Bacterial respiratory infections leave horses open to EHV-1 – study

Equine herpesvirus-1 takes advantage of epithelial thinning caused by bacterial endotoxins, researchers show.
Image by Krzysztof Niewolny

Horses with bacterial respiratory infections caused by Staphylococcus aureus or Bordetella bronchiseptica may be more susceptible to infection by equine herpevirus-1 (EHV-1), researchers suggest.

Eline Van Crombrugge and her fellow researchers in Belgium found that endotoxins produced by the two bacteria increase the sensitivity of horses to EHV-1 infections.

The Ghent University study team, writing in the journal Viruses, said respiratory disease in horses can be caused by a range infectious agents and involve many environmental factors.

The respiratory tract of the horse has developed multiple antiviral barriers in response. However, these can become compromised by environmental threats. Pollens and mycotoxins are known to increase mucosal susceptibility to EHV-1 by interrupting cell junctions, allowing the virus to reach its basolateral receptor.

Whether bacterial toxins also play a role in this impairment has not been studied yet, they noted.

The researchers evaluated the role of the toxins α-hemolysin from S. aureus and adenylate cyclase from Bordetella bronchiseptica.

Equine respiratory mucosal tissue samples from horses were used in the laboratory experiment. The samples were innoculated with the toxins and the effects observed.

The study team found that the toxins decreased epithelial thickness. Additionally, α-hemolysin induced detachment of epithelial cells and a partial loss of cilia (microtubule-based hair-like organelles).

These changes were linked by the research team to increased EHV-1 replication in the epithelium.

“In view of these results, we argue that the adenylate cyclase and α-hemolysin toxins increase the susceptibility of the epithelium to EHV-1 by disrupting the epithelial barrier function.”

The study, they said, is the first to report that bacterial exotoxins increase the horse’s sensitivity to EHV-1 infection.

“Therefore, we propose that horses suffering from infection by S. aureus or B. bronchiseptica may be more susceptible to EHV-1 infection.” In effect, the bacterial infections could “pave the way” for a primary EHV-1 infection to occur.

“Our findings,” they said, “are directly relevant for the veterinary practitioner. So far, no successful curative therapies are available against EHV-1, and the current commercial vaccines have not been overly effective in the prevention of clinical disease.

“We therefore argue that the prevention of EHV-1 infections should always be combined with the control of environmental factors that may promote the onset of EHV-1 replication.”

Acknowledging the importance of predisposing factors and identifying them is a prerequisite in the prevention of EHV-1, they said.

“Based on our findings, it should be recognized that bacterial infections with S. aureus or B. bronchiseptica could possibly precede an EHV-1 infection. These bacteria should thus at least be incorporated early on in the diagnostic approach of clinical respiratory disease. Upon diagnosing a predisposing infection with these bacteria, it may be beneficial to administer antimicrobial therapy early on in the course of the disease.”

The study team comprised Van Crombrugge, Emma Vanbeylen, Jolien Van Cleemput, Wim Van den Broeck, Kathlyn Laval and Hans Nauwynck.

Van Crombrugge, E.; Vanbeylen, E.; Van Cleemput, J.; Van den Broeck, W.; Laval, K.; Nauwynck, H. Bacterial Toxins from Staphylococcus aureus and Bordetella bronchiseptica Predispose the Horse’s Respiratory Tract to Equine Herpesvirus Type 1 Infection. Viruses 2022, 14, 149.

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

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