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A heart murmur is an abnormal sound heard during the heartbeat, caused by blood flowing in an unusual pattern or location within the heart. It’s not uncommon for newborn foals to have heart murmurs for several days after birth, as their heart transitions to life outside of the uterus.

However, some heart murmurs in foals can indicate underlying problems. Problematic heart murmurs typically arise due to the heart’s failure to properly transition from fetal to neonatal circulation. They can also occur due to malformation of the heart during its development.

Veterinarians describe these murmurs as congenital, meaning the horse was born with the murmur.

Causes of congenital heart murmurs in foals include ventricular septal defects, atrial septal defects, patent ductus arteriosus and valvular malformations. Ventricular septal defects are the most common type of heart malformation in horses.

Veterinarians assess heart murmurs in foals by their sound, timing in the cardiac cycle, and other characteristics to determine their significance and whether they are indicative of a congenital heart condition. The management and prognosis of a heart murmurs in foals depend on their underlying cause and the severity of the heart’s malformation.

Heart Murmurs in Foals

A heart murmur is an abnormal sound heard during the heartbeat cycle, detected through auscultation (listening with a stethoscope). In horses and foals, heart murmurs may be described as high-pitched, squeaking sounds or as a loud murmur with an audible thrill. [1][2]

Heart murmurs in foals can be caused by various factors and do not always indicate a heart defect. Some murmurs are benign (not harmful) and resolve as the foal matures, while others are signs of a more serious underlying condition.

Benign heart murmurs are frequently observed in newborn foals. The heart of a foal while it is still in the uterus serves a different function compared to its role after birth. The transition between fetal and neonatal circulation is critical, and failure of this process can result in structural abnormalities that produce heart murmurs.

Normal Function of the Heart

In all mammals, the function of the heart is to deliver oxygenated blood to the tissues. To facilitate this, blood passes through small vessels in the lungs where it can take up oxygen that the animal breathes in. This blood is then distributed throughout the body, where the cells remove oxygen from the blood to use in their cellular processes.

The equine heart is divided into four major quadrants:

  • The right atrium
  • The right ventricle
  • The left atrium
  • The left ventricle

Blood Circulation

Blood circulation through the heart occurs in the following four steps:

  1. The right atrium receives deoxygenated blood from the tissues, then passes it to the right ventricle.
  2. The right ventricle pumps this blood towards the lungs for oxygenation. [3]
  3. After picking up oxygen, the blood returns to the heart and enters the left atrium.
  4. The left atrium then passes the blood into the left ventricle, which pumps the blood out towards the tissues.

Through this process, blood is constantly oxygenated and deoxygenated as it moves through the animal’s circulation. [3]

Anatomy of the Fetal Heart

In the fetus, the function of the heart is to deliver oxygenated blood entering through the umbilical cord to the foal’s developing body. The lungs are non-functional, as they are not necessary to oxygenate blood. As a result, the function of the right side of the heart can be bypassed. [3]

To bypass the lungs, the fetal heart has a small hole called the foramen ovale within the walls of the atria. This hole allows oxygenated blood entering the right side of the heart to flow directly into the left side of the heart. [3]

There is also a hole between the pulmonary artery (the main vessel that carries blood to the lungs) and the aorta (the major vessel carrying oxygenated blood to the body). This hole, called the ductus arteriosus, serves as a backup for any blood pumped towards the lungs by the heart. [3]

Changes after Birth

Immediately after birth, the foramen ovale and ductus arteriosus in the foal’s heart must close to allow blood to enter the lungs for oxygenation.

The foramen ovale has a small flap covering the hole on the left side of the heart. Once the foal is born, increased pressure in the left side of the heart pushes the flap closed, preventing blood flow through the foramen ovale. [3]

Some foals may have blood flow through the foramen ovale for several weeks after birth. [3] Eventually, the foramen ovale closes off completely.

The ductus arteriosus closes when the foal takes its first breath after delivery. As the lungs inflate, the ductus arteriosus constricts and prevents blood flow. Over time, the hole fills in with muscular tissue, making a permanent closure. [3]

If any part of this process is disrupted or does not properly occur, it can lead to abnormal blood flow within the heart, potentially resulting in heart murmurs.

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Risk Factors

Overall, congenital cardiac malformations are uncommon in horses, particularly when compared to other species. [4] Estimates suggest that between 1 to 5 foals out of 1000 will have a congenital malformation. [4]

Because this condition is so rare, the underlying causes or risk factors have not been thoroughly studied. Possible causes, based on human medicine, include: [5]

  • Drugs used during pregnancy
  • Viral infections
  • Toxins in the environment
  • Nutritional disorders
  • Genetics

Arabian horses have a higher prevalence of congenital cardiac malformations compared to other breeds, but the reason for this increased risk is unknown. [4][6]

One study identified a chromosomal region associated with the development of ventricular septal defects in Arabians, however the precise gene has not been confirmed. [7]

Symptoms of Heart Murmurs

In foals, heart murmurs are very common up to three days of age, as the foramen ovale and ductus arteriosus begin to close. These murmurs can be heard when listening the foal’s heart with a stethoscope and do not necessarily indicate a medical condition.

Problematic heart murmurs show additional symptoms, such as: [3]

  • Precordial thrill, where the murmur can be felt on the foal’s side
  • Swelling of the lower abdomen
  • Enlarged jugular veins
  • Visible pulse in the jugular veins
  • Weakness
  • Fatigue
  • Collapse

Diagnosis

Initial diagnosis of a heart murmur involves listening to the foal’s heart with a stethoscope. The veterinarian can classify the heart murmur based on when during the heartbeat cycle the murmur occurs, which allows them to make an initial list of possible causes.

Confirming the cause of a heart murmur involves ultrasound of the heart to identify abnormal blood flow. Ultrasound of the heart, called echocardiography, can examine the structure of the heart and show the direction of blood flow through the structures. [6] Examination by a veterinary cardiology specialist may be necessary for a confirmed diagnosis.

Other diagnostic tests that may be helpful include: [3]

  • Blood pressure testing
  • X-rays
  • Electrocardiograms
  • Blood oxygen testing

Ventricular Septal Defects

Ventricular septal defects (VSDs) are the most common heart defect in horses. [3][8] Welsh Mountain ponies are particularly predisposed to VSDs compared to other breeds. [3]

These defects result from incomplete development of the wall between the right and left ventricles of the heart. [3] The resulting hole causes oxygenated blood entering the left side of the heart to flow into the right side of the heart, resulting in overload of the right ventricle. [3]

Symptoms

Most minor VSDs show no symptoms in foals, however a murmur can be heard when listening to the heart with a stethoscope. [5][8] As the foal ages, they may develop exercise intolerance or poor performance. [5]

Foals with large VSDs may exhibit signs of heart failure, such as: [6]

  • Rapid heart rate
  • Enlarged jugular veins
  • Increased respiratory effort
  • Edema of the skin, particularly on the lower abdomen

Prognosis

Generally, treatment of ventricular septal defects is not feasible, although there is one case report describing surgical correction of a VSD. [3][5]

Horses that develop heart failure from a VSD can be reasonably managed with medications to improve quality of life. [5] Some small VSDs in young foals close spontaneously as the horse grows, eliminating the murmur. [6]

The overall prognosis of VSDs depends on the size of the hole, whether there are other structural changes in the heart resulting from the VSD, and whether symptoms of heart failure are present. [9] In general, horses with minor cases are considered safe to ride and compete. [9]

Horses with small VSDs can have successful athletic careers, including low level racing. [3] One study reported that only 15% of horses with VSDs showed symptoms of cardiac disease. [10]

Larger VSDs can cause symptoms such as exercise intolerance, and eventually may cause congestive heart failure. [3] VSDs are also commonly found alongside other cardiac malformations, which increases the severity of cardiac disease and worsens the prognosis. [3][10]

The American College of Veterinary Internal Medicine recommends annual cardiac examinations for horses diagnosed with VSDs, particularly if they are used for athletic performance. [9] For horses with large VSDs, exercise testing may be necessary to confirm their safety of performance. [9]

Affected horses should not be used for breeding, as there is thought to be a genetic component in VSD development. [9][10]

Atrial Septal Defects

Atrial septal defects (ASDs) are holes between the two atria of the heart. These holes are due to incomplete formation of the wall between the left and right atria during development, similar to VSDs. [3] ASDs are uncommon in horses.

The hole in the atrial wall causes blood from the left atrium to enter the right atrium, resulting in overload of the right side of the heart. [3][11]

The severity of atrial septal defects depends on the size of the hole. With large holes, ASDs often result in heart failure with an overall poor prognosis. [3][8] Smaller holes may not cause any symptoms. [5]

Patent Ductus Arteriosus

In some foals, the ductus arteriosus does not close properly after birth and remains patent. This leaves a hole between the pulmonary artery and the aorta, allowing blood from the aorta to enter the pulmonary artery. [3]

The excess blood entering the pulmonary artery overloads the lung circulation and the left side of the heart, resulting in heart failure and increased blood pressure in the lungs (pulmonary hypertension). [3][11]

Small PDAs may not affect the horse at all. Larger holes can cause heart failure with a poor prognosis. Some cases also report rupture of the pulmonary artery due to high blood pressure, resulting in sudden death of the horse. [3][5]

Valvular Defects

Compared to other species, heart valve malformations are uncommon in horses. The heart valves control blood flow between the atria and the ventricles, and between the ventricles and the pulmonary artery or aorta.

Malformations of the heart valves either result in incomplete closure of the valve, allowing continuous blood flow, or stenosis of the valve, where the valve opening is too small for adequate blood flow.

Pulmonic Stenosis

In foals, pulmonic stenosis is the most common malformation. [3] This malformation affects the pulmonic valve, which is the heart valve between the right ventricle and the pulmonary artery.

Since the valve opening is too small, blood accumulates within the right ventricle and can rapidly result in heart failure. [3] Severely affected foals will not have enough blood entering their lungs for oxygenation, resulting in hypoxemia (inadequate blood oxygen). [3] These foals die within several days of birth.

Symptoms of severe pulmonic stenosis in newborn foals include: [3][6]

  • Stunted growth
  • Blue mucous membranes even after oxygen administration
  • Loud heart murmurs
  • Collapse
  • Exercise intolerance

Mildly affected horses can have a normal life, though they typically have some degree of exercise intolerance. [6]

Complex Cardiac Defects

Many foals that experience cardiac defects have multiple defects, rather than a single isolated defect. These foals have an overall poor prognosis, due to significant compromise to the heart’s function.

Tetralogy of Fallot

Tetralogy of Fallot is a very uncommon condition, however it is notable for veterinarians as the main cause of right-to-left shunting.

In a right-to-left shunt, deoxygenated blood from the right side of the heart enters the left side of the heart and distributes to the body. Most other heart malformations cause left-to-right shunting, due to the higher blood pressure in the left side of the heart. [3]

The main features of tetralogy of Fallot are: [3]

  • Pulmonic stenosis
  • Large ventricular septal defect
  • Overriding of the aorta, where the aorta receives blood directly through the VSD rather than from the left ventricle
  • Thickening of the right ventricular wall

This combination of cardiac malformations is thought to arise from incomplete development of the heart walls and the walls of the major vessels leaving the heart. [6] Arabian horses appear predisposed to tetrology of Fallot development. [6]

Clinical Signs

Symptoms of tetralogy of Fallot include: [3][5]

  • Blue mucous membranes
  • Exercise intolerance
  • Growth stunting
  • Abnormally viscous blood

Most foals with tetralogy of Fallot die shortly after birth or are euthanized. [3][5]

Summary

  • Heart murmurs are abnormal sounds that occur due to disruptions to normal blood flow through the heart.
  • In newborn foals, heart murmurs can be normal for several days after birth and typically resolve on their own.
  • Some heart murmurs can result in heart failure and a poor prognosis for the foal, while others may show few or no symptoms.
  • The most common heart malformation in horses are ventricular septal defects.
  • Consult with your veterinarian if you suspect a medical condition or have any questions pertaining to your foal’s cardiovascular health.

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References

  1. Tou, S. Congenital and Inherited Disorders of the Cardiovascular System in Horses. Merck Veterinary Manual. 2019.
  2. Heart Murmurs in Horses. University of Florida Large Animal Hospital.
  3. Marr. C. M. and Bowen. I. M., Eds., Cardiology of the horse, 2nd ed. Edinburgh; New York: Saunders, 2010.
  4. Marr. C. M., The Equine Neonatal Cardiovascular System in Health and Disease. Veterinary Clinics of North America: Equine Practice. 2015. doi: 10.1016/j.cveq.2015.09.005. View Summary
  5. Reed. S. M. et al., Equine internal medicine, 3rd ed. St. Louis, Mo: Saunders Elsevier, 2010.
  6. Scansen. B. A., Equine Congenital Heart Disease. Veterinary Clinics of North America: Equine Practice. 2019. doi: 10.1016/j.cveq.2018.11.001. View Summary
  7. Fousse. S. L. and Stern. J. A., Unraveling the Genetics Behind Equid Cardiac Disease. Veterinary Clinics of North America: Equine Practice. 2020. doi: 10.1016/j.cveq.2020.03.004. View Summary
  8. McKinnon. A. O. et al., Equine Reproduction. Wiley-Blackwell, 2010.
  9. Reef. V. b. et al., Recommendations for Management of Equine Athletes with Cardiovascular Abnormalities. Journal of Veterinary Internal Medicine. 2014. doi: 10.1111/jvim.12340. View Summary
  10. De Lange. L. et al., Prevalence and Characteristics of Ventricular Septal Defects in a Non-Racehorse Equine Population (2008-2019). Journal of Veterinary Internal Medicine. 2021. doi: 10.1111/jvim.16106. View Summary
  11. Buergelt. C. D., Equine Cardiovascular Pathology: An Overview. Animal Health Research Reviews. 2003. doi: 10.1079/AHRR200353. View Summary