For thousands of years, humans have cared deeply for dogs. We’ve cared for them so deeply that we selectively bred them until they became deformed little monsters. Over generations of artificial selection, domestic dogs have developed a range of physical health problems — hip dysplasia in German shepherds, breathing issues in bulldogs, and heart disease in Cavalier King Charles spaniels — because of the genes we’ve chosen for them. But on Thursday, scientists identified an unusual risk factor for dogs that aren’t normally considered at-risk for heart disease.

In a paper published in the journal Scientific Reports, scientists showed in a large-scale study that dogs without a predisposition to heart disease born between June and August have higher rates of cardiovascular disease than dogs born at other times of the year. This effect peaked in July, as researchers found dogs born then were 47 percent more likely to have heart problems at any point in their lives than those born at other times during the year. The strange exception to this trend were dog breeds with a genetic predisposition to heart problems — outliers that may be key to understanding what’s actually going on here.

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Cavalier King Charles Spaniels have a higher risk of congenital heart disease, so they're usually monitored closely for this issue. 

The results of the study, which used Orthopedic Foundation of Animals data on 129,778 dogs from 253 different breeds, mirror the team’s previous findings that certain lifetime diseases — including heart disease — were more likely to occur in people born during certain months. That study, as well as others on the same topic, suggest that the month of conception is associated with certain gene mutations. (As you may have realized by now, this study is actually more about humans than dogs, as the dog heart is often used as a research model for the human heart.)

The dog breeds that bucked this trend can help explaining the paper’s findings. The study’s authors suspect that the gene mutation involved in heart disease in dogs born during the summer may be a gene that isn’t normally involved in heart disease, which could explain why the summer peak doesn’t affect dogs that are already prone to heart problems. Of course, there might be simpler explanations. One might be that dogs already genetically predisposed to heart problems are more closely monitored and therefore are less likely to suffer from undiagnosed issues.

Another has to do with breeding. “Dogs that show signs of cardiovascular disease that come from high-risk breeds are often prevented from breeding along with other disease-related conditions,” the authors write. Humans, however, don’t simply decide to take their genes out of the gene pool because they have a family risk of disease, though, so if scientists can better understand the reasons behind this strange summer phenomenon, they can better understand how to help humans live healthier lives.

The researchers report that the monthly heart disease risks for dogs in this study are similar to those exhibited by humans. So, it sees that dogs are not only humans’ best friend; they might also be our best chance at understanding the risks that people with summer birthdays face.