Clinical signs of FPL include depression, abdominal pain, vomiting, diarrhea that may be bloody, and fever. Owners often report
finding their cats with their heads hanging over their water dishes.6 The fevers are diphasic, peaking at 24 hours and again at 48 hours.10 Clinical laboratory findings include neutropenia, often with lymphopenia and anemia, and electrolyte and total protein concentrations
that reflect dehydration, vomiting, and diarrhea.6
1A. Gross lesions of FPL virus infection. Figure 1A shows a flaccid small intestine from a 3-month-old kitten.
At necropsy, there may be nasal and ocular exudation and tacky mucous membranes. A consistent and particularly striking finding
in cases of FPL is flaccidity of the intestine, often with mucosal sloughing and hemorrhage (Figures 1A & 1B). Normal feline small intestine is turgid and slightly coiled. A ground-glass appearance to the intestinal serosa, lymph
node enlargement and edema, and a gelatinous, greasy, or liquid appearance to the bone marrow may also be present.10 Histologic examination of the intestine reveals dilated crypts lined by irregular, hyperplastic epithelial cells; fusion
and blunting of the villi; and epithelial necrosis. In some cases, eosinophilic intranuclear inclusions are identifiable in
infected crypt epithelium (Figure 2). In lymphoid tissues, edema, hyperemia, and lymphoid necrosis are seen frequently, and occasional intranuclear inclusion
bodies are found in histiocytes.10
Figure 1B shows a hemorrhagic and flaccid small intestine from an 8-month-old kitten. These cases are from two separate FPL
outbreaks that occurred in animal shelters in Wisconsin in 2003.
FPL can be presumptively diagnosed based on a consistent history, clinical signs, and clinical pathology findings. FPL can
also be presumptively diagnosed postmortem based on the gross and histopathologic lesions discussed above. However, there
are many other FPL diagnostic methods.
2. This photomicrograph from the small intestine of a cat with FPL shows dilatation of the intestinal crypts, hyperplasia
and necrosis of the crypt epithelium, and a viral intranuclear body in an infected epithelial cell (arrow) (40X).
Two serologic tests, serum neutralization and hemagglutination inhibition, can be helpful in determining the activity of FPL
in a feline population. Both tests should be performed on paired sera to determine whether there is a rise in titer between
the acute and convalescent samples.7 Both sera should be submitted to the same laboratory and should be assayed in the same test run to obtain reliable data.
Fecal antigen test kits such as the SNAP Parvo Antigen Test (Idexx) are only approved and licensed for detecting canine parvovirus,
but it is generally known that they also detect FPL viral antigen in feline feces.6,11 These tests are used extralabel because they allow rapid, inexpensive, in-house detection of the virus. They are sensitive
and specific but can detect FPL antigen in cats vaccinated with a modified live FPL vaccine within five to 12 days. Another
type of bench-top test, a one-step immunochromatographic test not currently marketed in the United States, has been reported
with a similar sensitivity to the SNAP Parvo Antigen Test and was developed for use with feline, canine, and mink fecal samples.11