Uncontrolled dogs, those with or without proper owners, may pose great threat to the bodily well-being of the people, other animals and the surrounding environment. The trouble of uncontrolled dogs may range from excrement on public sidewalks to fatal dog bites. Negligence of dog owners and law enforcement officials contribute to the problem of vicious and unpredictable dog behaviour. The popularity of organized dog fighting may reinforce the mystique of owning a wild dog, as well may an individual's need to protect his home and family from harmful intruders. Abandoned dogs and those that are allowed to roam freely are real problems to wildlife as they compete with other predators and kill wild animals, disturb the ecosystem, transmit diseases to wildlife and also interbreed with closely related species.
There are an estimated one billion domestic dogs worldwide and their conditions range from feral and free-ranging to entirely dependent on humans. It is estimated that 80 percent are feral and even stray dogs often rely on humans for their survival. As the human population rises, so will the number of dogs.
In 1988, eight Americans were killed by pit bull terriers. At a District of Columbia Council hearing on the fortification of dog-bite laws during the same year, one member attempted to shield a dangerous breed by insisting all dogs are covered by the equal protection clause of the USA Constitution. The American pit bull terrier is the most ferocious dog known, according to the RSPCA. The Japanese Tosa, almost as large as a mule, is known as the "sumo wrestler" of the dog fighting world. Other dogs considered dangerous and unpredictable include rottweilers, bull terriers, Staffordshire terriers, wolf and Alsatian hybrids, Japanese Akita and Neapolitan Mastiffs.
In Belgium, ten thousand dogs are put down each year because of behavioural problems associated with inbreed aggression.
Organized dog fighting is in some countries an ancient tradition, acting as both entertainment and sport. Despite the number of fatalities, serious injuries, dog-bite laws and animal protection acts incurred, the breeding of uncontrolled dogs remains a reality. A 1991 British report shows 125 attacks by pit bulls within one year, which included the disfigurement of a 6-year-old girl who was bitten 25 times by one dog. The British government's outrage at these figures prompted a ban on the import of "Very Dangerous Fighting Dogs", which included American pit bull terriers and Japanese Tosa. In 1991, there were an estimated 30,000 pit bull terriers in the UK, some of which were a part of 50 organized dog fighting syndicates in the country (15 of which were based in London).
Feral and free-ranging dogs have received surprisingly little attention, but conservationists say they have contributed to the extinction of nearly one dozen wild bird and animal species. Dogs are said to threaten nearly 200 species worldwide, some of which are critically endangered. As such, they have become the third worst human-introduced predators after cats and rats.