ST. LOUIS -- Humans are at greater risk of being infected by diseases from the animal world, according to researchers who have documented 38 illnesses that have made that jump over the past 25 years.
That's not good news for efforts to curb the spread of bird flu, which scientists fear could mutate and be transmitted easily among people.
There are 1,407 pathogens -- viruses, bacteria, parasites, protozoans, and fungi -- that can infect humans, said Mark Woolhouse, professor of epidemiology at the University of Edinburgh in Scotland. Of those, 58 percent come from animals.
Scientists consider 177 of the pathogens to be emerging forms not previously linked to human disease. Others are reemerging as infective agents. Most of the pathogens will never cause pandemics, but some researchers believe bird flu could prove an exception.
As the lethal H5N1 strain of bird flu continues to be discovered in more countries, fears of a worldwide pandemic have increased. The virus has spread across Asia into Europe and Africa.
Controlling bird flu will require renewed focus on the animal world, including the chickens, ducks, and other poultry that have been sacrificed by the tens of millions to stem the progress of the virus, infectious-disease researchers said at a news conference yesterday at a meeting of the American Association for the Advancement of Science.
''The strategy has to be looking at how to contain it in the animal world, because once you get into the human side, you're dealing with vaccines and antiretrovirals, which is a whole new realm," said Nina Marano, a veterinarian and public health specialist with the National Center for Infectious Diseases.
Bird flu has killed at least 91 people worldwide, most of them in Asia, since 2003, according to the World Health Organization. It appears to kill about half the people it infects. However, should it mutate so it can pass from human to human, the virus is expected to grow less deadly, said Dr. Stanley Lemon of the University of Texas Medical Branch at Galveston. ''It is very unlikely that it would maintain that kind of case mortality rate if it made the jump," Lemon said.
Each year, one or two new pathogens and multiple variations of existing threats infect humans for the first time. That pace appears to be unsustainable in the long run because it would imply that people run the risk of being overrun, Woolhouse told reporters.
''Humans have always been attacked by novel pathogens. This process has been going on for millennia. But it does seem to be happening very fast in these modern times," Woolhouse said.
Woolhouse says that either many of those diseases and other afflictions will not persist in humans or that there is something peculiar today allowing so many of them to take root in humans.
One explanation may be the recent and wide-scale changes in how people interact with the environment in a more densely populated world that is growing warmer and in which travel is faster and more extensive, Marano said. Those changes can ensure that pathogens no longer stay restricted to animals, she added. Examples from recent human history include HIV, Marburg, SARS, and other viruses.
That prospect leaves open the question of what threats await humans.
''It always surprises us. We think that avian flu will be the next emerging disease. My guess is something else might come out before that," said Alan Barrett, of the University of Texas Medical Branch at Galveston.