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Last Updated

15 Jun 2003

Source: Washington Post, June 15, 2003

Infections Now More Widespread

Animals Passing Them to Humans

By Rob Stein, Washington Post Staff Writer

It happens again and again.

Strange and frightening new infections seem to appear out of nowhere, such as Lyme disease, Ebola and, of course, AIDS. With monkeypox coming on the heels of SARS, which emerged not long after West Nile, it's a phenomenon that seems to be happening at an accelerating rate.

Scientists agree that the trend is alarming and does appear to be gathering speed. In the coming years, they say, dangerous new pathogens could arise more frequently, multiplying the misery at a minimum and, in the most dire scenarios, increasing the chances of a cataclysmic plague.

"There are probably hundreds, if not thousands -- maybe even millions -- of viruses out there," said Robert G. Webster, a leading virologist at the St. Jude Children's Research Hospital in Memphis. "We don't even know they're there until we disturb them. SARS is probably just a gentle breeze of what one of these big ones is going to do someday."

All the new diseases have one thing in common: Animals passed them to people. Fears of new pathogens and bioterrorism have prompted more intense efforts to detect new infections, uncovering some that may have been there unrecognized all along. But many leading experts agree that animals are transmitting viruses, bacteria and parasites to humans more rapidly than ever before, spawning ailments known as zoonotic diseases.

"Influenza is a zoonotic disease. HIV is a zoonotic disease. Monkeypox. SARS," said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases. "You can go on and on."

The increasing pace is being caused by a confluence of factors that bring people into contact with a greater diversity of creatures than ever before, experts say.

As Earth becomes more populated, people are increasingly living, farming and hunting in previously undisturbed parts of the world, such as rainforests and jungles in Asia and Africa that teem with creatures humans barely know. Similarly, suburban sprawl is pushing into undeveloped areas, where populations of deer, other animals and insects are exploding because of conservation measures. At the same time, climate change is driving birds, mosquitoes and other living things into new zones, carrying microbes with them.

Meanwhile, people are bringing a widening array of rodents, amphibians and other exotic animals out of the wild as pets, and as food. People and animals can fly to new places while they are still infectious. And modern industrialized agriculture distributes contaminated food more widely than ever before, exposing people to dangers such as mad cow disease and E. coli.

"There are a multitude of factors that allow people to come in contact with what may once have been infections that were buried deeply into the ecology," said Stephen S. Morse of Columbia University in an interview. "With the global traffic of people and goods . . . there are many more pathways, or highways, for what I called some years ago, 'viral traffic.' "

This phenomenon, coupled with the threat of bioterrorism, has triggered a surge of initiatives to try to protect against these new threats. The United States and the World Health Organization are trying to improve early-warning systems for infectious diseases. Many states, and Congress, are considering new laws to restrict importation of exotic animals.

"The exotic animal trade is enormous, and it continues to spiral out of control. Some say the exotic animal trade is second only to the drug trade. And at the moment, it's virtually unregulated," said Wayne Pacelle, vice president for communications and government affairs of the Humane Society of the United States.

There are laws to protect farm animals and crops from disease, wild plants and animals from invasive exotic species and endangered species. But there are no federal laws or agencies, and just a patchwork of state laws and regulations, specifically targeted at protecting people from animal-borne diseases.

"What's the next plague or scourge that is going to be introduced as a derivative of the exotic animal trade?" Pacelle asked.

Animals have spread diseases to people throughout history. Smallpox may have jumped to humans from camels. All the roads that led to Rome may have carried new diseases there, contributing to its downfall. Rats spread the Black Death. The deadly Spanish flu, like all influenza viruses, originated in waterfowl.

"You could argue hypothetically that many infections we recognize as familiar to humans could have originated as zoonoses," Morse said. "Many of the infections that we recognize today as being important human infections have relatives in other species, which suggests that they may well have gotten to them from those other species. That's plausible, as we continue to see that happen today."

Diseases move from animals to people in many ways. Flu viruses originate in wild waterfowl, such as ducks. Every once in a while, they mutate and gain the capacity to infect people. This occurs most frequently in southern China, probably because of the large number of people living close to large numbers of chickens and pigs. Pigs can be infected by bird and human flu viruses, providing a natural mixing bowl for the pathogens.

Of all the known pathogens, influenza viruses elicit the most concern because of their proclivity to mutate and spread easily. Many say it is just a matter of time before the next Spanish flu, which killed approximately 50 million people in 1918 and 1919.

Other times viruses jump from animals to people because of what humans eat. The leading theory for the origin of AIDS is that the virus probably arose about 75 years ago in central Africa when a hunter cut himself while butchering a chimp for food. The animal was infected with simian immunodeficiency virus (SIV), which mutated into the human immunodeficiency virus (HIV).

Something similar probably happened to produce the hemorrhage-causing Ebola virus in 1976. It can infect primates, though its main hiding place remains a mystery.

Lyme disease got its name in 1977 after a cluster of children in Lyme, Conn., developed arthritis. Scientists eventually determined their illness was caused by a bacterium transmitted to humans by black-legged ticks that had proliferated along with the deer on which they feed.

In 1993, otherwise healthy, relatively young people started dying in the American Southwest. It turned out they were being killed by the hantavirus, spread in the feces and urine of field mice. The virus probably had been around for decades, but heavy rains led to a huge increase in the field mouse population that year.

Until 1999, West Nile virus was only found in Africa and the Middle East. But it was discovered in New York when birds started dying. No one knows for sure how it arrived, but it could have been when an infected bird got blown off course during migration, or an infected bird was brought into the country as a pet. The virus is spread by infected mosquitoes, which are now found nationwide.

This year, severe acute respiratory syndrome (SARS) spread globally, triggering an unprecedented international health emergency. The infection is believed to have jumped to humans after restaurants in southern China started serving small nocturnal mammals called civets as a delicacy. The theory is that food preparers became infected while handling or butchering the animals.

Before last month, the monkeypox virus, which is related to the dreaded smallpox virus, was only found in central and western Africa. Investigators traced the current outbreak, the first in the Western Hemisphere, to prairie dogs, which have become popular pets. The prairie dogs may have become infected by Gambian giant pouched rats imported from Africa by a Texas exotic pet dealer.

But monkeypox is just one of countless viral diseases carried by animals around the world, and among the least dangerous. Lassa fever, for example, sparks periodic epidemics in West Africa that kill as many as half of all its victims. It spreads very easily and is carried by a small rat.

"Someone could decide to make cute pocket pets out of those critters," said Peter B. Jahrling, a senior scientific adviser to the U.S. Army Medical Research Institute of Infectious Diseases. "Then we'd have an outbreak of Lassa fever here to worry about."