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Respiratory syncytial virus (RSV) is the principal agent that causes bronchiolitis in infants (human RSV) and pneumonia in calves (bovine RSV). At present, no vaccine is available for use in humans, and those designed for cattle are little effective in the field. However, this is a particularly contagious disorder that affects almost all children before they are two, and represents the leading cause of hospitalisation among infants aged less than 6 months. In cattle units, this virus is responsible for more than 60% of the respiratory diseases observed in dairy herds, and up to 70% in suckler herds (meat production). These viruses therefore constitute major human and animal health issues.

INRA scientists, working with a team from AP-HP and the University of Versailles-Saint-Quentin-en-Yvelines, have developed a new method that enables the study over several days of replication of this virus in living mice. The first step in their work consisted in inserting a gene expressing a luminescent protein (luciferase) in the viral genome. After being infected by the virus, the mice received an intranasal dose of luciferin. This substance reacts with luciferase to produce bioluminescence that can be detected using an ultra-sensitive camera. The scientists were thus able to monitor development of the infection in the nose and lungs, with the intensity of the bioluminescent signal indicating the degree of viral replication.

As a second stage, the scientists administered an antiviral agent to the mice. Elimination of the virus resulted in a weakening luminous signal that disappeared after a few days. 

The evaluation of candidate vaccines or antiviral agents is based on animal models, the most widely used being the mouse. However, RSV does not make the mice "sick", and measuring viral replication requires the euthanasia of numerous animals and recourse to laborious and costly techniques. This new method represents a major step forwards regarding the in vivo study of how antiviral agents act on Respiratory Syncytial Virus. It is also relatively easy to implement, which will contribute to accelerating the search for solutions to control this virus.