Patent Ductus Arteriosus in Horses: PDA in Foals Explained

Patent ductus arteriosus, or PDA, is a heart defect present at birth. Before a foal is born, the ductus arteriosus is a normal blood vessel that lets blood bypass the lungs, which are not yet being used. After birth, that vessel is expected to close as the foal starts breathing on its own.

In foals, that closure is not always immediate. Merck notes that complete closure may be delayed for up to about a week after birth, and a transient murmur can be heard during that period. That means a murmur in a very young foal is not automatically a serious defect. Still, if the ductus stays open beyond the expected newborn period, it can create abnormal blood flow from the aorta into the pulmonary artery.

When PDA persists, extra blood is pushed back through the lungs and left side of the heart. Over time, that can enlarge the left heart chambers, increase strain on the lungs, and raise the risk of arrhythmias or left-sided congestive heart failure. Some foals look normal at first, while others become weak or develop breathing problems as the circulation becomes more abnormal.

This is considered a rare congenital heart defect in horses. Early evaluation matters because the outlook depends on how large the shunt is, whether the foal is showing clinical signs, and whether heart damage has already started.

PDA is a congenital defect, which means the foal is born with it. The underlying problem is failure of the ductus arteriosus to close after birth. In the fetus, this vessel has an important job. After delivery, once the lungs expand and normal circulation begins, the vessel should shut down.

Why that closure fails in some foals is not fully understood. In horses, congenital heart defects are uncommon overall, but PDA is listed by Merck as one of the more common congenital cardiovascular defects seen within that already rare group. Breed-related patterns are not well defined for PDA specifically, although congenital heart defects in general have been reported more often in Arabians than in many other breeds.

It is also important to separate normal delayed closure from a true persistent PDA. Very young foals can have a temporary murmur while the ductus is still closing. That is why timing matters. A murmur heard in the first several days of life may be physiologic, while a murmur that persists beyond the expected closure window deserves a more complete cardiac workup.

Because congenital heart disease may have a heritable component in some horses, your vet may advise caution with future breeding decisions if a true structural defect is confirmed.

Diagnosis starts with a careful physical exam. Your vet will listen for a murmur, assess heart rate and rhythm, check breathing effort, and look for signs of poor perfusion or fluid overload. In a bright newborn foal, your vet may recommend a short recheck interval if the murmur could still fall within the normal closure period.

The key test for confirming PDA is echocardiography, which uses ultrasound to visualize the heart and major vessels. This helps identify the abnormal connection, estimate the direction and size of blood flow, and look for enlargement of the left heart chambers. Merck also notes that evaluation of congenital heart defects in horses may include electrocardiography, chest x-rays, and other imaging to judge severity and complications.

Additional tests may include an ECG if arrhythmias are suspected, bloodwork to assess the foal's overall health, and chest imaging if there is concern for pulmonary overcirculation or heart failure. In referral settings, serial exams may be used to track whether the ductus is closing on its own or whether the defect is causing progressive strain.

Because a murmur alone does not tell the whole story, the goal of diagnosis is not only to name the defect but also to understand how much it is affecting the foal. That severity assessment guides whether monitoring, medical support, or referral-level care makes the most sense.

There is no guaranteed way to prevent PDA in an individual foal because it is a congenital structural defect that develops before birth. Good broodmare care supports overall foal health, but it cannot fully prevent congenital heart abnormalities.

What you can do is focus on early detection. Have newborn foals examined promptly, especially if they seem weak, breathe faster than expected, tire while nursing, or have poor growth. A murmur heard in the first days of life may be temporary, but follow-up matters if it persists.

Breeding decisions also play a role. Because congenital cardiovascular defects may have a heritable component in some horses, your vet may advise against breeding a horse with a confirmed structural heart defect. That recommendation is meant to reduce the chance of passing along a problem, even when the exact inheritance pattern is unclear.

For pet parents, the most practical prevention strategy is really prevention of complications: timely exams, appropriate rechecks, and referral when needed. Catching a significant PDA before heart failure develops gives your vet the best chance to guide care that matches your foal's condition and your goals.