Virus isolation of feline calicivirus (FCV) can require several days. The positive predictive value of serologic testing is poor because of widespread vaccination and natural exposure. Reverse transcription polymerase chain reaction (RT-PCR) can be used to rapidly amplify FCV RNA but will also amplify the vaccine strain.
Feline herpesvirus type-1 (FHV-1) can be detected by direct fluorescent staining of conjunctival scrapings, virus isolation, or PCR; current PCR tests also detect vaccination strains. PCR also failed to correlate with the presence of stomatitis in cats; quantitative PCR, although considered superior for FHV-1 detection, failed to correlate with the presence of conjunctivitis. PCR cannot distinguish between a carrier and a clinically ill cat for Mycoplasma species, Chlamydophila felis, and Bordetella bronchiseptica. PCR testing is hampered by the presence of PCR inhibitors in the stool for Giardia species, so false-positive tests may occur. However, PCR may be useful for identification and diagnosis of Cryptosporidium felis, as the species is more sensitive than immunofluorescence assays in cats. PCR is the test of choice for diagnosis of M haemofelis, M haemominutum, and M turicensis. PCR has also been useful in the diagnosis of Anaplasma phagocytophilum and an Ehrlichia canis-like organism that affects cats. The usefulness of PCR for Rickettsia felis is unclear. Testing healthy cats for Bartonella is not recommended; testing should be reserved for cats with suspected clinical bartonellosis. A PCR-positive result does not prove clinical bartonellosis because this organism is so common in cats. Serology and PCR are recommended diagnostic approaches. Cytauxzoon felis is easily detected by cytologic examination of blood smears or aspirates; PCR can be used to amplify the organism’s DNA from the blood of cytologically negative cats. There are no commercially available serologic tests for this organism. RTPCR is not very accurate in distinguishing feline immunodeficiency virus (FIV)–vaccinated cats from those with naturally acquired infections because FIV is not present in the blood at high levels. The stage of feline leukemia virus infection has been characterized by quantitative PCR; however, these assays are not widely available. Feline enteric coronavirus and feline infectious peritonitis (FIP) virus can both be amplified from cat blood; however, positive results do not necessarily correlate with clinical disease. Toxoplasma gondii, Bartonella species, FHV-1, and coronavirus have all been amplified from aqueous humor in cats with uveitis.

Commentary: Diagnosis of infectious diseases was based on cytologic techniques, histopathologic techniques, and microbiological cultures to demonstrate the organism prior to development of molecular assays. Serologic assays were used to demonstrate an immune response to infection. Molecular assays are superior because of the inherent weaknesses in “traditional tests.” For example, cytologic and histologic techniques require organisms large and numerous enough for visualization by microscope. Culture requires knowledge of the organism’s growth needs and its preservation en route to the laboratory. Serologic testing is problematic because it can take weeks for the host to mount an immune response and only demonstrates exposure. However, molecular testing is not without its problems, the most immediate of which is laboratory quality control. In addition, many molecular tests may fail to discriminate between vaccine and infection strains. Finally, clinicians must be knowledgeable about the positive and negative predictive value of a test.—Karen A. Moriello, DVM, Diplomate ACVD

Molecular diagnostic assays for infectious disease in cats. Veir JK, Lappin MR. VET CLIN NORTH AM SMALL ANIM PRACT 40:1189-1200, 2010.