It was during his retirement that the events for which he became best known took place. Having read of the effectiveness of the myxomatosis virus in dealing with rabbit plagues in Australia, in 1952 Armand-Delille decided to introduce the virus onto his 3 km² private estate of Chateau Maillebois in Eure-et-Loir. He believed that the enclosed nature of the estate would prevent its spread. Inoculating two rabbits with virus acquired from a laboratory in Lausanne, Armand-Delille succeeded in rapidly eradicating the population on his estate, with 98% of the rabbits being dead within 6 weeks. However, within 4 months it became clear that the virus had inevitability escaped from his estate, the corpse of an infected rabbit having been found 50 km away.[1][2]
Within a year of the initial release, an estimated 45% of the wild rabbits of France had died of the disease, along with 35% of domestic rabbits, and the disease had spread to the rest of western Europe, destroying rabbit populations in the Netherlands, Belgium, Italy, Spain, Britain, and beyond. The effect on the rabbit population of France was dramatic. In the hunting season covering the year of the release of the virus, 1952–53, the total number of rabbits killed in 25 hunts exceeded 55 million. The figure for 1956-57 was just 1.3 million, a 98% reduction.
Armand-Delille found himself both condemned by rabbit hunters and showered with praise by farmers and foresters. He was prosecuted, and in January 1955 he was convicted and fined 5,000 francs. However, he was later honored; in June 1956 he was awarded a gold medal to commemorate his achievement by Bernard Dufay, honorary director-general of the French Department of Rivers and Forests. The medal depicts Armand-Delille on one side, and a dead rabbit on the other.
The disease has affected predators dependent on rabbits, in particular the Iberian lynx, a rabbit specialist which is unable to significantly adapt its diet. It is not uncommon for shooters to specifically target infected rabbits, viewing the act as being merciful. However, in 2005 the UK Land Registry conducted a survey of 16,000 hectares of its land and reported that the rabbit population had increased three-fold every two years – likely a product of increasing genetic resistance to the virus.