Anthrax

Anthrax is a disease caused by the spore-forming bacterium  Bacillus anthracis, which can be used as a lethal biological weapon. In the fall of 2001, lethal anthrax bacteria were spread deliberately through the U.S. mail. Twenty-two people became ill, and five died. The perpetrator has not been caught.

Anthrax infects livestock far more often than people, but it can cause three forms of human disease:

• Cutaneous (skin) - infected when the bacterium enters a cut or abrasion on the skin.
• Intestinal - infected after eating contaminated food.
• Inhalation - infected by inhaling the bacterial spores.

Symptoms and Diagnosis

Symptoms of disease vary depending on how the disease was contracted, but symptoms usually occur within 7 days.

Cutaneous: Most (about 95%) anthrax infections occur when the bacterium enters a cut or abrasion on the skin, such as when handling contaminated wool, hides, leather or hair products (especially goat hair) of infected animals. Skin infection begins as a raised itchy bump that resembles an insect bite but within 1-2 days develops into a vesicle and then a painless ulcer, usually 1-3 cm in diameter, with a characteristic black necrotic (dying) area in the center. Lymph glands in the adjacent area may swell. About 20% of untreated cases of cutaneous anthrax will result in death. Deaths are rare with appropriate antimicrobial therapy.

Intestinal: The intestinal disease form of anthrax may follow the consumption of contaminated meat and is characterized by an acute inflammation of the intestinal tract. Initial signs of nausea, loss of appetite, vomiting, fever are followed by abdominal pain, vomiting of blood, and severe diarrhea. Intestinal anthrax results in death in 25% to 60% of cases.

Inhalation: Initial symptoms may resemble a common cold. After several days, the symptoms may progress to severe breathing problems and shock. inhalation anthrax is usually fatal.

Anthrax is diagnosed by isolating B. anthracis from the blood, skin lesions, or respiratory secretions or by measuring specific antibodies in the blood of persons with suspected cases.

The Bacterium

Bacillus anthracis is a bacterium that lives in soil and has developed a survival tactic that allows it to endure for decades under the harshest conditions. An anthrax bacterial cell can transform itself into a spore, a very hardy resting phase, to withstand extreme heat, cold, and drought, without nutrients or air. When environmental conditions are favorable, the spores will germinate into thriving colonies of bacteria. For example, a grazing animal may ingest spores that begin to grow, spread, and eventually kill the animal. The bacteria will form spores in the carcass and then return to the soil to infect other animals in the future.

While its spore form allows the bacteria to survive in any environment, the ability to produce toxins is what makes the bacteria such a potent killer. Together, the hardiness and toxicity of B. anthracis make it a formidable bioterror agent. Its toxin is made of three proteins: protective antigen, edema factor, and lethal factor.

Protective antigen binds to select cells of an infected person or animal and forms a channel that permits edema factor and lethal factor to enter those cells.

Edema factor causes fluid to accumulate at the site of infection. Edema factor can contribute to a fatal build-up of fluid in the cavity surrounding the lungs. It also can inhibit some of the body's immune functions.

Lethal factor also works inside the cell, disrupting a key molecular switch that regulates the cell's functions. Lethal factor can kill infected cells or prevent them from working properly.

Treatment

Antibiotics

If diagnosed early, anthrax is easily treated with antibiotics. Unfortunately, infected people often confuse early symptoms with more common infections and do not seek medical help until severe symptoms appear. By that time, the destructive anthrax toxins have already risen to high levels, making treatment difficult. Antibiotics can kill the bacteria, but antibiotics have no effect on anthrax toxins.

Vaccines

An existing anthrax vaccine is licensed for limited use. The vaccine is currently used to protect members of the military and individuals most at risk for occupational exposure to the bacteria, such as slaughterhouse workers, veterinarians, laboratory workers, and livestock handlers. The vaccine does not contain the whole bacterium; rather, it is made mostly of the anthrax protective antigen protein.

The vaccine is reported to be 93% effective in protecting against anthrax. Mild local reactions occur in 30% of recipients and consist of slight tenderness and redness at the injection site. Severe local reactions are infrequent and consist of extensive swelling of the forearm in addition to the local reaction. Systemic reactions occur in fewer than 0.2% of recipients.

References:

Centers for Disease Control and Prevention, USA.
National Institute of Allergy and Infectious Diseases, USA.