The most commonly-diagnosed heart condition in dogs, PDA is treated surgically in puppies with great success. But, writes Dr Peter Higgins, preventing this hereditary disease is more important than curing it.
At birth, mammals must adapt from living in a fluid environment (the amniotic fluid), where they acquire oxygen through the mother’s blood, to breathing air and acquiring oxygen through their own lungs. Changes to the ductus arteriosus are very important in this adaptation process. The ductus arteriosus is a small blood vessel that connects the pulmonary artery (which carries blood to the lungs) and the aorta (which carries blood to the rest of the body).
Before birth, most of the blood from the foetal heart bypasses the foetal lungs via the ductus arteriosus. The lungs gradually become functional late in foetal development. At birth, the blood supply from the mother is of course cut off. The dog (or other mammal) then begins breathing on its own and blood flow through the ductus arteriosus decreases dramatically so that the blood is no longer bypassing the lungs. Within a few days, the ductus closes completely so that blood becomes oxygenated before being pumped around the body.
Where the ductus does not close, the dog is left with a patent ductus arteriosus (PDA), the most commonly diagnosed congenital heart defect in dogs. The extent to which this affects the dog depends on the degree of patency, or opening, of the ductus. The more open the ductus, the more the dog’s health will be affected.
The PDA can range from a small blind pocket off the aorta, which doesn’t cause any problems, right through to varying degrees of abnormal blood flow through the ductus between the aorta and the pulmonary artery. Most commonly, there is a shunt from the left to the right side of the heart, with blood from the aorta (where there is more pressure) continuously shunted to the main pulmonary artery (where there is less pressure).
This means an increased volume of blood travels to the lungs, which results in fluid buildup (pulmonary oedema) and volume overload to the left side of the heart. Dogs with this form of PDA may experience coughing, reduced tolerance of exercise, loss of weight and, eventually, congestive heart failure. Without surgery, premature death is likely. Less commonly, there is a right-to-left shunt. In this case, the dog’s circulation is the same as that of a foetus.
That is, some of the blood leaving the right side of the heart bypasses the lungs entirely. This results in circulation of poorly oxygenated blood. Dogs with this type of PDA may experience shortness of breath, as well as weakness or collapse of the hind limbs.
Usually, a PDA is first suspected when a veterinarian hears the characteristic continuous ‘machinery’ heart murmur when examining a pup at the time of vaccination. Sometimes, this murmur is enough for a conclusive diagnosis, but occasionally vets may use x-rays, an ECG or an MRI. At this point, the murmur may be the only sign of a PDA that the puppy shows; the owner may not notice any difference between this pup and its healthy litter mates.
Surgery is recommended in all dogs under two years of age in which a left-to-right shunting PDA is diagnosed. Surgical treatment consists of tying off the patent ductus and is usually successful.
Surgery should be performed as soon as possible (as early as 8 to 16 weeks of age), before changes have occurred as the heart tries to compensate for the defect. The prognosis for a normal life after early surgery is usually very good.
Where the dogs show signs of heart disease, there are increased risks associated with surgery, so veterinarians generally recommend medical stabilisation before surgery. The problems associated with the less common right-to-left shunt can be managed medically. Treatment includes rest, exercise restriction and avoidance of stress.
PDA occurs in many breeds and is seen more often in females. The hereditary mapping is complex; as a single pair of genes does not cause the condition, it is difficult to predict the likelihood of a puppy being affected by the condition if one or both of its parents have it.
Responsible breeders, therefore, remove any dogs that show signs of the condition from breeding programs, as well as any dogs whose descendants have PDA.
Hereditary disease is a fact of life. All species can acquire some sort – horses, dogs, cats, pigs and, yes, even humans. Hereditary diseases are not confined to purebred animals and in breeding animals of any type, genetics makes up one of the most vexed and emotional areas of animal husbandry and veterinary science.
The keys to minimising the incidences and effects of hereditary disease are your knowledge of breeding practices and how you apply them. Generally speaking, the genetics of most inherited disorders are well understood and so is their treatment, but the breeding out of genetic disorders depends on breeders and judges understanding the medical and philosophical implications of hereditary diseases. Once these are understood, breeders and judges can ensure that the health of the next generation of breeding stock is better than the previous generation’s.
To improve the health of dogs, we must all stay up to date with the latest techniques and the latest veterinary and husbandry breakthroughs, and have the intestinal fortitude to apply them in breeding programs and the show ring.