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Salmonella Infections

by Matthew E. Levison, MD

Salmonella infections are caused by the gram-negative bacteria Salmonella.

  • People are usually infected when they eat contaminated food, such as undercooked chicken or eggs.

  • The bacteria usually infect the digestive tract but can travel through the bloodstream and infect other parts of the body.

  • People have nausea and crampy abdominal pain, followed by watery diarrhea, fever, and vomiting.

  • Identifying the bacteria in a sample, usually of stool, confirms the diagnosis.

  • Lost fluids are replaced, and antibiotics are usually not helpful for salmonellal intestinal infections.

Two species of Salmonella cause typhoid fever: Salmonella typhi and Salmonella paratyphi. Salmonella typhi resides only in people. Salmonella paratyphi resides mainly in people but sometimes in domestic animals. Other species of Salmonella normally reside in the digestive tract of many wild and domestic animals, such as cattle, sheep, pigs, fowl, and reptiles (including snakes, lizards, and turtles). Many of these can cause infections in people. One species, Salmonella enteritidis, causes most Salmonella infections in the United States.

Salmonella bacteria are excreted in the feces of infected animals and people, leading to contamination. In the United States during the 1970s, many infections were spread by pet turtles, so their sale was prohibited, resulting in fewer infections. Recently, the legal and illegal sale of pet reptiles has increased. Up to 90% of pet reptiles are infected with Salmonella.

People are infected usually by eating undercooked poultry or eggs but sometimes by eating undercooked beef and pork, unpasteurized dairy products, or contaminated seafood or fresh produce. Salmonella bacteria can infect the ovaries of hens and thus infect the egg before the egg is laid. Other foods may be contaminated by animal feces (for example, in slaughterhouses) or by infected food handlers who do not adequately wash their hands after using a toilet.

Because stomach acid tends to destroy Salmonella, a large number of these bacteria must be consumed for infection to develop, unless people have a deficiency of stomach acid. Such a deficiency may occur in children under 1 year old, in older people, and in people taking antacids or drugs that inhibit stomach acid production, including histamine-2 (H 2 ) blockers (such as ranitidine) or proton pump inhibitors (such as omeprazole).

The bacteria cause inflammation of the intestine (gastroenteritis) and thus are a common cause of diarrhea. Sometimes the bacteria enter the bloodstream (causing bacteremia) and spread, causing infections or collections of pus (abscesses) at distant sites, such as the bones, joints, urinary tract, and lungs. Bacteria may collect and cause infection on artificial (prosthetic) joints or heart valves, on a blood vessel graft, or on tumors. The lining of arteries, usually the aorta (the largest artery in the body), may be infected. Abscesses and infected arteries can cause chronic bacteremia. The infection is more likely to spread through the bloodstream in the following:

  • Infants

  • Older people

  • People with disorders that affect red blood cells such as sickle cell anemia or malaria

  • People with a disorder that weakens the immune system, such as human immunodeficiency virus (HIV) infection

  • People who take drugs that suppress the immune system, such as those used to treat cancer or prevent rejection of an organ transplant


When the intestine is infected, symptoms usually start 12 to 48 hours after the bacteria are ingested. Nausea and crampy abdominal pain occur, soon followed by watery diarrhea, fever, and vomiting. Symptoms resolve within a week. Long after symptoms are gone, a few people continue to excrete the bacteria in their stool. Such people are called carriers.

Up to about 30% of adults develop reactive arthritis weeks to months after diarrhea stops. This disorder causes pain and swelling, usually in the hips, knees, and Achilles tendon (which connects the heel bone and calf muscle).

Other symptoms may develop if bacteremia develops and infection spreads. For example, if a bone is infected, the area over it is often tender or painful. If a heart valve is infected, people may feel short of breath. If the aorta is infected, the back and abdomen may be painful.

Did You Know...

  • In the United States, up to 90% of pet reptiles are infected with Salmonella.


Doctors take a sample of stool, pus, or blood or use a swab to obtain a sample from the rectum. The sample is sent to a laboratory where bacteria can be grown (cultured). Identifying the bacteria in the sample confirms the diagnosis.


The intestinal infection is treated with fluids given by mouth or, for severe infection, intravenously.

Antibiotics do not shorten recovery time and may result in bacteria being excreted in the stool longer. Therefore, antibiotics are usually not given. However, people at risk of bacteremia and people with implanted devices or materials (such as an artificial joint or heart valve or a blood vessel graft) are given antibiotics. They may be given ciprofloxacin or azithromycin for 1 week. Antibiotics are given to people who continue to excrete the bacteria in the stool after symptoms are gone.

People with bacteremia are given ciprofloxacin or ceftriaxone intravenously for 2 weeks.

Abscesses are drained surgically, and antibiotics are given for 4 weeks.

If the aorta, a heart valve, or other areas (such as joints) are infected, surgery is usually required, and antibiotics are given for weeks or months.

Typhoid Fever

Typhoid fever (enteric fever) is caused by the bacteria Salmonella typhi or the related bacteria, Salmonella paratyphi.

  • Typhoid fever can be spread by consuming food or water contaminated with the stool or urine of an infected person.

  • People have flu-like symptoms, sometimes followed by delirium, cough, exhaustion, occasionally rash, and diarrhea.

  • Samples of blood, stool, other body fluids, or tissues are sent to a laboratory to grow (culture) the bacteria.

  • To prevent infection, people traveling to areas where typhoid fever is common should be vaccinated and, when there, should avoid certain foods and not drink unbottled water.

  • Infected people with or without symptoms are treated with antibiotics.

Typhoid fever is common in developing countries (mostly in the Indian subcontinent, the Philippines, and Latin America) where sanitary conditions are poor. Most cases in the United States are acquired while traveling in these countries.

Salmonella typhi is present only in people. People who are infected excrete the bacteria in stool and, rarely, in urine. A few infected people develop chronic infection of the gallbladder or urinary tract. They continue to excrete the bacteria in stool or urine, even though they no longer have any symptoms. Such people are called carriers. Thus, they do not know they can spread the infection. During the early 20th century, one such woman, a cook named Mary Mallon, spread typhoid fever to many people and became known as Typhoid Mary.

The bacteria may contaminate food or drink when hands are inadequately washed after defecation or urination. Water supplies may be contaminated when sewage is inadequately treated. Flies may spread the bacteria directly from stool to food.

Like all Salmonella bacteria, many of these bacteria must be consumed for infection to develop, unless the immune system is impaired or people have a deficiency of stomach acid.

The bacteria spread from the digestive tract to the bloodstream (causing bacteremia) and may infect distant organs such as the liver, spleen, gallbladder, lungs (causing pneumonia), joints (causing infectious arthritis), kidneys (causing pyelonephritis), heart valves (causing endocarditis), genital tract, the space within the tissues covering the brain and spinal cord (causing meningitis), and bone (causing osteomyelitis).

Did You Know...

  • Mary Mallon, so-called Typhoid Mary, was a cook who spread typhoid fever to many people during the early 20th century.


Typically, a flu-like illness begins about 8 to 14 days (up to 30 days) after infection. People may have a fever, headache, muscle and joint pains, abdominal pains, and a dry cough. They may lose their appetite. After a few days, the temperature peaks at about 103° to 104° F (39° to 40° C). Often the heartbeat is slow, and people feel exhausted and sometimes become delirious. During the second week, a rash of flat, rose-colored spots develops in about 10% of people. People may be constipated at first, but after 2 weeks, diarrhea may occur. In less than 5% of people, the intestine is torn (perforated) or bleeds. If infection spreads to other organs, symptoms of those infections may also develop.

Without treatment, people may have fever for 3 to 4 weeks. Up to 20% may relapse after initial recovery, and up to 20% may die. Most people who die are malnourished, very young, or very old. Stupor (unresponsiveness that requires vigorous stimulation to be aroused), coma, and shock are signs of severe infection and a poor prognosis.


To confirm the diagnosis, doctors take samples of blood, stool, other body fluids, or tissues and send them to a laboratory where the bacteria can be grown (cultured). The samples are examined and tested to determine whether the bacteria are present.


People who travel to areas where typhoid fever is common should avoid eating raw vegetables and other foods served or stored at room temperature. Generally, people can safely consume foods that are served very hot immediately after cooking, bottled or canned beverages that are sealed, hot tea or coffee, and fruit that they have peeled themselves. People should assume that ice and water (unless it is boiled or chlorinated before use) are unsafe. Sealed bottled water should be used for brushing teeth.

A vaccine given by mouth and a polysaccharide vaccine given by injection can help prevent typhoid fever. Both vaccines have few side effects. Vaccination is recommended for

  • Travelers to regions where typhoid fever is common

  • People who live in a household with carriers

  • Laboratory workers who work with the bacteria

People are usually protected for at least 2 years after vaccination by injection and for 5 years after taking the vaccine by mouth. However, they can be infected if many bacteria are ingested. In the United States, 75% of cases of typhoid fever occur in travelers who were not vaccinated or who did not keep their vaccinations up to date.


When antibiotics are used, fever lasts only 3 to 5 days, rather than 3 to 4 weeks, and the risk of death is reduced to less than 1%. Complete recovery may take weeks or months.

The antibiotic chloramphenicol is used worldwide. However, it can damage cells in bone marrow that make blood cells. Also, Salmonella typhi bacteria are becoming increasingly resistant to it. Thus, other antibiotics (such as ceftriaxone, ciprofloxacin, or azithromycin) are often used. People who are delirious, comatose, or in shock may also be given corticosteroids.

In 10 to 20% of people given antibiotics, the infection recurs, typically about 1 week after treatment is stopped. This infection is milder than the initial illness and is treated the same way.

Carriers must report to the local health department and are prohibited from working with food. Taking antibiotics for 4 to 6 weeks may eradicate the bacteria in many carriers. However, if carriers have abnormalities of the biliary or urinary tract (for example, kidney or gallbladder stones), surgery to correct these abnormalities is needed before bacteria can be eradicated.

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