The genus Salmonella includes thousands of bacterial groups called serovars, which are divided into two main categories: non-typhoidal and typhoidal. Non-typhoidal Salmonella are a frequent cause of food poisoning (gastroenteritis) and can infect a wide range of animals. These infections are localized to the intestines and often resolve without medical treatment.
In contrast, Salmonella enterica serovar Typhi (S. typhi) is a typhoidal serovar adapted specifically to humans, its only natural host. Unlike its non-typhoidal relatives, S. typhi causes a severe, systemic illness that extends beyond the gut. The bacterium has mechanisms to evade the initial inflammatory response in the intestines. Its ability to survive and multiply within host cells allows it to spread throughout the body, leading to a more serious condition.
Causation of Typhoid Fever
Infection with S. typhi bacteria leads to the systemic illness known as typhoid fever. After ingestion, the bacteria penetrate the intestinal wall and are engulfed by macrophages, a type of immune cell. S. typhi multiplies within these cells and spreads through the lymphatic system to the liver, spleen, and bone marrow. This incubation period lasts one to two weeks, during which the infected person may not show any signs of illness.
The onset of symptoms is gradual, beginning with a sustained high fever that can reach 39-40°C (103-104°F). This fever is accompanied by weakness, stomach pain, headache, and a loss of appetite. Some individuals may also experience constipation or, less commonly, diarrhea. As the illness progresses, a rash of small, rose-colored spots may appear on the chest and abdomen.
If typhoid fever is not treated, it can lead to serious complications. The persistent infection can cause inflammation of the small intestine, which may result in intestinal perforation or hemorrhage—holes or bleeding in the gut. Other potential complications include inflammation of the heart muscle (myocarditis) and inflammation of the brain (encephalopathy).
Transmission and High-Risk Areas
S. typhi spreads through the fecal-oral route, meaning the bacteria are passed in an infected person’s feces and ingested by another. This transmission occurs most commonly through consuming food or water contaminated with sewage. Direct person-to-person contact is less common but can happen if personal hygiene, such as handwashing, is inadequate.
A factor in the transmission of typhoid fever is the existence of chronic, asymptomatic carriers. These are individuals who have recovered but continue to harbor S. typhi bacteria in their gallbladder, shedding them in their feces for years without symptoms. These carriers can unknowingly contaminate food and water, spreading the infection. The case of “Typhoid Mary” Mallon, a cook in the early 20th century, is a well-documented example of a carrier who caused multiple outbreaks.
Typhoid fever is most prevalent in parts of the world with inadequate sanitation systems and a lack of access to safe drinking water. High-risk regions include South Asia (particularly India, Pakistan, and Bangladesh), Africa, and parts of Latin America. In these areas, deficiencies in public water treatment and sanitation infrastructure make it easier for the bacteria to contaminate the water supply and food chain.
Diagnosis and Medical Treatment
Diagnosis is based on isolating S. typhi from a patient’s body fluids or stool. The primary diagnostic method is a blood culture, where a blood sample is placed in a medium to encourage bacterial growth, though it can take a few days for results. Bone marrow cultures are more sensitive but are also more invasive and reserved for complex cases. Stool and urine cultures may also be used, but they are more likely to be positive later in the illness.
Treatment for typhoid fever relies on antibiotics to kill the S. typhi bacteria. Patients must complete the entire prescribed course to ensure all bacteria are eliminated from their system. Common antibiotics used include fluoroquinolones like ciprofloxacin for susceptible strains, and third-generation cephalosporins such as ceftriaxone. Azithromycin is also an effective option, particularly in regions with high rates of resistance.
A growing global health concern is the emergence and spread of antibiotic-resistant S. typhi strains. Some strains have become resistant to multiple types of antibiotics, including the most common first-line treatments. This extensive drug resistance (XDR) complicates treatment, forcing doctors to use less common, sometimes more expensive, or less effective antibiotics. The rise of resistant strains makes it harder to treat infections, increases the risk of complications, and underscores the need for ongoing surveillance and responsible antibiotic use.
Prevention and Vaccination
Preventing typhoid fever involves a combination of vaccination and practicing careful food and water safety measures, especially when traveling to high-risk areas. Two main types of vaccines are available to provide protection against S. typhi. One is an injectable conjugate vaccine, which can be given to children from 6 months of age and adults, and provides longer-lasting immunity. Another option is an oral live, attenuated vaccine, which consists of four capsules taken every other day and is suitable for individuals over the age of six. Vaccination is recommended for people traveling to endemic regions.
Beyond vaccination, adhering to strict food and water precautions is a primary defense against infection. A guiding principle is to “boil it, cook it, peel it, or forget it.” This includes taking the following steps:
- Drink only water that has been boiled or is commercially bottled.
- Avoid ice unless it is made from safe water.
- Eat food that is thoroughly cooked and served hot.
- Avoid raw, unpeelable fruits and vegetables.
Good personal hygiene, particularly frequent and thorough handwashing with soap and water, also prevents the spread of the bacteria. This is especially important after using the bathroom and before handling or eating food.