Penn Medicine researchers have some bad news for summer puppies and the people who love them:  Certain breeds of dogs born during June, July, and August have a higher risk of developing heart disease than pups born at other times of the year.

The research, published recently in Scientific Reports, has implications for people, whose hearts are remarkably similar to those of dogs, said Mary Regina Boland, an assistant professor of informatics in the biostatistics, epidemiology, and informatics department at the Perelman School of Medicine.  Previous research has shown seasonal differences in human births and risk for heart disease.

Boland's hypothesis is that the culprit in canines is exposure to outdoor air pollution — fine air particulates, including dust and pollen — early in a puppy's development. Pollution peaks in summer and winter, though time spent outdoors is much less then.  Some genetic interaction with an environmental factor is likely, she said.

Mary Regina Boland is an assistant professor of informatics at the Perelman School of Medicine at the University of Pennsylvania.
Courtesy of Penn Medicine
Mary Regina Boland is an assistant professor of informatics at the Perelman School of Medicine at the University of Pennsylvania.

Boland's team analyzed data from nearly 130,000 dogs tracked by the Orthopedic Foundation of Animals.   It looked separately at breeds known to be at higher risk for heart diseases — that was 71 percent of the dogs — and breeds not predisposed.  The summer effect was seen only in the breeds that are not predisposed, which include Norfolk terrier, Berger Picard, American Staffordshire terrier, Bouvier des flandres and Havanese, plus others.  The risk of heart disease was twice as high for puppies born in July than for those born in May, when the risk of heart disease was lowest.  July pups of these breeds were 47 percent more likely to get heart disease than average.

The good news is that heart disease is less common in dogs than in people.  The overall risk for dogs was low, starting at 0.3 to 2 percent, depending on the breed.

Among breeds already at risk for heart disease, the risk was more consistent across the months.  It was 24 percent higher for both September and December puppies.  The team theorized that such breeds are monitored much more closely for heart problems, which likely affects which animals are chosen to reproduce, but Boland said she doesn't know why risk might be higher in some months.