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Germs that infect humans

Bacillus anthracis

Bacillus anthracis

© Dennis Kunkel Microscopy, Inc.

Microorganism: the bacterium Bacillus anthracis

Disease: anthrax, or charbon

Occurrence of the disease

History: anthrax has a very interesting history. In the past, this disease was often associated with people who worked with sheep’s wool. Around 1850, more careful research enabled John Bell, an English doctor, to identify the bacterial origin of the disease. The bacterium responsible, Bacillus anthracis, was isolated for the first time by Robert Koch in 1876. In 1881, three researchers including Louis Pasteur demonstrated the possibility of immunization against anthrax using weakened cultures of the bacterium.

Current situation: this disease is most widespread in tropical countries, among persons who work with livestock. Fewer than ten cases per year are reported in the United States.

Mechanism of action of the microorganism: the bacteria can infect the skin, the lungs, and even the intestines. The symptoms of the disease are caused by an exotoxin produced by the bacteria that poisons surrounding tissue.

Anthrax bacteria have a very resistant form known as spores; this enables them to remain alive for many decades in soil or animal products.

Symptoms of the disease: the disease usually develops in a cut or scratch on the skin, which blackens as the infection develops. Symptoms appear such as headache, fever, and nausea. Death occurs in 20% of untreated cases. Pulmonary anthrax can also occur, following inhalation of the anthrax bacterium. Symptoms of pulmonary anthrax are fever and anaphylactic shock; death occurs within 48 hours. If the bacteria are swallowed, nausea and vomiting cause death in 60% of cases.

Incubation period: usually between one and seven days.

Contagious period: person-to-person transmission is very rare. However, soil contaminated with spores of the bacterium can remain infectious for decades.

Hosts: infected animals. Carcasses of infected animals can end up in the soil, thereby shedding anthrax bacteria. This is why B. anthracis can be found in soil in many parts of the world. The bacterium can also be found in products derived from animal carcasses.

Transmission: transmission to humans can occur through direct contact or through an intermediary made from infected animals, such as sheep’s wool. Anthrax can also be spread through soil contaminated by infected animals, or through plant fertilizers that contain the bones of infected animals.

Discoverer of the microorganism: Koch in 1876.

Treatment: some antibiotics such as penicillin G in combination with streptomycin.

Geographical distribution of the microorganism: generally speaking, anthrax is rare in industrialized countries. Regions where the bacterium is considered endemic are found in South and Central America, eastern and southern Europe, in Asia, and in Africa.

Prevention: vaccination of livestock and of workers at risk, such as members of the armed forces.

Vaccine: acellular vaccine. This vaccine does not contain complete bacteria, but antigenic components of the bacterium.