Clinical microbiologists are sentinels for recognition of a bioterrorist event, especially involving microorganisms such as B. anthracis. Even though this organism is rarely found, sentinel laboratory protocols require ruling out the possibility of anthrax before reporting any blood, CSF, or wound cultures in which a large gram-positive aerobic rod is isolated. During the 2001 terrorist attacks on the United States, the index case associated with the anthrax distribution was discovered by an astute clinical microbiologist who identified large gram-positive rods in a patient’s cerebrospinal fluid. B. anthracis should be suspected if typical nonhemolytic “Medusa head” or ground glass colonies are observed on 5% sheep blood agar. The Red Line Alert Test (Tetracore, Inc., Gaithers burg, Maryland) is a Food and Drug Administration (FDA)-cleared immunochromatographic test that presumptively identifies B. anthracis from blood agar (Figure 1). The sentinel laboratory anthrax protocol was revised in 2005 and again in 2010 to use FDA-cleared tests in order to rule out nonhemolytic, nonmotile Bacillus spp. as potential isolates of B. anthracis.

Fig1. Red Line Alert Test. A red line appears on the cassette  if the culture isolate is presumptive Bacillus anthracis. (Courtesy  Tetracore, Inc., Gaithersburg, Maryland.)

Epidemiology

 Anthrax remains the most widely recognized bacillus in clinical microbiology laboratories. It is primarily a disease of wild and domestic animals including sheep, goats, horses, and cattle. The decline in animal and human infections is a result of the development of veterinary and human vaccines as well as improvements in industrial applications for handing and importing animal products. The organism is normally found in the soil and primarily causes disease in herbivores. Humans acquire infections when inoculated with the spores, either by traumatic introduction, ingestion, or inhalation during exposure to contaminated animal products, such as hides (Table 1). Bacillus anthracis produces endospores, which are highly resistant to heat and desiccation. The spores remain viable in a dormant state until they are deposited in a suitable environment for growth, including moisture, temperature, oxygenation, and nutrient availability. Because of the ability to survive harsh environments, infectiousness, ease of aerosol dissemination, and high mortality rate, the spores may be effectively used as an agent of biologic warfare .

Table1.  Epidemiology

Pathogenesis and Spectrum of Disease

 B. anthracis is the most highly virulent species for humans and is the causative agent of anthrax. The three forms of disease are cutaneous, gastrointestinal (ingestion), and pulmonary (inhalation) or woolsorters’ disease (Table 2). The cutaneous form accounts for most human infections and is associated with contact with infected animal products. Infection results from close contact and inoculation of endospores through a break in the skin. Following inoculation and incubation period of approximately 2 to 6 days in most cases, a small papule appears that progresses to a ring of vesicles. The vesicles then develop into an ulceration. The typical presentation is of a black, necrotic lesion known as an eschar. The mortality rate for untreated cutaneous anthrax is low, approximately 1%.

Table2. Pathogenesis and Spectrum of Disease

Ingestion anthrax results from ingestion of spores and is presented in two forms: oral or oropharyngeal with the lesion in the buccal cavity, on the tongue, tonsils, or pharyngeal mucosa and gastrointestinal anthrax with the lesions developing anywhere in the gastrointestinal tract. Oropharyngeal symptoms may include sore throat, lymphadenopathy, and edema of the throat and chest.

The initial symptoms on gastrointestinal anthrax may be nonspecific with progression to abdominal pain, bloody diarrhea, and hematemesis. The mortality rate is much higher than that of cutaneous anthrax and usually attributed to toxemia and sepsis.

Pulmonary (inhalation) anthrax is due to inhalation of the spores. The endospores are ingested by macro phages and taken to the lymph nodes where the infection develops into a systemic infection. The disease develops from flulike symptoms to respiratory distress, edema, cyanosis, shock, and death. Patients typically demonstrate abnormal chest x-rays with pleural effusion, infiltrates, and mediastinal widening. Woolsorters’ disease and ragpickers’ disease are used to describe respiratory infections that result from exposure to endospores during the handling of animal hides, hair, or fibers and other animal products.

Complications often follow all three forms of anthrax disease. Patients often develop meningitis within 6 days after exposure. Recovery results in long-term immunity to subsequent infections.

Virulence is attributed to the production of anthrax toxin. The toxin consists of three proteins. One of these proteins, protective antigen (PA), facilitates the transport of the other two proteins into the cell. Edema factor, EF, is responsible for edema, whereas lethal factor, LF, is primarily responsible for death.

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