Salmonella enterica serovar Typhi is the bacterium that causes typhoid fever, a systemic illness that can be severe. The bacterium infects humans by establishing itself in the intestinal tract and gallbladder. The resulting infection, also known as enteric fever, can lead to serious health complications if not treated promptly.
The Cause of Typhoid Fever
Salmonella is a genus of bacteria with many varieties, known as serovars, that can cause illness. Salmonella enterica serovar Typhi (S. Typhi) is distinct from the more common non-typhoidal Salmonella serovars, which are a frequent cause of gastroenteritis, or food poisoning. While non-typhoidal strains cause a localized infection in the gut, S. Typhi is unique in its ability to cause a more invasive and systemic illness.
After being ingested, the bacteria penetrate the intestinal wall and are carried throughout the body via the bloodstream. This systemic spread allows the bacteria to infect organs like the liver, spleen, and bone marrow, leading to the full-body illness of typhoid fever. The bacterium possesses specific genetic features, including virulence plasmids and regions called Salmonella pathogenicity islands (SPIs), which enable it to evade the host’s immune system and establish a widespread infection.
Transmission and Contamination Sources
The primary mode of transmission for S. Typhi is the fecal-oral route, where the bacterium is passed in the feces of an infected person and enters a new host through ingestion. This spread is almost exclusively from human to human, as humans are the only known natural host and reservoir for the bacteria.
Common vehicles for transmission include drinking water polluted with untreated sewage or consuming food prepared by an infected person who has not practiced proper hand hygiene. Raw fruits and vegetables irrigated with contaminated water can also harbor the bacteria. In regions with adequate sanitation, transmission is more frequently linked to food handlers, while in areas with poor sanitation, contaminated water is a more common source.
A significant factor in the spread of typhoid fever is the existence of asymptomatic chronic carriers. These are individuals who have recovered from the illness but continue to shed S. Typhi in their feces for a year or longer without showing symptoms. The most famous historical example is Mary Mallon, or “Typhoid Mary,” a cook in the early 1900s who unknowingly transmitted the infection to dozens of people through the meals she prepared.
Progression of Typhoid Fever Symptoms
After an incubation period of 6 to 30 days, the illness progresses through distinct stages if left untreated. The initial phase is characterized by the slow onset of a fever that increases over several days. This is accompanied by general feelings of weakness, headache, muscle aches, and a dry cough.
As the infection advances into the second week, the fever becomes high and sustained, often reaching 103-104°F (39-40°C). Abdominal pain becomes more pronounced, and patients may experience either constipation or diarrhea. A characteristic, though not universal, sign that can appear is the development of “rose spots,” which are small, faint pink spots on the chest and abdomen that last for two to five days.
In the third week, the illness can become severe and lead to serious complications without antibiotic treatment. The individual may become delirious or enter a state of apathy known as the “typhoid state.” Life-threatening complications such as intestinal bleeding or perforation can occur as the bacteria damage the intestinal wall. If the patient survives into the fourth week, the fever and other symptoms will begin to slowly subside, though recovery can be prolonged.
Diagnosis and Medical Treatment
Confirming a case of typhoid fever requires more than a clinical assessment, as its symptoms can overlap with other illnesses. A definitive diagnosis is made through laboratory testing to detect S. Typhi bacteria. The most reliable method involves taking a sample of blood, stool, or bone marrow and placing it in a culture where the bacteria can grow and be identified. A bone marrow culture is considered the most sensitive test for detecting the bacteria.
The primary treatment for typhoid fever is a course of antibiotics. The choice of antibiotic depends on the specific strain and its susceptibility to different drugs. The emergence of antibiotic-resistant strains has made treatment more complex, sometimes requiring alternative medications like azithromycin.
Patients must complete the entire course of antibiotics as prescribed to ensure the infection is fully cleared and to prevent a relapse. Supportive care includes adequate fluid intake to prevent dehydration and eating regular meals to maintain strength. In severe cases, hospitalization for intravenous fluids and antibiotics may be necessary.
Global Impact and Prevention
Typhoid fever disproportionately affects developing regions with inadequate sanitation and limited access to clean water. The disease is endemic, meaning it is consistently present, in many parts of the world, particularly South Asia, sub-Saharan Africa, and parts of Latin America. In 2019, it was estimated that typhoid caused over 9 million illnesses and 110,000 deaths worldwide, with children and adolescents being the most affected.
Preventing typhoid fever relies on a combination of vaccination and safe practices. For individuals traveling to high-risk areas, vaccines are recommended. Two main types are available: an oral, live-attenuated vaccine and an injectable polysaccharide vaccine. More recently, typhoid conjugate vaccines (TCVs) have been recommended for routine use in children living in highly endemic countries.
Beyond vaccination, practicing safe hygiene and food and water habits is essential for prevention. A useful mantra for travelers is “boil it, cook it, peel it, or forget it,” which serves as a reminder to:
- Consume foods that are thoroughly cooked and served hot.
- Eat fruits and vegetables that you have washed and peeled yourself.
- Drink water that has been boiled or is commercially bottled.
- Avoid any food or drink that does not meet these standards.