A new study has revealed that pet owners who feed their dogs bully sticks may unknowingly be adding excessive calories and potentially harmful bacteria to their dogs' diets.

American and Canadian researchers from the Cummings School of Veterinary Medicine at Tufts University and the University of Guelph conducted the study to examine the caloric density and bacterial contamination of bully sticks, also known as “pizzle sticks," according to a news release from Tufts.

In addition to the analysis of the treats, which are made from the uncooked, dried penis of a bull or steer, researchers surveyed pet owners and pet owners to evaluate their knowledge about bully sticks.

Treats packed with calories

Researchers bought 26 bully sticks made by different manufacturers from retail locations in the U.S. and Canada. They then analyzed the caloric content of a random subset of the 26 treats.

The caloric content range for the treats they tested ranged from 9 to 22 calories per inch, which researchers said works out to 88 calories contained in the average 6-inch bully stick. For a 50-pound dog, the 88 calories amounts to 9 percent of its daily diet. A 10-pound dog would get a whopping 30 percent of its daily calorie requirements from one bully stick, researchers said.

Dr. Lisa M. Freeman, who was the first author of the study, said owners could be inadvertently increasing their dogs’ obesity risk by regularly feeding them bully sticks.

“While calorie information isn’t currently required on pet treats or most pet foods, these findings reinforce that veterinarians and pet owners need to be aware of pet treats like these bully sticks as a source of calories in a dog’s diet,” Freeman said in the news release.

Risk of bacterial contamination

During the study, researchers tested all 26 bully stick treats to learn whether they contained bacterial contaminants. Researchers reported that their analysis found:

• One stick was contaminated with Clostridium difficile.
• One contained methicillin-resistant Staphylococcus aureus (MRSA).
• Seven sticks were contaminated with Escherichia coli - one of which was resistant to tetracycline.

Despite the limited sample size and the knowledge that not all of the bacterial strains are known to infect humans, researchers recommended that people wash their hands after handling treats like bully sticks that are uncooked, according to the news release.

Surprising misconceptions by pet owners and veterinarians

Researchers were surprised to learn through their survey of veterinarians and pet owners that many people aren’t aware bully sticks are made from bull penis. According to their survey of 852 adults, 62 percent of veterinarians knew the true source of bully sticks compared to 44 percent of general respondents.

Another surprising misconception revealed by the study, Freeman said, was that “... 71 percent of people feeding bully sticks to their pets stated they avoid by-products in pet foods, yet bully sticks are, for all intents and purposes, an animal by-product.”

Read the full study in the January edition of the Canadian Veterinary Journal (CVMA members only).