin-to-globulin ratio.²⁸

Serum protein electrophoresis also is often performed in cats suspected to have FIP (Figure 7-12); the main rationale behind the test is to distinguish a polyclonal from a monoclonal hypergammaglobulinemia in order to differentiate FIP (and other chronic infections) from tumors such as multiple myelomas or other plasma cell tumors (see Chapter 65). However, in cats with FIP, both polyclonal and monoclonal hypergammaglobulinemia can occur, and the same is found in tumors. Therefore the value of electrophoresis is limited.

Figure 7-12 Electrophoresis of a cat with hypergammaglobulinemia caused by FIP.

Other laboratory parameters, including liver enzymes, bilirubin, blood urea nitrogen, and creatinine, can be elevated variably depending on the degree and localization of organ damage but are not helpful in establishing an etiological diagnosis. Hyperbilirubinemia and icterus (Figure 7-13) often are observed and frequently are a reflection of hepatic necrosis. Sometimes bilirubin concentration is increased in cats with FIP without evidence of hemolysis, liver disease, or cholestasis; this unusual finding is otherwise observed only in septic animals. Bilirubin metabolism and excretion into the biliary system is compromised in these cats, likely due to high levels of TNFα that inhibit transmembrane transport. Thus, high bilirubin in the absence of hemolysis and elevation of liver enzyme activity should raise the suspicion of FIP.²⁸

Figure 7-13 Yellow mucous membranes in a cat with liver insufficiency and hepatic icterus due to FIP.

Recent research has focused on the diagnostic value of acute-phase reaction parameters including α1-acid glycoprotein (AGP), a serum acute phase protein that is elevated in cats with FIP.^29,30 AGP is not only overexpressed in cats with FIP but also undergoes several modifications in the sialic acid content, including decreased expression of both α(2,6)-linked and α(2,3)-linked sialic acid.³¹ High serum AGP levels (>3 mg/mL) can support the diagnosis of FIP,³² but levels also rise in other inflammatory conditions, and thus these changes are not specific. Additionally, AGP also may be high in asymptomatic cats infected with FCoV, especially in households in which FCoV is endemic.³³

EFFUSION FLUID EXAMINATION

If there is effusion, the most important diagnostic step is to get a fluid sample because tests on effusion have a much higher diagnostic value than tests that can be performed on blood. Fluid can be obtained through ultrasound-guided fine-needle aspiration (Figure 7-14) or using the “flying cat technique” in case of ascites (Figure 7-15). Only about one half of all cats who are presented with effusion have FIP.³⁴ Thus, although effusions of clear yellow color (Figure 7-16) and sticky consistency often are called “typical,” the presence of this type of fluid in body cavities alone is not diagnostic. Sometimes the fluid caused by FIP has a totally different appearance, and even some cases with pure chylous effusion have been reported.³⁵ Usually, the effusion’s protein content is very high (>3.5 g/dL) and consistent with an exudate, whereas the cellular content is low (<5000 nucleated cells/mL) and resembles more that of a modified transudate or even pure transudate.

Figure 7-14 Ultrasound-guided puncture to obtain ascites for diagnostic tests in a cat with FIP.

Figure 7-15 “Flying cat technique” to obtain ascites without ultrasound by puncturing at the lowest point of the “flying” cat.

Figure 7-16 “Typical” effusion of clear yellow color and sticky consistency obtained from a cat with FIP.

Major differential diagnoses for these effusions include inflammatory liver disease, lymphoma, heart failure, and bacterial peritonitis or pleuritis. Lactate dehydrogenase (LDH) activity typically is high (>300 IU/L) in effusions due to FIP as it is released from inflammatory cells. Cytological examination of the effusion in cats with FIP shows a variable picture, but often consists predominantly of macrophages and neutrophils (Figure 7-17). Cytological changes may appear similar in cats with bacterial serositis or sometimes with lymphoma; these effusions, however, usually can be differentiated by the presence of malignant cells or intracellular bacteria in cytological specimens and by bacterial growth on culture, respectively. Electrophoresis on effusion fluid is a diagnostic tool with a relatively high positive predictive value (if albumin-globulin ratio is <0.4) and a relatively high negative predictive value (if the ratio is >0.8).

Figure 7-17 Cytological examination of the effusion with predominance of macrophages and neutrophils in a modified transudate of a cat with FIP.

“Rivalta’s test” is a very simple, inexpensive method that does not require special laboratory equipment and can be performed easily in private practice. This test was developed originally by an Italian researcher (named “Rivalta”) around 1900 and was used to differentiate transudates and exudates in human patients. However, other methods have replaced this test in human medicine. It is not diagnostically helpful in dogs with effusion. However, this test is very useful in cats to differentiate between effusions due to FIP and effusions caused by other diseases.²⁸ The high protein content and high concentrations of fibrin and inflammatory mediators lead to a positive reaction (Box 7-1). To perform this test (Figure 7-18), a transparent reagent tube (volume 10 mL) is filled with approximately 8 mL distilled water, to which one drop of acetic acid (highly concentrated vinegar, 98 per cent) is added and mixed thoroughly. On the surface of this solution, one drop of the effusion fluid is layered carefully. If the drop disappears and the solution remains clear, the Rivalta’s test is defined as negative. If the drop retains its shape, stays attached to the surface, or floats slowly down to the bottom of the tube (droplike or jellyfish-like), the Rivalta’s test is defined as positive (Figure 7-19). The Rivalta’s test had a high positive predictive value (86 per cent) and a very high negative predictive value for FIP (96 per cent) in a study in which cats who presented with effusion were investigated (prevalence of FIP 51 per cent).²⁸ Positive Rivalta’s test results sometimes can occur in cats with bacterial peritonitis or lymphoma. Those effusions, however, usually are easy to differentiate through macroscopic and cytological examinations, and/or bacterial culture.

Figure 7-18 Reagents necessary to perform the “Rivalta’s test” including a transparent reagent tube (volume 10 mL), distilled water, acetic acid (highly concentrated vinegar, 98 per cent).

Figure 7-19 Positive “Rivalta’s test” with a drop retaining its shape and staying attached to the surface. Performed with the effusion of a cat with FIP.

CEREBROSPINAL FLUID EXAMINATION

Analysis of cerebrospinal fluid (CSF) from cats with neurological signs due to FIP lesions may reveal elevated protein (50 to 350 mg/dL with a normal value of less than 25 mg/dL) and pleocytosis (100 to 10,000 nucleated cells/mL) containing mainly neutrophils, lymphocytes, and macrophages.^36–38 This is, however, a relatively nonspecific finding. Many cats with neurological signs caused by FIP have normal CSF.

MEASUREMENT OF ANTIBODIES

Antibody titers measured in serum are an extensively used diagnostic tool. However, a high percentage of healthy cats are FCoV antibody–positive, and most of those cats will never develop FIP. There is no “FIP antibody test”; all that can be measured is antibodies against FCoV. Thus, antibody titers must be interpreted extremely cautiously; the inadequacies and pitfalls of this test have been the topic of continuous discussions and controversies, and it has been assumed that more cats have died of misleading FCoV antibody test results than of FIP.³⁹ Although criticized frequently, antibody testing still has a certain role in the diagnosis and, more importantly, in the management of FIP, when done by appropriate methodologies and when results are interpreted properly. However, antibody testing can only be useful if the laboratory is reliable and consistent, because methodologies and antibody titer results may vary significantly. A single serum sample, divided and sent to five different laboratories in the United States, yielded five different results.⁴⁰

It is important to understand that the presence of antibodies does not indicate FIP and the absence of antibodies does not exclude FIP. Low or medium titers have no diagnostic value.²⁸ It has been shown that in cats with fulminant FIP, titers decrease terminally,³⁹ and approximately 10 per cent of cats with clinically manifest FIP have negative results.²⁸ This is either because large amounts of virus in the cat’s body bind to antibodies and render them unavailable to bind the antigen in the antibody test, or because the antibodies are lost into the effusion when protein is translocated during vasculitis. If interpreted carefully, very high titers can be of certain diagnostic value.

It also has been shown that the height of the antibody titer correlates directly with the amount of virus that is shed in the feces: cats with high antibody titers are more likely to shed FCoV and to shed more consistently higher amounts of the virus.⁴¹ Therefore the height of the titer is correlated directly with virus replication rate and the amount of virus in the intestines.

Antibody measurement also may be important for the common situation in practice, in which a cat is presented because it had been in contact with a cat with FIP or a suspected or known virus excreter. Either the owner wants to know the prognosis for the exposed cat, or the owner wishes to obtain another cat and needs to know whether the exposed cat is shedding FCoV. Also, cat breeders may request testing with the goal of creating a FCoV-free cattery. Screening a cattery for the presence of FCoV or screening a cat before introduction into a FCoV-free cattery also are important indications.

Measuring antibodies in fluids (e.g., effusion, CSF) other than blood has been investigated. Presence of antibodies in effusion is correlated with the presence of antibodies in blood,⁴² and thus, antibody titers in effusions also are not very helpful. Cats in a recent study had medium antibody titers irrespective of whether they had FIP or not.⁴³ One study looking into the diagnostic value of antibody detection in CSF found a very good correlation to the presence of FIP when compared to histopathological examination⁴⁴; however, two recent studies of a large number of cats presented to veterinary teaching hospitals revealed no significant difference in antibody titers in CSF from cats with neurological signs due to FIP compared to cats with other neurological diseases confirmed by histopathological examination.^45,46