Equine Strangles Management

When speaking with local veterinarians, Amy Stieler Stewart, DVM, PhD, DACVIM-LA, has noticed that equine strangles is a common topic.

“It comes up a lot,” said Dr. Stewart, an assistant teaching professor of physiology at North Carolina State University’s College of Veterinary Medicine.

Strangles is one of the most common contagious bacterial diseases affecting horses worldwide. Caused by Streptococcus equi subsp. equi, it spreads easily through direct contact as well as contaminated water buckets, tack and even human hands or clothing. In the U.S., thousands of cases are reported each year, with hundreds of confirmed outbreaks. The disease is considered endemic in many regions across all 50 states. Young horses — particularly weanlings, yearlings and those under 5 — are most at risk, especially in high-contact environments such as boarding barns, training facilities, show circuits and sales barns. However, she said it is important to note that older horses are also susceptible.

While strangles is widely recognized in equine practice, some of its most challenging aspects are still underestimated. One key concern is the ability of S. equi subsp. equi to persist outside the horse. The organism can survive in the environment — particularly in cold, wet conditions — and may linger on commonly used equipment such as twitches and endoscopes, creating ongoing opportunities for transmission if biosecurity lapses.¹

Equally important, Dr. Stewart noted, is the misconception around how long horses remain infectious. While many animals recover within two to three weeks, some may continue shedding bacteria for up to six weeks after clinical signs resolve. Others can become silent carriers, showing no outward symptoms while still spreading disease, reinforcing the need for careful quarantine and follow-up testing.

Diagnostic limitations add another layer of complexity. Different testing methods — such as nasal swabs, pharyngeal washes or guttural pouch lavage — vary in sensitivity, and intermittent shedding means a single negative result may not definitively rule out infection. Together, these factors highlight why strangles remains a persistent and sometimes difficult disease to fully control in equine populations.

When complications occur

What distinguishes a routine case of strangles from a full-scale medical or biosecurity crisis often comes down to complications and management, Dr. Stewart said. In its typical form, strangles presents with fever and swollen lymph nodes that resolve with supportive care, including anti-inflammatories and local treatment of abscesses.

However, up to 20% of cases can escalate. “A major medical event would likely be a horse that had severe complications,” Dr. Stewart said, pointing to risks such as persistent guttural pouch empyema and chondroid formation, which can lead to chronic shedding. Other complications include immune-mediated conditions such as purpura hemorrhagica and myositis, as well as “bastard strangles,” where abscesses form internally in organs such as the liver or spleen. In severe instances, horses may develop respiratory distress requiring emergency intervention, including a temporary tracheotomy.²

Even seemingly mild cases demand vigilance. “All cases should be treated as if they could become major outbreaks,” Dr. Stewart emphasized, noting that while mortality is low, the disease spreads easily — especially in large barns with frequent horse and human movement. Strict biosecurity measures, including temperature monitoring, isolation and limiting transport, remain critical.

Diagnostic approaches have also evolved. Quantitative PCR (qPCR) has become the gold standard, replacing traditional culture due to improved sensitivity, while endoscopic evaluation of the guttural pouches is now considered essential for identifying carrier animals.

Carriers remain a persistent challenge. Horses may appear healthy yet intermittently shed S. equi subsp. equi, and “one negative nasal swab” is often not enough to confirm clearance, Dr. Stewart noted. Roughly one in 10 horses may become long-term carriers if not properly screened and managed, prolonging the risk of transmission within equine populations.

Best practices for biosecurity

Looking ahead, Dr. Stewart said one simple shift in biosecurity practices could have an outsized impact on controlling strangles across the industry: more rigorous screening of new arrivals. Specifically, she advocates for guttural pouch endoscopy and lavage at the end of quarantine — before a horse is introduced to the broader population. Because silent carriers may show no outward signs, they can easily pass through standard quarantine protocols undetected. “They routinely are allowed to enter a new barn or herd after a two- to three-week quarantine without any concern when they really could be harboring infection,” she said.

During active outbreaks, Dr. Stewart emphasized that early detection is critical. Twice-daily temperature monitoring can help identify infected horses before they begin shedding bacteria, as fever often precedes nasal shedding by two to three days. That window, she noted, is key to isolating cases and preventing wider spread.

For barns hesitant to test or disclose cases, Dr. Stewart frames the conversation in terms of long-term risk. Undetected carriers can fuel recurring outbreaks, creating ongoing health challenges and financial strain. Transparency, paired with thorough testing to confirm horses are disease-free before leaving quarantine, is essential to protecting both herd health and a facility’s reputation.

What’s next in strangles detection and prevention

Emerging advances in diagnostics and prevention are offering new hope in the fight against strangles, according to Dr. Stewart.

One promising development is the introduction of new serologic tests, including the Protein 92 ELISA, which has shown improved sensitivity and specificity compared with traditional assays. This could allow veterinarians to more accurately identify horses that have been exposed to S. equi subsp. equi, while reducing issues with cross-reactivity.

Vaccine innovation is also evolving. Researchers are exploring novel vaccines that target recombinant proteins, with the goal of improving protection while minimizing adverse reactions — an area that has historically limited broader vaccine adoption.

At the same time, advances in point-of-care (POC) PCR testing could significantly change how outbreaks are managed. Rapid, stall-side diagnostics would enable veterinarians to make immediate decisions about isolation and treatment, helping to reduce the window for disease transmission.

For now, strangles vaccination is considered “risk-based,” Dr. Stewart said. While current vaccines may reduce disease severity and limit infection in some cases, they are not a substitute for strong biosecurity.

“Additionally, there are reported complications that can occur following vaccine administration, so not every horse is a candidate for the strangles vaccine,” she said. “This is an area of ongoing research and a space to watch!”

References:

1. Durham AE, Hall YS, Kulp L, Underwood C. A study of the environmental survival of Streptococcus equi subspecies equi. Equine Vet J. 2018;50(6):861-864. doi:10.1111/evj.12840
2. Svonni E, Andreasson M, Fernström LL, Rydén A, Pringle J, Riihimäki M. Potential for residual contamination by Streptococcus equi subspp equi of endoscopes and twitches used in diagnosis of carriers of strangles. Equine Vet J. 2020;52(6):884-890. doi:10.1111/evj.13248
3. Valberg S. Myopathies associated with Streptococcus equi equi infection. Equine Veterinary Education. 2024;36(8):400-402. https://doi.org/10.1111/eve.13954
4. Boyle AG, Timoney JF, Newton JR, Hines MT, Waller AS, Buchanan BR. Streptococcus equi Infections in Horses: Guidelines for Treatment, Control, and Prevention of Strangles-Revised Consensus Statement. J Vet Intern Med. 2018;32(2):633-647. doi:10.1111/jvim.15043

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