The recent outbreaks of bird flu and mad cow disease have raised awareness of the danger of diseases spread by animals to humans, known as zoonotic viruses, the researchers say.
Although those two new diseases have affected a relatively small number of people, more common zoonotic diseases like dengue fever and rabies kill tens of thousands of people each year worldwide and appear to be on the rise, the researchers write.
For example, during the study period from 2000 to 2005:
- Rabies, spread by infected dogs, bats, rats, and other animals, killed an estimated 30,000 people.
- Dengue virus affected 50 million people and killed approximately 25,000.
- Japanese encephalitis virus, transmitted by mosquitoes, was responsible for an estimated 50,000 illnesses and 15,000 estimated deaths.
- Lassa fever, a serious viral infection spread by contact with the feces or urine of infected rodents, affected up to 300,000 people and killed about 5,000.
- SARS, believed to have originated in palm civets and/or horseshoe bats, killed 774 of the 8,102 people infected.
"There has been a global resurgence in the dengue virus — which is transmitted between monkeys in the jungle by the mosquitoes that feed on them," says researcher Jonathan Heeney, in a news release.
"The cycle can move into nearby urban areas, where it can then be transmitted from person to person by mosquitoes," says Heeney, chairman of the department of virology at the Biomedical Primate Research Centre in the Netherlands. "This has been attributed to regional population growth around large cities, increased transportation, and failing public control measures."
Even zoonotic viruses for which there are preventive vaccines — like yellow fever — continue to pose a threat and affect an estimated 200,000 people, according to the World Health Organization.
Viruses spread among animals become a serious threat to humans when they adapt for human-to-human transmission.
About a quarter of the diseases originally spread only among animals are capable of readily spreading from human to human, such as measles and HIV.
In the study, published in the Journal of Internal Medicine, the researchers analyzed the number of deaths and illnesses worldwide caused by diseases spread by animals from 2000 to 2005.
The results showed one of the most widely publicized emerging zoonotic diseases, H5N1 bird flu, killed nearly half of the 145 people infected with the virus during the study period.
Though most of the attention has focused on the potential for human-to-human transmission of the bird flu virus, researchers say the risk posed by diseases currently spread from animals to humans is also great. Many have no known vaccine or cure.
Preventive Effort Needed
To prevent emerging zoonotic diseases from becoming major public health threats, Heeney calls for greater cooperation between health and medical experts from various fields.
"The early identification, control, and prevention of re-emerging viral zoonotics lie not only with clinicians and public health experts but, more importantly, with veterinarians, animal scientists and wildlife ecologists," he says.
"They are in the best position to identify trends and patterns, such as increases in the number of deaths of wild or domestic animals. Awareness and surveillance of eco systems will play a key role in identifying and controlling new, emerging, and re-emerging viral zoonotics," Heeney says.
SOURCES: Heeney, J. Journal of Internal Medicine, November 2006; vol 260: pp 399-408. News release, Journal of Internal Medicine.
By Jennifer Warner
Reviewed by Brunilda Nazario