An intact female 8-week-old domestic shorthair cat was taken into a foster/rescue home and cohabited a house with approximately nine other cats. The facility had a periodic history of FIP cases, including two deaths in the previous 4 months. The cat was co-housed in a large open sunroom containing seven litterboxes, which were cleaned once daily. The diet consisted of commercially available dry and canned food, which was separately offered in individual dishes. The cat was not rabies vaccinated, but had obtained two feline viral rhinotracheitis, calicivirus and panleukopenia vaccinations.
At 14 weeks of age, the cat was presented to a general practitioner for evaluation of poor weight gain, soft stool and upper respiratory tract infection. The cat was underconditioned and weighed 2.5 lb (1.1 kg), with a body condition score (BCS) of 2/5, despite being active, alert and having a good appetite. Conjunctivitis and a yellow mucopurulent discharge from the nares were noted, and the cat had a fever of 102.6 F (39.2 C). A fecal flotation was performed owing to the soft but formed stool, and no ova or parasites were detected. A complete blood count (CBC) and chemistry profile were performed (Tables 1 and 2). The chemistry profile showed marked elevations in alkaline phosphatase, alanine transferase and phosphorus levels. A decrease in creatinine and albumin was also noted (Table 1), along with mild anemia and monocytosis. Amoxicillin clavulanic acid (Clavamox drops; Zoetis) 62.5 mg/ml was dispensed and administered at 15.6 mg (12 mg/kg) PO q12h for 10 days. Blood parameters were re-evaluated at 20 weeks of age using a less defined panel and values were within the normal range (Table 1).
At approximately 6 months of age, the cat returned to the general practitioner for evaluation of pelvic limb gait abnormalities that had progressed over the previous 2 weeks. Examination revealed symmetric pulses in both hindlimbs and the presence of a pain response; however, less of a response was noted on the right side. Paresis was observed in the right hindlimb. When the forelimbs were lifted, the cat was able to walk minimally on the hindlimbs. The cat had severe non-ambulatory paraparesis, with more severe deficits on the right side. No information about spinal reflexes was available. A CBC and chemistry panel were performed (Tables 1 and 2). The chemistry panel revealed hypoalbuminemia, a decrease in the albumin:globulin (A:G) ratio, low creatinine values and hyperphosphatemia. The CBC revealed a slight anemia, monocytosis and thrombocytopenia. Platelet clumping was noted upon microscopic evaluation. Meloxicam (Metacam oral suspension) was dispensed and a single 0.2 mg dose was administered orally.
At 8 months of age, the cat was returned to the general practitioner due to progression of the paraparesis. The client noted further deterioration of the pelvic limb paresis, and now identified 'stiffness' in the thoracic limbs. There was no information about the pelvic limb reflexes; however, the cat had started to have occasional urinary and fecal incontinence. Appetite seemed normal; however, the cat remained thin. Physical examination revealed a temperature of 101.5 F (38.6 C), heart rate of 170 beats per min and respiratory rate of 30 breaths per min. BCS was 3/9; however, the weight was not noted. Abdominal palpation revealed a large, easily expressible urinary bladder. Neurologic examination findings revealed normal mentation with no involuntary movements such as tremors. The cat was still very ataxic and ambulatory but now tetraparetic, which was much worse in the pelvic limbs. There was no information about cranial nerve abnormalities, and the eyes and retinas were within normal limits. From a video provided by the owner (see supplementary material), tail paresis was identified. The lesion was considered to affect the CNS and was localized as multifocal. A fecal flotation and direct smear were evaluated, with no ova or parasites seen. Cryptococcus and Toxoplasma antibody titers were performed and were negative.
The tetra-ataxia and paresis significantly worsened over the next few months. Additionally, the cat now had titubation, tail plegia (see video in the supplementary material) and consistent urinary and fecal incontinence. Owing to the grave prognosis, the client elected humane euthanasia, at which time the cat was 10 months (40 weeks) of age. A necropsy was performed at the Animal Health Diagnostic Center, Cornell University College of Veterinary Medicine, and this revealed no significant gross abnormalities outside of mild mesenteric lymphadenomegaly. Representative sections of all organs, including the entire brain and spinal cord, were fixed in 10% neutral buffered formalin from which sections were cut, stained with hematoxylin and eosin, and analyzed via light microscopy. Immunohistochemistry for FCoV was carried out using monoclonal antibody FIPV3-70 (1:1000), AP-Anti-Mouse IgG and Bond Polymer Refine Red Detection (Leica Microsystems).
Histologic examination revealed lesions typical of FCoV infection within the CNS. In the spinal cord, the leptomeninges were diffusely expanded by moderate numbers of predominantly plasma cells, admixed with fewer lymphocytes and macrophages, and surrounded by a moderate amount of edema. The underlying white matter was multifocally vacuolated with numerous dilated myelin sheaths, digestion chambers and rare spheroids (Figure 1a). At the level of the lateral aperture, the choroid plexus was expanded by large numbers of plasma cells, lymphocytes and macrophages (Figure 1b). The ependyma lining the ventricular system was effaced by a similar inflammatory population, admixed with fibrin, edema and was also forming thick perivascular cuffs often disrupting the sub-ependymal parenchyma (Figure 1c). Immunohistochemistry revealed strong intracytoplasmic immunoreactivity within macrophages associated with the inflammation (Figure 1d). No FIP-associated lesions were present in other organs. Non-FCoV comorbid histologic findings were chronic enteritis with mid-mucosal fibrosis and mesenteric lymphoid hyperplasia.
Molecular analysis of the viral spike protein was performed at several time points during the study. Fecal samples were collected at 5 months of age (feces #1) and at 8 months of age (feces #2). Following euthanasia (at 10 months of age) tissue samples were collected, along with a fecal sample (feces #3). A central 156 base pair region of the spike protein gene, including the critical S1/S2 activation site of the virus, was PCR amplified and sequenced as described in Licitra et al, with the following modifications: 25 mul reverse transcription PCRs were performed with qScript XLT 1-Step RT PCR kit (Quantbio). PCR conditions were 20 mins at 50 C, 3 mins at 95 C and 40 cycles of 10 s at 95 C, 20 s at 55 C, 40 s at 72 C, then 10 mins at 72 C. PCR products were purified using Diffinity RapidTips (Diffinity Genomics).
PCR and sequencing showed the presence of a type I FCoV, based on a sequence alignment with reference genomes. The sequence information obtained from this cat is shown in Figure 2. The viral sequences from the CNS (brain and spinal cord) contained specific amino acid changes compared with other samples (feces, small intestine, mesenteric lymph node and kidney). The most notable change was an arginine to methionine (R-M) substitution at the critical P1 activation position, corresponding to residue 793. Other changes that correlated with viruses present in the CNS were present in two other positions: 770 alanine to valine (A-V); and 786 threonine to alanine (T-A).