It started with a late-night cough. "He was otherwise fine, but … something was weird and different," Verai Ramsammy said of her miniature schnauzer, Louie. The ICU physician was a meticulous dog person, the kind who bought special food for her pets.

Within months, Ramsammy's second dog, Mico, fell ill with the same problem. This made Ramsammy's veterinarians sit up. The dogs, both mini-schnauzers, were unrelated.

Their cases helped link a serious, sometimes fatal, heart condition with the latest dog food fad. As more cases were reported, veterinarians and the Food and Drug Administration began investigating a potential link between boutique, grain-free diets and a heart disease called canine dilated cardiomyopathy (DCM), which had been known primarily as a genetic disorder. This summer, the FDA issued a caution against grain-free diets. Since then, more reports have poured in — nearly 300 overall.

Canine DCM weakens the dog's heart, said veterinary cardiologist Darcy Adin, preventing it from pumping enough blood, so it enlarges to compensate. After a certain point, fluid backs up from the heart into the lungs causing congestion and coughing. Other symptoms include difficulty breathing, weakness and lethargy. It can eventually "lead to congestive heart failure signs and, in some cases, sudden death," Adin said.

Across the country at the University of California at Davis, Joshua Stern, another veterinary cardiologist, started to see surprising signs of heart disease in his golden retriever patients. Multiple veterinary groups also started to notice this disturbing trend. They alerted the FDA. The vets started to find that many of the sick dogs had been on grain-free diets, high in legumes. The length of exposure to the diet ranged from months to years.

The condition is linked to a taurine deficiency. Taurine is an essential amino acid that most animals, including humans, can create their own. Dogs get a lot of it from their diet. Chicken and beef are high in taurine, while rabbit, lamb, legumes, pea-protein and other ingredients found in some grain-free foods have little or no taurine.

In these cases, adding taurine to the dogs' diet (and taking them off legumes) can reverse the disorder if caught early enough, Stern and Adin said.

The industry response has been varied. Mars Petcare, the manufacturers of brands like Pedigree and Whiskas, said it was working to "better understand any potential link between ingredients and DCM."