France Galop has confirmed one horse at Cagnes-sur-Mer is being treated for the potentially deadly neurological variant of equine herpes virus.
A further three horses from the same stable have been isolated after spiking a fever, although none has shown any further symptoms associated with EHV-1.
Cagnes is coming towards the end of its traditional winter jumps meeting, with the Flat meeting – popular with many foreign-based trainers – due to start on January 13.
Extra bio-security measures have been introduced by authorities, primarily designed to minimise the amount of contact between horses from different strings.
France Galop has also made available quarantine boxes at Maisons-Laffitte for those trainers wishing to monitor horses that have been in Cagnes ahead of their return home.
EHV-1 struck one barn at Jean-Claude Rouget's yard in Pau back in April, claiming the lives of two horses. Yorkshire-based trainer Kevin Ryan was also forced to shut his yard down temporarily in June after an isolated case of the virus was discovered.