Vaginal microbiome screening reveals vaccine strain reactivation at lambing in ewes

Researchers have shown that sequencing the vaginal microbiome of pregnant ewes can detect the reactivation of a Brucella vaccine strain at parturition, offering a non-invasive window into infection dynamics in livestock. The study, published in the journal Animal Microbiome, followed vaccinated pregnant sheep longitudinally and found that one candidate vaccine, 16MΔwzm, reactivated at lambing, while another, Rev1Δwzm, remained undetectable throughout.

The team profiled the vaginal microbiomes of pregnant ewes vaccinated with attenuated strains of Brucella melitensis, using 16S rRNA amplicon sequencing of the V3–V4 region. This approach reads the genetic barcodes of bacteria present in vaginal samples, allowing researchers to track both the overall microbial community and the presence of Brucella itself over the course of gestation and delivery.

Brucella was undetectable during pregnancy in the vaccinated animals, but in ewes that had received the 16MΔwzm strain the bacterium reappeared at parturition. This reactivation coincided with significant disruption of the vaginal microbiome, suggesting that the peripartum period is a critical moment when a previously controlled infection can resurface and potentially be shed.

In contrast, ewes vaccinated with Rev1Δwzm maintained stable vaginal microbiomes that resembled those of unvaccinated controls, and no Brucella was detected in these animals at any point. The authors note that this pattern supports the superior safety profile of Rev1Δwzm in pregnant ewes, which had been reported in previous work.

The vaginal microbiome is known to influence reproductive health and susceptibility to infection in livestock, but its dynamics during gestation, particularly in relation to intracellular pathogens such as Brucella, have remained underexplored.

According to the researchers, the findings provide a proof of concept that vaginal microbiome profiling can accurately detect Brucella in shedder sheep. Because sampling is non-invasive, the approach could in principle be used to monitor infection dynamics in natural hosts without disturbing animals, aiding brucellosis control within a sustainable One Health framework that links animal, human and environmental health.

The authors are careful to stress the limitations of the study. The work was carried out under controlled conditions rather than in the field, and the cohorts involved were small. They state that studies under field conditions with larger cohorts are necessary before the approach can be translated into routine practice, and the article is being shared early as a peer-reviewed, accepted manuscript that is subject to further editorial edits before the final version of record.

The study also highlights the differing behaviour of the two vaccine candidates. Live attenuated strains 16MΔwzm and Rev1Δwzm exhibit differing safety profiles in pregnant ewes, and the longitudinal microbiome data now provide an independent line of evidence consistent with those earlier safety observations, linking bacterial reactivation to measurable changes in the host’s vaginal microbial community.

Beyond brucellosis, the work suggests that microbiome sequencing could become a general tool for monitoring intracellular pathogen dynamics in livestock, an area the authors identify as underexplored.

Subject of Research: Vaginal microbiome profiling to detect Brucella reactivation in vaccinated pregnant ewes

Article Title: Pregnancy-associated vaginal microbiome profiling detects Brucella reactivation in vaccinated pregnant ewes

Article References: Zabalza-Baranguá, A., Llorens-Rico, V., Tafat-Arriaga, A., Lluch-Senar, M., & Grilló, M.-J. (2026). Pregnancy-associated vaginal microbiome profiling detects Brucella reactivation in vaccinated pregnant ewes. Animal Microbiome. https://doi.org/10.1186/s42523-026-00639-5

Image Credits: AI Generated

DOI: 10.1186/s42523-026-00639-5

Keywords: Brucella, vaginal microbiome, pregnant ewes, vaccination, 16S rRNA sequencing, brucellosis, parturition, livestock, One Health, vaccine safety, 16MΔwzm, Rev1Δwzm