Pathogenicity and virulence of Rhodococcus equi in foals following intratracheal challenge

Twelve foals, between 27 and 83 days old, were infected with 2 strains of Rhodococcus equi by intratracheal administration. Ten of the 12 foals were inoculated with 10 4–10 10 colony forming units (cfu) of ATCC 33701 strain. The other 2 foals were inoculated with 10 9 cfu of a plasmid-cured derivative of the ATCC 33701 strain (ATCC 33701P −). All of the 10 foals challenged with the ATCC 33701 strain showed clinical signs of pulmonary disease within 5–13 days, such as gross lesions associated with acute bronchopneumonia and microscopic lesions associated with granulomatous pneumonia. The two foals challenged with the ATCC 33701P − strain showed neither clinical signs of disease nor gross lesions. Apparently, when lacking plasmid, the virulent Rhodococcus equi lost its pathogenicity.