Biofilm in the Mare's Uterus: Why Breaking It Up Is Not Enough

Breeders and vets increasingly talk about "biofilm" when a mare will not get in foal. It is a real phenomenon, but the common response, breaking the biofilm up, often misses the actual problem. Here is what a biofilm is, how it relates to the dormant infection behind many problem mares, and why a surface treatment is rarely enough.

What a biofilm actually is

A biofilm is a community of bacteria that settles on the surface of the uterine lining and builds a protective layer around itself. The bacteria form a micro-colony on the mucosa and secrete an extracellular matrix, a slimy substance rich in proteins that shields them. The bacteria most often involved, Streptococcus equi subspecies zooepidemicus, typically arrive when the cervix is open around foaling and the uterus is exposed to the normal microbiota of the lower reproductive tract. Over repeated seasons they can establish on the lining and, in some cases, slip into a dormant state.

Biofilm and dormant infection are not the same thing

This distinction is the one that matters. A biofilm is superficial: it sits on the surface of the lining. The dormant infection behind many problem mares sits deeper, inside the mare's own uterine cells and as far as about 10 millimetres into the tissue. Some bacteria within a biofilm do become dormant, so the two can coexist, but they are not the same problem. You can have a biofilm on the surface while the more stubborn dormant bacteria are hidden in the tissue below.

Why acetylcysteine is not enough

The usual way to tackle a biofilm is acetylcysteine, a mucolytic agent that breaks up mucus, the same kind of compound used when a cold fills the airways. It does break up biofilm on the surface to some extent. The limitation is simple: acetylcysteine works on the surface and does not reach into the tissue, and it does not bring dormant bacteria back into an active state. As Professor Anders Miki Bojesen explains, since much of the problem is in the tissue, fixing the surface is "not even halfway." It is an easy, inexpensive treatment that feels like progress, which is often why both vet and owner reach for it, but it leaves the deeper infection untouched.

This is also why a mare can seem to improve after a biofilm treatment, get in foal, and then fail again the following season. The surface was cleared. The dormant reservoir in the tissue was not.

Reaching the infection the biofilm hides

To address the dormant bacteria, you first have to make them detectable. Standard diagnostics depend on bacteria growing on a culture plate, and dormant bacteria do not grow, which is why a standard swab catches only about a third of positive mares, and even a flush or biopsy reaches only roughly 60 to 70 percent. bActivate is a diagnostic growth medium that brings dormant Streptococcus zooepidemicus back into an active, growing state. Once the bacteria are active, a culture can identify them and the veterinarian can apply targeted treatment. Activation reaches what breaking up a biofilm cannot: the dormant infection deep in the tissue.

What this means for your problem mare

If your mare has been treated for biofilm, perhaps with acetylcysteine, and still fails to conceive, a dormant infection deeper in the uterine lining is a likely explanation. The practical step is to talk to your veterinarian about activating and culturing the uterus rather than treating the surface alone. You can hear Professor Bojesen and Dr. Morten Ronn Petersen discuss this in depth on the bActivate podcast, or review the clinical evidence.

Written by the Bojesen & Petersen Biotech ApS team. Medical oversight: Prof. Anders Miki Bojesen DVM PhD (University of Copenhagen) and Dr. Morten Ronn Petersen DVM PhD Dipl. ACT.