From Rats To Humans: Around Thirty Europeans Infected With Cowpox Virus By Their Pet Rats

This is the first time that transmission of this type of virus, cowpox, from pet rats to humans has been described. This new disease underlines the risks of zoonoses (animal diseases that can be transmitted to humans) linked to the adoption of new types of pet.

In early 2009, four cases of cutaneous infection were reported in northern France. The patients had blackish lesions on their skin, which led to their initially being diagnosed as rickettsioses, which are diseases caused by intracellular pathogenic bacteria called rickettsia. In order to confirm this hypothesis, samples were sent for testing to the national center of reference for rickettsioses, the Joint Research Unit for Emerging Infectious and Tropical Diseases in Marseille, directed by Didier Raoult. The researchers' conclusion was unequivocal: this was not a case of rickettsiosis.

Their investigation did not stop there. Raoult and his team contacted their virologist colleagues at the Emerging Viruses Unit at La Timone hospital. The virologists suspected that this was a case of infection by the cowpox virus, which, astonishingly, was rapidly confirmed. The viruses were observed by means of electron microscopy, and molecular analysis revealed that all the cases were caused by the same strain of cowpox virus.

The virus is endemic in western Europe, including France, in wild rodents, which make up its main reservoir. It belongs to the same family (Poxviridae) as the now extinct smallpox virus, and is potentially pathogenic in humans. It becomes apparent as a viral infection after a period of incubation of about one week. The cutaneous lesion heals spontaneously after six weeks. It is therefore not serious for humans, except for immunodeficient individuals such as the elderly or people who have received transplants, in whom the infection can spread.

Although cowpox infection is rare in humans, it is not unheard of. Sporadic cases have regularly been reported in Europe since 2002. However, until now, only a few cases of infection had been described, all of which were transmitted in one of two ways: either as a result of handling infected wild rodents, or by being scratched by a cat which had itself become infected by a wild rodent.  Since none of the patients with cutaneous lesions had a cat, this type of transmission was ruled out. However, all of them had recently adopted a rat as a pet. This is thus the first time that transmission of cowpox virus from pet rats to humans has been observed. 

Changing human practices, such as the adoption of new types of pet, can therefore create conditions that are favorable to the emergence of new animal diseases that can be transmitted to humans. This work also underlines the importance of setting up a diagnostic capability at national level for the rapid identification of emerging pathogens.