Feline infectious peritonitis in a cat presented because of papular skin lesions

Abstract

A 19-week-old neutered male domestic shorthair cat was examined because of multiple raised pruritic skin lesions along the dorsal head and back. Histopathology of biopsies of the lesions detected nodular pyogranulomatous dermatitis with vasculitis and necrosis, leading to a suspicion of feline infectious peritonitis (FIP). Postmortem examination revealed gross lesions consistent with FIP. Histopathologic lesions and positive immunohistochemical staining for feline coronavirus in multiple tissues, including the skin, confirmed the diagnosis of FIP. The current case was similar to previous cases, except for the initial presentation with cutaneous lesions and no other clinical signs, which had not been reported previously.

Introduction

Feline infectious peritonitis (FIP) is a chronic disease of cats that is caused by a mutated form of feline coronavirus (FCoV) (1). This disease is characterized by gross lesions that may include pyogranulomas in multiple organs, polyserositis, and high protein cavitary effusions. Histologic lesions include pyogranulomatous inflammation and vasculitis in affected organs (2–4). Presence of FIP virus in lesions suggestive of FIP is often confirmed through immunohistochemistry (5).

Although FIP is a relatively common disease that has been reported since the 1950’s, cutaneous lesions associated with this disease are rare (2,5–11). Clinical and gross skin lesions have included papular nodules, pitting edema, or skin fragility with papular nodules being the most common (5 of 7 reported cases) (5,7,8,11). All cats with papular lesions displayed multisystemic signs of FIP suggesting that these skin lesions appear later in the course of the disease.

In this report, we describe a unique case of FIP in which papular skin lesions were the cause for the cat’s initial presentation to the referring veterinarian and were found before other multisystemic signs of infection were identified.

Case description

A 19-week-old neutered male domestic shorthair cat was first presented to its veterinarian on August 15, 2015, for multiple raised pruritic skin lesions along the dorsal head and back. The cat was febrile at this time, so therapy was initiated with cephalexin and diphenhydramine. The cat was again presented 5 d later with poor appetite and spreading of the skin lesions, and treatment was modified to consist of prednisone and methylprednisolone acetate. Biopsies were taken and submitted to the dermatopathology service of Prairie Diagnostic Services (PDS) at the Western College of Veterinary Medicine (WCVM), Saskatoon, Saskatchewan. Histopathology revealed nodular pyogranulomatous dermatitis with vasculitis and necrosis, and there was a strong suspicion of FIP. The cat’s health continued to decline, with progression to seizures. Euthanasia was performed on August 24, 2015 and the cat was received for necropsy at PDS a day later.

The cat was in good body condition and there were multiple 3- to 5-mm diameter skin nodules along the dorsal surface of the head and neck, as well as the lateral aspects of the cranial thorax, similar to those described in the clinical history (Figure 1A). The abdomen, thorax, and pericardium contained abundant viscous serofibrinous effusions. The lungs, liver, and kidney contained multiple white nodules ranging from 3 to 7 mm in diameter. Multiple mesenteric lymph nodes were enlarged and firm. There were no other significant findings on gross examination. A variety of tissues were fixed in 10% buffered formalin, routinely processed and stained with hematoxylin and eosin (H&E) for histopathology.

Figure 1.

A — Multiple round raised skin nodules along the dorsal surface of the head and neck. B — Histological section from an affected area of the skin displaying a locally extensive area of necrosis and few inflammatory infiltrates; H&E. C —Immunohistochemical staining of the skin displaying positive brown staining for feline infectious peritonitis virus within an affected area.

Histologically, the liver, lungs, kidneys, and mesenteric lymph nodes featured multifocal to coalescing areas of pyogranulomatous inflammation composed of lymphocytes, plasma cells, macrophages, and neutrophils that were often surrounding and infiltrating the wall of veins (phlebitis). Many larger foci had a central core of necrosis. Similar inflammatory cells often infiltrated the wall of veins in the meninges and extended into the meninges and, occasionally, the neuropil. Histologic examination of the eye revealed pyogranulomatous inflammation and phlebitis in the periocular tissue, as well as pyogranulomatous optic neuritis. Sections of the skin also featured multiple vessels with walls infiltrated by mixed inflammatory cells and surrounding large pale areas of dermal necrosis characterized by loss of collagen fibers and replacement with karyorrhectic and necrotic debris and surrounded by low numbers of lymphocytes, plasma cells and macrophages (Figure 1B).

Microscopic sections of the duodenum, jejunum, ileum, and colon featured multifocal crypt necrosis and mild lymphoplasmacytic infiltrates within the lamina propria.

Immunohistochemical staining for intracellular feline coronavirus N protein (FIPV3-70, 1:500; Custom Monoclonals, West Sacramento, California, USA) was performed and showed positive staining within the cytoplasm of macrophages in the lung, kidney, skin, and brain (Figure 1C).

Immunohistochemistry of the colon, ileum, and spleen for feline parvovirus and feline leukemia virus yielded negative results. A cause of the crypt necrosis was not determined. Feline infectious peritonitis with associated cutaneous lesions was diagnosed in this cat based on gross and histologic lesions along with immunohistochemistry results.

Discussion

This report presents a case of systemic feline coronavirus infection in a cat causing cutaneous lesions along with classic lesions of feline infectious peritonitis (FIP). While FIP is a common disease in domestic cats, cutaneous lesions associated with this disease are rare; there have only been 7 previous reports (2,5–9,11). The most common cutaneous manifestation is formation of papular nodules, as observed in the current case, but there are also single reports of swollen limbs with pitting edema and development of skin fragility syndrome in separate cases related to FIP virus infection (2,5–9,11). The current case is of particular interest because the cutaneous lesions were the primary reason for initial presentation, while all other cases, save for the case of pitting edema, were presented because of other systemic signs.

Examination of previous reports of FIP cases with skin involvement revealed interesting patterns that fit with many of the findings in the current case (Table 1). Younger animals seemed to be at higher risk for cutaneous lesions, as 4 of the 7 previous cases were less than 3 y old and the cat in the current case was only 19 wk old. There was no gender predilection, as 3 of the cats in previous cases were intact males, 3 were spayed females, 1 was of unspecified gender, and the cat in the current case was a neutered male. Domestic shorthair (4/7 previous and the current case) and sphinx cats (2/7 previous cases) were overrepresented in FIP cases with cutaneous lesions, but conclusions cannot be drawn based on the low number of cases. Finally, all cases included some gross evidence of granuloma formation.

Table 1.

Data summary of feline infectious peritonitis cases with cutaneous involvement.

Reference	Breed	Signalment	Age
(7)	DSH	Male intact	Adult
(9)	Sphinx	Male intact	1 y
(2)	DSH	Not specified	1 y
(11)	DSH	Female spayed	6 y
(6)	DSH	Male intact	7 m
(5)	Sphinx	Female spayed	2 y
(8)	DSH	Female spayed	13.5 y
Current case	DSH	Male neutered	5 mo

DSH — Domestic shorthair.

Despite the variety of gross manifestations of cutaneous disease in previous reports, histologically, all cats in the 7 reported cases had similar pyogranulomatous vasculitis and necrosis within the dermis. All cases also showed positive immunostaining for FCoV antigen in macrophages in the dermis and other affected tissues (2,5–9,11). It appears that histologic examination of skin lesions related to FIP yields consistent findings and should raise suspicion for FIP, with IHC needed for definitive diagnosis.

There has yet to be an explanation for the development of cutaneous lesions in few cases of FIP with absence in most cases. The reason may be host-specific; one proposed explanation is the selectiveness of endothelial cell reactivity to systemic cytokines. There may be individual variability in the cellular adhesion molecules and integrins expressed on endothelial cells of different tissues and on leukocytes (2). Alternatively, tissue specificity may be related to viral variability. While this has not been investigated thoroughly, the gene for the surface glycoprotein S of feline coronavirus differs between non-mutated coronavirus and the mutated FIP virus and may also take part in determining the tissues to which virus particles may disseminate (12,13).

Regardless of the mechanism of tissue specificity of FIP viral infection, cutaneous lesions do occur occasionally. The reason for rarity of dermal involvement needs further exploration, and pathologists should keep FIP on their differential diagnosis list when considering biopsies from cats with skin lesions. CVJ