Global resurgence in zoonotic viral diseases

Nov 8 2006

Doctors and veterinarians need to work together to tackle the increasing global threat of zoonotic viral diseases spread by non-human vertebrate hosts - including dogs, cattle, chickens and mosquitoes - according to a review in the November issue of Journal of Internal Medicine.

An estimated 50 million people acquired zoonotic diseases between 2000 and 2005 and up to 78,000 have died, reports Dr Jonathan Heeney, Chair of the Department of Virology at the Biomedical Primate Research Centre in the Netherlands.

And the diseases responsible for the majority of zoonotic illnesses, and a third of the deaths in the study period, appear to be increasing. This is particularly worrying because there are no effective vaccines for some of the most common zoonotic viruses.

"For instance 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. The cycle can move into nearby urban areas where it can then be transmitted from person to person by mosquitoes" says Dr Heeney.

"This has been attributed to regional population growth around large cities, increased transportation and failing public control measures."

Recent publicity about the risk of an H5N1 (bird flu) epidemic - which killed just over half of the 145 people infected during the study period - has centered on the risk of human-to-human transmission. It has also stressed the increased risk that humans face from living in close proximity to large concentrations of birds.

"Viral infections with zoonotic potential can become serious killers once they are able to establish the necessary adaptations for efficient human-to-human transmission under conditions sufficient to reach epidemic proportions" says Dr Heeney.

"That is why it is so important for experts from all walks of medicine to work more closely together.

"Vaccines have been very successful at eradicating devastating human disease such as smallpox. But we need to be vigilant and ensure that emerging diseases such as monkeypox don't find immunological niches in generations who are no longer vaccinated against diseases like smallpox.

"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.

"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."

Zoonotic killers between 2000 and 2005 included:

• Rabies, which killed an estimated 30,000 people
• Dengue Virus, which affected 50 million people and killed approximately 25,000
• Japanese Encephalitis Virus, with up to 50,000 estimated cases and up to 15,000 estimated deaths
• Lassa Fever, which affected up to 300,000 people and killed about 5,000
• SARS Corona virus, which killed 774 of the 8102 people infected

Even a zoonotic virus like Yellow Fever - for which an effective vaccine exists - is estimated to affect 200,000 people, according to the World Health Organization.

"It should be stressed that only about a quarter of zoonotic pathogens are readily spread from humans to humans" says Dr Heeney.

"But it is believed that in extreme situations when certain animal viruses transmit to humans they may mutate and adapt to the new host so effectively that they may become almost exclusively spread from humans to humans. This seems to have already happened with measles and the HIV virus.

"Although the number of cases of human bird flu deaths is relatively small when compared with diseases such as Rabies and Dengue, the publicity generated by those deaths has helped to raise awareness of zoonotic diseases.

"We now need to build on that awareness and ensure that the international medical community monitors changes in animal and human health very carefully to ensure that we identify and control any new, emerging and re-emerging zoonotic viruses."