The underbelly of the trading world known as the black market usually conjures up images of diamonds, drugs and guns -- not lions and tigers and bears.

But the exotic pet trade is thriving, with everything from large cats and wild monkeys to prairie dogs and boa constrictors being bought and sold.

"It's a multi-faceted problem," said Alan Green, author of Animal Underworld: Inside America's Black Market for Rare and Exotic Species. "There are public health issues, public safety issues and animal protection issues."

Because pet ownership isn't regulated by the federal government and most exotic animals aren't sold through pet stores, it's virtually impossible to track how many potentially dangerous animals are living in the homes of private citizens.

And owning an exotic pet is actually legal at the federal level and in many states, though some counties have outlawed it.

"Once it becomes a pet, it's no longer under our jurisdiction," said Jim Rogers, a spokesman for the USDA's Animal and Plant Health Inspection Service (APHIS). "We don't track it. You can have any kind of pet you want."

But these animals are wild, so trying to keep them as house pets often leads to trouble. Many people who bring home a tiger, monkey, bear, kangaroo or poisonous snake don't know what they're getting into.

"Most people buy them on a whim," Green said. "The people who are buying these animals have no business owning them because they don't have the expertise, proper facilities or knowledge of how to care for them."

Usually once the animals mature they become unmanageable and their owners want to get rid of them -- only to find there aren't many places that will take unwanted exotic pets.

"They may be jeopardizing the health and safety of their kids," Green said. "Then it becomes this endless search for a place to send them -- but there's no answer."

Zoos won't accept exotic pets, though there are some SPCA shelters and other sanctuaries that will. But Green said the animals often don't receive adequate care at such facilities.

Not only is it legal to own exotic pets, it's also legal to trade them as long as the buyer or seller has a USDA-issued license. But a number of the animals are laundered -- bought and sold so many times that they vanish on paper -- which makes it impossible to trace their history, according to Green.

Though zoos argue that they sell their surplus animals only to reputable dealers, Green said evidence suggests that they, along with circuses and university labs, help supply the exotic pet black market.

"There's a system of middlemen who sell and resell the same animals so no one can tell where they came from," he said. "It's all anonymous. Legitimate organizations have concocted a network."

Though the USDA doesn't regulate companion animal ownership, a number of maulings by pet lions and tigers led the APHIS to issue a written opinion on the matter.

"Large wild and exotic cats are dangerous animals," the statement warns. "Because of these animals' potential to kill or severely injure both people and other animals, an untrained person should not keep them as pets."

In addition to the cat maulings, there have been numerous instances of venomous snakes turning on their owners.

Last month, a Duluth, Minn., man was bitten by his pet African cobra. Though nearby Minneapolis forbids keeping a pet that is "wild by nature," the city's animal care and control manager told the Associated Press that the risk posed by poisonous snakes is growing -- especially since they're used in drug houses to deter intruders.

"These are potentially very deadly situations," said Bob Marotto. "We're dealing with more reptiles than we've ever dealt with before."

Green said it's not uncommon to see a certain species on the black market spike because the exotic pet trade is fueled by what's "in." A few years ago, lions were all the rage. Then tigers, until the market was flooded with them.

"Certain monkeys come into vogue too," he said. "The exotic pet trade is basically a fad phenomenon, looking for the next big thing. It’s like the fashion industry."