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 where cases of "pigeon fever" and "ulcerative lymphadenitis" have been recorded.
This disease is spread between infected animals by vectors and through contamination of the environment with exudate from abscesses. Major bacterial spread occurs when superficial abscesses are broken open and release discharge that is then contacted by other animals during grooming, or contaminates feed, water, and bedding that other animals consume. These abscesses may break open spontaneously or be broken open on surfaces or during shearing. C. pseudotuberculosis can survive in soil for up to 8 months and contaminate bedding and indoor handling facilities for several weeks. In horses, arthropod vectors are considered a significant source of infection. As vector patterns change with warming temperatures, C. pseudotuberculosis in horses is re-emerging in the United States.
This bacterial species has caused occasional cases of infection in people who work closely with infected small ruminants, resulting in similar swellings of the lymph nodes in the neck and groin. The most likely route of infection is direct contact with the infected animal or its raw products. Therefore, mostly farm workers and animal health workers are at risk.
Cellular morphology, biochemistry, and identification
C. pseudotuberculosis is a Gram-positive bacterium that can be either coccoid or filamentous rods, which can be organized into palisades. Metachromatic volutin granules containing phosphate can be seen in the rod form, but not the coccoid form when stained using Albert's or Neisser's methods. Other characteristics of this bacterium include being nonsporulating, noncapsulated, and possessing fimbrae, but it is immobile.