Two-legged Americans aren't the only ones battling an obesity epidemic. According to new data from the Association for Pet Obesity Prevention, more than half of the country's dogs and cats are overweight, and one-fifth are so overfed as to be considered obese. Such weight issues could lead pets to develop arthritis, diabetes, high blood pressure, and kidney disease, says APOP founder Dr. Ernie Ward. Indeed, "the problem is reaching epidemic proportions," says Gwendolyn Bounds in The Wall Street Journal. Here, a brief guide, by the numbers:

Percent above normal weight at which an animal is considered obese

Percent of cats that were identified as obese in 2010, compared to 19 percent in 2007

Percent of dogs that were identified as obese in 2010, compared with 10 percent in 2007

171 million
Approximate number of dog and cats kept as pets in the United States

$25 million
Amount the nation's largest pet insurer paid out to treat obesity-related conditions, like asthma and ligament ruptures, in 2010

Percent increase in sales of specialty "DogTread" treadmills since they hit the market in 2007

Percent of veterinarians who consider at least some of their dog patients obese, according to a Pfizer study

Percent of owners who consider their dogs obese, according to the same study. "People's idea of what constitutes a fat pet often differs from clinical reality," says Bounds.

Percent of humans in the United States who were considered obese, as of 2008, according to a World Health Organization report. "As pet owners exercise less, so do their pets," says Sonia Hyde at Daily Health Report.

Sources: Wall Street Journal, San Francisco Chronicle, Yahoo!, Daily Health Report