What is the Salmonella Bacteria?

Salmonella is a bacterium that causes one of the most common enteric (intestinal) infections in the United States – salmonellosis. The term Salmonella refers to a group or family of bacteria that variously cause illness in humans. Salmonella serotype typhimurium and Salmonella serotype enteritidis are the most common in the United States.

What is the Incidence of Salmonella Infections?

In 2009, over 40,000 cases of Salmonella (13.6 cases per 100,000 persons) were reported to the Centers for Disease Control and Prevention (CDC) by public health laboratories across the nation, representing a decrease of approximately 15% from the previous year, but a 4.2% increase since 1996. [1] Overall, the incidence of Salmonella in the United States has not significantly changed since 1996. Only a small proportion of all Salmonella infections are diagnosed and reported to health departments. It is estimated that for every reported case, there are approximately 38.6 undiagnosed infections. The CDC estimates that 1.4 million cases, 15,000 hospitalizations, and 400 deaths are caused by Salmonella infections in the U.S. every year.

Salmonella can be grouped into more than 2,400 serotypes. The two most common serotypes in the U.S. are S. Typhimurium and S. Enteritidis. S. Typhi, the serotype that causes typhoid fever, is uncommon in the U.S. But globally, typhoid fever continues to be a significant problem, with an estimated 12-33 million cases occurring annually. Moreover, outbreaks in developing countries have a high death rate, especially when caused by strains of the bacterium that are resistant to antibiotic treatment.

Salmonella are found in the intestinal tract of wild and domesticated animals and humans. Some serotypes of Salmonella, such as S. Typhi and S. Paratyphi are only found in humans. For ease of discussion, it is generally useful to group Salmonellae into two broad categories: typhoidal, which includes S. Typhi and S. Paratyphi, and non-typhoidal, which includes all other serotypes.

What is the Prevalence of Salmonella in Food and Elsewhere?

Most Salmonella infections are caused by eating contaminated food, especially food from animal origins. One study found that 87% of all confirmed cases of Salmonella were foodborne, with 10 percent from person-to-person infection and 3% caused by pets. Food remains the most common vehicle for the spread of Salmonella, and eggs are the most common food implicated. Chicken is also a major cause of Salmonella. Beginning in 1998, the publisher of Consumer Reports magazine has conducted surveys and tested chicken at retail for Salmonella and Campylobacter. Its 2009 study found 14% of broiler chickens at grocery stores to contain Salmonella. A USDA Baseline Data Collection Program report done in 1994 documented Salmonella contamination on 20.0% of broiler-chicken carcasses. However, in 2009 the same USDA data collection survey showed the prevalence of Salmonella in broiler chickens at 7.5%. Additionally, turkey carries a lower risk with a prevalence of 1.66%.  

While Salmonella comes from animal feces, fruits and vegetables can become contaminated. A common source is raw sprouts, which have been the subject of at least 30 reported outbreaks of foodborne illnesses since 1996. The U.S. Department of Health and Human Services cautions against consuming raw sprouts under any circumstances: “Unlike other fresh produce, seeds and beans need warm and humid conditions to sprout and grow. These conditions are also ideal for the growth of bacteria, including Salmonella, Listeria, and E. coli.”

What are the Symptoms of Salmonella Infection?

Salmonella infections can have a broad range of illness, from no symptoms to severe illness. The most common clinical presentation is acute gastroenteritis. Symptoms include diarrhea, and abdominal cramps, often accompanied by fever of 100°F to 102°F (38°C to 39°C). Other symptoms may include bloody diarrhea, vomiting, headache and body aches. The incubation period, or the time from ingestion of the bacteria until the symptoms start, is generally 6 to 72 hours; however, there is evidence that in some situations the incubation can be longer than 10 days. People with salmonellosis usually recover without treatment within 3 to 7 days. Nonetheless, the bacteria will continue to be present in the intestinal tract and stool for weeks after recovery of symptoms—on average, 1 month in adults and longer in children. 

What are the Complications of Salmonella Infection?

In approximately 5% of non-typhoidal infections, patients develop bacteremia. In a small proportion of those cases, the bacteria can cause a focal infection, where it becomes localized in a tissue and causes an abscess, arthritis, endocarditis, or other severe illness. Infants, the elderly, and immune-compromised persons are at greater risk for bacteremia or invasive disease. Additionally, infection caused by antimicrobial-resistant non-typhoidal Salmonella serotypes appears to be more likely to cause bloodstream infections. 

Overall, approximately 20% of cases each year require hospitalization, 5% of cases have an invasive infection, and one-half of 1% die. Infections in infants and in people 65 years of age or older are much more likely to require hospitalization or result in death. There is some evidence that Salmonella infections increase the risk of developing digestive disorders, including irritable bowel syndrome.

Although most persons that become ill with diarrhea caused by Salmonella recover without any further problems, a small number of persons develop a complication often referred to as reactive arthritis. The terminology used to describe this type of complication has changed over time. The term “Reiter’s Syndrome” was used for many years but has now fallen into disfavor. The precise proportion of persons that develop reactive arthritis following a Salmonella infection is unknown, with estimates ranging from 2 to 15%. Symptoms of reactive arthritis include inflammation (swelling, redness, heat, and pain) of the joints, the genitourinary tract (reproductive and urinary organs), or the eyes.

More specifically, symptoms of reactive arthritis include pain and swelling in the knees, ankles, feet and heels. It may also affect wrists, fingers, other joints, or the lower back. Tendonitis (inflammation of the tendons) or enthesitis (inflammation where tendons attach to the bone) can occur. Other symptoms may include prostatitis, cervicitis, urethritis (inflammation of the prostate gland, cervix or urethra), conjunctivitis (inflammation of the membrane lining the eyelid) or uveitis (inflammation of the inner eye). Ulcers and skin rashes are less common. Symptoms can range from mild to severe.

One study showed that on average, symptoms developed 18 days after infection. A small proportion of those persons (15%) had sought medical care for their symptoms, and two thirds of persons with reactive arthritis were still experiencing symptoms 6 months later. Although most cases recover within a few months, some continue to experience complications for years. Treatment focuses on relieving the symptoms.

How to Diagnose a Salmonella Infections?

Salmonella bacteria can be detected in stool. In cases of bacteremia or invasive illness, the bacteria can also be detected in the blood, urine, or on rare occasions in tissues. The test consists of growing the bacteria in culture. A fecal, blood or other sample is placed in nutrient broth or on agar and incubated for 2-3 days. After that time, a trained microbiologist can identify the bacteria, if present, and confirm its identity by looking at biochemical reactions. Treatment with antibiotics before collecting a specimen for testing can affect bacterial growth in culture, and lead to a negative test result even when Salmonella causes the infection.

What is the Treatment for Salmonella Infection?

Salmonella infections usually resolve in 3 to 7 days, and many times require no treatment. Persons with severe diarrhea may require rehydration, often with intravenous fluids. Antimicrobial therapy (or treatment with antibiotics) is not recommended for uncomplicated gastroenteritis. In contrast, antibiotics are recommended for persons at increased risk of invasive disease, including infants younger than 3 months of age. 

What are the steps a person can take to prevent a Salmonella infection?

In general, safe cooking and preparation of food can kill existing Salmonella bacteria and prevent it from spreading. Additionally, safe choices at the grocery store can greatly reduce the risk of Salmonella. 

  • Always wash your hands before you start preparing food.
  • Cook poultry until it reaches an internal temperature of 165 ºF.
  • Cook beef and pork until they reach 160ºF. High quality steaks (not needle or blade tenderized) can be safely cooked to 145ºF.
  • Cook eggs until they reach 160ºF or until the yoke is solid. Pasteurized eggs are available in some grocery stores.
  • Do not eat or drink foods containing raw eggs. Examples include homemade eggnog, hollandaise sauce, and undercooked French toast.
  • Never drink raw (unpasteurized) milk. 
  • Avoid using the microwave for cooking raw foods of animal origin. Microwave-cooked foods do not reach a uniform internal temperature, resulting in undercooked areas and survival of Salmonella.
  • If you are served undercooked meat, poultry, or eggs in a restaurant don’t hesitate to send your food back to the kitchen for further cooking.
  • Avoid cross-contamination. That means that you should never allow foods that will not be cooked (like salads) to encounter raw foods of animal origin (e.g., on dirty countertops, kitchen sinks, or cutting boards). Wash hands, kitchen work surfaces, and utensils with soap and water immediately after they have been in contact with raw foods of animal origin.
  • Wash hands with soap after handling reptiles, amphibians or birds, or after contact with pet feces. Infants and persons with compromised immune systems should have no direct or indirect contact with such pets.
  • Reptiles, amphibians or birds, or any elements of their housing (such as water bowls) should never be allowed in the kitchen.
  • Avoid eating in animal barns and wash your hands with soap and water after visiting petting zoos or farm settings.
  • Always wash your hands after going to the bathroom. The hands of an infected person who did not wash his or her hands adequately after using the bathroom may also contaminate food. 

References

1. CDC, “Salmonella Annual Summary Tables 2009,” 2009, available online at

http://www.cdc.gov/ncezid/dfwed/PDFs/SalmonellaAnnualSummaryTables2009.pdf

2. CDC, “Preliminary FoodNet Data on the Incidence of Infection with Pathogens Transmitted Commonly through Food—10 States, 2008,” MORBIDITY AND MORTALITY WEEKLY REPORT, Vol. 58, No. 14, pp. 333-37 (April 10, 2009), online at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5813a2.htm

3. Voetsch, Andrew, et al., “FoodNet Estimate of the Burden of Illness Caused By Non-Typhoidal Salmonella Infections in the United States,” CLINICAL INFECTIOUS DISEASES, Vol. 15, No. 38, Supplement 3, pp. S127-34 (April 15, 2004) available online at http://cid.oxfordjournals.org/content/38/Supplement_3/S127.long

4. American Academy of Pediatrics, “Salmonella infections,” RED BOOK: 2006 Report of the Committee on Infectious Diseases, edited by L. K. Pickering, pp. 581–584 (27th ed. 2006).

5. Miller, S. and Pegues, D., “Salmonella Species, Including Salmonella Typhi,” in Mandell, Douglas, and Bennett’s PRINCIPLES AND PRACTICE OF INFECTIOUS DISEASES, Sixth Edition, Chap. 220, pp. 2636-650 (2005). 

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9.  Varma, Jay K., et al., “Antimicrobial-Resistant Non-typhoidal Salmonella is Associated with Excess Bloodstream Infections and Hospitalizations, JOURNAL OF INFECTIOUS DISEASES, Vol. 191, No. 4,  pp. 554-61 (Feb. 15, 2005) available online at http://jid.oxfordjournals.org/content/191/4/554.long

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