Strangles Complications in Horses: Muscle Disease, Purpura and Warning Signs

By Dr Duncan Houston

Strangles usually starts as a respiratory infection, but rare complications can turn it into a serious whole body disease.

Most horse owners think of strangles as fever, nasal discharge and swollen lymph nodes under the jaw. That is the classic picture, and many horses recover with careful isolation, monitoring and supportive care.

The problem is that strangles is not always simple.

In some horses, infection with Streptococcus equi subspecies equi can trigger serious complications, including purpura hemorrhagica, metastatic abscesses, immune mediated muscle disease, rhabdomyolysis, rapid muscle atrophy, kidney complications and even myocarditis. These are uncommon, but when they happen, they can progress quickly and become life threatening. AAEP lists purpura hemorrhagica, myositis and immune mediated myopathies among recognised strangles complications.

Quick Answer

Strangles can rarely cause serious immune mediated muscle complications in horses. Warning signs include severe stiffness, painful muscle swelling, reluctance to move, inability to rise, rapid muscle loss over the back or hindquarters, dark urine, limb swelling, fever, colic signs, bleeding spots on the gums, or sudden deterioration after recent strangles infection or exposure. (MSD Veterinary Manual)

If a horse develops muscle pain, weakness, marked swelling, recumbency, dark urine, colic, breathing difficulty or rapidly worsening signs during or after strangles, treat it as urgent and call a veterinarian immediately.

What Is Strangles?

Strangles is a contagious upper respiratory disease of horses caused by Streptococcus equi subspecies equi. It commonly causes fever, depression, nasal discharge, pharyngitis, swollen lymph nodes and abscessation, especially around the submandibular and retropharyngeal lymph nodes. Fever often appears before the more obvious nasal discharge and lymph node swelling.

Classic strangles signs can include:

• Fever
• Depression
• Reduced appetite
• Thick nasal discharge
• Cough
• Pain when swallowing
• Swollen lymph nodes under the jaw
• Abscesses that may rupture and drain
• Difficulty swallowing in more severe cases
• Noisy breathing if enlarged lymph nodes compress the airway

Merck Veterinary Manual notes that the incubation period is usually 3 to 14 days, and horses may begin shedding bacteria within a few days of fever. (Merck Veterinary Manual)

In uncomplicated cases, strangles is usually managed with isolation, supportive care, pain relief where appropriate, abscess management and careful monitoring. Antibiotics are not automatically used in every uncomplicated case, because treatment decisions depend on disease stage and severity. (Merck Veterinary Manual)

Why Can Strangles Affect the Muscles?

The muscle complications linked with strangles are usually not just the bacteria “moving into the muscles.”

The bigger issue is the immune system.

After infection or exposure to Streptococcus equi, some horses can develop an abnormal immune response. This can damage blood vessels, muscle tissue or both. The main muscle related complications include:

• Purpura hemorrhagica with muscle infarction
• Strangles associated myositis
• Immune mediated myositis
• Nonexertional rhabdomyolysis
• Rapid muscle atrophy, especially in genetically susceptible horses
• Rare inflammatory muscle diseases with severe systemic illness

A review of immune mediated muscle diseases in horses describes Streptococcus equi infection or vaccination as a trigger for infarctive purpura hemorrhagica and immune mediated myositis in some horses. (Sage Journals)

In plain English: the horse’s immune system can overreact, and the collateral damage can be muscle, blood vessels, gut, skin, kidneys or other organs.

That is why these cases can become dangerous so quickly.

The Main Strangles Related Muscle Complications

1. Purpura hemorrhagica

Purpura hemorrhagica is an acute immune mediated vasculitis. That means the immune system attacks blood vessels, causing inflammation, leakage, swelling and sometimes tissue damage. MSD Veterinary Manual describes purpura hemorrhagica as a type III hypersensitivity reaction associated with immune response to Streptococcus equi proteins, often after strangles. (MSD Veterinary Manual)

Signs may include:

• Swelling of the limbs
• Swelling under the belly
• Swelling of the head or muzzle
• Petechiae or small red bleeding spots on gums or mucous membranes
• Larger bruised or purple areas on mucous membranes
• Fever
• Depression
• Stiffness
• Muscle pain
• Skin oozing or sloughing in severe cases
• Colic signs if intestinal blood supply is affected

AAEP notes that purpura hemorrhagica can cause swelling of the head, limbs and trunk, and severe swelling may lead to skin oozing or sloughing. It also notes that rhabdomyolysis and infarctive myositis can be associated with purpura hemorrhagica.

This is not “just swelling.”

It can be a serious blood vessel disease.

2. Infarctive myositis

Infarctive myositis means areas of muscle are damaged because their blood supply is compromised. This can happen with severe purpura hemorrhagica.

The muscle can become:

• Firm
• Swollen
• Painful
• Stiff
• Hot or tender
• Associated with severe lameness or reluctance to move

The immune mediated muscle disease review describes infarctive purpura hemorrhagica as a severe vasculitis that can result in muscle and organ infarction. It notes that painful swellings may appear in muscles that contact the ground when horses lie down, including pectoral, hind limb adductor and gaskin muscles, and that colic signs are a poor prognostic feature when gut infarction is present. (Sage Journals)

This is one of the reasons a horse with strangles exposure plus muscle swelling and colic signs is a serious emergency.

3. Immune mediated myositis

Immune mediated myositis is an inflammatory muscle disease that can cause rapid muscle loss, particularly over the back and hindquarters.

It is especially recognised in Quarter Horse related breeds and is associated with the MYH1 genetic mutation. UC Davis describes immune mediated myositis and nonexertional rhabdomyolysis as part of myosin heavy chain myopathy, with triggers including Streptococcus equi infection or exposure, respiratory viruses, pigeon fever and certain vaccinations. (Center for Equine Health)

Signs can include:

• Rapid muscle atrophy
• Loss of muscle over the topline
• Loss of gluteal muscle
• Stiffness
• Malaise
• Reluctance to move
• Pain
• Weakness

EquiManagement notes that immune mediated myositis can present with acute tying up, muscle pain, stiffness, reluctance to move and rapid muscle atrophy, often visible in the epaxial and gluteal muscles within 24 hours of onset. (EquiManagement)

That speed is the frightening part.

This is not normal loss of condition. This is rapid muscle disease.

4. Nonexertional rhabdomyolysis

Rhabdomyolysis means muscle breakdown. Nonexertional rhabdomyolysis means the horse develops tying up type muscle damage without exercise being the trigger.

UC Davis describes nonexertional rhabdomyolysis as severe muscle pain, stiffness and reluctance to move. Horses may lie down and struggle to rise, urine may become brown, and blood tests can show very high creatine kinase activity. (Center for Equine Health)

Red flags include:

• Severe stiffness
• Reluctance to walk
• Sweating from pain
• Trembling
• Recumbency
• Dark brown or coffee coloured urine
• Very painful muscles
• Markedly increased CK and AST on blood tests

This can become a kidney risk if severe muscle breakdown releases myoglobin into the bloodstream and urine, especially if the horse is dehydrated.

Strangles Muscle Disease Risk Framework

The key decision point is this: a horse with strangles signs plus muscle pain, swelling, dark urine, colic or inability to rise is no longer a routine strangles case.

When Is This an Emergency?

Call a veterinarian immediately if a horse with current or recent strangles exposure develops:

• Severe stiffness
• Painful swollen muscles
• Reluctance to move
• Inability to rise
• Dark brown urine
• Rapid muscle loss over the back or hindquarters
• Marked swelling of the limbs, head, sheath, udder or belly
• Red spots or bruising on gums, lips or eyes
• Skin oozing or sloughing
• Colic signs
• Breathing difficulty
• Difficulty swallowing
• Severe depression
• Persistent high fever
• Sudden worsening over hours

AAEP states that strangles complications can lead to severe disease and euthanasia, and lists immune mediated complications including purpura hemorrhagica, myositis, glomerulonephritis and myocarditis.

The real concern is not just the original respiratory infection.

The real concern is that the immune response has started damaging tissues that the horse needs to survive.

How Do Vets Diagnose Strangles Related Muscle Disease?

Diagnosis depends on the stage of disease, signs present and whether the horse is actively infected, recently recovered, recently vaccinated, or exposed to a known strangles case.

Your vet may use:

• Full physical examination
• Temperature monitoring
• Lymph node palpation
• Respiratory examination
• Muscle palpation
• CBC and fibrinogen or SAA
• CK and AST muscle enzyme testing
• Kidney values
• Electrolytes
• Urinalysis for myoglobin
• Nasopharyngeal wash, guttural pouch wash or abscess sample
• PCR and culture for Streptococcus equi
• SeM antibody titres
• Ultrasound of painful muscles or abscesses
• Skin or muscle biopsy in selected cases
• Genetic testing for MYH1 in Quarter Horse related breeds

AAEP notes that PCR of nasal secretions is recommended for pyrexic horses not draining from an abscess, and that guttural pouch lavage with PCR and endoscopy is the preferred approach for determining recovered carrier status. It also notes that SeM antibody testing may support diagnosis of purpura hemorrhagica or metastatic disease, but does not prove protection, active infection or carrier status.

For muscle disease, the blood tests that matter most are often CK and AST.

CK can rise quickly with active muscle damage.

AST rises and falls more slowly, helping show the broader muscle injury pattern.

If the urine is dark, kidney protection becomes a major concern.

What Else Can Look Like Strangles Related Muscle Disease?

A horse that is stiff, sore, weak or losing muscle after respiratory disease does not automatically have strangles related myopathy.

Important rule outs include:

• Exertional rhabdomyolysis
• Polysaccharide storage myopathy
• Recurrent exertional rhabdomyolysis
• Myosin heavy chain myopathy
• Tetanus
• Botulism
• Laminitis
• Severe cellulitis
• Trauma
• Tying up from electrolyte imbalance
• Vitamin E or selenium related muscle disease
• Anaplasmosis
• Equine herpesvirus infection
• Equine influenza
• Pigeon fever
• Sepsis
• Metastatic abscessation
• Kidney disease
• Severe colic causing reluctance to move

Merck lists infectious and immune mediated causes among non exercise associated rhabdomyolysis differentials, including Streptococcus equi, influenza and other causes. (Merck Veterinary Manual)

This is where veterinary judgement matters.

The history points you in a direction, but bloodwork, testing and clinical examination decide how worried we should be and what treatment is safest.

How Are These Complications Treated?

Treatment depends on whether the horse has active infection, immune mediated disease, muscle breakdown, purpura hemorrhagica, kidney risk, colic, respiratory compromise or abscessation.

This is not a home treatment situation.

Veterinary treatment may include:

• Isolation and biosecurity
• Supportive care
• Pain relief and anti inflammatories where appropriate
• Antibiotics when indicated
• Corticosteroids for immune mediated disease where appropriate
• IV fluids if dehydration, kidney risk or myoglobinuria is present
• Monitoring CK, AST, kidney values and electrolytes
• Urine monitoring
• Management of abscesses
• Guttural pouch evaluation where carrier status is suspected
• Nursing support for recumbent horses
• Referral hospital care in severe cases

Merck states that antimicrobial treatment is indicated in complicated strangles cases, including metastatic strangles, purpura hemorrhagica and myositis, while uncomplicated abscessed strangles cases are managed differently. (Merck Veterinary Manual)

MSD Veterinary Manual describes treatment of severe purpura related muscle infarction as requiring early detection, penicillin and aggressive corticosteroid therapy, with poor outcomes if severe disease progresses to intestinal infarction. (MSD Veterinary Manual)

That is why these cases need a vet making decisions in real time.

Steroids may be essential in immune mediated disease, but they can also be dangerous if used incorrectly in the face of uncontrolled infection. Antibiotics may be indicated in complicated disease, but they are not a casual preventive treatment for every strangles exposure.

This is the veterinary balancing act.

Will the Horse Recover?

The prognosis depends on the complication and how quickly treatment starts.

Many horses with uncomplicated strangles recover well. Merck notes that most horses with uncomplicated strangles recover with supportive care, although complications can be severe. (Merck Veterinary Manual)

The outlook becomes more guarded when there is:

• Severe purpura hemorrhagica
• Marked limb or body swelling
• Skin sloughing
• Severe muscle infarction
• Very high CK and AST
• Dark urine
• Kidney compromise
• Recumbency
• Colic signs
• Suspected intestinal infarction
• Severe respiratory compromise
• Rapid progressive muscle wasting

UC Davis notes that horses with MYH1 related immune mediated myositis or nonexertional rhabdomyolysis can recover, but almost half may experience recurrent episodes, and severe cases may require euthanasia because of quality of life concerns. (Center for Equine Health)

The earlier these cases are identified, the better the chance of controlling the immune response before the damage becomes irreversible.

What Should You Do Right Now?

1. Isolate the horse

If strangles is suspected, isolate the horse from others immediately.

Do not move the horse around the yard unless needed for safety or veterinary care.

AAEP recommends separating horses into clean, exposed and sick groups during an outbreak, with twice daily temperature monitoring and immediate isolation of horses that develop fever.

2. Check temperature twice daily

Fever can appear before nasal shedding and before obvious lymph node swelling. Twice daily temperature checks help identify exposed horses early.

3. Watch for complication signs

Monitor for:

• Limb swelling
• Body swelling
• Muscle pain
• Stiffness
• Reluctance to move
• Dark urine
• Colic
• Rapid muscle loss
• Bleeding spots on gums
• Breathing difficulty
• Difficulty swallowing

These signs change the urgency.

4. Do not vaccinate during an active outbreak

Vaccination decisions around strangles need veterinary guidance. AAEP states that vaccination during an outbreak increases the risk of complications, including purpura hemorrhagica, and is not recommended.

5. Do not start random medication

Do not give human medication, leftover antibiotics, steroids or high dose anti inflammatories without veterinary direction.

This is especially important because complicated strangles cases may need very specific treatment, and incorrect treatment can mask signs, worsen risk or delay proper care.

6. Prepare key information for your vet

Have these details ready:

• When fever started
• When nasal discharge started
• Whether lymph nodes are swollen
• Whether any abscesses have ruptured
• Recent strangles exposure
• Vaccination history
• Recent travel or shows
• Whether other horses are sick
• Any limb or body swelling
• Any muscle stiffness or pain
• Whether urine is dark
• Whether the horse can stand
• Appetite and water intake
• Current medications already given

The clearer the history, the faster your vet can sort the risk.

Common Mistakes With Strangles Complications

Mistake 1: Thinking strangles is always mild

Many horses recover, but strangles can cause severe complications. Muscle disease, purpura hemorrhagica, metastatic abscesses, airway obstruction and carrier states are all real concerns.

Mistake 2: Ignoring swelling after the respiratory signs improve

Purpura hemorrhagica often appears after exposure or recovery. New limb swelling, head swelling, body swelling or gum bruising after strangles should not be brushed off.

Mistake 3: Calling rapid muscle loss “weight loss”

Rapid loss of muscle over the back or hindquarters, especially in a Quarter Horse related breed, is not normal conditioning loss. It may be immune mediated muscle disease.

Mistake 4: Vaccinating at the wrong time

Strangles vaccination is risk based and should not be given casually during an outbreak or to high risk horses without veterinary advice. AAEP notes that SeM titres may help identify horses at risk of vaccine complications, but these titres do not indicate protection from infection.

Mistake 5: Assuming the horse is safe once the nasal discharge stops

AAEP states that shedding often persists for 2 to 3 weeks after recovery, but intermittent shedding can continue for months to years if bacteria persist in the guttural pouches or sinuses. Without testing, horses should be considered potentially infective for up to 6 weeks after all clinical signs resolve.

Mistake 6: Not testing carrier horses

Recovered carriers can keep outbreaks going. AAEP states that guttural pouch endoscopy and lavage PCR are recommended to detect persistent infection, and that testing is the only way to determine whether a horse is no longer shedding.

How To Reduce the Risk of Severe Strangles Complications

You cannot prevent every complication, but you can reduce risk with better control, monitoring and timing.

Use strict outbreak biosecurity

During a suspected outbreak:

• Separate clean, exposed and sick horses
• Handle clean horses first, then exposed horses, then sick horses
• Use dedicated equipment where possible
• Wear gloves and protective clothing for sick horses
• Disinfect hands, buckets, hoses and shared surfaces
• Avoid sharing tack, grooming gear or feed tubs
• Restrict horse movement on and off the property
• Monitor temperatures twice daily

AAEP highlights separation of groups, temperature monitoring, hygiene and barrier precautions as key outbreak control measures.

Quarantine new arrivals

AAEP recommends quarantining new arrivals for 3 weeks while monitoring temperatures, and performing guttural pouch lavage PCR and endoscopy before introduction into the farm population where appropriate.

Do not rush vaccination decisions

Strangles vaccines can be useful in selected horses and populations, but timing matters. They are not usually given during active outbreaks, and horses with recent exposure or high SeM titres may have increased complication risk.

Take post strangles swelling seriously

If a horse develops swelling after strangles, especially with fever, stiffness or bruising on mucous membranes, call your vet early.

Waiting several days can be the difference between treatable immune disease and advanced tissue damage.

Consider genetic testing in at risk breeds

For Quarter Horses, Quarter Horse crosses and related breeds, UC Davis recommends testing for the MYH1 mutation to support diagnosis and breeding decisions around myosin heavy chain myopathy. (Center for Equine Health)

Myth vs Reality

Frequently Asked Questions

Can strangles cause muscle disease in horses?

Yes. Rarely, strangles can trigger immune mediated muscle complications, including myositis, rhabdomyolysis, infarctive myositis and rapid muscle atrophy. These are uncommon but can be serious or life threatening.

What are the warning signs of purpura hemorrhagica after strangles?

Warning signs include limb swelling, head or body swelling, small red or purple spots on the gums or eyes, fever, depression, stiffness, skin oozing, colic signs and painful muscle swelling. These signs need urgent veterinary attention. (MSD Veterinary Manual)

Is post strangles muscle disease contagious?

The muscle disease itself is usually immune mediated and not directly contagious. However, the original strangles infection is contagious, and horses may continue shedding Streptococcus equi after clinical signs improve.

Should I vaccinate my horse after a strangles outbreak?

Not without veterinary guidance. Vaccination during an outbreak is not recommended, and SeM antibody titres may be used to help identify horses at higher risk of vaccine complications. Titres do not prove protection from infection.

Can horses recover from strangles related myopathy?

Some horses can recover, especially with early diagnosis and treatment. Prognosis is more guarded if the horse becomes recumbent, develops very high muscle enzymes, dark urine, kidney compromise, severe purpura, colic or rapid progressive muscle wasting. (MSD Veterinary Manual)

The Bottom Line

Most horses with strangles do not develop severe muscle disease.

But when they do, it can move fast.

The signs that matter most are not just nasal discharge and swollen lymph nodes. Watch for what happens during recovery: limb swelling, body swelling, bruising on the gums, muscle pain, stiffness, dark urine, rapid muscle loss, colic or inability to rise.

Those are not normal recovery signs.

They are warning signs that strangles may have triggered a serious immune mediated complication.

If your horse has current or recent strangles and suddenly becomes stiff, swollen, weak, painful, dark in the urine, colicky or unable to stand, do not wait. Early veterinary treatment gives the best chance of survival and may reduce irreversible muscle or organ damage.

If you are unsure whether your horse’s signs are normal strangles recovery or a possible complication, ASK A VET™ can help you organise the history, track the warning signs and decide when urgent veterinary care is needed.