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Feline Immunodeficiency Virus (FIV) Testing 1341

• Quantitative mRNA RT-PCR results Causes of Abnormally Low Levels tissue aspirates/biopsies in special sample
reported as negative, high positive (>50 False-negative serologic test results may arise buffer (obtained from Auburn University,
VetBooks.ir • RT-PCR results negative or positive with antibody production (especially in immuno- uploads/2015/03/FIP-virus.pdf). Keep samples
from peracute infection, low level or lack of
copies/specimen), or low positive (<50
www.vetmed.auburn.edu/wp-content/
copies/specimen)
suppressed cats), or antibody bound within
for RT-PCR at room temperature (do not
biotyping
immune complexes. False-negative RT-PCR
idexxcom-live-b02da1e51e754c9cb292133b-
results due to low nuclei acid load or unknown freeze). For FIPV RealPCR™, refer to https://
Causes of Abnormally High Levels strain variation. 9c56c33.aldryn-media.com/filer_public/dd/81/
Positive FCoV titer (IFA or ELISA) indicates dd8192c9-ae9b-4d1b-841a-06748adc75ac/
nonspecific exposure to any of the following: Next Diagnostic Steps to Consider feline-infectious-peritonitis-virus.pdf.
FIPV, FECV, other coronaviruses, or FIP if Levels are Low
vaccine. Very high titers (>1 : 16,000) may If FIPV is still suspected, consider same diag- Relative Cost: $$ (IFA serum antibody,
increase suspicion of FIP when compatible nostic steps as described above or submitting PCR), $$$ (IFA tissue, RT-PCR)
clinical signs are present, but the result is not alternative specimen type for RT-PCR.
confirmatory. Pearls
Lab Artifacts • A negative titer does not rule out FIP, nor
Next Diagnostic Steps to Consider RT-PCR: false-positive results due to speci- does a positive titer confirm FIP. Multiple
if Levels are High men contamination; false-negative results due site sampling and evaluation via the RT-PCR
Correlate with history, physical exam findings, to degradation of nucleic acids via improper increases sensitivity and specificity.
and other laboratory data. Cytologic evaluation, sample handling or storage • FCoV testing is not useful to predict future
PCR, or IFA on effusion fluid may be sup- illness in healthy cats.
portive but not definitive. RT-PCR biotyping Specimen Collection and Handling
or simultaneous testing of blood, feces, and 0.5-1 mL serum (red top tube) or CSF for AUTHOR: Patty J. Ewing, DVM, MS, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
effusion or tissue aspirates may assist in dif- IFA or ELISA. Samples stable for days to
ferentiating FIP from FECV. Histopathologic weeks at 4°C or for months at −20°C. For
and immunohistochemical analysis of biopsy mRNA RT-PCR, submit ≥ 0.5 mL of EDTA
or necropsy tissue remains the gold standard blood (lavender top tube) and/or body cavity
for FIP diagnosis. See p. 327. fluid, fecal swab (for exclusion of FECV),

Feline Immunodeficiency Virus (FIV) Testing

Definition Next Diagnostic Steps to Consider Specimen Collection and Handling
Feline immunodeficiency virus (FIV) is a len- if Levels are High • ELISA, Western blot, IFA: 1.0 mL serum
tivirus that attacks the immune system of cats, • Confirmatory tests: Western blot or IFA may (red top tube); stable at 2°C-8°C for 7 Laboratory Tests
resulting in progressive immunosuppression. or may not differentiate between maternal days or weeks to months if frozen. Most
antibody, vaccination, or infection. ELISA tests are run as point-of-care kits per
Physiology • RT-PCR test may be a useful test for manufacturer’s directions.
FIV is inoculated via saliva or blood from bite differentiating some vaccine exposures or • RT-PCR: 2 mL whole blood (lavender top
wounds. There are three phases of infection. maternal antibody from true infection. tube), 0.5 mL CSF; stable for 10 days at
Acute: FIV initially infects T lymphocytes 2°C-8°C or weeks to months frozen
and salivary glands, then other mononuclear Causes of Abnormally Low Levels
cells; cats may have fever, leukopenia, and Negative ELISA, IFA, or Western blot indicates Relative Cost: $$ (IFA, ELISA); $$$
lymphadenopathy. Subclinical: cats may be lack of exposure, acute or subacute infection, or (Western blot)
without clinical signs for years. Chronic: undetectable levels of antibody due to immu-
progressive immunosuppression with various nosuppression. Negative RT-PCR indicates Pearls
clinical disorders (see p. 325) lack of infection, nucleic acid below limit of • Seroconversion may take up to 3 months,
detection of assay, or uncommon strain not so when screening healthy cats via ELISA,
Reference Interval detected by assay. retesting is recommended > 2 months after
ELISA, IFA, Western blot or RT-PCR: positive potential exposure.
or negative Next Diagnostic Steps to Consider • FIV-vaccinated cats test positive as early as
if Levels are Low 28 days postvaccination and remain positive
Causes of Abnormally High Levels If clinical signs suggest FIV infection, test via a for at least 1 year. RT-PCR may help identify
ELISA and immunochromatogenic assays are second method or retest in 4-12 weeks. FIV-infected cats that have been vaccinated
screening tests for antibody. Positive result or have unknown vaccination history and
indicates exposure to FIV or FIV vaccine. Lab Artifacts test positive on ELISA, IFA, or Western blot.
Due to maternally transferred antibody, RT-PCR: false-positive results due to contami-
FIV testing of kittens ≤ 6 months old born nation of samples; false-negative results due AUTHOR: Patty J. Ewing, DVM, MS, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
to FIV-positive queens is unreliable (false- to degradation of nucleic acids via improper
positives). sample handling and storage

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