Page 1518 - Small Animal Internal Medicine, 6th Edition

1490 PART XIV Infectious Diseases

result. If the test used is very sensitive, a negative test result
helps exclude the diagnosis of FIP because most cats with
VetBooks.ir effusive or noneffusive disease are serum antibody-positive.
However, cats with FIP are occasionally serologically nega-
tive because of rapidly progressive disease, with a delayed
rise in titer, disappearance of antibody in terminal stages of
the disease, or immune complex formation. Positive coro-
navirus serologic test results are more difficult to use for
a number of reasons. Infection of cats by any coronavirus
can cause cross-reacting antibodies; therefore a positive anti-
body titer does not diagnose FIP, protect against disease,
or predict when a cat may develop clinical FIP. Presence
of maternal antibodies can confuse the diagnosis in very
young kittens. However, this is age dependent as maternal
antibodies decline to undetectable concentrations by 4 to 6
weeks of age; kittens infected in the postnatal period become
seropositive at 8 to 14 weeks of age. Because coronavirus
antibody tests are not standardized, results from differ- FIG 96.2
ent laboratories commonly do not correlate (Addie et al., Abdominal effusion consistent with the effusive form of feline
2015). infectious peritonitis identified on necropsy of an affected
cat.
Immunocytochemistry has been developed to detect
coronavirus antigen in CSF, aqueous humor, and effusions
(Doenges et al., 2016; Felten et al., 2017; Felten et al., 2018). antigens are commonly detected by immunocytochemistry
The assay can be very sensitive, but the specificity is low in in the effusions of cats with FIP, but the sensitivity is less
most studies because some cats without FIP can be positive. than 100% as antigens have also been detected in the effu-
Immunohistochemistry documenting the presence of coro- sions of some cats with other diseases (Felten et al., 2017).
navirus in tissues concurrently with characteristic histopath- If molecular assays are available, they should be performed
ologic findings can be used to prove the diagnosis of FIP on cats with effusions consistent with FIP becasue viral RNA
(Harvey et al., 1996). can be amplified from effusions in most cats with FIP by
Because virus isolation is not practical clinically, RT-qPCR RT-qPCR and is unlikely to be amplified from effusions from
is used most frequently to amplify coronaviruses in feces, other causes.
blood, effusions, CSF, aqueous humor, and tissues. Multiple The most definitive diagnosis of FIP is based on combina-
different molecular targets have been employed with varying tion of characteristic histopathologic findings with the pres-
sensitivity and specificity. Use of these assays on blood or ence of coronavirus by virus isolation, immunocytochemical
feces generally do not differentiate FIPV from FECV and or immunohistochemical staining, or amplification of viral
so positive results to not prove FIP. Use of these assays on RNA in effusions, other fluids like CSF or aqueous humor or
effusions appear to have the most clinical utility. If effusions tissues by RT-qPCR.
are not present, use of an RT-qPCR on circulating periph-
eral blood mononuclear cells was more sensitive than use of Treatment
serum (Doenges et al., 2017). In cats with neurologic and/or Because an antemortem diagnosis of FIP is difficult to make,
ocular signs, the sensitivity of a real-time RT-PCR on CSF assessment of studies reporting successful treatment is virtu-
was 85.7% in one study (Doenges et al., 2016) ally impossible. Treatments for FIP have been reviewed, and
Effusions from cats with FIP are sterile, colorless to there is no protocol that is consistently effective (Hartmann
straw-colored, may contain fibrin strands, and may clot and Ritz, 2008; Pedersen et al., 2014b). A small percentage
when exposed to air (Fig. 96.2). The protein concentration of cats have spontaneous remission, adding to the confusion
on fluid analysis commonly ranges from 3.5 g/dL to 12 g/ concerning therapeutic response. Supportive care, including
dL and is generally higher than that associated with other correction of electrolyte and fluid balance abnormalities, and
diseases. Mixed inflammatory cell populations of lympho- lessening stress should be provided to cats with FIP as
cytes, macrophages, and neutrophils occur most commonly; needed.
neutrophils predominate in most cases, but in some cats Feline coronaviruses are dependent on 3C-like protease
macrophages are the primary cell type seen. In some cats, the for replication (Kim et al., 2015). The use of one 3C-like
coronavirus antibody titers are greater in the effusion than in protease inhibitor showed promise in the treatment of FIP
serum. Measurement of protein concentrations in effusions but is not currently on the market (Pedersen et al., 2017). In
and calculation of the albumin/globulin ratio (AGR) can vitro inhibition of FIP virus replication has also been dem-
aid in the diagnosis of effusive FIP. In one study, an AGR onstrated with a number of drugs, including ribavirin,
of 0.5 had a positive predictive value of 89%, and an AGR human interferon-α, feline fibroblastic interferon-β, adenine
of 1.0 had a negative predictive value of 91%. Coronavirus arabinoside, and amphotericin B. However, to date no