Facts You Should Know About Salmonella
- Salmonella are a group of closely related rod-shaped, Gram-stained negative (Gram-negative or G-) bacteria that have flagella (tail-like structures used for movement). Salmonella types are further characterized by specific proteins found on the bacterial and flagellar surface. Each different combination of protein coats is termed a serovar. Serovars are distinguished usually by special laboratories with immunologic tests.
- The nomenclature of specific types of Salmonella (a member of the Enterobacteriaceae sometimes noted as an enterobacterium) has changed in recent decades. Currently, many investigators consider the over 2,500 serovars (different types or strains) to be members of only two species, S. enterica or S. bongori. However, many serovars were considered to be and named as individual species in the past before more sophisticated genetic methods to characterize separate species were available. Consequently, many of the old serovar names are still seen in the medical literature, such as S. enteritidis, S. typhimurium, S. typhi, S. newport, and S. choleraesuis. In other cases, doctors simply avoid the name problem and identify all isolates as Salmonella spp (species) since the bacteria of this group are so closely related.
- Salmonella infections cause diseases in humans (for example, salmonellosis, gastroenteritis, typhoid fever, and paratyphoid fever), animals, and birds.
- They are one of the major causes of gastroenteritis in both industrialized and third-world countries and are considered to be the most common cause of food-borne disease in the U.S. Salmonella were first isolated from infected pigs in 1885 by Theobald Smith and were named after his lab director, D.E. Salmon.
What Are Signs and Symptoms of Salmonella Infection?
- The signs and symptoms of Salmonella infections depend on the overall health of the infected person (for example, normal or with a suppressed immune system) and the particular serovar infecting the patient.
- Signs and symptoms usually begin about 12-72 hours after ingestion of the bacteria. In general, people contract S. spp (for example, serovars S. enteritidis, S. cholerasuis, or non-typhoidal types of the bacteria) that usually cause a self-limiting diarrhea, nausea, abdominal cramps, and vomiting termed salmonellosis or Salmonella gastroenteritis (Salmonella poisoning).
- Fever may be present but usually lasts only one to three days, with all the signs and symptoms resolving in about three to seven days. This gastroenteritis is not always traced to the bacterial source and is sometimes simply termed "food poisoning," a term used to describe similar symptoms and signs caused by several different bacterial, parasitic, and viral organisms (for example, E. coli, Giardia, and rotavirus). Those with a suppressed immune system, the elderly, neonates, and young children may develop more severe symptoms (for example, bacteremia or sepsis).
- Fever and the above-mentioned symptoms lasting over seven to 10 days suggest infection with the more virulent serovars, S. typhi or S. paratyphi. S. typhi causes typhoid fever, which includes symptoms of a high fever (104 F), abdominal pains, sweating, and confusion; some of those affected may develop swollen lymph nodes.
- About half of patients develop a slow heartbeat (bradycardia), and some get slightly raised red or rose-colored spots (rose spots) on the chest and abdomen. S. paratyphi causes paratyphoid fever, a disease similar to but with less severe symptoms than typhoid fever.
- Some untreated patients who become infected with S. typhi or S. paratyphi and are otherwise healthy will resolve the infection in about one month, but others can suffer complications (for example, becoming a carrier of the organism, developing organ infections, sepsis, and potentially death).
Bacterial Infections 101: Types, Symptoms, and Treatments
How Common Are Salmonella Infections?
The most common infection caused by Salmonella is salmonellosis (also termed salmonella gastroenteritis). Over 1.4 million cases per year reportedly occur in the U.S. Other countries that are industrialized have similar occurrence rates in their populations. However, many cases go unreported, so some experts suggest that the actual number in the U.S. could be over 20 million cases per year. About 500-1,000 cases per year may result in death. In contrast, developing countries have a much higher rate of salmonellosis, but accurate estimates of its prevalence are lacking. Nonetheless, S. spp is considered by some researchers to cause the majority of food-borne infections in the U.S.
Typhoid fever occurs infrequently in industrialized countries; only about 500 cases per year are reported in the U.S., and the majority of these occur in people returning from a visit to a developing country. The U.S. Centers for Disease Control and Prevention (CDC) estimate that 21.7 million cases occur worldwide per year with about 217,000 deaths.
Paratyphoid fever, like typhoid fever, occurs infrequently in industrialized countries. About 100-400 cases per year occur in the U.S., with a majority originating in people who travel to a developing country. Fortunately, paratyphoid fever is not as severe an infection as typhoid. Deaths result in less than 1% of diagnosed patients. Both typhoid and paratyphoid fevers have been termed "enteric fever," but this term is not specific and a few authors use the term for any Salmonella infection.
How Do Salmonella Bacteria Cause Disease(s)?
The majority of Salmonella infections are due to ingestion of food or water, although direct contact with animals has become more common as a source of the organisms to cause infection. In people with normal gastrointestinal tracts and immune systems, researchers have estimated that about 1 million to 1 billion organisms need to be ingested to cause infection, because normal human stomach acid can kill large numbers of these bacteria. If some bacteria reach the intestine, the organisms can attach to intestinal cells where Salmonella toxins (cytotoxin and enterotoxin) can damage and kill cells. The intestinal cell damage results in the inability of the body to normally retain and adsorb fluids, so diarrhea results. In some people, the diarrhea can cause serious dehydration. However, the majority of Salmonella-caused infections are then eventually eliminated by the person's immune defenses. Some Salmonella are not eliminated; these bacteria survive the initial immune response by living inside cells (macrophages) of the immune system. The bacteria can sometimes spread to the blood (bacteremia). Some Salmonella (for example, S. typhi) can also enter the gallbladder and remain there. The patient can recover from the disease but still sheds bacteria through the gallbladder secretions (bile) into the feces. This person thus becomes a carrier of Salmonella and potentially can infect many others, especially if the person lives in unsanitary conditions or works in the food-processing industry.
Is Salmonella Contagious?
- Yes, both non-typhoidal and typhoidal strains are contagious but many outbreaks initiate from food-borne illness outbreaks.
- The organisms can be shed in feces and/or cross-contaminate the hands of food handlers and be passed on to other people.
- The incubation period usually is about six to 72 hours, although some people may not show signs of these infectious diseases for two weeks.
What Are Risk Factors for Salmonella Infection?
The greatest risk factor for getting a Salmonella infection is ingesting the bacteria in contaminated food or water. Another high risk factor for people in industrialized countries is eating and drinking when visiting in a developing country where sources of food and drink may be contaminated. However, even industrialized countries like the U.S. can have outbreaks of Salmonella infection if a food or water source is not properly sanitized or screened for contamination. An example was a 2008-2009 outbreak of Salmonella infections that were traced to a peanut processing plant that sold processed peanut material (paste) that was subsequently put into many food products (candy, cookies, ice cream, cereals, dog food). This contaminated paste caused about 600-700 cases of salmonella in 46 states. More than 125 products were eventually recalled and the company plant closed. The most common source of contamination in the U.S. is from eggs (especially handling or eating raw eggs) and poultry products. In 2018, the CDC warned the public not to eat any Kellogg's Honey Smacks cereal. About 100 people have been infected and 30 hospitalized with Salmonella.
People older than 70 years of age and less than 20 years of age are in the highest risk groups. A large percentage of pediatric typhoid infections occur in children under 5 years of age. Older individuals may have underlying diseases and compromised immunity that lead to higher risk while children may have less gastric acidity which allows bacteria to better survive passage through the stomach and into the GI system beyond the stomach.
Eating raw meat, poultry, tuna, eggs, or eating unwashed vegetables or fruits, including seeded vegetables, sprouts, and nuts/seeds, also increases the risk of Salmonella infection. In 2008, raw tomatoes were linked to over 160 Salmonella infections. Salmonella can be cultured from animals, especially birds and reptiles; outbreaks have been linked to exposure to turtles, snakes, and other house pets, including iguanas, chicks, and ducklings. Many amphibians and reptiles contain Salmonella in their feces. Any dusty, dirty material may contain Salmonella as well as mycotic (fungal) organisms. Most amphibian, reptile, and bird feces may contain these infectious agents and may be the primary source of the bacteria for individuals who have close contact with these animals or their cages. Infections, person to person, can happen by fecal/oral contamination. Consequently, hand-washing and eating well-cleaned and thoroughly cooked foods decrease the risk of exposure to Salmonella and other infectious agents.
When Should Someone Seek Medical Care for a Salmonella Infection?
- Most cases of salmonellosis are self-limited and require no medical intervention.
- Some result in dehydration or have other complications that should be treated with medical help.
- If fever lasts longer than three days, if the diarrhea is bloody, if the person becomes weak and dizzy, or if abdominal pain is intense, medical help is probably needed.
- Patients who are compromised in any way (cancer, AIDS, the elderly, infants, people with weakened immune systems) should seek medical care soon after symptoms arise, especially if they have had any recent travel to a developing country.
- Anyone with initial symptoms of typhoid or paratyphoid fever (fever over four to seven days, diarrhea, weakness, abdominal pain), especially if they have had recent travel to developing countries, should seek medical care immediately.
How Do Doctors Diagnose a Salmonella Infection?
- Many cases are first suspected when a physician or public-health official recognizes a group of patients who ate food from a similar source has developed fever and diarrhea. The definitive diagnosis of all Salmonella infections is based on a culture of the organisms from the patient and occasionally from a source (for example, food or water).
- In most cases, the feces of the patient is cultured on agar media that are selective for Salmonella spp. Unfortunately, these culture results can take three to seven days to obtain.
- Although rapid tests such as polymerase chain reaction (PCR) that detect the genetic material of the bacteria have been attempted, PCR does not seem sensitive enough to detect the organisms in feces. Investigators suggest that the sensitivity of PCR is good when performed on a blood sample rather than feces, but this test is not widely available. Blood cultures are sometimes done in patients suspected of having typhoid and paratyphoid fevers. They are positive in about more than half of these patients. Culture results, when positive, help the physician to differentiate Salmonella infections from other organisms or infections that may have similar initial symptoms (for example, E. coli, Campylobacter, shigellosis, brucellosis, amebic dysentery, botulism, Listeria).
- Often rectal or fecal exams are done to determine if the patient has blood in the feces. Many physicians will empirically treat patients with antibiotics if they show blood in the feces because some physicians consider the infection more likely to be caused by an aggressive bacterial agent.
- If the patient's infection is severe, other ancillary tests may be done (CT scan, ultrasound, CBC count, liver function tests, biopsy) to help determine the extent of the infection.
Bowel regularity means a bowel movement every day.
What Is the Treatment for a Salmonella Infection?
Treatment for salmonellosis is controversial. Since the disease is often self-limited and some studies show no difference in outcomes between antibiotic-treated and untreated patient populations, many doctors recommend no antibiotics and cite evidence that antibiotics may prolong a carrier state.
Other physicians disagree and treat with antibiotics for up to 10-14 days. Most physicians will treat patients with antibiotics if the patients are immunosuppressed (for example, patients that have AIDS, cancer, or the elderly). Usually, the antibiotics (fluoroquinolones or cephalosporins) are given orally for a few days until the fever stops. If Salmonella is isolated from the patient, the bacteria should be studied to determine if they are resistant to certain antibiotics so the physician can choose the most effective treatment. This needs to be done especially for patients with typhoid and paratyphoid fever because all these patients should be treated with antibiotics that are effective against this Salmonella spp.
Just like the problems seen with MRSA worldwide, Salmonella types are being reported. Resistance to fluoroquinolone antibiotics has been reported to be as high as 41% in some S. typhi types or strains. The use of an effective antibiotic is important since antibiotic therapy for typhoid and paratyphoid may be lengthy (five to seven days for uncomplicated cases, 10-14 days for severe infections, four to six weeks for septic and focal infections, and one to three months in Salmonella carriers).
Occasionally, surgery is required to treat a site of focal infection. For example, the infected gallbladder is removed in some patients who are carriers.
What Are Complications of a Salmonella Infection?
The majority of patients with salmonellosis have no complications. However, patients who are immune compromised may develop complications similar to those for typhoid fever. Complications that may occur with typhoid fever (and less frequently for paratyphoid fever) are numerous and occur in about 30% of untreated or undertreated infections. Patients may exhibit one or more complications, with a low mortality rate (death). The following is a partial list of some of the most serious complications of typhoid fever that may become life-threatening far less frequently than food poisoning:
- intestinal perforation,
- myocarditis, abscesses,
- an aortic aneurysm,
- reactive arthritis,
- osteomyelitis (especially in patients with sickle cell disease), and
- a persistent carrier state.
There are many other problems that can occur with most organ systems in the body. Early diagnosis and appropriate antibiotic therapy greatly reduce the complication rate.
What Is the Prognosis for Patients Infected With Salmonella?
The prognosis for salmonellosis is very good since it is a self-limiting disease in most patients. Even immunosuppressed patients can do well if the disease is diagnosed and treated promptly. Complications occur if patients become dehydrated or if the disease is caused by aggressive or drug-resistant serovars.
Patients diagnosed with paratyphoid fever usually do well and develop fewer complications than patients with typhoid fever. If patients with paratyphoid or typhoid fever are diagnosed early and appropriate antibiotics are given, few or no complications develop and the patients recover. Untreated cases of typhoid result in some patients developing complications that can be severe and result in permanent disability or death.
Patients infected with S. typhi serovars that are resistant to multiple drugs have a worse prognosis and can develop more complications.
Is It Possible to Prevent Salmonella Infections?
For all Salmonella diseases, the key to prevention is proper hygiene and enforcement of public health rules and regulations. Proper hygiene starts with hand washing with soap and water before eating and especially after handling any raw foods such as eggs, meat, or produce. People can reduce bacterial infections by preventing cross-contamination of other foods and by not serving undercooked foods. Avoiding direct contact with carriers of Salmonella (for example, small turtles, snakes, chickens, pigs, and typhoid patients) reduces the chance of infection. Public-health practices such as chlorination of drinking water, enforcing restaurant cleanliness and employee hand washing, and not allowing human carriers of Salmonella to work in the food-handling industry further reduce the chance of Salmonella exposure. Perhaps the most famous failure of public health measures was in 1907 when a cook named Mary Mallon (Typhoid Mary) was suspected of infecting hundreds of individuals with a S. typhi serovar.
The CDC issues recalls of items, usually processed foods or vegetables, found to be contaminated with S. spp or other infectious or poisonous agents. People that take heed of these warnings and the accompanying advice reduce their chance of infection. In the past several years, recalls and reports of S. spp contamination of ground turkey (reportedly a drug-resistant strain in 2011), eggs, parsley, dog food, and other items have been publicly announced. Most recently, mangoes, cantaloupes, and Wawa Fruit Cups have been cited or recalled because of S. spp contamination. The source of cantaloupe contamination has been traced in August 2012, to Chamberlain Farms Produce; the company has suspended all melon shipments. As of August 2017, the U.S. Food and Drug Administration (FDA) and CDC recommends that Maradol papayas from Caribena, Cavi, and Valery be avoided. About 141 individuals (with 45 hospitalized) in 19 states so far have been diagnosed with the disease. The source is suspected to be contaminated papayas from Mexico. Cereal (for example, Kellogg's Honey Smacks) is a new source of Salmonella bacteria in foodborne illness.
Currently, there is no vaccine available to prevent salmonellosis, and the CDC does not recommend the general population be vaccinated against S. typhi serovars. However, the CDC does recommend that individuals going to developing countries where typhoid fever is endemic (some regions in Africa, Asia, and Latin America) be vaccinated with a typhoid vaccine. There are two types of vaccines currently available to individuals. Ty21a is an oral vaccine that requires four doses administered two weeks before travel, while ViCPS vaccine is injected once and requires only one dose administered one week before travel. The Ty21a immunization requires a booster every five years with the minimum vaccination age of 6 years, while ViCPS requires a booster every two years with a minimum vaccination age of 2 years. Work is in progress to develop additional vaccines for all Salmonella infections.
Reviewed on 4/4/2020
Klochko, A. "Salmonella Infection (Salmonellosis)." Dec. 20, 2019. <https://emedicine.medscape.com/article/228174-overview>.
United States. Centers for Disease Control and Prevention. "Salmonella infections linked to Kellogg's Honey Smacks cereal." Sept. 26, 2018. <https://www.cdc.gov/salmonella/mbandaka-06-18/advice.html>.