Nocardia spp. are strictly aerobic, ubiquitous soil-dwelling organisms that are largely responsible for the decomposition of organic plant material. Infection usually occurs via inhalation of these organisms in airborne dust particles, leading to pulmonary disease.
The increasingly relative importance of the atypical mycobacteria, many of which are ubiquitous in the environment, was recognized with the decline in tuberculous disease. Generally, atypical mycobacteria are unusual causes of disease in patients who are immunocompetent but can in immunocompromised hosts such as AIDS and cancer patients.
Although not a common problem in the United States, it is in other parts of the world. With the advent of effective antimicrobial agents, the number of cases of leprosy worldwide has fallen from 12 million in 1982 to 6 million in 1991. It remains a significant problem, however, because the incidence of new cases has not yet declined, and much of the affected population lives in areas where effective medical treatment is difficult to obtain.
Mavium is the most common atypical mycobacterium to cause disease in humans. In immunocompetent patients, M avium can cause pulmonary disease (Box 1). It is the most common pulmonary pathogen of all the atypical mycobacteria. There are several risk factors for pulmonary M avium infection besides AIDS.
Approximately one-third of the world’s population is infected with M tuberculosis, according to World Health Organization estimates, resulting in 2.9 million annual deaths. In the United States, tuberculosis is on the rise, after several decades of steady decline.
Symptoms of infection consist of fatigue, weight loss, fever, night sweats, and a productive cough. Most children who are infected with TB usually have no symptoms.
Brucellosis (also called undulant fever, Mediterranean fever, Malta fever) is an infection that causes abortion in domestic animals. It is caused by one of six species of Brucella coccobacilli. It may occasionally be transmitted to humans, in whom the disease could be acute or chronic with ongoing fever and constitutional symptoms without localized findings.
Pasteurella multocida has been recovered from cultures of specimens from the nasopharynx and the gastrointestinal tract of a large number of asymptomatic wild and domestic animals. The highest carriage rates occur in cats (50%-90%), dogs or swine (50%), and rats (15%). Infections are usually preceded by a cat or dog bite or scratch on an extremity.
Conditions that are associated with increased risk for Yersinia spp. infections (yersiniosis) include iron overload states (such as in patients who receive chronic blood transfusions or those with hemochromatosis) and the use of desferrioxamine, a bacterial siderophore. Infections caused by Y enterocolitica are more common in children than adults.
The genus Yersinia, named after Alexander Yersin (1863-1943), includes Y pestis, Y enterocolitica, and Y pseudotuberculosis. Y pestis is the cause of plague, a disease that has left its mark on human history since the medieval time. Y enterocolitica and Y pseudotuberculosis also cause mesenteric lymphadenitis. Plague occurs worldwide.