David Frum's Newsweek piece on America the Anxious has many sobering statistics about the weak economy: Americans makes less money, owe more debt, and are having fewer marriages. Yet it seems one very important metric was left out: pet ownership.

According to the American Veterinary Medical Association, fewer families are replacing pets once they die and fewer pets are getting bought:

"It's clearly the economy," says Karen Felsted of Felsted Veterinary Consultants, in Richardson, Texas. She presented the findings at the vets association's national meeting in San Diego this week. "The percentage of households that owned at least one pet was down 2.4%." That's 2.8 million households that became petless. "It's a significant number," she says.

The number of pets of all kinds had been rising steadily since at least 1986, when AVMA began doing its twice-a-decade count. But from 2006 to 2011 it declined.

One factor: When older pets die, people are less likely to replace them, possibly because they can't afford to, says Ron DeHaven, CEO of the vets association.

Changing demographics also plays a role, says Stephen Zawistowski, science adviser to the American Society for the Prevention of Cruelty to Animals in New York.

Pet ownership tends to be more common in families that include two parents and children. Single people, couples without children and older people are less likely to have pets. As America moves away from the mom-dad-two-kids household, he says, pet numbers decline.

One data-point for those who care about the war between cats and dogs. In the weak economy, both cat and dog ownership is declining but there is a greater decline in the number of cats as pets (7.6 million fewer vs. 2 million fewer).