The finding reveals the global urgency for doctors to stay aware of zoonotic illnesses that are transmitted by non-human animals.

    "This comes on the heels of other major zoonotic viral epidemics in the last decade," said virologist Jonathan Heeney of the Biomedical Primate Research Center in The Netherlands.

    By reviewing past studies, Heeney found the diseases responsible for the majority of zoonotic illnesses seem to be increasing. These include severe acute respiratory syndrome (SARS), West-Nile virus, Ebola virus and monkeypox.

    Zoonotic killers during 2000 to 2005 included rabies (estimated 30,000 killed), dengue virus (affected 50 million people, killed around 25,000), Japanese encephalitis virus (estimated 15,000 died), Lassa fever (affected up to 300,000, killed about 5,000) and SARS Corona virus (774 died).

    Rabies is spread by host animals such as dogs, cats, bats and horses. Dengue virus and Japanese encephalitis is spread by mosquitoes. Lassa fever is spread by a rodent known as the multimammate rat, and the host for SARS is yet to be discovered.

    There are no effective vaccines for some of the most common zoonotic viruses. Heeney said doctors and veterinarians need to work together to tackle this increasing global threat.

    "They are in the best position to identify trends and patterns, such as increases in the number of deaths of wild or domestic animals," Heeney said. "Awareness and surveillance of ecosystems will play a key role in identifying and controlling new, emerging and re-emerging viral zoonotics."

    The bird flu, or H5N1, is the current No. 1 attention getter because of its potential to spread from chickens and other birds to humans, and because the virus may mutate in a way that allows it to spread between humans. During the study period, bird flu killed just over half of the 145 people infected with the virus.

    An estimated 700,000 to 2.7 million people -- 75 percent of them African children -- die of malaria each year. However, malaria is not a zoonotic disease, because the virus depends on a human host for part of its life cycle.

    Over time, viruses can develop the needed "machinery" for efficient transmission not only from the animal host to humans, but from human to human. When this happens, Heeney said, zoonotic illnesses can become serious human killers with potential to reach epidemic proportions.

    While vaccines have eradicated devastating human diseases, such as smallpox, other related viruses, such as monkeypox, could hit people whose smallpox vaccines have expired.

    (Agencies)