Strangles in Horses: Prevention, Signs, and Treatment

Strangles is one of those diseases that can turn a barn upside down in a matter of days. One horse comes back from a show with a runny nose. Within two weeks, half the barn has swollen lymph nodes and thick nasal discharge, and you're canceling lessons, quarantining horses, and sanitizing everything you own. It spreads fast, it's messy, and it's expensive. Not because of treatment costs necessarily, but because of the downtime, the disruption, the lost board income, the canceled events, the frantic phone calls from owners demanding answers.

The bacterium responsible is Streptococcus equi subspecies equi, or S. equi for short. It's been around for centuries. Historical records from the 1200s describe outbreaks that sound exactly like what we see today, which tells you something about how tenacious this organism really is. Despite our modern understanding of infectious disease, strangles remains one of the most common bacterial infections in horses worldwide. And it keeps circulating because of carrier horses that show no symptoms whatsoever but shed the organism into shared water, onto fencing, into the general environment where naive horses pick it up without anyone realizing what happened until the fevers start rolling through the barn (Cornell University College of Veterinary Medicine).

How It Spreads

Transmission is primarily through direct contact with nasal discharge or pus from ruptured abscesses. The bacterium is tough. Remarkably tough. S. equi can survive in the environment for weeks under favorable conditions, persisting in water troughs, on fence rails, on shared equipment, and in contaminated soil. Shared water buckets are one of the fastest ways to spread it through a group of horses. One sick horse drinks, another healthy horse drinks ten minutes later, and the deal is done.

People spread it too. If you handle a sick horse and then go pet the one in the next stall without washing your hands and changing clothes, you've just become a vector. Farriers, veterinarians, feed delivery drivers, barn visitors, the kid who comes to pet the ponies on Saturday mornings. Any of them can carry the organism from farm to farm on boots, hands, and equipment. Biosecurity isn't paranoia. It's common sense. A 2018 study cited by UC Davis found that S. equi remained viable on wooden fencing for up to 63 days under cool, damp conditions, which should terrify anyone running a boarding operation.

The incubation period is typically 3 to 14 days after exposure. Younger horses, under five, tend to be more susceptible and often develop more severe clinical signs. Older horses with previous exposure may carry some degree of immunity and show milder symptoms or avoid clinical disease altogether. But "some immunity" is not "bulletproof immunity," and plenty of mature horses get walloped by strangles when a new strain rolls through.

Clinical Signs

The classic progression starts with fever, often 103 to 106 degrees Fahrenheit. The horse becomes depressed, goes off feed, and develops a serous nasal discharge that quickly turns thick, purulent, and bilateral. You'll see it crusted around the nostrils, sometimes dripping in ropes. Appetite craters. Energy vanishes. The eyes go dull.

Then the lymph nodes swell. The submandibular and retropharyngeal lymph nodes, under the jaw and behind the throat, enlarge dramatically. They become hot, firm, and exquisitely painful. The horse may hold its head extended, neck stretched out flat, trying to relieve pressure on the throat. Swallowing becomes difficult or nearly impossible. Some horses sound like they're breathing through a cocktail straw, which is where the name "strangles" originates. In severe cases the swollen lymph nodes can genuinely compromise the airway, and according to the Merck Veterinary Manual, emergency tracheotomy is occasionally necessary to keep the horse alive while abscesses mature.

The abscesses eventually ripen and rupture, draining thick yellow-green pus. This is actually a turning point in terms of disease progression. Once they drain, the horse usually starts improving within 24 to 48 hours. Relief is almost visible. But that discharge? Wildly infectious. Everything it contacts needs to be treated as biohazard material. Bedding, buckets, your jacket sleeve, your boots.

Most horses recover in three to four weeks. The immune response following natural infection is fairly robust. About 75 percent of recovered horses develop solid immunity lasting at least five years according to research reviewed by Texas A&M. But not every case follows the textbook, and the ones that don't can get ugly fast.

Bastard Strangles

In roughly 1 to 2 percent of cases the infection metastasizes. Instead of staying contained in the head and throat lymph nodes, S. equi spreads to lymph nodes elsewhere in the body: the abdomen, the lungs, the liver, the brain, the kidneys. This is called bastard strangles. The name fits.

Abscesses forming in the abdomen or chest are fiendishly difficult to diagnose and even harder to treat. Horses with bastard strangles may show vague signs. Weight loss despite adequate feed. Intermittent fever that comes and goes without explanation. Low-grade colic episodes. Generalized malaise. Poor performance. These symptoms don't immediately scream strangles, especially if the initial respiratory phase was mild or went unnoticed entirely. Some cases aren't diagnosed until ultrasound or postmortem examination reveals internal abscesses scattered across organs like buckshot.

Another serious complication is purpura hemorrhagica, an immune-mediated vasculitis that can erupt one to four weeks after infection or, frustratingly, after vaccination. The horse's own immune system mounts a catastrophic overreaction to streptococcal antigens, causing widespread inflammation of blood vessels throughout the body. Clinical signs include severe limb edema so dramatic the legs look like tree trunks, petechial hemorrhages splashed across the gums and inner eyelids, and painful skin swelling that can slough in sheets. The AAEP notes that purpura hemorrhagica carries a guarded prognosis and demands aggressive immunosuppressive therapy, IV fluids, and meticulous supportive care, often over weeks of hospitalization.

Treatment

Treatment of strangles is somewhat controversial, and the right approach depends entirely on the stage of disease. Here's the general thinking among equine practitioners:

Early stage (fever only, no abscess formation yet): Some veterinarians will prescribe antibiotics, usually penicillin, to try to knock out the infection before abscesses develop. S. equi is typically exquisitely sensitive to penicillin. However, there's a legitimate school of thought that early antibiotic use can prevent the horse from developing full immunity, essentially leaving it susceptible to reinfection later. This is a real dilemma. Cornell's diagnostic lab has published recommendations acknowledging both sides of the argument. Discuss the pros and cons with your vet, who knows your specific situation.

Abscess stage: Once abscesses are forming, the general recommendation is to step back and let them mature. Antibiotics at this stage may slow abscess maturation without eliminating the infection, effectively prolonging the disease and the quarantine period. Hot packing the swollen lymph nodes three or four times daily can help bring abscesses to a head faster. Your vet may lance mature abscesses to facilitate drainage, which provides immediate visible relief. Keep the drainage site clean and flush it with dilute chlorhexidine or povidone-iodine as directed.

Supportive care: Soft feeds like soaked hay cubes, beet pulp mashes, or even grass if available for horses that cannot swallow normally. NSAIDs for fever management and pain control. Fresh water always available, positioned at a comfortable height since some horses with throat swelling struggle to lower their heads to ground-level buckets. Monitor vital signs closely, including temperature twice daily. Watch for any indication of respiratory distress, worsening depression, or signs that suggest the disease is moving beyond the straightforward version.

Quarantine and Management

This is where strangles gets really, really inconvenient. But cutting corners on quarantine is exactly how isolated cases become barn-wide catastrophes that drag on for months.

• Isolate sick horses immediately. Separate barn, separate paddock, separate equipment, separate everything. Ideally a dedicated caretaker for sick horses who doesn't handle healthy ones at all. If that's impossible, handle healthy horses first thing in the morning, change clothes completely, then deal with the quarantine group last.
• New arrivals should be quarantined for 2 to 3 weeks before joining the general population. This is good practice regardless of strangles. It applies to all infectious diseases. Period. No exceptions. No "but she just came from a nice barn." Nice barns get strangles too.
• Stop all horse movement on and off the property during an active outbreak. No shows, no trail rides, no visitors bringing horses, no horses leaving for any reason. Your farrier and vet should be made aware and should implement their own decontamination protocols between your property and their next call.
• Disinfect everything. S. equi is killed by most common disinfectants. Chlorhexidine, dilute bleach solutions, and phenolic compounds all work well. Water troughs, feed buckets, grooming tools, cross-ties, stall walls, door latches, trailer ties, halters. Anything a sick horse has contacted or breathed on. UC Davis recommends a 1:10 bleach solution with a 10-minute contact time for hard surfaces.
• Test before releasing from quarantine. Horses should be clinically recovered AND have three consecutive negative nasopharyngeal cultures or guttural pouch washes taken at weekly intervals before being cleared. This is non-negotiable. Some horses become persistent carriers, harboring the organism in chondroid formations within the guttural pouches for months without showing a single clinical sign. These silent shedders are responsible for a staggering number of outbreaks.

Vaccination

Strangles vaccines exist, but they're not part of the AAEP core vaccine recommendations. They're considered risk-based, meaning vaccination makes sense in certain situations but isn't universally recommended for every horse everywhere.

The intranasal modified-live vaccine (Pinnacle I.N.) is the most commonly used in the United States. It delivers the antigen directly to the mucosa at the site of natural infection, which is a theoretical advantage over injectable vaccines because it stimulates local IgA antibody production. However, it has notable limitations. It should never be given to horses with active strangles or recent exposure. It can occasionally cause abscess formation at the site if accidentally injected rather than administered intranasally, which has happened and is as bad as it sounds. And the level of protection is not 100 percent. Vaccinated horses can still develop strangles, though clinical signs tend to be less severe.

The injectable M-protein extract vaccine (Strepvax II) is another option but carries a meaningfully higher risk of triggering purpura hemorrhagica, especially in horses with high existing antibody titers from prior natural infection. It has fallen out of favor somewhat for this reason, and the AAEP recommends checking SeM antibody titers before using injectable strangles vaccines in any horse with an unknown or positive strangles history.

Vaccination is most appropriate for horses at high risk of exposure. Frequent travelers, competitors, horses living on properties with high turnover. Talk to your vet about whether it makes sense for your specific situation. And remember always that vaccination is not a substitute for good biosecurity practices. A vaccine reduces disease severity in most cases. It does not build a force field around your horse.

The Carrier Problem

Up to 10 percent of recovered horses become persistent carriers, harboring S. equi in chondroids, which are basically inspissated pus balls, within the guttural pouches. These horses look completely healthy. They eat well. They perform well. Their temperature is normal. Their bloodwork looks fine. And they periodically shed the organism into the environment, sparking new outbreaks that nobody can trace back to the source because the source doesn't look sick.

Guttural pouch endoscopy and lavage can identify and treat carriers, but it requires specialized veterinary expertise and equipment. The veterinarian passes an endoscope into the guttural pouches, visualizes the chondroids, and flushes them out or breaks them up. Sometimes repeated lavage sessions are necessary. Sometimes a course of topical or systemic antibiotics follows. It's involved, it's not cheap, but it works. And given that a single unidentified carrier horse can seed outbreaks across multiple barns over months or even years, the investment in identifying and clearing these animals is absolutely justified.

This is why the "test out" protocol matters so profoundly. Releasing a horse from quarantine based solely on clinical recovery, the whole "well, she looks fine now" approach, is playing Russian roulette with every other horse on the property. Three clean cultures. Weekly intervals. No shortcuts. The Texas A&M Veterinary Medical Teaching Hospital, Cornell, UC Davis, and the AAEP all agree on this point. Three clean cultures or you're gambling.

Frequently Asked Questions

What causes strangles in horses?

Strangles is caused by the bacterium Streptococcus equi subspecies equi. It spreads through direct contact with infected horses or contaminated equipment, water sources, and surfaces. The bacteria can survive in the environment for weeks under the right conditions.

How long is a horse with strangles contagious?

Horses shed the bacteria for a minimum of 3-6 weeks after clinical signs resolve. Some horses become chronic carriers, harboring the bacteria in their guttural pouches for months or even years without showing symptoms. Post-infection guttural pouch endoscopy and lavage are the only way to confirm a horse is truly clear.

Is there a vaccine for strangles?

Yes. Both intranasal and intramuscular vaccines exist. The intranasal modified-live vaccine generally produces stronger mucosal immunity. However, strangles vaccines are not considered core vaccines by the AAEP and carry a higher risk of adverse reactions than most equine vaccines. Discuss with your vet whether the risk profile at your barn warrants vaccination.

Can strangles kill a horse?

The mortality rate for uncomplicated strangles is about 1-2%. However, complications like bastard strangles (metastatic abscess formation in internal organs) and purpura hemorrhagica (immune-mediated vasculitis) can be life-threatening. Young, old, or immunocompromised horses face the highest risk of serious complications.

How do you treat strangles?

Most cases resolve with supportive care: anti-inflammatories, hot compresses on abscesses to encourage drainage, and rest. Antibiotics are controversial because they can delay abscess maturation and prolong recovery. Antibiotics are typically reserved for horses with systemic illness, difficulty breathing, or bastard strangles. Isolation of infected horses is critical.

Sources

• "Strangles in Horses" - Texas A&M Veterinary Medical Teaching Hospital vetmed.tamu.edu
• "Strangles (Streptococcus equi)" - Merck Veterinary Manual merckvetmanual.com
• "Strangles: Biosecurity and Prevention" - AAEP aaep.org
• "Streptococcus equi Infections" - Cornell University College of Veterinary Medicine vet.cornell.edu
• "Strangles FAQ" - UC Davis Center for Equine Health ceh.vetmed.ucdavis.edu