Study: Does salmon pose a health risk to dogs?

Everyone knows cats love fish, but these days, dogs are eating their fair share, too. Especially salmon.

Salmon is an increasingly common ingredient in commercial dog food because manufacturers are looking for unconventional protein sources and they want to include more omega-3 polyunsaturated fatty acids. Salmon fits the bill on both counts.

Sadly, it’s also a great source of mercury.

Excess mercury exposure is a health hazard for people and animals. Depending on the method of exposure—such as skin contact, ingestion, or inhalation—it can lead to respiratory and gastrointestinal disease, kidney injury, impacted fetal development, and neurologic issues.

The most common way people and pets in the US are exposed to mercury is by eating fish containing methylmercury, a highly toxic organic compound formed when mercury combines with carbon. Salmon contains some of the highest levels of methylmercury of any fish species. And while previous studies have measured total mercury in commercial pet foods, additional testing for methylmercury had not been done.

So is dog food that contains salmon—and more specifically, methylmercury—dangerous for dogs?

In a recent study published in Topics in Companion Animal Medicine, researchers at the University of California, Davis (UC Davis), investigated levels of methylmercury in a small sampling of 24 commercial dog food diets. Of the 24 diets tested, half contained some kind of salmon product (e.g., salmon, salmon meal, and salmon oil). 12 of the diets were wet and 12 were dry.

This study marks the first time methylmercury has been tested in commercial dog diets.

Researchers detected low concentrations of total mercury in 3 of the 24 diets, and only one of those contained a detectable level of methylmercury.

“The concentrations detected are unlikely to pose a risk to healthy adult dogs,” said lead author Rae Sires, DVM, a nutrition resident at the UC Davis Veterinary Medical Teaching Hospital. “These results should be reassuring to dog owners.”

Surprisingly, two of the three diets that tested positive didn’t contain any fish products.

That left the researchers scratching their heads a bit: “We need more data to determine where the total mercury detected in dog foods is coming from,” Sires said.

Large ranges in mercury concentrations in pet foods have been reported previously, with cat diets often being higher than those for dogs. It can also depend on the kind of food: A Brazilian study evaluating 88 local dog and cat foods found that mercury concentrations were significantly higher in canned pet food than in dry pet food.

The National Research Council provides maximum tolerable levels for mercury for several species, including cats; however, there is a lack of data for dogs, which precludes recommendations for safe exposures in canines.

Additionally, there are currently no regulatory guidelines for mercury concentrations in US pet foods.

Despite the lack of official guidelines, the researchers remain upbeat on the subject of salmon: “Our study doesn’t support avoiding fish- or salmon-based diets,” Sires said.