Salmonella is a bacterium that causes one of the most common enteric (intestinal) infections in the United States – Salmonellosis. In some states (e.g. Georgia, Maryland), salmonellosis is the most commonly reported cause of enteric disease, and overall it is the second most common bacterial foodborne illness reported (usually slightly less frequent than Campylobacter infection).
The reported incidence of Salmonella illnesses is about 14 cases per each 100,000 persons (MMWR Weekly, 2006), amounting to approximately 30,000 confirmed cases of salmonellosis yearly in the U.S. (CDC, 2005, October 13). In 2005, just over 36,000 cases were reported from public health laboratories across the nation, representing a 12 percent decrease compared with the previous decade, but a 1.5 percent increase over 2004 (CDC, 2007).
As only about 3 percent of Salmonella cases are officially reported nationwide, and many milder cases are never diagnosed, the true incidence is undoubtedly much higher (Mead, 1999). The CDC estimates that 1.4 million cases occur annually (CDC, 2005, October 13). Approximately 600 deaths are caused by Salmonella infections in the U.S. every year, accounting for 31 percent of all food-related deaths (CDC, 2005, October 13; MMWR Weekly, 2001).
Theobald Smith, research-assistant to Daniel E. Salmon, discovered the first strain of Salmonella – Salmonella cholerae suis – in 1885. Since that time, the number of strains (technically termed serotypes or serovars) of Salmonella known to cause salmonellosis has increased to over 2,300. Salmonella typhi, the strain that causes typhoid fever, is uncommon in the U.S., while Salmonella enterica serotype Typhimurium and Salmonella enterica serotype Enteritidis have been the most frequently reported illnesses since 1993. Salmonella enterica serotypes Newport, Mississippi and Javiana have been increasingly identified as the source of illness (MMWR Weekly, 2006).
Symptoms of Salmonella Infection
An infectious dose of Salmonella is small, probably from 15 to 20 cells. Typically, non-typhoidal Salmonella produces a self-limiting febrile gastrointestinal illness that is indistinguishable from that caused by other bacterial enteric pathogens. Dehydration is the principal clinical concern. The incubation period – the time between ingestion of Salmonella bacteria and the onset of illness – varies from six to 72 hours (Mayo Clinic, 2007, April 12; MMWR Recomm Rep, 2001).
Salmonella can cause three different kinds of illness: gastroenteritis, typhoid fever, and bacteremia.
Symptoms of Salmonella gastroenteritis include diarrhea, abdominal cramps, fever, nausea, and/or vomiting. In mild cases diarrhea may be non-bloody, occur several times per day, and not be very voluminous; in severe cases it may be frequent, bloody and/or mucoid, and of high volume.
Fever generally occurs in the 100°F to 102°F (38°C to 39°C) range. Vomiting is less common than diarrhea. Headaches, myalgias (muscle pain), and arthralgias (joint pain) are often reported as well. Whereas the diarrhea typically lasts 24 to 72 hours, patients often report fatigue and other nonspecific symptoms lasting 7 days or longer.
Complications of Salmonella Infection
Typhoid fever, also known as enteric fever, is caused by Salmonella serotype typhi. The onset of symptoms usually occurs between 5 and 21 days after ingestion of Salmonela typhi bacteria. Symptoms may include constipation, cough, sore throat, headache, and a rash on the infected individual’s chest, as well as the slowing of the heartbeat and enlargement of the liver and spleen (Mayo Clinic, 2007, April 12).
Bacteremia is characterized by infection of tissues surrounding the brain and spinal cord (meningitis) and infection within the bloodstream (sepsis). This condition occurs when Salmonella enter and circulate within an infected individual’s bloodstream, and is accompanied by few symptoms (Mayo Clinic, 2007, April 12).
Reiter’s syndrome, which includes and is sometimes referred to as “reactive arthritis” is an uncommon, but debilitating, result of a Salmonella infection. Reiter’s syndrome is a disorder that causes at least two of three seemingly unrelated symptoms: reactive arthritis, eye irritation, and urinary tract infection (Hill Gaston & Lillicrap, 2003). The reactive arthritis associated with Reiter’s develops when a person eats food that has been tainted with bacteria. Reactive arthritis is characterized by the inflammation of one or more joints following an infection localized in another portion of the body, commonly the gastrointestinal tract. The symptoms of Reiter’s Syndrome usually occur between one and three weeks after the infection.
The three most common symptoms of Reiter’s syndrome are arthritis, eye irritation, and urinary tract symptoms. The arthritis associated with Reiter’s syndrome typically affects the knees, ankles, and feet, causing pain and swelling. Wrists, fingers and other joints can be affected, though with less frequency. Patients with Reiter’s syndrome commonly develop inflammation where the tendon attaches to the bone, a condition called enthesopathy. Some patients with Reiter’s syndrome also develop heel spurs, bony growths in the heel that cause chronic or long-lasting foot pain. Arthritis from Reiter’s syndrome can also affect the joints of the back and cause spondylitis, inflammation of the vertebrae in the spinal column. The duration of reactive arthritis symptoms can vary greatly. Most of the literature suggests that the majority of patients recover within a year. The condition, can, however, be permanent. One study found nearly 50 percent of patients with post dysenteric reactive arthritis continued to have symptoms roughly one year after onset (Inman, et al., 1998).
The involvement of the eye in Reiter’s syndrome is most commonly manifested as conjunctivitis, inflammation of the mucous membrane that covers the eyeball, or uveitis, an inflammation of the inner eye. Conjunctivitis and uveitis can cause redness of the eyes, eye pain and irritation, and blurred vision.
The third site for Reiter’s syndrome symptoms is the urogenital tract. This includes the prostate, urethra, and penis in men and the fallopian tubes, uterus, and vagina in women. Men may notice an increased need to urinate, a burning sensation when urinating, and a discharge from the penis. Some men also develop prostatitis. Symptoms of prostatitis include fever, chills, increased need to urinate, and a burning sensation when urinating.
How is Salmonella Infection Diagnosed?
Salmonella bacteria can be detected in stool or blood cultures, and a diagnosis of salmonellosis is confirmed by culture. Specimens of blood or feces are placed in nutrient broth or on agar and incubated for 2-3 days. After that time, a trained microbiologist can recognize Salmonella bacteria, if present, by its unique characteristics.
In the stool, the laboratory is challenged to pick out Salmonella from many other similar bacteria that are normally present. Many persons submit specimens for testing after they have started antibiotics, which may make it even more difficult for a lab to grow Salmonella. So, the diagnosis of salmonellosis may be problematic and many mild cases are culture-negative.
Blood cultures are often not performed and in most cases the blood stream is not infected. Bacteremia does occur in 5 percent of adults with Salmonella gastroenteritis and can result in hematogenous spread to the heart (endocarditis), spleen, bone (osteomyelitis), and joints (reactive arthritis), although blood cultures are rarely positive.
Treatment for Salmonella Infection
Salmonella infections usually resolve in five to seven days, and many times require no treatment unless the patient becomes severely dehydrated or the infection spreads from the intestines. Persons with severe diarrhea may require re-hydration, often with intravenous fluids.
Treatment with antibiotics is not usually necessary; however, if the infection spreads from the intestines, or otherwise persists, the infection can be treated with ampicillin, gentamicin, trimethoprim/sulfamethoxazole, ceftriaxone, amoxicillin, or ciprofloxacin. The length of treatment varies depending on the extent of a patient’s illness, and can range from 14 days for enteric fever to six weeks for bactremia (Mayo Clinic, 2007, April 12). Some Salmonella bacteria have become antibiotic-resistant.
Consult your healthcare provider if you believe you have become ill with salmonellosis.
Antimicrobial Resistance in Salmonella Bacteria
Antimicrobial resistance in bacteria is an emerging and increasing threat to human health. Physicians should be aware that antimicrobial resistance is increasing in foodborne pathogens and that patients who are prescribed antibiotics are at increased risk for acquiring antimicrobial-resistant foodborne infections. In addition, “…increased frequency of treatment failures for acute illness and increased severity of infection may be manifested by prolonged duration of illness, increased frequency of bloodstream infections, increased hospitalization or increased mortality,” (Angulo, Nargund, & Chiller, 2004).
The use of antimicrobial agents in the feed of food animals is estimated by the FDA to be over 100 million pounds per year. Estimates range from 36 to 70 percent of all antibiotics produced in the United States are used in a food animal feed or in prophylactic treatment to prevent animal disease. In 2002, the Minnesota Medical Association published an article by David Wallinga, M.D., M.P.H. who wrote:
According to the [Union of Concerned Scientists], 70 percent of all the antimicrobials used in the United States for all purposes—or about 24.6 million pounds annually—are fed to poultry, swine, and beef cattle for nontherapeutic purposes, in the absence of disease. Over half are “medically important” antimicrobials, identical or so closely related to human medicines that resistance to the animal drug can confer resistance to the similar human drug. Penicillins, tetracyclines, macrolides, streptogramins, and sulfonamides are prominent examples. (Wallinga, 2002).
The National Antimicrobial Resistance Monitoring System (NARMS) reported that Campylobacter has been recovered from 47 percent of chicken breasts tested in recent studies. In the same NARMS studies, five multi-drug resistant strains of Salmonella Newport were recovered from ground beef, ground turkey, and pork chops.
According to the report, “…antimicrobial resistance among these foodborne bacteria is not uncommon and often associated with the use of antimicrobial agents in food animals,” (Stevenson, et al., 2002). Ceftriaxone-resistant Salmonella has also been reported (Fey et al., 2000). The emergence of multidrug-resistant Salmonella typhimurium in the United States is another example of a drug-resistant bacteria spreading from animals to humans (Glynn et al., 1998).
A large proportion of serotype Typhimurium isolates were resistant to multiple antimicrobial drugs; in a 2003 national survey, 45% were resistant to one or more drugs and 26% had a five-drug resistance pattern characteristic of a single phage type, DT104 (2). Similarly, serotype Newport has emerged as a major multidrug-resistant pathogen (CDC, 2007).
The use of antibiotics in feed for food animals, on animals prophylactically to prevent disease, and the use of antibiotics in humans unnecessarily must be reduced. European countries have reduced the use of antibiotics in animal feed and have seen a corresponding reduction in antibiotic-resistant illnesses in humans (Angulo, Baker, et al., 2004).
How to Prevent a Salmonella Infection
Persons most at-risk for contracting Salmonellosis include:
- Individuals sharing households with infected persons
- Owners of pet reptiles, such as an iguanas, lizards or turtles
- Individuals with compromised immune systems, such as people with AIDS, cancer patients, and transplant recipients
- Individuals with inflammatory bowel disease
- Individuals with sickle cell disease
- Individuals who have illnesses such as malaria or sickle cell anemia
- Individuals using corticosteroids
- Individuals who use antacids (Mayo Clinic, 2007, April 12)
To prevent Salmonella bacterial infection:
- Cook poultry, ground beef, and eggs thoroughly before eating. In order to insure that eggs do not contain viable Salmonella they must be cooked at least until the yoke is solid and meat and poultry must reach 160ºF or greater throughout. Pasteurized eggs are also available in some areas and for foodservice organizations (MMWR Weekly, 1996).
- Do not eat or drink foods containing raw eggs. Examples include homemade eggnog, hollandaise sauce, and undercooked French toast.
- Never drink raw (unpasteurized) milk.
- 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.
- Wash hands, kitchen work surfaces, and utensils with soap and water immediately after they have been in contact with foods of animal origin.
- Be particularly careful with foods prepared for infants, the elderly, and those with a compromised immune system.
- 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.
* 2008 Cantaloupe Salmonella Outbreak
* 2008 Malt-O-Meal Salmonella Agona
* Alamosa, Colorado Municipal Water System Salmonella Outbreak
* Banquet (ConAgra) Pot Pie Salmonella Outbreak
* Black Forest Bakery Salmonella Outbreak
* Brook-Lea Salmonella Outbreak
* Cafe Santa Fe Salmonella Outbreak
* Chili’s Salmonella Outbreak
* Corky & Lenny’s Deli Salmonella Outbreak
* Golden Corral Salmonella Outbreak
* Harmony Farms Salmonella Outbreak
* KFC Salmonella Outbreak
* Kunick Cantaloupe Salmonella Outbreak
* Linh’s Bakery Salmonella Outbreak
* Malt-O-Meal Salmonella Outbreak
* Old South Restaurant Salmonella Outbreak
* Orchid Island Orange Juice Salmonella Outbreak
* Paramount Farms Salmonella Outbreak
* Peter Pan and Great Value Peanut Butter Salmonella Outbreak
* Quality Inn and Bogey’s Restaurant Salmonella Outbreak
* San Antonio Taco Salmonella Outbreak
* Seasons on the Pond Salmonella Outbreak
* Sheetz and Coronet Foods Salmonella Outbreak
* Shipley Sales Cantaloupe Salmonella Outbreak
* Sun Orchard Orange Juice Salmonella Outbreak
* Sunset House Salmonella Outbreak
* Sushi King Salmonella Outbreak
* Taste of Chicago Salmonella Outbreak
* Veggie Booty Salmonella Outbreak
* Wal-Mart Salmonella Outbreak
* Western Sizzlin’ Salmonella Outbreak
* Angulo FJ, Baker NL, Olsen SJ, Anderson A, Barrett TJ. (2004). Antimicrobal Use in Agriculture: Controlling the Transfer of Antimicrobal Resistance to Humans. Seminars in Pediatric Infectious Diseases. 15(2): 78-85.
* Angulo FJ, Nargund VN, and Chiller TC. (2004). Evidence of an Association Between Use of Anti-microbial Agents in Food Animals and Anti-microbial Resistance Among Bacteria Isolated from Humans and the Human Health Consequences of Such Resistance J. Vet. Med. B Infect. Dis. Vet. Public Health. 51(8-9):374-9.
* CDC. (2005, October 13). Salmonella: Technical Fact Sheet. Retrieved August 29, 2007 from Centers for Disease Control and prevention Web site, http://www.cdc.gov/ncidod/dbmd/diseaseinfo/salmonellosis_t.htm.
* CDC. 2007. Salmonella Surveillance: Annual Summary, 2005. Atlanta, Georgia: US Department of Health and Human Services.
* Glynn MK, Bopp C, Dewitt W, Dabney P, Mokhtar M, Angulo FJ. (1998). Emergence of multidrug-resistant Salmonella enterica serotype typhimurium DT104 infections in the United States. N Engl J Med 338:1333–1338.
* Hill Gaston JS, Lillicrap MS. (2003). Arthritis associated with enteric infection. Best Practices & Research Clinical Rheumatology 17(2):219-239.
* Inman RD, Johnston MEA, Hodge M, Falk J, and Helewa A. (1988). Postdysenteric Reactive Arthritis, A Clinical and Immunogenic Study following an Outbreak of Salmonellosis. Arthritis and Rheumatism 31:1377-1383.
* Mayo Clinic. (2007, April 12). Salmonella. Retrieved August 29, 2007 from Mayo Clinic Web site, http://www.mayoclinic.com/health/salmonella/DS00926.
* Mead PM, Slutsker L, Dietz V, McCaig LF, Bresee JS, Shapiro C, Griffin PM, and Tauxe RV. (1999). Food-related Illness and Death in the United States. Emerg. Infect. Dis. 5:607-625.
* MMWR Recomm Rep. (2001). Diagnosis and management of foodborne illnesses: a primer for physicians. January 26, 2001 / 50 (RR-2):1-69. Atlanta, GA: Centers for Disease Control and Prevention.
* MMWR Surveillance Summaries, Appendix B. (1996). Guidelines for confirmation of foodborne-disease outbreaks. October 25, 1996 / 45:(SS-5) 59-66. Atlanta, GA: Centers for Disease Control and Prevention.
* MMWR Weekly. (1996). Guidelines for confirmation of foodborne-disease outbreaks. October 25, 1996 / 45(S-5): 58-66. Atlanta, GA: Centers for Disease Control and Prevention.
* MMWR Weekly. (2001). Summary of Notifiable Diseases, United States, 1999. April 06, 2001 / 48(53):1-104. Atlanta, GA: Centers for Disease Control and Prevention.
* MMWR Weekly. (2006). Preliminary FoodNet Data on the Incidence of Infection with Pathogens Transmitted Commonly Through Food – 10 States, United States, 2005. April 14, 2006 / 55(14);392-395. Atlanta, GA: Centers for Disease Control and Prevention.
* Stevenson JE, White DG, Torpey III DJ, Craig AS, Smith KE, Park MM, Pascucilla MA, Anderson AD, and the NARMS Working Group. (2002). Enhanced Surveillance for Antimicrobial Resistance Among Enteric Bacteria: NARMS Retail Food Study. International Conference on Emerging Infectious Diseases. Atlanta, GA, March 2002.
* Wallinga, D. (2002, October). Antimicrobial Use in Animal Feed: an Ecological and Public Health Problem. Minnesota Medicine. 85. Retrieved January 16, 2008, from http://www.mmaonline.net/publications/MNMed2002/October/Wallinga.html.