The unique and mysterious microbial world of horse skin revealed in study

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The healing of experimentally induced wounds on one of the horses at different body sites. A: 24 hours, B: 7 days, C: 14 days, D: 21 days, E: The head wound, with scar present.
The healing of experimentally induced wounds on one of the horses at different body sites. A: 24 hours, B: 7 days, C: 14 days, D: 21 days, E: The head wound, with scar present. Image: Kamus et. al https://doi.org/10.1371/journal.pone.0206989

A wound study in horses has found that the make-up of their skin microbiota is unique, with researchers noting that the most common bacteria found is usually quite difficult to grow.

The University of Montreal researchers say their findings suggest that using humans or laboratory animals as models may not prove accurate when it comes to horse skin and wound care.

Louis Kamus, Christine Theoret and Marcio Costa used DNA-based next-generation sequencing to reveal some fascinating insights into the microbes that populate horse skin.

Acidobacteria comprised the vast majority of bacteria present on the skin of control horses,” the trio reported in the open-access journal PLOS ONE.

“This difficult-to-grow bacterium has been reported to be of low abundance as identified by next-generation sequencing in the skin of humans and mice,” they continued.

Acidobacteria is associated with the occurrence of psoriasis in humans, they added.

“This demonstrates that the skin microbiota of horses is unique and extrapolation of data from humans or laboratory animals may not be accurate for the equine species.”

The researchers noted that wound management was an important part of equine practice because the species’ flight instinct predisposed horses to injury.

Skin trauma often needed labour-intensive treatment. Closing wounds was seldom successful, they noted. Healing wounds in horses often become chronic, especially when on a limb, where persistent inflammation is associated with development of excessive granular tissue.

The study devised an experiment to learn more about the microbiota of horses during healing of skin wounds in two locations, on the side of the body and on the lower limbs.

They chose these areas because they are known to present different healing patterns.

They also wanted to investigate how bandaging affected the bacterial communities around the skin wounds on the limbs.

In their experiment, small full-thickness skin wounds were created on four healthy horses under sedation. Each horse received two lower-leg wounds and one wound on their side.

The limb wounds on each horse were randomly assigned to bandaging or not. The bandaged limbs received a traditional half-limb bandage consisting of a low-adherent gauze-like dressing covered by cotton wool roll held in place with an adhesive tape.

Full-thickness samples were saved for analysis when the wounds were created. Full-thickness biopsy punches were then taken from the margins of the wounds after one week, two weeks, and after the wounds were assessed as being fully healed.

Thoracic skin samples obtained from three healthy horses were included in the analysis as controls.

No signs of discomfort nor changes in vital signs were recorded in any of the horses during the study period, and no pain-killers were required.

The great similarity in the skin microbiota of the three control horses suggested that this was a stable environment consistent across individuals, they said.

Virtually all of the most abundant genera (more than 1%) found in the skin of healthy horses from the control group were unclassified at the genus level and most of them belonged to the Acidobacteria phylum.

“The fact that most bacteria found in this study were unclassified at the genus level along with 20% unclassified bacteria at the phylum level, reinforces the need of further efforts to investigate this unexplored environment,” they said.

“It is unlikely that those unclassified DNA sequences are the consequence of sequencing errors. Therefore, this finding may truly reflect the presence of a high number of unknown bacteria in the skin of horses, which might be of special importance for the development of new therapies.”

The researchers found that the location of the wound significantly influenced bacterial composition of equine skin and healing wounds.

Fusobacterium and Actinobacillus were strongly associated with limb wounds during the initial phases of healing, they reported.

Bandaging was found to have a significant impact on the microbiota during the healing process.

A richer and more diverse community was seen in unbandaged limb wounds one week after the wound was established, suggesting that bandaging altered environmental conditions, possibly via an effect on temperature, moisture, debris, oxygen tension and decreasing interaction with the ground microbiota.

“It appears that the effect of bandaging on wound microbiota may cause long-term changes, since several genera present after full healing were associated only with wounds that had been left unbandaged.

“However, the consequences associated with such bacterial changes remain to be investigated before formulating management recommendations.”

Generally, the skin microbiota after healing was more similar to samples from controls, demonstrating the resilience and stability of the environment.

The study team described the equine skin microbiota as a rich and stable environment that was disturbed by wounding, but returned to its previous stage after full healing.

“Results of this study provide new insight for a better understanding of the contribution of bacteria to wound healing in horses and may facilitate the future development of therapeutic strategies using commensal bacteria.”

Kamus LJ, Theoret C, Costa MC (2018) Use of next generation sequencing to investigate the microbiota of experimentally induced wounds and the effect of bandaging in horses. PLoS ONE 13(11): e0206989. https://doi.org/10.1371/journal.pone.0206989

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

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