Packed with well-protected pathogenic (disease-causing) bacteria, biofilms can form spontaneously on wound surfaces and in deeper wound layers, creating significant challenges for both the equine patient and the treating veterinarian.

A biofilm’s resistant matrix makes it tough to treat with antimicrobials and allows it to evade the body’s immune response, resulting in wounds that get larger instead of healing, said Lynn Pezzanite, DVM, MS, PhD, Dipl. ACVS-LA, assistant professor of equine emergency and critical care at Colorado State University’s (CSU) Veterinary Teaching Hospital, in Fort Collins.

“Development of infection involving biofilms has important implications for the management of chronic wounds in horses, as they present unique challenges in diagnosis and are more resistant to typical treatment methods,” Pezzanite said during the 2021 American Association of Equine Practitioners’ convention, held Dec. 4-8, in Nashville, Tennessee.

“Wounds with biofilms may not exhibit signs typically associated with infection, although biofilms still prolong and impair healing as bacteria compete for metabolic resources and suppress the host inflammatory response in healing,” she said. “Early recognition of the presence of biofilms in nonhealing wounds and targeted treatment are key to the successful management of biofilms in equine practice.”

Biofilms are organized communities of bacteria enveloped in a three-dimensional matrix known as an extracellular polymeric substance (EPS), which comprises water, proteins, polysaccharides, glycolipids, and bacterial DNA. They can include one or several species of bacteria, usually from the Pseudomonas, Enterococcus, and Staphylococcus genera, and can form in as little as 24 to 48 hours.

Thanks to their organization and EPS, biofilms are highly resistant to topical and systemic antimicrobials, said Pezzanite. “For example, Staphylococcus aureus has been demonstrated to be up to 100-fold more resistant to antimicrobial therapy when growing in biofilms versus in planktonic (free-floating) status,” she said. Furthermore, the use of antiseptics can damage native tissues, further delaying wound healing, and is therefore not recommended.

Bacterial biofilms might be present in most chronic (long-term) as well as some acute wounds, Pezzanite said. In some cases the associated bacteria can’t be grown in traditional cultures after swabbing the wound. “Biofilms can only be definitively diagnosed using scanning electron or confocal micrography or molecular techniques, which are not usually available to clinicians,” Pezzanite said.

Therefore, deep tissue biopsy—especially from any wound pockets or fissures—just after debridement is the best way to confirm the horse is dealing with biofilms, Pezzanite said. If the horse has a wound that isn’t healing well, looks like it’s too moist and/or always inflamed, or otherwise appears infected but the cultures keep coming back negative, veterinarians should perform deep tissue biopsy. That involves drawing the swab across the surface of the wound with enough pressure to pick up the biofilm but carefully enough to avoid drawing blood. “Blood itself contains antimicrobial elements that may affect culture results,” she said.

She added that informing the laboratory if the horse is already on systemic antimicrobials is important. While biofilms are only responsive to systemic antimicrobials at very high doses, the drugs  have the potential to skew lab findings at any dose.

Treating biofilms is a mechanical process—at least to start. Veterinarians must shave, scrape, wipe, or jet-spray the entire surface of the affected area, including the devitalized tissue that “serves as nutrients to the microbes,” she said. This breaks up the biofilm and its protective matrix. Consequently, that’s the prime time to then treat with topical antimicrobials—ideally matched to the culture results—while the bacteria are vulnerable, said Pezzanite.

“Topical treatments are recommended to be applied within four hours of debridement or less, before biofilm re-formation,” she said.

Such debriding and antimicrobial treatment should continue daily until the wound shows signs of proper healing, said Pezzanite. The horse might need to be  anesthetized for the first debridement and perhaps follow-up sessions, if the animal is difficult to work with or the wound is complex or near joints or other important anatomical structures. Any foreign bodies and, in some situations, orthopedic implants would need to be removed.

Veterinarians and assistants should safeguard themselves from the potentially antimicrobial-resistant bacteria by wearing proper protective equipment, she added.

Depending on the wound’s age and size and the persistence of the biofilm coverage, recovery could take an extended period, typically weeks to months, said Pezzanite. However, veterinarians should see improvement within a few days.

Following debridement and antimicrobial therapy, veterinarians might perform skin grafting in some wounds to speed the healing process once the biofilm issue has been fully resolved and granular tissue is healthy, she said. When treated correctly, prognosis is very good, and owners are often happy with the healed area’s cosmetic appearance.

Treatment costs can be high in biofilm cases, with horses hospitalized for weeks or months and the possibility of one or more surgeries under general anesthesia, Pezzanite said. However, veterinarians should inform owners early on that faster and more aggressive treatment will lead to less expense overall, because wounds with biofilms don’t heal on their own.

Horses at the greatest risk of developing biofilms are older; malnourished; fighting systemic infections; chronically stressed; have weakened immune systems, pituitary pars intermedia dysfunction (PPID), or other pituitary-adrenal axis diseases; have extensively contaminated wounds; or have had surgical implants (e.g., plates, screws) placed at the wound site previously, she said. Even so, any horse can develop biofilms at a wound site. Further research for better diagnostics and treatment in cases of biofilm in horses is underway, Pezzanite said.