Other Names
What is Equine Myocarditis (Viral)?
Equine viral myocarditis is an inflammatory condition of the heart muscle (myocardium) triggered by viral infection. When viruses invade the myocardium, they cause direct cellular damage and provoke an immune response that results in inflammation, degeneration, and sometimes death of heart muscle cells. Over time, this can severely compromise the heart’s ability to pump blood efficiently, leading to reduced cardiac output and, in serious cases, heart failure.
The condition can affect horses of any age, breed, or sex, though performance horses — particularly those in active training — face the greatest clinical risk because the demands placed on an already-compromised heart are far higher. In mild cases, horses may recover fully with rest; in severe cases, myocarditis can be fatal or result in permanent cardiac damage that ends an athletic career.
Causes
Several viruses have been associated with equine myocarditis, either as primary pathogens infecting the heart directly or as secondary complications following systemic illness:
- Equine herpesvirus (EHV-1 and EHV-4): Most commonly linked to respiratory illness, EHV can occasionally extend to cardiac tissue.
- Equine influenza virus: Systemic influenza infection may lead to myocardial inflammation, particularly during severe disease or inadequate rest.
- Equine arteritis virus (EAV): Causes equine viral arteritis (EVA) and can affect the vascular endothelium adjacent to cardiac tissue.
- African horse sickness virus: A notifiable disease with recognised cardiac manifestations.
- Coxsackievirus-like agents and other enteroviruses: Less well-characterised in horses, but implicated in some cases.
Myocarditis often follows a respiratory viral illness, appearing days to weeks after the primary infection has apparently resolved. This delayed presentation means owners may not immediately connect a horse’s poor performance or cardiac signs with a recent illness episode.
Symptoms and Signs
Clinical signs vary considerably depending on the severity and stage of infection:
- Exercise intolerance or sudden drop in performance
- Irregular heartbeat (cardiac arrhythmias) — audible as an irregular rhythm on auscultation
- Elevated resting heart rate (tachycardia)
- Weakness, lethargy, or malaise
- Oedema, particularly of the lower limbs or ventral abdomen
- Pale or congested mucous membranes
- Muscle trembling or sweating without exertion
- Collapse or sudden death in severe cases
- Elevated body temperature during active viral infection
Some horses present with relatively subtle signs — a horse that simply “doesn’t feel right,” loses enthusiasm for work, or shows minor gait abnormalities. These non-specific presentations make early diagnosis challenging and reinforce the value of thorough cardiac assessment following any febrile illness.
Diagnosis
Diagnosis of equine viral myocarditis typically requires a combination of clinical examination and diagnostic testing:
- Auscultation: Detection of murmurs or arrhythmias on physical examination.
- Electrocardiography (ECG): Identifies specific arrhythmias such as atrial fibrillation, ventricular premature contractions, or conduction abnormalities.
- Echocardiography: Ultrasound examination of the heart to assess myocardial wall motion, chamber dimensions, and overall function. A key tool in grading severity.
- Serum cardiac troponin I (cTnI): An elevated troponin level is a sensitive marker of myocardial cell damage and is increasingly used in equine cardiac diagnostics.
- Haematology and biochemistry: May reveal leucocytosis, elevated inflammatory markers, or creatine kinase (CK) elevation.
- Viral serology or PCR: Can identify the causative virus, though results may not alter immediate management.
Treatment
There is no specific antiviral treatment for most cases of equine viral myocarditis. Management is largely supportive:
- Complete rest: Absolute stall rest is essential. Continued exercise with active myocarditis risks fatal arrhythmias and sudden death.
- Anti-inflammatory therapy: Non-steroidal anti-inflammatory drugs (NSAIDs) may reduce myocardial inflammation, though use requires careful monitoring given the potential for gastrointestinal side effects.
- Antiarrhythmic drugs: Horses with significant arrhythmias may require treatment with digoxin, quinidine, or other antiarrhythmic agents — always under veterinary supervision.
- Supportive care: Maintenance of hydration, appropriate nutrition, and management of secondary conditions.
- Serial monitoring: ECG and echocardiographic monitoring to assess resolution or progression before any return to work.
The duration of rest required varies, but most veterinary guidelines recommend a minimum of four to six weeks of complete rest following clinical resolution, with repeat cardiac evaluation before resuming exercise. For more information on equine cardiac conditions and management, the British Equine Veterinary Association (BEVA) provides professional resources and guidelines.
Prevention
- Maintain an up-to-date vaccination programme against equine influenza, EHV-1, and EHV-4.
- Enforce strict biosecurity during and after respiratory outbreaks — isolate affected horses promptly.
- Allow full recovery from any febrile illness before returning to work; never rush a horse back into training.
- Monitor horses closely in the days following a viral illness for subtle changes in behaviour or exercise tolerance.
- Regular health monitoring and veterinary checks for performance horses, particularly during competition season.
Prognosis
Prognosis depends heavily on the severity of myocardial damage and the presence of ongoing arrhythmias. Horses with mild, transient myocarditis and no significant arrhythmias often recover well with rest and can return to their previous level of work. Those with persistent arrhythmias, significant echocardiographic abnormalities, or markedly elevated troponin levels carry a more guarded prognosis. Some horses recover to a lower level of performance, while a small proportion develop chronic dilated cardiomyopathy. Sudden cardiac death during exercise remains a risk until myocarditis has definitively resolved and cardiac function is confirmed normal.
When to Call the Vet
Contact your vet promptly if your horse:
- Has recently recovered from a viral illness and now shows exercise intolerance or lethargy
- Has an irregular or very rapid heartbeat detected at rest or post-exercise
- Shows signs of weakness, sweating, or collapse without obvious cause
- Develops oedema of the limbs or abdomen
- Is underperforming significantly without a musculoskeletal explanation
Do not work a horse you suspect may have a cardiac issue until cleared by a veterinary surgeon. Early intervention substantially improves outcomes.
Frequently Asked Questions
Can a horse fully recover from viral myocarditis?
Yes — many horses with mild-to-moderate myocarditis make a full recovery and return to previous performance levels, provided they receive adequate rest and monitoring. Severe cases with persistent arrhythmias or significant structural changes carry a more guarded prognosis.
How long does a horse need to rest with myocarditis?
A minimum of four to six weeks of complete rest is generally recommended following clinical resolution, but this varies with severity. Your vet should confirm via ECG and echocardiography that cardiac function has normalised before the horse returns to any exercise.
Is viral myocarditis contagious?
The myocarditis itself is not contagious, but the underlying viral infection (such as equine influenza or EHV) is. If a horse is diagnosed with myocarditis secondary to a respiratory virus, other horses on the yard may also be at risk from the viral illness and should be monitored.
Can myocarditis be prevented by vaccination?
Vaccination against common equine respiratory viruses (influenza, EHV) reduces the risk of primary infections that can lead to myocarditis. It does not guarantee prevention, but significantly lowers the likelihood of severe viral illness and its cardiac complications.