The Role of Ultrasound in the Diagnosis of Feline Infectious Peritonitis

Dr. Lauren Pike

Patient Information:

Species: Feline
Breed: Domestic Shorthair
Gender: Neutered Male
Age: 10 Months

History:

Presented for vomiting and diarrhea of one week duration. Approximately one month prior, had experienced fever of unknown origin, with mildly increased white blood cells. At that time a fever of unknown origin panel had been performed and was negative (this included PCR tests for Analplasma spp., Bartonella spp., Cryptococcus spp., Cytauxzoon felis, Ehrlichia spp., feline calicivirus, feline coronavirus (FCoV), feline hemotropic mycoplasma (FHM), feline panleukopenia virus, FeLV, FIV, Salmonella spp. and Toxoplasma gondii).
The day of presentation, patient was noted to be febrile again, with possible bunching of the intestines on abdominal palpation.

Ultrasound Findings:

In the left kidney, there is a poorly-defined, heterogenous, irregularly shaped lesion causing the capsule at the lesser curvature to be irregular in contour, measuring 1.3x0.8 cm.
The ileocecocolic junction and transverse colon are severely thickened (up to 9.8 mm), with hypoechoic loss of layering. Within this thickening, there is a nodular area measuring 12.6 mm, defined by severely rounded thickening, at the junction of transverse and descending colon. The descending colon contains gas and liquid stool.
The mesenteric lymph nodes are moderately enlarged (up to 1.0 cm depth), with heterogenous, moderately-severely hypoechoic echogenicty.
Scant to mild anechoic peritoneal effusion is appreciated.

Abdominal Ultrasound Interpretation:

ICCJ/Colon - the findings are severe - DDX: inflammatory bowel disease/food intolerance vs. infiltrative neoplasia (small-cell lymphosarcoma vs. mast cell tumor) vs. parasitism (cestode) vs. dry FIP vs. fungal (histoplasmosis).
Lymph nodes - the findings are moderate - DDx: FIP vs. infiltrative neoplasia (lymphoma vs. mast cell vs. other) vs. IBD vs. infection vs. reaction vs. metastatic neoplasia
Kidney Lesion - the findings are mild -DDx: FIP vs. primary renal carcinoma, primary renal TCC, renal lymphosarcoma - may appear as diffuse disease or focal mass, renal malignant histiocytosis or mastocytosis, malignant osteosarcoma, hemangiosarcoma - metastatic VS. fungal infection (Cryptococcus or Aspergillosis)
Ascites- this finding is mild - DDx: transudate vs. hemorrhagic vs. exudate

Additional Tests:

Fine-needle biopsies of the ICCJ/Colon and lymph nodes were submitted to EasternVetPath, Casey J. LeBlanc DVM, PhD, Diplomate, ACVP (Clinical Pathology). Cytology results showed pyogranulomatous inflammation and neutrophilic lymphadenitis, respectively. While these are nonspecific inflammatory findings, with this signalment and presentation, they supported a strong suspicion for FIP. Therefore, the slides were sent to Auburn University for Feline Infectious Peritonitis Virus mRNA PCR, which was positive in both locations.

Discussion:

Feline Infectious Peritonitis is thought to be an immune-mediated disease caused by a mutated form of Feline Coronavirus (FCoV). Due in part to the fact that the exact mutation is unknown, and because Feline Coronavirus is relatively common (approximately 80-90% of cats in multi-cat households are affected), but FIP is seen in only 5-10% of those infected with FCoV, FIP is classically difficult to diagnose.
In the above case, clinical suspicion was high, but serum PCR for Coronavirus was not supportive. Ultrasound was elected as the next step due to the presence of GI signs and abdominal palpation; however, ultrasound is frequently used to examine for causes of fever of unknown origin.
The Coronavirus PCR which had been performed 1 month prior to the above scan was negative. Possible explanations for this include mutations of the virus or use of inappropriate primers. The mRNA PCR assay that was used to diagnose FIP in this patient detects messenger RNA of the M gene of coronavirus, which is expressed only during viral replication. As traditional FCoV replicates only in the intestinal tract, this test is highly specific for FIP if positive when performed on extra-intestinal tissues or effusions.
Traditionally, FIP has been considered to be essentially 100% fatal, with prognosis being days to weeks with the effusive form and up to months or possibly longer in the dry form. Due to the immune-mediated nature of the disease, immunosuppressive therapy has been described (prednisolone, cyclophosphamide, or chlorambucil), with limited success. Recently, a nucleoside analogue GS-441524 has been released in some countries,but has not been approved in the US by the FDA. This medication has shown some promising results.