Strangles in Horses: Symptoms, Diagnosis and Carrier Control

By Dr Duncan Houston

Strangles is common, contagious and often underestimated, especially when a horse looks better but may still be shedding bacteria.

Strangles is one of the classic horse diseases most owners have heard of, but hearing the name and managing an outbreak safely are two very different things.

The obvious cases are usually easy to worry about: fever, thick nasal discharge, swollen lymph nodes under the jaw, and abscesses that may rupture and drain pus. The harder cases are the quiet ones. A horse can recover, look normal, and still carry Streptococcus equi in the guttural pouches, silently restarting the whole problem later.

That is why modern strangles control is not just about treating the sick horse. It is about isolation, testing, temperature monitoring, carrier detection, movement control, and knowing when vaccination helps or makes risk worse.

Quick Answer

Strangles is a highly contagious bacterial respiratory disease of horses caused by Streptococcus equi subspecies equi. It spreads through nasal discharge, abscess drainage, contaminated buckets, water troughs, tack, hands, clothing, grooming tools and close horse contact. Fever is often the first sign, followed by thick nasal discharge, swollen lymph nodes, difficulty swallowing and abscess formation. (Equine Disease Communication Center)

If strangles is suspected, isolate the horse immediately, stop horse movement on and off the property, check temperatures twice daily, and contact your veterinarian for testing and outbreak control advice.

What Is Strangles in Horses?

Strangles is an upper respiratory tract infection caused by Streptococcus equi subspecies equi. The bacteria enter through the nose or mouth, infect lymphoid tissue, and commonly cause painful swelling and abscess formation in lymph nodes around the head and throat. The name “strangles” comes from severe cases where enlarged lymph nodes can obstruct the airway. (Center for Equine Health)

Classic sites affected include:

• Submandibular lymph nodes under the jaw
• Retropharyngeal lymph nodes behind the throatlatch
• Guttural pouches if abscess material drains internally
• Upper airway structures if swelling becomes severe

Most uncomplicated cases recover, but strangles becomes much more serious when it causes airway obstruction, guttural pouch infection, internal abscesses, purpura hemorrhagica, muscle disease, or a long term carrier state. UC Davis notes that uncomplicated cases often recover fully, but complications can greatly increase the risk of death. (Center for Equine Health)

Why Strangles Spreads So Easily

Strangles spreads easily because infected horses shed bacteria in nasal discharge and abscess drainage. Other horses can become infected through direct contact or by touching contaminated surfaces such as buckets, water troughs, feed tubs, tack, brushes, stalls, trailers, clothing, hands, hoses and shared equipment. (Equine Disease Communication Center)

In practice, outbreaks usually spread because of one of four things:

• A new horse arrives without quarantine
• A horse with fever is not isolated early enough
• Buckets, water sources or equipment are shared
• A recovered carrier horse is never tested

The most dangerous time is not always when the horse looks obviously sick. Merck notes that horses may start shedding bacteria within 2 to 3 days of fever, and fever can appear before the classic nasal discharge and lymph node swelling. (Merck Veterinary Manual)

That means a horse with a fever and no snot yet can still be the start of a barn problem.

What Are the Symptoms of Strangles?

Strangles signs can vary from mild to severe. Young horses often show more obvious disease, but horses of any age can be affected. Merck describes the disease as most severe in horses around 1 to 5 years old, while still reporting cases in foals and older adults. (Merck Veterinary Manual)

Common signs include:

• Fever
• Depression
• Reduced appetite
• Thick white, yellow or green nasal discharge
• Coughing
• Pain when swallowing
• Swollen lymph nodes under the jaw
• Swollen lymph nodes behind the throatlatch
• Abscesses that rupture and drain pus
• Difficulty swallowing
• Noisy breathing
• Extended head and neck posture
• Respiratory distress in severe cases

A key clinical detail: fever often appears 24 to 48 hours before the more obvious signs. This is why temperature monitoring matters so much during an outbreak.

Strangles Risk Framework

Risk level	What it looks like	What it may mean	What to do
Low risk	Horse is bright, normal temperature, no discharge, no lymph node swelling, no known exposure	Routine prevention is the focus	Maintain biosecurity and quarantine new arrivals
Moderate risk	Fever, mild depression, reduced appetite, recent travel or possible exposure	Early strangles or another infectious disease is possible	Isolate and call your vet for testing advice
High risk	Fever plus thick nasal discharge, swollen lymph nodes, coughing, difficulty swallowing, or abscess drainage	Strangles is a major concern	Isolate, stop movement, begin outbreak precautions
Critical	Breathing difficulty, inability to swallow, severe swelling, colic, limb swelling, purple spots on gums, severe depression, collapse	Complicated strangles or immune disease is possible	Seek urgent veterinary care immediately

The decision point is simple: a horse with fever and possible strangles exposure should be isolated before waiting for the dramatic signs.

What Are Carrier Horses?

Carrier horses are one of the main reasons strangles outbreaks keep coming back.

A carrier horse appears healthy but continues to harbour S. equi, usually in the guttural pouches. The horse may intermittently shed bacteria and infect other horses without looking sick. Merck notes that asymptomatic carriers and recovering horses play an important role in maintaining outbreaks, while UC Davis describes carriers as a reservoir that can perpetuate disease in a population. (Merck Veterinary Manual)

This is the part that catches many properties.

The sick horse is obvious.

The carrier horse is the ghost in the system.

A recovered horse may look normal, eat normally, travel normally and still be the reason another outbreak starts two months later.

How Long Can Horses Shed Strangles?

Shedding often continues after the horse looks clinically better.

AAEP states that shedding typically persists for 2 to 3 weeks after recovery, but intermittent shedding can occur for months to years if bacteria persist in the guttural pouches or paranasal sinuses. AAEP also states that without diagnostic testing to detect chronic shedders, horses should be considered potentially infective for up to 6 weeks after all clinical signs resolve.

Merck gives similar practical control advice: screening for shedding or carrier status should not begin until 3 weeks after clinical signs resolve, and recovering horses can continue shedding for about 4 to 6 weeks after recovery. (Merck Veterinary Manual)

So no, “the snot stopped” does not mean “the horse is clear.”

That is barn optimism, not biosecurity.

How Do Vets Diagnose Strangles?

Strangles diagnosis is based on clinical signs, exposure history and laboratory testing.

Testing may include:

• Culture of pus from a draining abscess
• PCR testing of abscess material
• Nasopharyngeal wash
• Nasopharyngeal swab
• Guttural pouch wash
• Guttural pouch endoscopy
• Blood testing for SeM antibodies in selected cases
• Ultrasound or other imaging if internal abscesses are suspected

Merck states that diagnosis relies on culture or PCR of abscess material, guttural pouch washes or nasopharyngeal washes. It also notes that guttural pouch wash is the most sensitive approach for detecting carrier animals. (Merck Veterinary Manual)

PCR vs Culture: Why Both Matter

PCR and culture answer different questions.

PCR is very sensitive and detects S. equi DNA. That is useful, especially when only small amounts of bacterial material are present. But PCR cannot reliably tell whether the detected DNA came from live bacteria, dead bacteria, or modified live vaccine strain. Cornell specifically notes that PCR can detect DNA from live, dead or modified live vaccine origin bacteria. (Cornell Vet College)

Culture is used to grow live bacteria. A positive culture confirms viable bacteria, but culture can be falsely negative if the sample is poor, taken too early, taken when shedding is intermittent, or delayed in transport.

In practice, PCR is sensitive, culture adds context, and your vet chooses the sample site based on the stage of disease.

How Are Carrier Horses Tested?

The best carrier testing usually involves the guttural pouches.

AAEP states that guttural pouch lavage PCR combined with endoscopy is the test of choice for determining the status of recovered horses, because it allows examination for inflammation, chondroids and purulent material. AAEP’s diagnostic table also lists guttural pouch lavage as the best method for detecting carrier animals.

Merck notes that a single bilateral guttural pouch wash, ideally with endoscopy, is the most sensitive way to detect continued infection. If guttural pouch testing is not possible, three negative nasopharyngeal washes taken 7 days apart can be used for release from quarantine. (Merck Veterinary Manual)

Cornell also recommends endoscopic examination and washes of both guttural pouches for horses with positive PCR or culture, and notes that three negative consecutive guttural pouch washes 7 days apart can confirm negative status. (Cornell Vet College)

The key point: carrier control is not guesswork. The only way to know a horse has stopped shedding is to test.

What Is Internal Strangles?

Internal strangles is also called metastatic strangles. Some people still use the older term “bastard strangles.”

This occurs when S. equi spreads beyond the head and neck, causing abscesses in other parts of the body. Merck describes metastatic strangles as abscess formation in other lymph nodes, particularly in the abdomen and sometimes the thorax. AAEP lists possible internal abscess sites including the lung, liver, spleen, kidney, brain, mediastinum and mesentery. (Merck Veterinary Manual)

Possible signs include:

• Recurrent fever
• Weight loss
• Depression
• Poor appetite
• Colic signs
• Coughing or respiratory signs
• Chronic ill thrift
• Persistent inflammation on bloodwork
• Enlarged internal lymph nodes or abscesses on imaging

This form is usually not the classic snotty horse spreading pus around the yard. It is a deeper, more dangerous complication that needs veterinary investigation.

What Is Purpura Hemorrhagica?

Purpura hemorrhagica is an immune mediated complication that can occur after natural infection or vaccination exposure to streptococcal antigens. It causes blood vessel inflammation, swelling, bruising and sometimes severe tissue damage. AAEP’s vaccination guideline states that purpura can develop 2 to 4 weeks after natural or vaccinal exposure, and that severe head swelling can compromise breathing.

Warning signs include:

• Swelling of the limbs
• Swelling under the belly
• Swelling of the head or muzzle
• Red or purple spots on the gums or eyes
• Hives
• Fever
• Depression
• Skin oozing
• Skin sloughing
• Colic signs
• Muscle pain or stiffness

Purpura itself is not contagious, but the original strangles infection is contagious.

When Is Strangles an Emergency?

Call your veterinarian urgently if your horse has any of the following:

• Difficulty breathing
• Noisy breathing or stridor
• Severe swelling behind the throatlatch
• Difficulty swallowing
• Feed or water coming from the nose
• Severe depression
• Persistent high fever
• Unable to eat or drink
• Colic signs
• Limb, head or body swelling after strangles
• Red or purple spots on gums or eyes
• Skin oozing or sloughing
• Severe muscle pain or stiffness
• Rapid deterioration
• Foal or young horse with severe signs

AAEP notes that death in acute strangles can occur from airway compression caused by abscess formation, and that complications such as internal abscesses, purpura hemorrhagica, myositis, kidney disease and myocarditis can lead to severe disease.

This is the clinical rule: uncomplicated strangles is often manageable, but breathing difficulty, severe swelling, colic, purpura signs or rapid worsening changes everything.

How Is Strangles Treated?

Treatment depends on whether the case is uncomplicated or complicated.

Many uncomplicated cases are managed with supportive care, including:

• Isolation
• Rest
• Soft, palatable feed
• Water access
• Pain relief or anti inflammatory medication when appropriate
• Warm compresses for maturing abscesses
• Veterinary monitoring
• Abscess drainage when ready and appropriate

AAEP states that treatment consists of supportive care unless the horse is experiencing complications. It also states that antibiotics should be used at the discretion of the attending veterinarian, and should not be used as a preventative.

Antibiotics may be needed in selected cases, such as:

• Very high fever and severe malaise
• Profound lymph node swelling
• Respiratory distress
• Metastatic abscessation
• Purpura hemorrhagica being treated with corticosteroids
• Guttural pouch infection in carrier horses

This is why owners should not start leftover antibiotics “just in case.” In some uncomplicated cases, inappropriate antibiotic use may interfere with immune response or complicate disease progression.

Should You Vaccinate Against Strangles?

Strangles vaccination is risk based, not automatically required for every horse.

Vaccination may be considered for horses at higher risk of exposure, including young horses, horses on properties with repeated strangles issues, horses that travel frequently, horses attending shows or sales, and horses mixing with changing populations. AAEP describes strangles vaccines as risk based vaccines and notes that vaccination can help disease control in individuals and populations.

Available vaccine types include:

• Injectable killed vaccine
• Modified live intranasal vaccine

AAEP states that the modified live intranasal vaccine is designed to stimulate local immunity at the pharyngeal and lingual tonsils, and accurate delivery matters for efficacy.

Vaccination is not perfect. It does not guarantee a horse will never get strangles, but it may reduce disease severity and help control risk in appropriate populations.

When Should You Avoid Strangles Vaccination?

Do not vaccinate casually during an outbreak or immediately after recent exposure without veterinary advice.

AAEP states that vaccination during an outbreak increases the risk of complications, including purpura hemorrhagica, and is not recommended. AAEP also notes that SeM titres may be used before vaccination to help identify horses at higher risk of vaccine complications, but titres do not indicate protection from infection.

This matters especially for:

• Horses recently exposed to strangles
• Horses that recently recovered
• Horses on an active outbreak property
• Horses with high SeM titres
• Horses with previous vaccine reactions
• Horses with current fever or nasal discharge

A blood titre can help guide risk, but it does not prove the horse is protected, actively infected, or clear of carrier status. AAEP states the SeM antibody ELISA cannot differentiate natural infection from vaccination and is not a measure of protection, active infection or carrier status.

Strangles Vaccination Schedule Basics

Your veterinarian should choose the product and schedule based on the horse, property risk and manufacturer directions.

AAEP vaccination guidance lists:

• Previously vaccinated adult horses: annual vaccination based on risk
• Unvaccinated adults with killed vaccine: 3 dose primary series at 3 week intervals, then annual revaccination
• Unvaccinated adults with modified live intranasal vaccine: 2 dose primary series 2 to 3 weeks apart, then annual revaccination
• Foals with modified live intranasal vaccine: administration at 9 months, with a 2 dose primary series 3 weeks apart, then annual revaccination from 12 months
• Broodmares: vaccination based on risk and product type, with killed vaccine sometimes considered 4 to 6 weeks before foaling in high risk situations

The important practical point: vaccination should be planned before exposure, not in the middle of barn chaos.

Outbreak Control: What To Do Right Now

If strangles is suspected, act fast.

1. Isolate the horse immediately

Any horse with fever, nasal discharge, swollen lymph nodes, coughing or suspected exposure should be separated.

AAEP recommends separating horses into clean, exposed and sick groups during an outbreak, with immediate isolation of any horse showing fever.

2. Stop movement

Do not move horses on or off the property until your vet advises it is safe.

This includes:

• Shows
• Lessons
• Agistment moves
• Trail rides
• Sales
• Breeding visits
• Farrier appointments on other properties

Reporting and movement rules vary by country, state and region, so your vet can advise on local requirements. Merck notes that strangles reporting rules vary between jurisdictions. (Merck Veterinary Manual)

3. Check temperatures twice daily

Temperature monitoring is one of the best early detection tools because fever often precedes nasal shedding and visible discharge.

AAEP recommends twice daily body temperature monitoring in an outbreak.

4. Handle horses in the right order

Handle clean horses first, exposed horses second, and sick horses last.

Ideally, use separate staff for each group. If that is not possible, change clothing, gloves and boots between groups and disinfect hands and equipment properly. AAEP recommends barrier precautions and careful workflow to reduce personnel contamination.

5. Stop sharing equipment

Do not share:

• Buckets
• Feed bins
• Water troughs
• Hoses
• Brushes
• Halters
• Lead ropes
• Tack
• Nose cloths
• Grooming gear

People often think the sick horse spreads strangles. True. But the shared bucket can be the villain with a handle.

6. Clean, disinfect and dry

Organic material reduces disinfectant effectiveness, so remove dirt, pus, manure and feed residue before disinfecting.

AAEP states that contaminated surfaces should be aggressively cleaned, disinfected and dried, with special attention to shared surfaces such as water, hay and feed containers. It also notes that survival is longer in cold, wet conditions.

7. Test before releasing horses back to the herd

Do not rely only on a set number of days.

Testing is the safest way to confirm a horse is no longer shedding. AAEP states plainly that the only way to determine whether a horse is no longer shedding is to test.

How Long Should Quarantine Last?

There is no single perfect calendar rule for every outbreak.

Without testing, AAEP recommends considering horses potentially infective for up to 6 weeks after all clinical signs resolve. With testing, release decisions may be based on guttural pouch lavage PCR with endoscopy or repeated nasopharyngeal washes, depending on what is available and what your veterinarian recommends.

For new arrivals, AAEP recommends quarantine for 3 weeks while monitoring temperatures, with guttural pouch lavage PCR and endoscopy before introduction into the farm population.

The practical rule: time helps, but testing gives confidence.

What Else Can Look Like Strangles?

Not every fever and snotty nose is strangles.

Important rule outs include:

• Equine influenza
• Equine herpesvirus
• Equine rhinitis virus
• Streptococcus equi subspecies zooepidemicus infection
• Guttural pouch empyema from other causes
• Dental sinus disease
• Pneumonia
• Choke with nasal discharge
• Allergic airway disease
• Foreign body irritation
• Internal abscess disease
• Purpura hemorrhagica after previous exposure

This is why testing matters. A horse may look like it has strangles, but treatment, quarantine and herd decisions should be based on proper veterinary assessment.

Common Mistakes Owners Make With Strangles

Mistake 1: Waiting for nasal discharge before isolating

Fever often comes first. If you wait until the horse is snotty, you may already have missed the best early isolation window.

Mistake 2: Putting recovered horses straight back with the herd

Recovered horses may continue shedding. Without testing, AAEP advises treating horses as potentially infective for up to 6 weeks after signs resolve.

Mistake 3: Forgetting carriers

A normal looking horse can still harbour bacteria in the guttural pouches and restart an outbreak later.

Mistake 4: Sharing buckets and tools

Strangles is excellent at using human convenience against us. Shared buckets, troughs, brushes, halters and hoses can spread infection.

Mistake 5: Vaccinating during the wrong window

Vaccination during an outbreak is not recommended because it can increase complication risk, including purpura hemorrhagica.

Mistake 6: Using antibiotics without veterinary direction

Antibiotics are not a routine preventative. They may be needed in complicated cases, but inappropriate use can interfere with immune response or delay proper abscess maturation.

Mistake 7: Assuming PCR positive always means active infection

PCR detects DNA. It can detect live bacteria, dead bacteria or vaccine strain DNA after intranasal vaccination. Interpretation needs timing, clinical signs and sometimes culture. (Cornell Vet College)

How To Prevent Strangles on Your Property

Prevention is mostly about reducing exposure and catching cases early.

A strong prevention plan includes:

• Quarantine new arrivals for 3 weeks
• Monitor new horse temperatures daily, ideally twice daily
• Ask about recent strangles exposure before arrival
• Avoid shared buckets and troughs where possible
• Keep grooming gear horse specific
• Clean and disinfect transport equipment
• Avoid nose to nose contact at shows and events
• Bring your own buckets, feed tubs and gear when travelling
• Separate young horses from high traffic adult populations when practical
• Test recovered or suspicious horses before reintroduction
• Investigate recurring outbreaks for carrier horses
• Discuss risk based vaccination with your vet

AAEP specifically highlights limiting exposure as the best method of preventing an outbreak, quarantine of new arrivals, monitoring temperatures, and immediate isolation of horses with lymph node swelling, nasal discharge or fever.

Myth vs Reality

Myth	Reality
“If the horse looks better, it is safe.”	Recovered horses can keep shedding and may need testing before rejoining the herd.
“Only snotty horses spread strangles.”	Fever can occur before obvious nasal discharge, and carriers may appear normal.
“PCR positive always means live bacteria.”	PCR detects DNA and may detect dead bacteria or vaccine strain DNA.
“Vaccination stops every case.”	Vaccination can reduce risk or severity but does not guarantee full protection.
“Antibiotics should be given to every exposed horse.”	Antibiotics are not routine prevention and should be used only under veterinary direction.
“Purpura is contagious.”	Purpura is an immune reaction, but the original strangles infection is contagious.

Frequently Asked Questions

Is strangles contagious to humans?

Human infection with S. equi is uncommon, but it has been reported. Immunocompromised people should take extra precautions to avoid exposure, and everyone handling sick horses should use good hygiene.

Can a vaccinated horse still get strangles?

Yes. Strangles vaccines can reduce risk and severity, but they do not guarantee complete protection. Vaccination decisions should be based on exposure risk, outbreak status, previous infection history and veterinary advice.

How long should a recovered horse stay isolated?

Without testing, horses should be considered potentially infective for up to 6 weeks after all clinical signs resolve. With testing, your vet may use guttural pouch lavage PCR with endoscopy or repeated nasopharyngeal washes to help decide when the horse can safely return.

What is the best test for carrier horses?

A bilateral guttural pouch wash with PCR, ideally combined with endoscopy, is generally the most sensitive way to detect persistent infection or carrier status. (Merck Veterinary Manual)

Should I vaccinate during a strangles outbreak?

Usually no. AAEP states that vaccination during an outbreak increases the risk of complications, including purpura hemorrhagica, and is not recommended. Horses with no fever, no signs and no known contact may need individual veterinary assessment.

The Bottom Line

Strangles is not just a snotty nose disease.

It is a herd disease.

The horse with swollen lymph nodes matters. The fever horse matters. The bucket matters. The handler’s hands matter. The recovered horse that looks perfectly normal matters too.

The strongest strangles control plan is built around early isolation, twice daily temperature checks, strict separation of clean, exposed and sick groups, careful cleaning, smart testing, and carrier control before horses are allowed back into the group.

If you remember one thing, make it this:

Do not wait for the whole barn to look sick before acting. Fever plus possible exposure is enough to start isolation and call your vet.

That early decision can be the difference between one sick horse and a full property outbreak.

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If you are dealing with possible strangles, a recent exposure, a recovered horse, or uncertainty around carrier testing, ASK A VET™ can help you organise the signs, track temperatures, prepare the right history, and decide when urgent veterinary advice is needed.