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Pathological findings and patterns of feline infectious peritonitis in the respiratory tract of cats.

Slaviero M, Cony FG, da Silva RC, De Lorenzo C, de Almeida BA, Bertolini M, Driemeier D, Pavarini SP, Sonne L. Pathological findings and patterns of feline infectious peritonitis in the respiratory tract of cats. J Comp Pathol. 2024 Apr;210:15-24. doi: 10.1016/j.jcpa.2024.02.001. Epub 2024 Mar 12. PMID: 38479335.

* This publication is not an open access report.

The pathological findings of feline infectious peritonitis (FIP), an important viral disease with worldwide distribution, have been well-documented. However, there is a paucity of data related to respiratory tract pathology and FIP-associated respiratory disease. Though there is evidence of pyogranulomatous pneumonia, the lungs are not considered a primary target organ of FIP. In early stages of FIP, respiratory signs such as sneezing and nasal discharge have been described. A recent retrospective study from Brazil analyzed respiratory lesions identified in necropsies of cats diagnosed with FIP over a 12 year period from 2010 and 2022. The goals of the study were to review FIP pathological findings in the respiratory tract, identify the occurrence and distribution of FIPV antigen in lung tissue, and define lung injury patterns.

Case Selection and Description:
– 66 cats (from a total of 2,089 postmortem examinations, and 112 FIP-related deaths) were selected based on histological inflammatory lesions in the lung and pleura
– 89% of the cats were described as ‘mixed-breed’, with 91% of the cats listed as kitten or young adult
– Feline retroviral status: 62% of the cats were FeLV-positive, 4.5% FIV-positive, and 4.5% FeLV and FIV positive
– 35% (23/66) of cats had listed clinical signs as ‘respiratory’, which including sneezing and nasal discharge (8/66), and dyspnea related to pleural effusion (12/66).
– Cases were divided into three gross patterns: 1) Thoracic cavity filled with yellow to red liquid, fibrillar, gelatinous, or pasty material, which was adherent to the pleura; pulmonary parenchymal atelectasis without other parenchymal alterations. 2) #1 pattern with uncollapsed pulmonary parenchyma, multiple punctate white nodules randomly distributed throughout the lungs. 3) Little to no fluid in thoracic cavity, uncollapsed lung lobes with markedly pale lung parenchyma, and randomly distributed white nodules
– Histology was reviewed and six patterns were categorized: diffuse pleuritis, multifocal pleuritis, pleuritis and perivascular injury, pleuritis and pneumonia, perivascular injury, and pneumonia
– Immunolabeling for FIPV was evaluated according to regions that were positive: pleura, perivascular, lung parenchyma, peribronchial, and bronchial-associated lymphoid tissue. The intensity of immunolabeling was described as mild, moderate, and marked.

Gross Findings:
– Effusion was noted as such: 74% had effusion in the thoracic, pericardial, and/or abdominal cavity, 46% with thoracic effusion affecting both right and left sides, and 26% did not have any effusion
– The number of cases with the described gross patterns:
1) 18/66 – Thoracic cavity filled with yellow to red liquid, fibrillar, gelatinous, or pasty material, which was adherent to the pleura; pulmonary parenchymal atelectasis without other parenchymal alterations
2) 13/66 – Uncollapsed pulmonary parenchyma, multiple punctate white nodules randomly distributed throughout the lungs.
3) 23/66 – Little to no fluid in thoracic cavity, uncollapsed lung lobes with markedly pale lung parenchyma, and randomly distributed white nodules
4) 12/66 had no gross pulmonary lesions, with 5 of those having abdominal effusion

Histological and Immunohistochemical findings:
– 73% of cases had visceral pleura involvement (24% with only pleural involvement), with 40 of 48 cases possessing effusion
– 76% cases with lung parenchymal injury
– 53% cases with pneumonia
– 23% cases with only vascular lesions
– IHC was tested on the lung tissue of 59 out of the 66 patients, and 45/59 (89%) had detectable FIP antigen. 58% of cats has marked intensity of immunolabeling, with 17% moderate and 26% mild.

Conclusions:
At this diagnostic laboratory, lesions in the respiratory tract were found in 59% of FIP cases. In comparison to other reports, this study described pneumonia as a common finding when gross lesions were confined to the respiratory tract, and almost 30% of patients had marked fibrin deposition in the thoracic cavity without multifocal granulomas. The lungs and visceral pleura were also affected in a similar percentage of patients. Only some cats (35%) had reported clinical respiratory signs, but cats with respiratory signs had significant lesions of pneumonia. Greater than 70% of cats were infected with FeLV or FIV, which could be a risk factor for FIP infection. The study’s feline population was similar to other studies as the vast majority of the patients were kitten and young adults (91%).
Limitations include retrospective nature of the study, only 4 of the 66 cases had upper respiratory tissues to evaluate, no evaluation of antemortem clinical diagnostic tests, and no available information on bacterial isolation in the respiratory tracts.
This report brings to light the need for additional studies of the upper and lower respiratory tracts of cats infected with FIPV as the current literature is scarce. Respiratory lesions, including pneumonia, may be more common than once thought.  -BJP

For Further Reading:

Pedersen NC. A review of feline infectious peritonitis virus infection: 1963-2008. J Feline Med Surg. 2009 Apr;11(4):225-58. doi: 10.1016/j.jfms.2008.09.008. Epub 2009 Feb 28. PMID: 19254859; PMCID: PMC7129802.

André NM, Miller AD, Whittaker GR. Feline infectious peritonitis virus-associated rhinitis in a cat. JFMS Open Rep. 2020 Jun 23;6(1):2055116920930582. doi: 10.1177/2055116920930582. PMID: 32637147; PMCID: PMC7313338.

Thayer V, Gogolski S, Felten S, Hartmann K, Kennedy M, Olah GA. 2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines. J Feline Med Surg. 2022 Sep;24(9):905-933. doi: 10.1177/1098612X221118761. Erratum in: J Feline Med Surg. 2022 Sep 6;:1098612X221126448. PMID: 36002137.