Dutch study estimates silent strangles carrier prevalence at 2%: full analysis

A new Dutch study adds a concrete number to a long-running strangles question: how many apparently healthy horses are still carrying Streptococcus equi subsp. equi in the background. Writing in Equine Veterinary Journal, investigators from Utrecht University and Royal GD estimated the true prevalence of silent carriers in the Netherlands at 2.0%, based on PCR testing of repeated nasopharyngeal lavages from adult horses and ponies without obvious clinical signs. (research-portal.uu.nl)

That figure matters because strangles remains endemic globally and silent carriers have long been viewed as one of the main reasons the disease keeps resurfacing after apparent recovery. The Dutch team framed the work as a response to a European evidence gap, noting that carrier-prevalence data in horse husbandry settings have been limited. Their estimate also fits with the authors' broader body of work on S. equi transmission dynamics in the Netherlands, which has examined how persistent infection and horse movement patterns can sustain exposure opportunities in the wider equine network. (research-portal.uu.nl)

The study used a cross-sectional design and sampled 166 horses on 86 premises, with three nasopharyngeal lavages collected at weekly intervals and analyzed by PCR. Using Bayesian methods, the authors calculated a mean true-prevalence estimate of 2.0% and a median of 1.6%, with a 95% credible interval ranging from 0.1% to 5.9%. The paper was published online ahead of print on April 26, 2026. The authors also flagged that they did not reach their target enrollment of 200 horses, which widens uncertainty, and that the sensitivity of three repeated nasopharyngeal lavages for detecting carriership remains unresolved. (research-portal.uu.nl)

That last point is especially important in the context of the wider strangles literature. Multiple prior studies and consensus guidance have emphasized that carrier detection can depend heavily on where samples are taken. Work published in the Journal of Veterinary Internal Medicine and other sources has found that repeated nasopharyngeal lavage can be useful, but direct guttural pouch sampling remains central because persistent infection often localizes there. AAEP guidance similarly describes the guttural pouch as a frequent site of persistent infection and intermittent shedding. (academic.oup.com)

Industry and expert commentary around strangles has consistently focused on the same practical challenge: outwardly healthy horses can still be epidemiologically important. UK Surveillance of Equine Strangles data, for example, reported that about one-third of positive laboratory diagnoses in 2018 were confirmed in outwardly healthy carrier horses. Review and conference materials have also argued that qPCR has likely improved recognition of silent carriers, while leaving open an important clinical question: which PCR-positive horses pose the greatest real-world transmission risk to susceptible groups. (researchgate.net)

Why it matters: For veterinary professionals, the Dutch estimate is less a reassurance than a planning number. A 2% background prevalence in apparently healthy horses suggests that silent carriership is uncommon, but not rare enough to ignore, especially in referral settings, sales movement, competition circuits, or premises with a recent strangles history. It supports a risk-based approach to biosecurity: follow-through after outbreaks, targeted screening before reintegration or transport, and clear communication with pet parents and facility managers that absence of clinical signs doesn't fully rule out infection. It also underscores the limits of one-off testing and the need to match the sampling strategy to the question being asked. (research-portal.uu.nl)

The study may also help normalize a more evidence-based conversation about screening in endemic settings. If prevalence in the general adult population is indeed in the low single digits, broad untargeted testing may have different value than focused testing in horses with outbreak links, compatible history, or movement-associated risk. At the same time, the wide credible interval and unresolved test sensitivity mean the estimate shouldn't be treated as the final word. (research-portal.uu.nl)

What to watch: The next developments to watch are larger prevalence studies, work that pairs nasopharyngeal testing with guttural pouch evaluation, and research that connects PCR-positive carrier status to actual onward transmission risk, because that's what will most directly shape screening recommendations and day-to-day case management. (research-portal.uu.nl)