Deadly viruses mutating to infect humans at rate never seen before

By Mark Henderson

Our correspondent reports from the meeting in St Louis of the world's largest general scientific study

NEW DISEASES

AT least one new disease is jumping the species barrier from animals to human beings every year, exposing people to emerging germs at a rate that may be unprecedented.

The first work to catalogue the range of germs capable of infecting people has disclosed that 38 new human pathogens have emerged in the past 25 years. Three quarters of these, including Aids, avian flu, Sars and new variant CJD, originated as animal diseases.

HIV is thought to have started as a monkey or ape virus in Africa

The survey, led by Mark Woolhouse, of the University of Edinburgh, has identified more than 1,400 pathogens that can cause disease in human beings, at least 800 of which crossed the species barrier from animals.

While it is not known whether the rate at which diseases are jumping species is accelerating, Dr Woolhouse said it was impossible that human beings had been exposed to so many new pathogens so quickly through most of history.

Changes in human behaviour and the environment, such as bushmeat hunting, intensive agriculture, the ease of long-distance travel and global warming, were all likely to be helping animal germs to acquire the ability to infect people.

�The rate of accumulation we are seeing now is too fast to be supported over an evolutionary timescale,� he told the American Association for the Advancement of Science conference in St Louis yesterday. �We would be overrun with pathogens.

�Either many of these pathogens will not persist in humans, or there is something very unusual about the present time. The most obvious explanation is the pace and scale of change in the ways humans interact with their environment, providing new opportunities for humans to be exposed to and to transmit novel pathogens.�

The deadliest example of a germ that has recently crossed from animals to human beings is HIV, which is thought to have started out as a monkey or ape virus in Africa. Other conditions with an animal origin include the Ebola and Marburg haemorrhagic fevers, the coronavirus that causes Sars, the West Nile virus that is now endemic in the United States and H5N1 avian flu.

The class of pathogen most responsible for new human infections is the RNA viruses, which include HIV and influenza. These seem particularly adept at jumping species because they have small genomes that mutate easily, allowing them to adapt to new hosts.

The animal reservoir of new diseases with the potential to infect human beings requires a new approach to medicine, in which its human and veterinary branches become more closely allied to detect and fight the pathogens that present a threat, Dr Woolhouse said.

�We have to recognise that there is really only one medicine,� he said. �Pathogens don�t distinguish between humans and animals, but we do with our distinction between medicine and veterinary medicine. We can�t afford to go on like that. Vets and medics need to work together.

�This has to start with students. Veterinary students need to learn about the public health implications of their field of study, and medics really need to get their act together.

�In the medical textbooks, the discussion of any disease always starts with the first human case. No it didn�t. It was probably in animals first. We need to exploit this knowledge in the veterinary field if we are to have the best chance of containing new pathogens.�

His call was supported by Nina Marano, of the US Centres for Disease Control and Prevention, Atlanta. �We have to bring public health and veterinary medicine closer,� she said. �Eleven of the 12 top-category bioterror agents are of zoonotic (animal) origin, and we are seeing the spread of highly pathogenic H5N1 flu.�