ANTHRAX

What is anthrax?

Anthrax is an acute infectious disease of great historical interest which is making headlines as an agent of biological warfare. It is caused by the spore-forming, rod-shaped bacteria, Bacillus anthracis, which primarily infects herbivores. Humans can acquire anthrax by agricultural or industrial exposure to infected animals or animal products. Human to human transmission is unknown. Human anthrax is not common, however the potential for intentional release of anthrax spores in the environment has raised much concern and publicity.

What are the clinical manifestations?

There are 3 common clinical manifestations of anthrax - cutaneous disease, pulmonary disease from inhalation of anthrax spores, and gastrointestinal disease. Cutaneous anthrax, the most common form, is usually curable. The primary skin lesion is usually a nondescript, painless, pruritic papule that appears three to five days after the introduction of endospores. In 24 to 36 hours, the lesion forms a vesicle that undergoes central necrosis and drying, leaving a characteristic black eschar surrounded by edema and a number of purplish vesicles. The edema is usually more extensive on the head or neck than on the trunk or extremities.A small percentage of cutaneous infections become systemic, and these can be fatal.

Systemic infection resulting from inhalation of the organism has a mortality rate approaching 100 percent, with death usually occurring within a few days after the onset of symptoms. The course of inhalational anthrax is dramatic, from the insidious onset of nonspecific influenza-like symptoms to severe dyspnea, hypotension, and haemorrhage within days of exposure. A rapid decline, culminating in septic shock, respiratory distress, and death within 24 h is not uncommon. The high mortality seen in inhalational anthrax is in part due to delays in diagnosis.In most cases there is no infection in the lungs but the endospores are engulfed by alveolar macrophages and transported by them to the mediastinal and peribronchial lymph nodes, with the spores germinating en route. Anthrax bacilli multiply in the lymph nodes, causing hemorrhagic mediastinitis, and spread throughout the body in the blood. Classic findings on the chest radiograph include widening of the mediastinum and pleural effusions. Pneumonia is less common; key pathologic manifestations include severe haemorrhagic mediastinitis, diffuse haemorrhagic lymphadenitis, and oedema.

Gastrointestinal, oropharyngeal and meningeal anthrax are rare. GI anthrax can present with acute abdomen, malaena, ascites, diarrhoea and shock.

How is the diagnosis made?

Diagnosis requires a high index of suspicion since the initial symptoms are nondescript. Blood culture and serology may make the diagnosis but patients with systemic disease often die before positive blood cultures can be obtained, making early diagnosis and treatment crucial.

What is the treatment?

Treatment involves supportive care in an intensive care facility and high doses of penicillin. Resistance to third-generation cephalosporins has been noted. Ciprofloxacin and doxycycline are other drugs of choice in treatment. Antibiotic therapy should be continued for at least 14 days after symptoms abate. Despite early and vigorous treatment, the prognosis of patients with inhalational, gastrointestinal, or meningeal anthrax remains poor.

Prevention.

Prophylaxis for asymptomatic patients with suspected exposure to anthrax spores can be achieved with a six-week course of doxycycline or ciprofloxacin.

Three anthrax vaccines are commercially available but their comparative effectiveness and safety is not clear. Vaccines have been shown to be effective against aerosolized exposure in animal studies. Killed anthrax vaccines appear to be effective in reducing the risk of contracting anthrax with a relatively low rate of adverse effects. About 30% of recipients may experience a mild flu-like illness and soreness at the injection site, but systemic reactions to the vaccine are rare.

References

Anthrax. Dixon et al NEJM 1999, 341(11): 815-826

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