Uterine Swab vs Lavage vs Biopsy: Which Finds the Infection?
In short. The swab samples the surface and is the least sensitive; the low-volume lavage samples the whole lining and finds more; the biopsy samples the tissue itself, finds the most, and grades the lining as well. In the one direct comparison, swab culture found 34 % of infected mares and biopsy culture 82 %. None of the three finds bacteria that are dormant. For a mare that has failed two or more cycles with clean samples, the fourth step is an activation culture.
What each method collects
- Guarded swab. A cotton tip on a guarded rod is passed through the cervix and rolled against the lining, usually of the uterine body. It collects surface cells and whatever bacteria sit on them, from an area the size of the tip. Cheap, quick, familiar, and done in every practice. It is also the reason so many problem mares are called clean.
- Low-volume lavage. 60 to 150 ml of saline is run into the uterus, the uterus is massaged, and the fluid is recovered and spun down. The pellet is cultured and examined for cells. The sample comes from the whole surface of both horns and the body, not from one spot, and it collects the fluid the mare was holding. It takes a few minutes longer than a swab and needs a clean technique.
- Endometrial biopsy. A small piece of the lining, a few millimeters, is taken with biopsy forceps through the cervix. One sample gives three answers: culture from the tissue, cytology from an impression of it, and histology, the Kenney score, that grades fibrosis, inflammation and cystic change. In the mare the lining is thick and the procedure is safe and quick; it is the most information per visit.
What each method finds
| Guarded swab | Low-volume lavage | Endometrial biopsy | Activation culture (bActivate, then any of the three) | |
|---|---|---|---|---|
| Sample | Surface of one spot | Surface of the whole lining plus retained fluid | Tissue, a few millimeters deep | Bacteria brought to the surface from the tissue |
| Culture sensitivity in the direct comparison | 34 % of infected mares (Nielsen 2005) | More than the swab; no direct comparison figure | 82 % of infected mares (Nielsen 2005) | 64 % activation-positive vs 8 % with saline (Petersen 2015, placebo-controlled) |
| Specificity | 1.0 (Nielsen 2005) | Good with clean technique | 0.92 (Nielsen 2005) | Not applicable; it is a pre-step |
| Cytology | Yes, from the tip | Yes, from the pellet | Yes, impression smear | As for the method used after activation |
| Grades the lining (Kenney score) | No | No | Yes | No |
| Finds dormant bacteria | No | No | No | Yes, that is its purpose |
| Best use | Routine check of a mare with no history | Fluid mares, problem mares, first work-up | Problem mares, older mares, any mare where the lining's condition matters | Problem mares with clean samples, two or more failed cycles |
The figures come from different studies and measure different things; the table is a decision aid, not a head-to-head trial.
Why the swab misses so much
Two reasons, and they stack. First, place: in chronically infected mares, Streptococcus zooepidemicus was found in foci 300 to 500 micrometers below the epithelium, with nothing on the surface the swab touches (Petersen et al., Clinical Theriogenology 2009). Second, state: part of those bacteria are dormant, with their metabolism shut down, and a culture only shows bacteria that grow. That is why even the lavage and the biopsy, which reach further, still miss the dormant fraction. After activation with a culture medium the organism was detectable in 64 % of mares, after saline in 8 % (Petersen et al., Veterinary Microbiology 2015).
Which method should you ask your vet for?
- A young mare, no history, routine pre-breeding check: a swab with cytology is defensible. A clean result means what it says in a mare with no reason to be infected.
- A mare that held fluid after covering, or failed one cycle: low-volume lavage or biopsy, not a swab. The extra minutes are cheaper than the next cycle.
- An older mare, a mare with foals, a previous endometritis: biopsy. You need the Kenney score anyway to know what you are working with.
- A problem mare with clean samples and two or more failed cycles: activation culture. The vet instills 10 ml of bActivate in early estrus, takes the second sample after 48 hours with whichever method is preferred, and cultures with an antibiogram. bActivate is a diagnostic aid, not a treatment. In the Hagyard series, 47 % of 64 problem mares that had been called clean were culture-positive after activation, and 83 % were pregnant after treatment and covering, without a placebo group (Petersen and Bojesen, Clinical Theriogenology 2014).
Whatever grows is nearly always treatable with a standard antibiotic: in 28,887 German endometrial samples, beta-hemolytic streptococci made up 79.7 % of positive cultures and were penicillin-sensitive in 99.5 % of cases (Köhne et al., 2024).
Frequently asked questions
Is a uterine biopsy dangerous for the mare?
No. The lining of the mare's uterus is thick, the sample is a few millimeters, and the procedure is routine in reproductive practice. She can be covered in the same or the next cycle depending on the findings.
Can a low-volume lavage replace the biopsy?
For culture and cytology it comes close and it is gentler. It does not give the Kenney score, so for an older or problem mare the biopsy still adds something.
If the biopsy culture is clean, is the mare clean?
Probably, if she has no history. In a problem mare, no: the biopsy still depends on bacteria growing, and dormant bacteria do not. The activation culture is the step that answers that question.
Does bActivate replace the swab, lavage or biopsy?
No. It comes before them. The medium wakes the dormant bacteria; the sample and the culture are then taken as usual, with whichever method the vet prefers.
Sources
- Nielsen JM. Endometritis in the mare: a diagnostic study comparing cultures from swab and biopsy. Theriogenology 2005;64:510-518.
- Petersen MR et al. Clinical Theriogenology 2009;1:393-409.
- Petersen MR et al. Veterinary Microbiology 2015;179:119-125.
- Petersen MR, Bojesen AM. Clinical Theriogenology 2014;6(3):313-314.
- Köhne M et al. Journal of Equine Veterinary Science 2024;105008.
Written by the Bojesen & Petersen Biotech ApS team. Medical oversight: Prof. Anders Miki Bojesen DVM PhD (University of Copenhagen) and Dr. Morten Rønn Petersen DVM PhD Dipl. ACT.
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