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.