Conservationists have expressed outrage over the lack of US wildlife ownership laws after the slaughter of 49 animals, including 18 rare Bengal tigers, set free from a private Ohio farm.

"Quite frankly, nobody should have these animals in the first place, so we need to take steps to change laws to make that a reality," Adam Roberts, executive vice president of Born Free USA, told AFP.

"These animals belong in accredited facilities with people who can handle them appropriately."

Bears, lions, tigers, wolves and monkeys ran amok when owner Terry Thompson, 62, flung open the enclosures at his Muskingum County animal farm near the town of Zanesville on Tuesday evening and then shot himself.

Police following shoot-to-kill orders, some of them armed only with handguns, said they had no choice but to exterminate the animals to protect local residents -- and in some cases, themselves -- as darkness fell.

"Public safety was our number one concern," Sheriff Matt Lutz said. "We are not talking about your normal, everyday house cat or dog."

By late Wednesday 49 animals were dead. Only six were captured alive: a grizzly bear, three leopards and two monkeys. Another monkey was still thought to be on the loose, or eaten by a lion.

There had been at least three dozen complaints since 2004 about Thompson's exotic menagerie -- including a giraffe grazing by a highway and a monkey in a tree -- and he had faced more serious charges of animal mistreatment.

Conservationists have for years demanded strict US wildlife ownership laws, especially in Alabama, Idaho, Nevada, North Carolina, Ohio, South Carolina, West Virginia and Wisconsin, where no such laws exist.

"All eight states that don't have regulations should immediately have an executive order by the governor banning the keeping or sale of these animals," Roberts told AFP. "Stop people acquiring these animals full stop.

"I always ask myself, what is it going to take? Is it going to take a woman getting mauled nearly to death by a chimpanzee, as happened in Connecticut? Well, no. People around the country can still have primates.

"Is this going to open up the eyes of the people in Ohio, which is one of the worst states in the country on the exotic pets issues? I sure hope it does, because this could have been worse. People could have been killed."

His call found one advocate in Democratic Ohio congressman Dennis Kucinich, also a leading animal rights advocate.

"I am hopeful that in light of this most recent tragedy, Governor (John) Kasich will heed the calls of the Humane Society of the United States and the public and quickly enact appropriate restrictions on the ownership of exotic animals," he said in a written statement.

People for the Ethical Treatment of Animals (PETA) called on states to introduce a blanket ban on the private ownership of exotic animals.

"A ban is really the answer to this," Delcianna Winders, PETA's director of captive animal law enforcement, told AFP. "Private citizens just aren't capable of giving these animals what they need."

For the World Wildlife Fund, the loss of 18 Bengal tigers was particularly devastating as the number of tigers in the wild has declined rapidly, from around 100,000 at the beginning of the last century to as few as 3,200 today.

Leigh Henry, a leading WWF expert on captive tigers, told AFP there are thought to be an astonishing 5,000 tigers held in the United States, the vast majority of them, some 95 percent, in private hands.

"The current patchwork of laws in the United States regulating these captive tigers is inexcusable," she said. "In Ohio and seven other states you can just go and buy a tiger with no requirement for any kind of license or permit."

A tiny number of pure-bred tigers are protected at the federal level by the Endangered Species Act and a larger number, those used for commercial purposes such as circuses or road-side zoos, are regulated by the Department of Agriculture.

But the vast majority of tigers are either unregulated or regulated at the state level. WWF's principal concern is that their body parts could end up being traded on the traditional medicine market.

Rising wealth in Asia has seen demand soar, and the international trade in wildlife products is now worth an estimated $6 billion a year.

"Wild products are preferred because they are always seen as more pure and potent," Henry explained. "They always carry a premium on price. As long as that market is there, the threat to wild tigers will increase."