Anses Agence nationale de sécurité sanitaire Alimentation Environnement Travail

Different pathogenic agents are able to cross the species barrier. These zoonoses (animal diseases passed on to humans) may lead to problems ranging from imperceptible infection to a fatal disease.

The risks of humans being infected by pathogenic agents from animals or products of animal origin are of increasing concern. Epidemiological monitoring networks, study and research programmes have been set up to find out more about these diseases, establish their prevalence, identify sources and reservoirs of the infection and develop suitable prevention systems. The emergence of diseases such as West Nile virus, which have recently emerged or resurfaced in France, are proof of the interest in promoting research and epidemiological monitoring in this field.

Assess the risks of new diseases emerging

While the risks of transmission to humans are clearly defined for some viruses, for others, this situation is hypothetical but cannot be completely dismissed. The emergence of new infectious diseases is mainly the result of a change in interaction between a pathogenic agent and its host. This change corresponds to modifications in farming methods, marketing, tourism or the use of medicinal products of animal origin. For a zoonosis to be identified as such, either the animal infection has to be sufficiently widespread or the human disease has to correspond to identifiable symptoms developing in an acute manner. In this case, only the number of incidents of a disease can establish a link between the human and animal infection.
Risk assessment is carried out by comparing strains isolated in humans and animals and by setting up serological surveys. These approaches do have their limits, however. The comparison of human and animal strains rarely leads to a univocal conclusion of the transmissibility risks. Moreover, serological surveys provide information on an often previous circulation of similar viruses in humans and animals, but these may have different biotypes. Lastly, risk assessment is impossible or too late in the case of emerging diseases.

Specific conditions for crossing the species barrier

New pathogenic agents are regularly identified in different animal species or present modifications in their tissues or in the species affected. Of course, these modifications may concern humans. However, for a virus to be passed on from animal to human, the animal must first excrete this virus in sufficient quantity. Newcastle disease, for example, has always been a rare zoonosis in humans, while it is relatively common in animals. Humans must also be in close proximity to be contaminated by a sufficient concentration of the pathogenic agent. Lastly, the immune response of the contaminated individual must be insufficient. The notion of the species barrier therefore covers both epidemiological mechanisms defining a probability of exposure to the risk and molecular mechanisms (receptiveness, phylogenic proximity, mutations) limiting the replication of the virus in a different host. Although these mechanisms generally prove effective, they can nevertheless be crossed under certain conditions involving modifications in ecosystems and the selection of new variants capable of multiplying in certain hosts. Analysis of the mechanisms causing these barrier crossings may prove to be very useful in providing a prospective definition of at risk epidemiological situations or viruses.