It has long been said that, in 1885, pioneering American veterinary scientist, Daniel E. Salmon, discovered the first strain of Salmonella. Though, Theobald Smith, research-assistant to Dr. Salmon, discovered the first strain of SalmonellaSalmonella Choleraesuis. But being in charge, Dr. Salmon received all the credit.[1] Today, the number of known serotypes of Salmonella bacteria totals over two thousand. And in recent years, concerns have been raised, as particular strains of Salmonella have become resistant to traditional antibiotics.

There are two Salmonella species: Salmonella enterica (S. enterica) and Salmonella bongori (S. bongori). S. bongori strains predominantly colonize cold-blooded reptiles, whereas S. enterica strains are capable of infecting both humans and mammals.[2] Based on factors such as morphology, structure, mode of reproduction, and other criteria, the two species are further classified into subgroups called serotypes or serovars. More than 2,600 serotypes have been described for Salmonella, and they are characterized by the type(s) of animal they are found in or by the clinical symptoms they cause.[3] Of these, less than 100 are responsible for most human Salmonella infections.[4]

Where Does Salmonella Come From?

Salmonellae are widely distributed in nature and are found in the intestinal tract of wild and domesticated animals and in humans. Salmonella poisoning can occur when a person ingests contaminated fecal particles transmitted by another infected human or animal.[5]

Salmonella enterica serotypes Typhi, Sendai, and Paratyphi A, B, or C are found exclusively in humans. These serotypes, collectively referred to as typhoidal Salmonella, cause enteric fever (also known as typhoid or paratyphoid fever if caused by serotypes Typhi or Paratyphi, respectively).[6] Most often, enteric fever is acquired through ingestion of food or water contaminated with human feces. Most U.S. residents who are diagnosed with typhoidal Salmonella are infected while traveling abroad in areas where typhoid fever and paratyphoid fever are common. Three types of vaccines against S. Typhi are commercially available, although there is still not a single licensed vaccine available against S. Paratyphi A.[7] Persons planning to travel outside of the United States are advised to find out if a vaccine for typhoid fever is recommended (see 

Most Salmonella infections are caused by eating contaminated food. One study found that 87% of all confirmed cases of Salmonella are foodborne. Foods of animal origin, including meat, poultry, eggs, or dairy products can become contaminated with Salmonella. Eating uncooked or inadequately cooked food—or food cross contaminated with uncooked or undercooked products—can lead to human infections. As explained in a comprehensive report issued by the U.S. Department of Agriculture’s Economic Research Service:

Salmonella contamination occurs in a wide range of animal and plant products. Poultry products and eggs are frequently contaminated with S. Enteritidis, while beef products are commonly contaminated with S.Typhimurium. Other food sources of Salmonella may include raw milk or other dairy products and pork.

In the past two decades, consumption of produce, especially sprouts, tomatoes, fruits, leafy greens, nuts, and nut butters, has been associated with Salmonella illnesses.[8] The surface of fruits and vegetables may be contaminated by human or animal feces. Changes in food consumption and production, as well as the rapid growth of international trade in agricultural products, have facilitated the transmission of Salmonella associated with fresh fruits and vegetables. 

In the United States, Salmonella is the second most commonly isolated bacterial pathogen when laboratory diagnosis of diarrhea is sought.[9] However, passive laboratory surveillance, which uses voluntary reporting by health care providers and facilities, captures only a fraction of illnesses that actually occur. Furthermore, only a small proportion of illnesses are confirmed by laboratory testing and reported to public health agencies. Thus, researchers rely on quantitative statistical modeling to estimate the incidence of foodborne illness. These estimates are used to direct policy and interventions.

What are the Symptoms of Salmonellosis?

Salmonella infections can produce a broad range of disease, from no symptoms to severe illness. The most common clinical presentation is acute gastroenteritis. Symptoms commonly include diarrhea and abdominal cramps, often accompanied by fever of 100°F to 102°F (38°C to 39°C). More serious infections may also involve bloody diarrhea, vomiting, headache, and body aches.[10]

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 three to seven days. Nonetheless, Salmonella bacteria can persist in the intestinal tract and stool for many weeks after the resolution of symptoms—on average, one month in adults and longer in children.[11]

Treatment of Salmonellosis

S. Typhi and S. Paratyphi can cause systemic illness if they invade the bloodstream (termed “bacteremia”). “Septicemia” or “sepsis” (bloodstream infection or “blood poisoning”) occurs if the bacteria multiply in the blood and cause the immune system to respond by activating inflammatory mechanisms. This may result in the development of “systemic inflammatory response syndrome,” or “SIRS.” SIRS includes tachycardia, tachypnea, fever, and abnormal white blood cell count. When the bacteria involved are S. Typhi or S. Paratyphi, this serious illness is called enteric typhoid, or paratyphoid fever. Symptoms may start gradually and include fever, headache, malaise, lethargy, and abdominal pain. In children, it can present seemingly innocuously as a non-specific fever. The incubation period for S.Typhi is usually 8 to 14 days, but it can range from three to 60 days. For S. Paratyphi infections, the incubation period is like that of nontyphoidal Salmonella—one to 10 days.[12]

Medical treatment is acutely important, though, if the patient becomes severely dehydrated or if the infection spreads from the intestines. Persons with severe diarrhea often require re-hydration, usually with intravenous (IV) fluids. But antibiotics are not necessary or indicated unless the infection spreads from the intestines, at which time the infection can be treated with ampicillin, gentamicin, trimethoprim/sulfamethoxazole, or ciprofloxacin. Unfortunately, though, some Salmonella bacteria have become resistant to antibiotics, largely because of the use of antibiotics to promote the growth of feed animals.[13]


A.        Reactive Arthritis

Formerly referred to as Reiter Syndrome, the term reactive arthritis refers to an inflammation of one or more joints, following an infection localized at a site distant from the affected joints. The predominant site of the infection is the gastrointestinal tract. And reactive arthritis can be post infection, meaning that the infection may not be active when diagnosed. Several bacteria, including Salmonella, can cause reactive arthritis.[14]  And although the resulting joint pain and inflammation can resolve completely over time, permanent joint damage can occur.[15]

The symptoms of reactive arthritis include pain and swelling in the knees, ankles, feet, and heels. Less frequently, the upper extremities may be affected, including the wrists, elbows, and fingers. 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 and can occur anywhere from three days to six weeks after the antecedent infection and may involve one or more joints, though usually six or fewer. Although most cases recover within a few months, some continue to experience complications for years. Treatment focuses on relieving the symptoms.[16]

B.        Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a functional disorder of the gastrointestinal tract. The hallmark symptoms of IBS are abdominal pain and altered bowel habits, ranging from constipation to diarrhea, or alternating diarrhea and constipation. Abdominal pain is usually crampy in nature, but character and sites can vary. In some patients, the pain is relieved by defecation but, in others, defecation may worsen the pain. Additional symptoms may include bloating, straining at stools, and a sense of incomplete evacuation.

The observation that the onset of IBS symptoms can be precipitated by gastrointestinal infection dates to the 1950s. Mechanisms are not known but include changes in the microbiome, use of antibiotics to treat the infection, and an increase in enteroendocrine cells.

Another consequence of infective gastroenteritis is the disruption of normal gut flora. Studies on postinfectious IBS have provided etiological insights into the pathogenesis of IBS. It is well documented that following infective gastroenteritis, more than 10% of affected individuals go on to develop postinfectious IBS.[17] The risk of postinfectious IBS appears greater with bacterial gastroenteritis compared to viral gastroenteritis.

[1]           Kass EH. (1987). A brief perspective on the early history of American infectious disease epidemiology. Yale J Biol Med. 60(4):341-8. 

[2]           Hernandez, A. K. C. Salmonella bongori. Poultry and Avian Diseases. Encyclopedia of Agriculture and Food Systems.

[3]           Boore AL, et al. (2015). Salmonella enterica Infections in the United States and Assessment of Coefficients of Variation: A Novel Approach to Identify Epidemiologic Characteristics of Individual Serotypes, 1996–2011. PloS One. 10(12): e0145416

[4]           Besser JM. (2018). Salmonella epidemiology: a whirlwind of change. Food Microbiol. 71:55-9.

[5]           Chiu, C.-H. (2019). Salmonella, Non-Typhoidal Species (S. Choleraesuis, S. Enteritidis, S. Hadar, S. Typhimurium). 

[6]           Ohad eGal-Mor, Erin C Boyle, & Guntram A. Grassl. (2014). Same species, different diseases: how and why typhoidal and non-typhoidal Salmonella enterica serovars differ. Frontiers in Microbiology, 5.

[7]           Id.

[8]           National Typhoid and Paratyphoid Fever Surveillance Annual Summary, 2015.” Centers for Disease Control and Prevention, 6 Nov. 2018. Available at:

[9]           “National Enteric Disease Surveillance: Salmonella Annual Report, 2016.” Centers for Disease Control and Prevention, 28 Feb. 2018. Available at:

[10]         “Salmonella.” Centers for Disease Control and Prevention, 24 Jun. 2020. Available at:

[11]         Id.

[12]         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. 2636-50 (2005).

[13]         Medalla, F., Gu, W., Mahon, B. E., Judd, M., Folster, J., Griffin, P. M., & Hoekstra, R. M. (2016). Estimated Incidence of Antimicrobial Drug-Resistant Nontyphoidal Salmonella Infections, United States, 2004-2012. Emerging infectious diseases23(1), 29–37.

[14]         See “Reactive Arthritis.” Questions and Answers About. N.p., n.d. Web. 12 Nov. 2015.

[15]         Id.

[16]         “Reactive Arthritis.” National Institute of Arthritis and Musculoskeletal and Skin Diseases, Oct. 2016. Available at:

[17]         Ng, Q. X., Soh, A., Loke, W., Lim, D. Y., & Yeo, W. S. (2018). The role of inflammation in irritable bowel syndrome (IBS). Journal of inflammation research11, 345–349.