LOS ANGELES, Jan. 7 (Xinhua) -- A system that allows zoos in the United States to avoid comprehensive government regulation has been questioned after an escaped tiger killed a teenager visitor and injured two others in San Francisco on Christmas Day.

    As the San Francisco Zoo tries to boost its image and security after the horrific accident, U.S. federal and state officials say there is no way to know how many animals escape from zoos every year because no single agency tracks the incidents.

Schoolchildren look at a Siberian Tiger at the San Francisco Zoo in May 2007 in California.  (Xinhua/AFP Photo) Photo Gallery>>>

    The U.S. Department of Agriculture's Animal and Plant Health Inspection Service enforces vaguely worded animal-safety laws, but the agency does not keep records of escapes.

    While some local governments may require zoos to report such incidents, it is a peace meal system that prevents any nationwide review of whether there are widespread patterns or problems in the industry.

    The lack of a comprehensive tracking system has some officials voicing growing concern that it might be time for the government to change the self-policing policy for the country's hundreds of public and private zoos, according to a report by the San Jose Mercury News daily.

    U.S. lawmakers so far have left much of any establishment of zoo industry standards and accreditation to the non-profit Association of Zoos and Aquariums (AZA), whose recommendations are strictly voluntary.

    The AZA defended the system after the San Francisco Zoo incident, saying that the Christmas Day tiger attack is the only time in its 84-year history that an escaped zoo animal has killed a visitor.

    "The San Francisco Zoo is a great zoo, it's an accredited AZA member in good standing, and it has our support during this difficult time," said the association's president in a statement the day after the fatal attack.

    Animals escapes were almost routine at the Los Angeles Zoo in 1990s, prompting lawsuits filed by the USDA, which resulted in nearly 40,000 dollars in fines for the zoo. The city-owned zoo has bolstered its staff-training programs and has been telling the USDA every time an animal get loose since 2000.

    But even as the Los Angeles Zoo tells the USDA about each of its escapes, the federal agency does not keep any record, according to a USDA spokeswoman.

    And private zoos, such as the San Diego Zoo and Wild Animal Park in California, could refuse to disclose any such information.

    "Like any other business organization, we provide the information that we're required to by law," San Diego Zoo spokesman Ted Molter said. "As a private not-for-profit, we're not required to make reports or distribute that information."

    Wayne Pacelle, president of the Humane Society of the United States, said zoo goers should have a right to know such information.

    The main zoo oversight association AZA does not release records of zoo escapes, and zoos are only required to report to the association incidents that result in injuries.

    California state assembly member Lloyd Levine said that more government regulation of zoos is needed, and he likened the zoo industry's currently oversight system -- essentially zoos watching zoos -- to "the fox guarding the hen house."