What is salmonella infection?

Causes and Symptoms

Salmonella are a group of bacteria that can cause gastrointestinal infections, blood infections, and various local infections. All types of salmonella can be carried in the gastrointestinal tract without symptoms after recovery from infection.

The clinical disease caused by salmonella depends on the type of bacteria, the amount of organisms ingested, and the age and immune status of the person infected. Infection with salmonella can take place with the ingestion of one or 100 million organisms. Increasing the dosage of bacteria decreases the incubation period and increases the severity of the resulting disease. After ingestion, the bacteria adhere to and invade the gastrointestinal tract. In the wall of the intestinal tract, salmonella survive and multiply in immune cells and then enter the bloodstream, where they proceed to any area of the body. Young infants and people with immune deficiencies and hemolytic anemia are at increased risk for severe and complicated infections.

One specific type of serious illness caused by salmonella is enteric or typhoid fever. Typhoid fever is rare in the United States, causing an estimated 5,700 cases per year, primarily in people who acquired it in other countries. This disease is caused by the Typhi variety of the Salmonella enterica bacterium. Symptoms during the first week of illness include progressively increasing fever with associated headache, muscle aches, abdominal pains, and lethargy. In the second week, the heart rate decreases, the liver and spleen enlarge, small red bumps form on the trunk, and the patient enters into a stupor. During the third to fourth week, intestinal hemorrhage and perforation are common. The fever begins to remit in the fifth to sixth week of illness. Diarrhea usually starts in the first week and resolves within six weeks. Without treatment, death can occur from gastrointestinal hemorrhage and perforation. Infants tend to have much more severe disease than older children.

Salmonellosis caused by nontyphoid salmonella is more common in the United States, causing about forty thousand reported cases per year; the number of unreported cases is thought to be far higher. The major reservoir of nontyphoid salmonella is the gastrointestinal tract of many animals, including mammals, reptiles, birds, and insects, although plants may also become contaminated. Farm animals and pet reptiles commonly carry salmonella. Some antibiotic resistance is caused by the use of antibiotics in animal feeds. Contaminated eggs and milk products are common sources of human infection.

Gastroenteritis
is the most common disease caused by nontyphoid salmonella. The incubation period for this disease is about one day, with a range from six hours to three days. Symptoms include nausea, vomiting, and abdominal pain. Diarrhea typically contains blood and mucus. Usually, symptoms disappear in less than a week in healthy children, but in young infants and in children with immune deficiencies, symptoms may persist for several weeks.

Bacteremia can occur in 1 to 5 percent of patients with salmonella gastroenteritis. Bacteremia is generally associated with fever, chills, and toxicity in the older child but may be asymptomatic in the infant. Children with an increased risk of bacteremia include those with acquired immunodeficiency syndrome (AIDS) or other immune deficiencies and hemolytic anemias such as sickle cell anemia.

Bacteremia can lead to infection of almost any organ. Children with sickle cell anemia are more prone to bone infections and meningitis. Salmonella may localize to areas of the body that have received trauma or that contain damaged tissue or a foreign body. Meningitis, inflammation of the covering of the spine and brain, is primarily seen as a complication of bacteremia in infants. Patients who have persistent bacteremia should also be evaluated for heart infection.

The diagnosis of a salmonella infection is best made by culturing stool and blood samples. With enteric fever, it is important to culture multiple sites multiple times. Antibiotic susceptibility testing must be performed routinely to guide therapy. Other bacterial causes of gastroenteritis can be confused with salmonella infection.

Treatment and Therapy

Treatment for gastroenteritis usually does not require antibiotics. Antibiotics do not speed the resolution of disease but instead lead to prolonged excretion of salmonella. Therapy is primarily focused on the correction of fluid and salt imbalances and on general supportive care. If the patient has indications of sepsis, shock, or chills, however, then antibiotics should be administered. Infants under three months of age and children with immune deficiencies should also be treated with antibiotics. Ampicillin is usually used as the initial treatment in uncomplicated cases, and third-generation cephalosporin antibiotics are used in severe and complicated cases. About 20 percent of nontyphoid salmonella in the United States is resistant to ampicillin as well as to other antibiotics. Antibiotic treatment should last ten days to two weeks in children with bacteremia and four to six weeks in children with bone infection or meningitis. Local infections may require surgical drainage.

Typhoid fever is treated for a minimum of two weeks. It is important to perform susceptibility testing for the possibility of resistance so that proper antibiotic therapy can be chosen. Chronic carriers of Salmonella enterica Typhi should be treated with antibiotics. If eradication is unsuccessful, surgical assessment of the biliary tract should be sought.

Prevention of the spread of salmonella requires a number of public health procedures. Hand washing is critical to the prevention of transmission. Persons who are carriers of salmonella should be excluded from food preparation and child-care settings. Hospitalized infants and children should be isolated. Proper sewage disposal, water purification, and chlorination are essential public health measures. In developing countries, the promotion of prolonged breast-feeding also reduces the infection rate.

There are two typhoid vaccines commercially available in the United States. The first is an oral, live attenuated vaccine that requires four doses given over a period of one week and a booster every five years. The second is a parenteral capsular polysaccharide vaccine given as a single intramuscular injection and a booster every two years. The vaccines are 50 to 80 percent protective. The live attenuated vaccine should not be given to patients who are pregnant, taking antibiotics, or immunocompromised, such as persons with the Human immunodeficiency virus (HIV).

Perspective and Prospects

Salmonella was identified as the cause of typhoid fever in 1880 and was first cultured in 1884. Since 1920, improvements in sanitation, water supplies, and sewage disposal have resulted in a marked decrease of typhoid fever in the United States. However, it is still a problem in the developing world, where it affects over 20 million people per year.

Recent research is focused on public health. Measures to decrease food contamination such as improved cleanliness, decreased use of antibiotics in animal feeds, and food irradiation are being evaluated and used to decrease transmission to humans. Research into alternate vaccines with fewer side effects and improved immune response is also being performed.

Nontyphoid salmonella causes as many as half a million infections per year in the United States. One-third of these infections are in children less than five years of age, and 40 percent are in adults over thirty years of age.

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