Rhodococcus Flashcards
Rhodococcus equi (formerly Corynebacterium equi)
Describe the morphology.
Describe what this organism would look like on culture.
Rather large rod with considerable pleomorphism ranging to cocci.
Cocci are more commonly seen in cultures growing on agar media and rods are more commonly observed in cultures growing in broth. Almost always heavily encapsulated.
Good on solid media. Colonies are quite distinct, moist, mucoid, flowing; where colonies coalesce they can become quite large (1cm). Salmon pink color often develops as the colonies age. Non hemolytic.
What is the general pathogenicity of Rhodococcus equi?
Causes purulent bronchopneumonia, enteritis and arthritis in foals.
Also a pulmonary infection in humans with AIDS.
Describe the what the culture plates of R. equi would look like.
Colonies are quite distinct, moist, mucoid, flowing; where colonies coalesce they can become quite large (1cm). Salmon pink color often develops as the colonies age. Non hemolytic.
What are some biochemical tests that will help you identify this organism?
Ferments no carbohydrates, urease pos (variable).
CAMP-like phenomenon is very strong and the clearing extends to the opposite side of the Staph streak.
Name the organism and the test.
R. equi
CAMP-test
Note: It is very strong and the clearing extends to the opposite side of the Staph streak
What are the specific antigens associated with R. equi?
Possesses a species specific antigen, with at least 7 capsular antigen types.
Virulence-associated protein A (VapA) is thought to be an important immunogen.
What is the natural habitat for R. equi?
Common commensal of the horse. Found in the GI tract, especially of younger animals.
Grows in soil and is commonly found in the soil of equine pastures and paddocks.
How resistant is R. equi to environmental stresses?
Seems to be reasonably resistant for a vegetative organism.
Apparently survives in dust for long periods although it is not actively growing in the dust.
Dust seems to be important in transmission of the agent to the foal respiratory tract
How is disease manifested in horses by R. equi?
Suppurative bronchopneumonia in foals; often secondary; see frequently in CID arabian foals.
Usually see in foals 2-5 months of age.
Mortality is usually high but not directly due R. equi.
Also see internal abscesses, pleurisy, metritis in mares, and ulcerative enterocolitis which may become chronic with abscesses in the mesenteric lymph nodes.
This is an example of a lesion caused by what organism?
R. equi
Explain the importance of gamma interferon and how it pertains to the immune system in a Rhodococcus equi infection.
In a murine model of disease, bacterial killing is dependent upon the presence of gamma interferon which activates macrophages to produce reactive nitrogen and oxygen intermediates.
These two radicals combine to form peroxynitrite, which kills R. equi.
Mice deficient in either NO synthase or the oxidative burst are more susceptible to lethal infection and have higher numbers of organisms in their tissues.
True/False: Rhodococcus equi can be found in pigs.
If so where in the pig?
TRUE
Frequently isolated from lymph nodes, thought to be the cause of abscesses, may also be secondary to tuberculosis
Are there any immunizations against R. equi?
What is the immunity like for animals having already been exposed to R. equi?
None available commercially but a killed bacterin for mares has been used to hyperimmunize mares. Bacterins based on the VapA protein might give better results. Plasma from hyperimmunized mares is commercially available. Immunization of foals has led to increased disease in some studies.
Immunity is poor, mares may carry it as a commensal organism for life.
What is the treatment for R. equi?
Erythromycin + rifampin used to be the standard treatment, however; long-acting macrolides such as azithromycin have been used to replace the erythromycin and give superior results.
Treatment needs to be initiated early in the disease in order to be successful.
