Spotlight falls on the role of Staphylococcus aureus in mud fever in horses

Share
Many contributing factors are believed to be involved in the development of equine pastern dermatitis, which is also known as scratches, mud fever, greasy heel, dew poisoning, grapes, and canker.
Many contributing factors are believed to be involved in the development of equine pastern dermatitis, which is also known as scratches, mud fever, greasy heel, dew poisoning, grapes, and canker. (File image)

Clear links between pastern dermatitis in horses and Staphylococcus aureus displaying a certain virulence profile are revealed in just-published research.

Equine pastern dermatitis is one of the most frequently encountered skin disorders in horses. Owners have several names for the condition, including scratches, mud fever, greasy heel, dew poisoning, grapes, and canker.

It is considered a syndrome rather than a disease, which complicates the scientific investigation of its cause.

Pastern dermatitis can result in a range of clinical signs, typically including skin redness, hair loss, scales, crusts, and thickening of the skin in the pastern area.

Many contributing factors are believed to be involved in the development of the condition. Among the most frequently suspected is the bacterium S. aureus, known for its pathogenic potential in skin and soft tissue infections.

Researchers with the University of Bern in Switzerland set out to investigate the association between S. aureus carriage and pastern dermatitis.

Sarah Kaiser-Thom and her fellow researchers took nasal and pastern swabs for analysis from 105 horses affected by the condition and 95 unaffected healthy animals.

The swab samples were cultured in a laboratory and checked for the presence of methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA).

The study team, reporting in the journal BMC Veterinary Research, found that a markedly higher proportion of samples from affected horses were positive for S. aureus, both from the pastern and the nose.

In horses with the condition, 59% of pastern samples were positive compared to 6.3% in in the healthy animals. The nasal swabs were also positive in 59% of the affected horses, compared with 8.4% of the healthy animals.

Molecular-based testing revealed further insights. The isolates belonged to 20 sequence types, with three lineages being predominant.

The high rates of S. aureus isolation from both pasterns and noses in affected horses, coupled with the fact that the respective isolates were frequently genetically closely related, may indicate it had spread from one area to the other.

The frequency of methicillin-resistant samples was comparatively low and did not differ significantly between affected and control horses, neither in the pasterns nor in the nose.

A total of 125 genes associated with bacterial virulence were identified in the genomes of the S. aureus strains.

The authors reported that antimicrobial resistance genes were almost equally frequent in pastern and in nasal samples, whereas some virulence factors, such as the beta-hemolysin, ESAT-6 secretion system, and some enterotoxins were more abundant in isolates from the pastern samples, possibly enhancing their pathogenic potential.

“The markedly higher prevalence of S. aureus containing specific virulence factors in affected skin suggests their contribution in the development and course of equine pastern dermatitis,” they said.

They said their study findings highlighted the potential importance of S. aureus in the development and progression of the condition.

"It remains open whether S. aureus plays a rather primary or secondary role in the development of equine pastern dermatitis," say researchers.
“It remains open whether S. aureus plays a rather primary or secondary role in the development of equine pastern dermatitis,” say researchers. (File image)

The genetic sequencing undertaken in the study provided new insights into the genetic features of the investigated S. aureus strains, particularly related to resistance and virulence factors, and framed their genetic relationships.

“Our observations suggest that dissemination of strains takes place between different sites within the same horses, and that spreading also occurs between horses living in the same stable.”

Some S. aureus lineages containing specific virulence factors were more present in affected pastern than in nasal cavities.

“It remains open, whether S. aureus plays a rather primary or secondary role in the development of equine pastern dermatitis, and what the clinical significance of these toxins is,” they said.

Further studies are required, they said, including investigations on colonization with S. aureus strains in affected horses to identify the key virulence factors contributing to the progression of the condition. “Our study underlines that the presence of S. aureus in horses should not be neglected in the diagnostic, prevention and treatment of equine pastern dermatitis following antimicrobial susceptibility testing.”

The study team comprised Kaiser-Thom, Vinzenz Gerber, Alexandra Collaud, Joel Hurni and Vincent Perreten.

Kaiser-Thom, S., Gerber, V., Collaud, A. et al. Prevalence and WGS-based characteristics of Staphylococcus aureus in the nasal mucosa and pastern of horses with equine pastern dermatitis. BMC Vet Res 18, 79 (2022). https://doi.org/10.1186/s12917-021-03053-y

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

Horsetalk.co.nz

Latest research and information from the horse world.

Leave a Reply

Your email address will not be published.