ZOONOSES

Fragile species barrier

There have been two alerts in recent years: outbreaks of new forms of Creutzfeldt-Jakob disease in the late 1990s and of severe acute respiratory syndrome in 2003. Both were casesof an animal pathogen being transmitted to humans. For the most part, it is a serious delusion to think that the famous species barrier can be protected. According to the European Med-Vet-Net network, more than 60% of the 1 400 or so microbes responsible for human infectious diseases may have come from animals.

Vaccination against avian influenza in Djakarta (Indonesia). © FAO/Arif Ariadi
Vaccination against avian influenza in Djakarta (Indonesia). © FAO/Arif Ariadi

Twenty-five years ago, general improvements in hygiene, the invention of antibiotics and the widespread use of vaccination lulled us into believing that the problem of infectious diseases had been resolved, or nearly so. The sudden emergence of AIDS in the early 1980s dampened this euphoria. A new virus that had suddenly migrated from great apes to humans has caused the worst pandemic the world has seen since the 1919 Spanish flu pandemic. For time immemorial, new diseases have emerged as a result of a pathogen passing from its animal reservoir to Homo sapiens. Rabies, West Nile virus and yellow fever are the most typical examples of this. The world ecological crisis (including deforestation and global warming), coupled with globalisation (trade in tropical animals, foodstuffs, tourism and so on), are multiplying the opportunities for new contacts between animals and humans and heightening this age-old threat.

Although zoonoses have been known for more than a century (the term ‘zoonosis' appears in Ernst Wagner's handbook of general pathology, Handbuch der allgemeinen pathologie, published in 1876), these diseases are still a long way from yielding all their secrets. To pass from animal to human, a virus, bacterium or parasite must cross a series of biological barriers before it can multiply on the surface of the human body, colonise its internal environment, multiply there after evading the human immune defence system and, in the worst scenario, be transmitted from person to person. Why are certain pathogens able to effortlessly negotiate these phases, each of which involves multiple modifications to their genetic programme? Nobody knows. We are still struggling to understand how the mysterious infectious prion of bovine spongiform encephalopathy managed to pass to humans without actually causing the feared epidemic.


Campylobacter infections

Advances in molecular genetics allow the problem to be tackled from a new angle. One of the basic research strands of Med-Vet-Net, a network of 300 researchers working to prevent and control zoonoses, concerns campylobacter infections (or campylobacteriosis) and gastro - intestinal infections, which are one of leading causes of bacterial food poisoning in Europe.

Campylobacter jejuni, a subtype of the campylobacter bacterium commonly found in poultry and livestock, is dangerous to humans. More than 100 strains have been listed, but it is almost impossible to determine their pathogenicity on the basis of their DNA gene pool. This has led to a research effort to identify the virulence factors in the bacterium's genome and to learn more about the mechanisms for contamination and foodborne transmission from animals to humans.


Keeping watch on two lists

Meanwhile, the only solution is to maintain close epidemiological surveillance of the health of humans, as well as of domestic and wild animals. Veterinarians, doctors and food safety specialists are all involved. Med-Vet-Net's primary task is to monitor the pathogens thought to cause outbreaks of the zoonoses listed by the European Council and Parliament in late 2003. List A includes eight diseases that are subject to continual surveillance. The most common ones cause foodborne gastrointestinal infections and can be serious in young children and elderly people. Apart from campylobacteriosis, such gastrointestinal infections can be caused by Listeria, Salmonella and certain Escherichia coli bacteria, as well as by a parasitic worm (trichinellosis). A much more serious disease is echinococcosis, which is contracted from eating wild fruit that has been soiled with the excrement of carnivores like foxes. Other diseases, such as brucellosis or tuberculosis caused by Mycobacterium bovis, mainly affect livestock producers and their contacts. List B diseases, which include rabies, West Nile fever and avian influenza, are zoonoses for which surveillance must commence as soon as a case is identified. So as to be at the ready...


Mikhaïl Stein


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