Plasmidial virulence of Rhodococcus equi and its implications of livestock infections for human health: a possible foodborne pathogen?

Rhodococcus equi, a versatile and adapted opportunistic in nature bacterium, infects animals and humans. This soil-borne microorganism widely occurs in farms. Its dissemination occurs by feces from domestic animals (particularly horses, cattle, and pigs). In the last decades, bacterial virulence has been strongly attributed to plasmid-encoded virulence-associated proteins (VAPs). To date, three virulence plasmid types have been recognized: pVAPA, pVAPB, and pVAPN, which are considered host- or livestock-adapted. The pVAPA type is related to equine isolates (horse-type), the pVAPB type is associated with pig isolates (porcine-type), whereas the pVAPN type occurs in domestic ruminants (bovine and caprine) (ruminant-type). Nonetheless, pathogenic R. equi possessing the three virulent plasmid types can infect humans. Inhaling aerosol particles from the environment of equids represents the first route traditionally considered for the transmission of R. equi to humans, although an epidemiological lack of transmission remains in human infections because some patients with rhodococcosis have no history of contact with livestock or their environment on farms. However, all pVAPs types have been found in humans infected by R. equi (predominantly living with HIV), which could be presumably transmitted to patients by the ingestion of contaminated undercooked or raw meat from slaughtered pigs, cattle and, occasionally, horses, representing a probably route of the transmission of the pathogen from livestock-to-humans that could partially explain infections in humans without a history of contact with cattle, pigs, horses, or their farm environments.