Mechanisms of viral emergence

Domingo, Esteban

Virus dissemination in a new host species occurs when the viral fitness in the recipient host is such that transmission from one infected individual into susceptible individuals of the same species attains a minimum required value. Here, again, several interconnected factors can play a role: the viral load achieved in the infected donor, the amount of infectious particles shed by the donor, the amount of virus in the different secretions and excretions (saliva, blood, semen, feces, etc.), transmission routes (contact, aerosol, blood, sexual, etc.) [22, 53] , the stability of the virus particles, etc. Thus, many influences that depend on the genetic make up of the virus and also on the nature of the virus-host relationship affect virus transmissibility. The capacity of expansion in the new host species is often expressed by a reproductive ratio of R 0 > 1, that quantitates the average number of infected contacts produced by each infected host [62, 63] . Some viruses, such as the SARS coronavirus, did not reach an average R 0 > 1 once introduced in the human population, preventing their continued expansion. Because of its epidemiologic parameters, SARS could be controlled by public health measures. In contrast, other viruses such as HIV-1 or the rhinoviruses are, unfortunately, highly efficient in sustained human-to-human transmission. In the case of HIV-1, asymptomatic donor individuals are capable of transmitting the virus to multiple recipient individuals [44] . In this case, a change of social habits (including sexual promiscuity, and travel) have contributed to the dissemination of HIV-1 in the human population. Obviously R 0 is not Vet. Res. (2010) 41:38 invariant, and may be modified by genetic change of the virus.