Equine asthma: Intriguing questions remain around the role of bacteria

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Do streptococcal bacteria play a role in cases of mild equine asthma? The intriguing possibility has been raised in Canadian research which found differences in the microbial communities of the lower respiratory tract between healthy horses and those with mild equine asthma.

University of Calgary researcher Stephanie Bond and her colleagues noted that although mild asthma in horses was primarily an inflammatory process, an infectious component was highly suspected.

The presence of Streptococcus zooepidemicus, Streptococcus pneumoniae, Actinobacillus spp., and Mycoplasma equihinis in tracheal samples was recently associated with the condition in work by other researchers. This indicated that the make-up of the lower respiratory tract microbiota − that’s the community of microorganisms − could contribute to development of the condition.

The main treatment is corticosteroids.

For their study, the researchers used 13 deconditioned Thoroughbred geldings engaged in chuckwaggon racing. Six were healthy, and seven were diagnosed with mild equine asthma.

The researchers set out to characterize the upper and lower respiratory tract microbiota associated with respiratory health and equine asthma, and to investigate the effects of the corticosteroid dexamethasone on these communities.

They found that the respiratory microbiota of horses was dominated by Proteobacteria (43.85%), Actinobacteria (21.63%), Firmicutes (16.82%), and Bacteroidetes (13.24%).

Fifty genera had a relative abundance greater than 0.1 percent, with Sphingomonas and Pantoea being the most abundant.

“This study demonstrates that the equine lung is not sterile, with the lower respiratory tract possessing a unique microbiota,” the study team wrote.

The researchers, writing in the journal BMC Microbiology, found differences between the upper and lower respiratory tract microbiota in healthy horses, with a decrease in richness in the lower airways.

Differences in the lower respiratory tract microbiota between healthy horses and those with mild asthma were also found. The abundance of Streptococcus was found to be greater in the trachea of horses with equine asthma than their healthy counterparts. In contrast, no differences were observed between disease status at the upper respiratory tract level.

Dexamethasone affected the lower respiratory tract microbiota of both healthy horses and those with asthma, with some microbial families increasing, including Streptococcus, and one decreasing.

“Further research on the role of Streptococcus in inflammatory airway disease is warranted,” they concluded.

“Dexamethasone treatment affected the lower respiratory tract microbiota, which suggests that control of bacterial overgrowth in inflammatory airway disease horses treated with dexamethasone could be part of the treatment strategy.”

The differences seen between healthy horses and those with mild asthma at the lower respiratory tract level concurred with the findings of previous studies which indicated bacteria could play a role in development of mild equine asthma, the authors noted.

However, in the current study, only the abundance of Streptococcus was found to have increased in horses with asthma, whereas the other work found increases in Actinobacillus spp., Acinetobacter spp. and Mycoplasma spp.

The study team comprised Bond, Edouard Timsit, Matthew Workentine, and Renaud Léguillette, all from the University of Calgary; and Trevor Alexander, from the Lethbridge Research Centre, part of Agriculture and Agri-Food Canada.

Upper and lower respiratory tract microbiota in horses: bacterial communities associated with health and mild asthma (inflammatory airway disease) and effects of dexamethasone
Stephanie L. Bond, Edouard Timsit, Matthew Workentine, Trevor Alexander and Renaud Léguillette
BMC Microbiology 2017 17:184 https://doi.org/10.1186/s12866-017-1092-5

The study, published under a Creative Commons License, can be read here

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