Pathology and viral antigen distribution of lethal pneumonia in domestic cats due to pandemic (H1N1) 2009 influenza A virus

Abstract

A novel swine-origin H1N1 influenza A virus has been identified as the cause of the 2009 influenza pandemic in humans. Since then, infections with the pandemic (H1N1) 2009 influenza virus have been documented in a number of animal species. The first known cases of lethal respiratory disease associated with pandemic (H1N1) 2009 influenza virus infection in house pets occurred in domestic cats in Oregon. A 10-year-old neutered domestic shorthair and an 8-year-old spayed domestic shorthair died shortly after developing severe respiratory disease. Grossly, lung lobes of both cats were diffusely firm and incompletely collapsed. Histologically, moderate to severe necrotizing to pyonecrotizing bronchointerstitial pneumonia was accompanied by serofibrinous exudation and hyaline membranes in the alveolar spaces. Influenza A virus was isolated from nasal secretions of the male cat and from lung homogenate of the female cat. Both isolates were confirmed as pandemic (H1N1) 2009 influenza virus by real-time reverse transcriptase polymerase chain reaction. With immunohistochemistry, influenza A viral antigen was demonstrated in bronchiolar epithelial cells, pneumocytes, and alveolar macrophages in pneumonic areas. The most likely sources of infection were people in the household with influenza-like illness or confirmed pandemic (H1N1) 2009 influenza. The 2 cases reported here provide, to the best of the authors' knowledge, the first description of the pathology and viral antigen distribution of lethal respiratory disease in domestic cats after natural pandemic (H1N1) 2009 influenza virus infection, probably transmitted from humans.

Figure 1

Lung; Cat No. 1. On cross section of a formalin-fixed lung lobe, the parenchyma is diffusely dark red to brown, incompletely collapsed, and has multifocal tan bronchocentric areas of increased tissue density (consolidation).

Figure 2

Lung; Cat No. 1. A bronchiole has diffuse epithelial loss, is lined by a dense layer of fibrin, and contains scattered luminal macrophages and cellular debris embedded in wispy fibrin strands. Interalveolar septa are severely thickened; alveolar spaces are flooded with wispy to dense fibrin and scattered macrophages and sloughed epithelial cells. HE.

Figure 3

Lung; Cat No. 1. Alveolar contain scattered round cells, wispy fibrin strands and hyaline membranes (arrowheads).

Figure 4

Lung; Cat No. 1. Peribronchiolar alveoli are lined by cuboidal epithelium consistent with type II pneumocyte hyperplasia. Alveolar spaces are filled with large round cells, dense fibrin accumulations, and small amounts of cellular debris. The lining of the alveolus on the left has a few multinucleated cells (arrowheads). The alveolus in the center has segmental epithelial loss (arrow). The bronchiole at the top of the figure has epithelial attenuation, and an eroded segment is covered with fibrin. HE.

Figure 5

Lung; Cat No. 2. The bronchial lumen on the left is filled with mucus and scattered neutrophils; bronchial epithelium is lost or flattened (arrowheads). The bronchial submucosa is severely expanded by hyperplastic glands and a lymphoplasmacytic infiltrate. Multifocally, degeneration of epithelial cells extends into bronchial glands (arrows). Bronchial cartilage is visible in the top right corner. HE.

Figure 6

Lung; Cat No. 1. Immunohistochemistry for influenza A viral antigen labels bronchiolar epithelium (left), peribronchiolar type II pneumocytes (middle), and scattered alveolar macrophages (right). ABC method with hematoxylin counterstain.

Figure 7

Lung; Cat No. 1. Immunhistochemistry for influenza A viral antigen. Individual and small clusters of bronchiolar epithelial cells have homogeneous to granular, red to dark red-brown, nuclear and cytoplasmic signal occasionally obscuring cellular detail. Immunoreactive epithelial cells are flanked by negative epithelial cells that appear histologically normal. ABC method with hematoxylin counterstain.

Figure 8

Lung; Cat No. 1. Immunhistochemistry for influenza A viral antigen labels the surface of bronchiolar epithelial cells (arrowheads). ABC method with hematoxylin counterstain.

Figure 9

Lung; Cat No. 1. The focally denuded bronchiolar mucosal surface is covered by debris with irregular, fibrillar to clumped immunoreactivity. ABC method with hematoxylin counterstain.

Figure 10

Lung; Cat No. 2. Immunhistochemistry for influenza A viral antigen labels the nucleus and cytoplasm of flat alveolar lining cells, interpreted as type I pneumocytes. ABC method with hematoxylin counterstain.

Figure 11

Lung; Cat No. 2. Immunhistochemistry for influenza A viral antigen obscures the detail of large round cells in alveolar spaces, interpreted as macrophages (arrowheads). ABC method with hematoxylin counterstain.