Salmonella typhi is a bacteria that is only known to cause disease in humans (i.e., typhoid fever).  Typhoid fever is uncommon in the United States (less than 300 cases per year) due to improved food-handling and treatement of drinking water.  Worldwide, however, typhoid fever affects millions of people each year and can be life-threatening.  Symptoms of typhoid fever include severe diarrhea, abdominal pain, and fever.  Typhoid fever can be treated with antibioitics.

Diseases

Typhoid fever (a.k.a. - enteric fever) - causes severe abdominal pain, fever, diarrhea, rash (in some)

Appearance (under a microscope)

Gram-negative, non–spore-forming, facultatively anaerobic bacilli (like pink hot dogs)

Habitat & Transmission

• Humans are the only known source
• Transmitted by close contact with an infected individual
• Also can be transmitted by stool-contaminated food or water

Weapons/Defenses

• Has developed resistance to commonly-used antibiotics (such as chloramphenicol, ampicillin, and trimethoprim)
• Fimbriae - string-like projections that help with attachment to intestinal wall before invasion of cells
• Inject proteins into intestinal cells, instructing them to allow entry of the bacteria
• Once inside intestinal cells, they trigger an inflammatory response
• Able to survive and replicate inside macrophages
• Can cause a symptom-free carrier state in some people - carriers who work as food preparers can spread the illness to many people

Weaknesses

• Eventually, the immune system can control infection.  T-cells and B-cell antibodies play a major role in defeating Salmonella typhi.
• A vaccine is available and is often used for people traveling to areas where typhoid fever is common
• Sensitive to antibiotics such as the fluoroquinolones, third-generation cephalosporins (i.e., rocephin), and azithromycin

Notes

• Typhoid fever causes an estimated 16 million cases worldwide and 600,000 deaths each year (mostly in developing countries)
• Before antibiotics were discovered, approximately 15% of patients with typhoid fever died.  Now with antibiotic treatment, approximately 0.4% of persons with typhoid fever in the United States will die from the illness.
• Time from exposure to illness - 7 to 14 days
• Duration of illness - 4 weeks (without antibiotics)

Photo 1 - Salmonella typhi bacteria (cause typhoid fever). United States Center for Disease Control (CDC)12/18/2006.  Used with permisssion.

Photo 2 - Mary Mallon (1870-1938) was nicknamed "Typhoid Mary".  This is an illustration that appeared in 1909 in The New York American.