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Germs that infect humans


Salmonella typhi

Salmonella typhi

© Dennis Kunkel Microscopy, Inc.

Microorganism: the bacterium Salmonella typhi

Disease: typhoid fever

Occurrence of the disease

History: "Typhoid Mary": Between 1896 and 1906, a cook infected with typhoid fever unknowingly infected 28 individuals while preparing their meals. Mary Mallon was a healthy carrier of the disease. In other words, she had no symptoms but could nevertheless transmit the disease. Mary had worked as a cook for seven different families in the New York area. When the 28 cases of typhoid fever were reported from these seven families, Mary was arrested by the Health Department of New York City. She was brought to a hospital where physicians examined her feces and found them to contain the typhoid bacteria. Mary was released but was prohibited from cooking for any other families. However, Mary changed her name and began to work again as a cook. During a period of five years she infected many other victims and was arrested a second time. The police kept her in a secure place until her death in 1938.

Current situation: currently, 17 million cases including 600,000 deaths are reported each year around the world. In the United States, 2,484 cases were reported in 1950 compared to fewer than 500 today. Most cases are from southeast Asia and northeast Africa. In Canada, 82 cases of typhoid fever were reported in 1998.

Mechanism of action of the microorganism: bacteria are taken in by mouth and travel to the intestine. The microorganisms infiltrate the cells lining the intestine and eventually end up in the blood and the liver.

Symptoms of the disease: as suggested by its name, typhoid fever causes fever but it also cause severe headache, abdominal pain, and weight loss.

Incubation period: from three days to a month

Contagious period: the contagious period lasts as long as there are bacteria in the feces, which usually corresponds to the period between a week after the infection begins until recovery (approximately three months after the onset of the infection). However, two to five per cent of infected individuals become chronic carriers.

Hosts: humans and less often domestic animals

Geographical distribution of the microorganism : many African and Asian countries as well as Central and South America

Transmission: spread is mostly through contaminated water. The bacteria are present in the feces of infected individuals until three months after the infection. Moreover, two to five per cent of infected individuals become healthy carriers, which means that they can transmit the disease during their entire life through their feces. These individuals have no symptoms.

Discoverer of the microorganism: Eberth in 1880

Treatment: typhoid fever bacteria are becoming more and more resistant to drugs. However, a drug called ciprofloxacin is still effective.

Prevention: recommended preventive measures include pasteurizing milk, purifying water, isolation of infected individuals, and prohibiting infected individuals from handling food. Regular hand washing and disposal of human feces in sanitary containers protected from flies are also effective prevention measures. In fact, flies can transmit the disease because they often land on human feces.

Vaccine: there are two types of vaccines against typhoid fever. One of them is live and attenuated; it is administered orally and requires a booster seven years later. The other is inactivated and a booster is required after three years.

Vaccination is recommended but not required.