Chlamydophila spp. infection in horses with recurrent airway obstruction: similarities to human chronic obstructive disease

Abstract

Background: Recurrent airway obstruction (RAO) in horses is a naturally occurring dust-induced disease mainly characterized by bronchiolitis which shows histological and pathophysiological similarities to human chronic obstructive pulmonary disease (COPD). In human COPD previous investigations indicated an association with Chlamydophila psittaci infection. The present study was designed (1) to clarify a possible role of this infectious agent in RAO and (2) to investigate the suitability of this equine disorder as a model for human COPD.

Methods: Clinico-pathological parameters of a total of 45 horses (25 horses with clinical signs of RAO and 20 clinically healthy controls) were compared to histological findings in lung tissue samples and infection by Chlamydiaceae using light microscopy, immunohistochemistry, and PCR.

Results: Horses with clinical signs of RAO vs. controls revealed more inflammatory changes in histology (p = 0.01), and a higher detection rate of Chlamydia psittaci antigens in all cells (p < 0.001) and bronchiolar epithelial cells alone (p < 0.001) by immunohistochemistry. The abundance of chlamydial inclusions increased with the severity of disease. PCR was positive in 60% of horses with RAO vs. 45% of the controls (p = 0.316). OmpA sequencing identified Chlamydophila psittaci (n = 9) and Chlamydophila abortus (n = 13) in both groups with no significant differences. Within the group of clinically healthy horses subgroups with no changes (n = 15) and slight inflammation of the small airways (n = 5) were identified. Also in the group of animals with RAO subgroups with slight (n = 16) and severe (n = 9) bronchiolitis could be formed. These four subgroups can be separated in parts by the number of cells positive for Chlamydia psittaci antigens.

Conclusion: Chlamydophila psittaci or abortus were present in the lung of both clinically healthy horses and those with RAO. Immunohistochemistry revealed acute chlamydial infections with inflammation in RAO horses, whereas in clinically healthy animals mostly persistent chlamydial infection and no inflammatory reactions were seen. Stable dust as the known fundamental abiotic factor in RAO is comparable to smoking in human disease. These results show that RAO can be used as a model for human COPD.

Figure 1

Immunohistochemistry of equine lung tissue in healthy horses. Animal showing no bronchiolitis (original magnification 10×).

Figure 2

Immunohistochemistry of equine lung tissue in healthy horses. Only occasionally some epithelial cells positive for Chlamydia psittaci antigens are detectable (brown staining, original magnification 40×).

Figure 3

Immunohistochemistry of equine lung tissue in RAO. In RAO, animals with severe bronchiolitis (note the striking intraluminal accumulation of neutrophils and macrophages) most of the bronchiolar epithelial cells carry CP antigens (brown staining, original magnification 20×).

Figure 4

Immunohistochemistry of equine lung tissue in RAO. Also in the respiratory bronchioli inflammation is found, chlamydial antigens (brown staining) are detectable in epithelial cells and macrophages (original magnification 40×).

Figure 5

Immunohistochemistry of equine lung tissue in RAO. Proliferating type II pneumocytes show chlamydial antigens as well (brown staining, original magnification 40×).

Figure 6

Immunofluorescence of equine lung tissue in RAO. In horses with RAO, the abundance of chlamydial inclusion bodies (red spots) in bronchiolar epithelial cells (stained green with an anti-cytokeratin pan antibody) and macrophages varies from high to low (compare Figure 7) (original magnification 40×).

Figure 7

Immunofluorescence of equine lung tissue in RAO. In horses with RAO, the abundance of chlamydial inclusion bodies (red spots) in bronchiolar epithelial cells (stained green with an anti-cytokeratin pan antibody) and macrophages varies from high (compare Figure 6) to low (original magnification 40×).