Corynebacterium pseudotuberculosis is a Gram-positive bacterium known globally to infect ruminants, horses, and rarely people. This bacterium is a facultative anaerobic organism that is catalase-positive and capable of beta-hemolysis. In small ruminants, C. pseudotuberculosis causes a disease called caseous lymphadenitis characterized by pyogranulomatous abscess formation. In general, this bacterium causes lesions of the skin, lymph nodes, and internal organs. A disease known as ulcerative lymphagenitis can also result from infection with C. pseudotuberculosis in the distal limbs of horses. This bacterium uses the virulence factors phospholipase D and mycolic acid to damage eukaryotic cell walls and resist phagocytic lysosomal degradation, respectively. Infection with this bacterium is often confirmed by bacterial culture of the purulent exudate. Once the diagnosis has been made, treatment of the infection can begin, but this is difficult due to the nature of the organism and the lesions it forms. Specifically, C. pseudotuberculosis is intrinsically resistant to streptomycin, with varying resistance to penicillin and neomycin depending on the strain. It has been shown to be susceptible to ampicillin, gentamicin, tetracycline, lincomycin, and chloramphenicol. Vaccines have also been produced to develop acquired immunity to this infection. History, epidemiology and zoonotic risk The first isolation of C. pseudotuberculosis came from a cow lymph node in 1888, by French bacteriologist Edmond Nocard. Shortly after, it was sampled from abscesses in a sheep by , and the bacterium was named the "Preisz–Nocard" bacillus until further work by German microbiologists in the mid-1900s, when it was recategorized into the Corynebacterium genus. It was finally renamed Corynebacterium pseudotuberculosis in 1948, to reflect that the clinical disease signs were similar to disease caused by M. tuberculosis species. Distribution of C. pseudotuberculosis is mostly traced by examining prevalence of caseous lymphadenitis, the bacterium's main pathological disease. Infection of domesticated sheep and goats has been found across the globe. Although few recent studies have been conducted into its prevalence, data from slaughterhouses in Australia in the late 1980s suggested that C. pseudotuberculosis was affecting 50–60% of sheep. Development of a vaccine was completed in 1983, and it was added to the recommended clostridial vaccines for sheep. The national average of prevalence in Australia was 5.2% in 2009, although this varies by region. Prevalence in Canadian sheep, mainly based on numbers from Alberta and Quebec abattoirs, has been reported between 8 and 36%. C. pseudotuberculosis also causes disease in horses, and should be considered prevalent in areas