Conference 25 - 2015 Case: 02 20160511

Signalment:

7-month-old spayed female domestic shorthair cat (felis catus)Chronic lethargy, anemia, icterus; presented with low body temperature.

Gross Description:

Lungs were diffusely reddened, firm and did not collapse. Throughout all liver lobes are 1mm to 3mm pale tan nodules. Similar pale tan nodules from lmm to 7mm diameter were scattered throughout the cortex of both kidneys. Mesenteric lymph nodes were enlarged.

Histopathologic Description:

Blood vessels in the lung are accentuated by inflammatory infiltrates that greatly expand the vessel wall. These infiltrates are co-mposed of macrophages, plasma cells, neu-trophils, lymphocytes and occasional Mott cells. Mild fibroplasia is present in the infiltrate also. Vessel lumens are narrowed by the infiltrates. Diffusely, alveoli contain edema fluid and low numbers of ma-crophages and occasional neutrophils.

Morphologic Diagnosis:

Lung: Vasculitis/perivasculitis, multifocal, pyogranulomatous with alveolar edema.

Lab Results:

Total protein - 10.0, albumin 2.1, globulins 7.5, phosphorous 6.8, potassium 3.4, elevated ALP, CK and total bilirubin (1.4).

Condition:

Feline infectious peritonitis

Contributor Comment:

Gross findings in this cat were suggestive of the dry form of feline infectious peritonitis (FIP), and the vascular lesions in the lung were suggestive of those described in cases of FIP. Coronaviral antigen was identified in the vessels by immunohistochemistry.

FIP is one of the leading infectious causes of death in cats from shelters and catteries.⁷ FIP is caused by infection with feline coronavirus (FeCoV) that has mutated to become a pathogenic virus (FIPV). FCoV infections are very common, with up to 90% seropositivity in cats, while FIP morbidity is low, rarely above 5% of infected cats. Most infections with feline coronavirus are sub-clinical, although mild diarrhea and vomiting can occur. The FCoV mutants that cause FIP are either generated within the individual cat, or possibly are acquired externally.

Persistently infected, healthy carriers are believed to be most important in the epidemiology of FIP.⁶Mutation of the FCoV in the carrier animal generates the FIP virus that is capable of replicating in mo-nocyte/macrophages, resulting in tra-nsportation to organs outside of the GI tract, and development of FIP disease. The FCoV mutants also apparently lose their ability to replicate in intestinal epithelium, as they are not recovered from intestinal tissues.² For this reason, it is believed that cat to cat transmission of FIP virus is infrequent; the virus does not readily infect cats under natural conditions because it does not replicate in enterocytes, even though the virus readily infects cats when they are experimentally inoculated by routes other than oral.

The host immune response largely dictates the lesions of FIP. The immune response to FIP virus is presently understood as follows: humoral immunity is not important for protection while protective immunity is largely cell mediated.⁷The type and strength of immunity determines the form that FIP virus infection will take. Cats that develop FIP will have either the wet or dry form depending on whether ineffective cell-mediated or humoral immunity dominates the clinical disease. Strong humoral im-munity with very weak or non-existent cellular immunity leads to effusive (wet) FIP. With the effusive form, cats will have up to 1 L of viscous abdominal fluid while pleural effusion is present in about 25% of cases. Humoral immunity with intermediate cellular immunity will manifest as the non-effusive FIP (dry form). With the dry form, the kidneys may be enlarged and nodular with white, firm nodules protruding from the cortex. Foci of inflammation may also be seen in other organs, including the liver and pancreas. The gross lesions of wet versus dry formsare often not distinctly separate, and much overlap occurs. Vasculitis and perivasculitis characterize the microscopic lesions of FIP. FIP-induced granulomatous vasculitis occurs in small to medium-sized veins predominantly in the leptomeninges, renal cortex, and eye, but also frequently in the lung and liver.⁴ Vasculitis is characterized as venous and perivenous, macrophage-dominated, cir-cular infiltrates in small veins, and focal infiltrates in larger veins. Neutrophils and T cells represented minorities among in-flammatory cells, and B cells mainly occur as peripheral rims around circular gra-nulomatous infiltrates. FIP virus-infected monocytes that become activated and emigrate from vessel lumens into perivenous locations are reported to be unique to the development of the periphlebitis that occurs.

JPC Diagnosis:

Lung: Vasculitis, necrotizing, lymphocytic and histiocytic, diffuse, severe with fibrinoid necrosis and alveolar and interstitial edema.

Conference Comment:

Feline coronavirus (FCoV) belongs to the genus Alphacoronavirus and species Alp-hacoronavirus-1, along with canine coronavirus and transmissible gastroenteritis virus of pigs. There are two serotypes of FCoV (based on antigenicity) types I and II, and while both may cause feline infectious peritonitis (FIP), type I is more common in the cat population. The serotypes differ primarily in growth characteristics in cell culture and in receptor usage, and it is notable that most, if not all, of the experimental work so far has been done on the Type II strains because they grow well in cell culture.

Type II FCoVs arose as a consequence of a double recombination between type I FCoV and CCoV. Along with homologous recombination, the pro-pensity for frequent variation and mutation of coronaviruses is also based on a high mutation rate (2.0×10?6 mutations per site per round of replication) and the sheer size of the genome (2632 kb).⁹ As is true with many viruses, even a single amino acid mutation and/or recombination events can change viral properties, host range and pathogenicity. Both feline enteric coronavirus (FECV) and feline infectious peritonitis virus (FIPV) can infect monocytes, but FIPVs are able to sustainably replicate in much higher numbers.⁵ However, not all monocytes are permissive to replication of FIPV⁸ and there is variation in individual cats regarding the susceptibility of monocytes to infection and replication, which influences disease sus-ceptibility. It has also been suggested that monocytes may potentially be the cells where mutation from FECV to FIPV occurs.^5,8 The precise genetic and me-chanistic differences that define changes in viral replication and virulence have not yet been clearly elucidated, but various proteins such as 3c, S, S1 fusion peptide and 7a/b, among others, appear to play a role.^5,8

In many cases, these various proteins appear to influence virus infection of, and replication in, monocytes. At least three key events are known in the development of FIP including systemic infection with FIPV, effective FIPV replication in monocytes and activation of those monocytes,⁵ highlighting the critical role of the monocyte response in development of FIP. There was conference discussion around the use of the terms wet and dry forms of FIP. Some researchers believe these are a temporal continuum, with the latter being a chronic manifestation, or a post-man-ifestation of the former. The terms are useful clinically, but we know little about what contributions of the virus and/or host are the bases of the two types of presentations. Lesion distribution in cases of FIP is rather consistent although some degree of in-dividual variation may be observed. Peritoneal involvement was seen in 75% of cases, often associated with abdominal effusion, and the kidneys, followed by eyes and brain, were most often affected according to one study, and ocular in-volvement was frequently bilateral.⁵ Antemortem diagnosis of FIP can be particularly challenging. Cytology of ab-dominal effusion suggestive of FIP contains nondegenerate neutrophils, macrophages, lymphocytes and few plasma cells on a proteinaceous background.

Using imm-unofluorescence or immunohistochemistry to visualize the virus within monocytes of the effusion is considered diagnostic, excepting cases of sequestered granulomas, such as this case. A positive reaction in the Rivaltas test, used to differentiate tran-sudates from exudates, may increase diagnostic sensitivity when accompanied by cytology of abdominal effusion.³ As discussed above, vascular lesions are generally limited to the small and medium sized veins in affected tissues due to interaction between activated mac-rophages and endothelium.⁵ In some cases, vascular lesions, which are generally dominated by macrophages, may be replaced by B cells and plasma cells.⁵This is commonly observed in ocular disease where plasma cells predominate. The clinical course of disease in the wet form is generally much faster than for the dry form and subclinical as well as a protracted or multiphasic course of disease may also be seen. The conference attendees discussed a recent publication that demonstrates theoretical reversal of viremia using pro-tease inhibitors, and clinical trials that are underway to establish whether a rigorous anti-viral therapy could be helpful.¹⁰ Conference participants described the inflammatory infiltrate as being vascular and perivascular in distribution, effecting small and medium sized vessels, and remarkably circumferential in most affected vessels. The loss of vascular architecture included the entirety of the vessel wall which was markedly thickened by fibrin, both hyalinized and fibrillar, edema, and a mixed population of inflammatory cells, predo-minantly macrophages. The endothelium is hypertrophic and vascular lumina are narrowed. The moderator stressed the importance of describing vascular changes in detail, specifically with reference to vasculitis; the description should ch-aracterize changes within each layer of the effected vessel. Edema fluid fills alveoli in this case, and has refluxed into small and terminal bronchioles, a change which needs to be distinguished from an airway-centric disease (i.e., bronchopneumonia). The alveolar septa and interstitium are mildly expanded by fibrin and edema. There are multifocal areas of alveolar emphysema and few areas of mild fibrinous pleuritis.

References:

1. Brown CC, Baker DC, Barker IK. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy and Palmer's Pathology of Domestic Animals. 5th ed. Vol 2. Saunders Elsevier: New York; 2007: 290-293.

2. Chang HW, deGroot RJ, Egberink HF, Rottier PJM. Feline infectious peritonitis: Insights into feline coronavirus pathobiogenesis and epidemiology based on genetic analysis of the viral 3c gene. J Gen Virol. 2010; 91:415-420.

3. Fischer Y, Sauter-Louis C, Hartmann K. Diagnostic accuracy of the Rivalta test for feline infectious peritonitis. Vet Clin Pathol. 2012; 41:558-567.

4. Kipar A, May H, Menger S, Weber M, Leukert W, Reinacher M. Morphologic features and development of granulomatous vasculitis in feline infectious peritonitis. Vet Pathol. 2005; 42: 321-330.

5. Kipar A, Meli ML. Feline infectious peritonitis: Still an enigma? Vet Pathol. 2014;51(2): 505-526.

6. Meli M, Kipar A, Muller C, Jenal K. High viral loads despite absence of clinical and pathological findings in cats experimentally infected with feline coronavirus (FCoV) type 1 and in naturally FCoV-infected cats. J Feline Med and Surg. 2004; 6:69-81.

7. Pedersen NC. A review of feline infectious peritonitis virus infection: 1963-2008. J Feline Med and Surg. 2009; 11:225-258.

8. Pedersen NC. An update on feline infectious peritonitis: Virology and immunopathogenesis. Vet J. 2014; 201(2):123-132.

9. Eckerle LD, Becker MM, Halpin RA, Li K, Venter E, et al. Infidelity of SARS-CoV Nsp14-exonuclease mutant virus replication is revealed by complete genome sequencing. PLoS Pathog. 2010;(5):e1000896.

10. Kim Y, Liu H, Galasiti Kankanamalage AC, Weerasekara S, et al. Reversal of the Progression of Fatal Coronavirus Infection in Cats by a Broad-Spectrum Coronavirus Protease Inhibitor. PLoS Pathog. 2016; 12(3):e1005531.