What is Salmonella?
The Salmonella bacterium causes one of the most common intestinal infections in the United States, salmonellosis. Salmonellosis is the second most common foodborne illness in the United States, and occurs only slightly less frequently than campylobacter infection (Craig and Zich, 2010). The Centers for Disease Control and Prevention (CDC) estimates that 1.4 million cases of nontyphoidal Salmonella occur yearly in the United States; of these cases, approximately 40,000 are culture-confirmed cases reported to the CDC (CDC, September 2010). For every single reported case of salmonellosis, there are 38 additional unreported cases (Maki, 2009). Each year, an estimated 15,000 hospitalizations and 500 deaths result from acute salmonellosis (Craig and Zich, 2010).
Salmonella, though named for Daniel E. Salmon, was actually first discovered in 1885 by Salmon’s research assistant, Theobald Smith. The genus Salmonella is subdivided into 50 serogroups (A, B, C1, C2, D, E, etc), which are further subdivided into over 2500 serotypes.
As medical technology evolved, researchers discovered more strains of the Salmonella virus; today, we know of about 2,450 serotypes of Salmonella that can cause salmonellosis (Bailey et al., 2010; D’Aoust, 2000). Despite the large number of serotypes that can cause human disease, more than half of salmonellosis cases are caused by only three serotypes: Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Newport (CDC, September 2010; Craig and Zich, 2010.).
Am I at risk for Salmonella infection?
Salmonella infects people of all age groups and backgrounds. cInfants, the elderly, and people who have compromised immune systems (for example, those with Human Immunodeficiency Virus) are at greater risk of severe or complicated disease. Children experience the highest risk of salmonellosis; children under five years old are diagnosed with the disease more frequently than all other people (CDC, September 2010; Voetsch et al, 2004).
How do people become infected with Salmonella?
The main channels of transmission can be characterized as “the five Fs”: food, feces, flies, fingers, and fomites (Giannella, 2010). Although direct person-to-person transmission of the infection can occur, more than 95% of Salmonella infections are transmitted by the consumption of foods contaminated with the bacteria. The bacteria live in the intestinal tracts of many animals, including cattle and poultry, wild animals, and pets, and are often found in these animals’ feces.
Salmonella bacteria are usually transmitted to humans who unknowingly consume food contaminated with the feces. In one study, almost half of the 500 outbreaks investigated over a 10-year period were related to animals or animal products (Giannella, 2010). Food may also become contaminated when someone infected with Salmonella handles or prepares food for someone else. The majority of the human infections stem from the widespread presence of the bacteria in lower-order animals. Beef and poultry products constitute the largest and most common sources of the bacteria, though Salmonella is also often found in unpasteurized milk, eggs, fish, and produce (Craig and Zich, 2010).
Salmonella infection may also occur through inadvertent contact with the feces of domesticated pets. About 10% of household cats and dogs can excrete the bacteria (Craig and Zich, 2010). In addition, pet reptiles – for example, iguanas, turtles, and snakes – have been the cause of multiple salmonellosis outbreaks.
How can I tell if my food is contaminated with Salmonella?
You can’t. Foods contaminated with Salmonella usually look, smell, and taste normal. Contaminated food sources typically associated with Salmonella are poultry, eggs, red meat, raw milk, and dairy products. More recently, infection has been associated with unpasteurized orange juice, cantaloupe, tomatoes, alfalfa sprouts, and other fresh produce.
What are the symptoms of a Salmonella infection?
The infectious dose of Salmonella is very small, and can be as small as 15-20 cells (US Food and Drug Administration (FDA), 2009). These bacteria subsequently multiply at an alarming rate, causing manifestation of symptoms. Salmonella can cause three different types of illness: gastroenteritis, typhoid fever, and bacteremia.
Gastroenteritis, the most common clinical manifestation of salmonellosis, presents in 75% of Salmonellainfections (Giannella, 2010). Defined as inflammation of the intestines or stomach resulting in some combination of diarrhea, nausea, and vomiting, gastroenteritis is typically characterized by fever, diarrhea (sometimes bloody), and abdominal cramps. The symptoms appear within 6 to 72 hours after consumption of the contaminated product; in some cases, however, the incubation period can be as long as seven to twelve days (Giannella, 2010).
Other common symptoms include vomiting, urinary tract infections, dysuria, pain in urination, nausea, headaches, myalgia (muscle pain), arthralgia (joint pain), fatigue, and dehydration. The illnesses usually last 5 to 7 days and most people recover without treatment, although some may require re-hydration with intravenous fluids.
Severe gastroenteritis may require hospitalization. In these patients, the Salmonella infection can spread from the intestines to the blood stream and into other body sites. Under these circumstances, the infection can cause death unless the patient is treated properly with antibiotics. Infants, the elderly, and people with impaired immune systems are more likely than others to become severely ill.
Typhoid fever occurs most commonly in developing countries. In the United States, cases of typhoid fever number fewer than 400 annually. In addition, the majority of these cases do not originate in the US but are brought into the country from overseas (National Institute of Health (NIH), 2009).
Individuals infected with Salmonella enterica serotypes typhi and paratyphi, and approximately 8% of individuals infected with strains of Salmonella other than typhi and paratyphi, develop an illness called Typhoid fever (less commonly known as enteric fever) (Giannella, 2010). The onset of symptoms usually occurs within 5 to 21 days after ingestion of the Salmonella Typhi bacteria.
Early symptoms of typhoid fever include abdominal pain, fever, and a “general ill-feeling” (NIH, 2009). As the disease progresses, severe diarrhea develops and the fever elevates to over 103 degrees. Some infected individuals experience a rash on the stomach and chest area; the rash takes the form of small red spots called “rose spots.” Other symptoms may include cough, sore throat, headache, abdominal tenderness, bloody stools, severe fatigue, agitation, nosebleeds, slowed heartbeat, enlarged liver, and enlarged spleen.
Generally, with early detection and treatment of typhoid fever, symptoms improve in 2-4 weeks and the prognosis for the infected individual is good. However, with the development of complications – often because of a failure to detect and treat the disease at an early stage – the outcome becomes poor. Such complications may include intestinal hemorrhage and severe gastrointestinal bleeding, kidney failure, intestinal perforation, and peritonitis (an inflammation of the thin tissue that lines the inner wall of the abdomen and covers most of the abdominal organs).
Bacteremia, the presence of bacteria in the blood, occurs when Salmonella bacteria enter and circulate within and individual’s bloodstream, which typically is a sterile environment. Salmonella serotype choleraesuis is the most common cause of Salmonella bacteremia, though it can result from exposure to and infection with a number of different Salmonella serotypes (Roy, 2010). This condition occurs in approximately 10% of Salmonella infections (Giannella, 2010), and can lead to sepsis and/or infection at sites other than the initial site of the Salmonella infection, which is typically the gastrointestinal tract. In cases of bacteremia, the infected individual may not experience any gastrointestinal symptoms (such as vomiting, nausea, or diarrhea) commonly associated with Salmonella infection.
How is a Salmonella infection detected?
A Salmonella infection is usually diagnosed by a positive culture from a stool sample of the infected person. In more severe illnesses, Salmonella bacteria can sometimes be detected in a contaminated person’s blood or urine. In most cases, doctors do not order a culture and simply treat the symptoms.
Even when a culture is ordered, detection of Salmonella can be very difficult, and as a result, only about 3% of Salmonella cases are officially detected and reported. When examining each of these cultures, the laboratory technician is asked to microscopically pick out Salmonella bacteria from thousands of other bacteria which are normally present in the cultures. In addition, cultures taken from people who have already started a course of antibiotics are less likely to produce positive results, even if those persons are infected with Salmonellabacteria.
How is a Salmonella outbreak detected?
The CDC explains that in order to find cases in an outbreak of Salmonella infections, public health laboratories serotype the Salmonella bacteria and perform a kind of “DNA fingerprinting” on laboratory samples. Investigators determine whether the “DNA fingerprint” pattern of Salmonella bacteria from one patient is identical to those from other patients in the outbreak and from the contaminated food. Bacteria with the same “DNA fingerprint” are likely to come from the same source.
Once an outbreak (defined as two or more identical Salmonella isolates) have been identified, public health officials then conduct intensive investigations, including interviews with ill people, to determine if people whose infecting bacteria match by “DNA fingerprinting” are part of a common source outbreak.
A series of events occurs between the time a patient is infected and the time public health officials can determine that the patient is part of an outbreak. This means that there will be a delay between the start of illness and confirmation that a patient is part of an outbreak. The timeline is as follows:
The duration of a community outbreak can run up to several months. For instance, the town of Riverside, California experienced an outbreak related to contamination of the municipal water supply; the epidemic lasted for months and involved approximately 16,000 individuals (Giannella, 2010).
Are there any long term complications of a Salmonella infection?
Persons with salmonellosis usually recover completely, although it may take several months before their bowel habits are entirely normal. Recent studies have indicated that 25% of patients reported altered bowel habits six months after a bacterial gastroenteritis illness.
Unfortunately, some people develop two more serious complications: Irritable Bowel Syndrome (IBS) and Reiter’s Syndrome.
Irritable Bowel Syndrome
Irritable Bowel Syndrome is a functional bowel disorder characterized by abdominal pain, diarrhea (with mostly loose stool passed urgently), and occurring more frequently than normal (Spiller and Garsed, 2009). In some, IBS manifests as alternating bouts of diarrhea and constipation. Other symptoms include whitish mucous in the stool, a feeling of incomplete evacuation (tenesmus), and bloating or abdominal distension. One study found that after an outbreak of salmonellosis, 31% of those infected subsequently developed new IBS symptoms that remained one year after infection (Mearin et al., 2005; McKendrick and Read, 1994).
Researchers have identified several medical conditions, or comorbidities, which appear more frequently in patients diagnosed with IBS. These include headaches, depression, fibromyalgia, inflammatory bowel disease, abdominal surgery, and endometriosis. For most people, IBS is a chronic condition, although the severity of symptoms varies over time.
IBS is commonly treated by attempting to relive its symptoms. Doctors routinely suggest one or more of the following for IBS patients: fiber supplements, anti-diarrheal medications, elimination of high-gas foods, anticholinergic medications, antidepressant medications, and counseling.
IBS can be extremely painful, embarrassing, costly, and can dramatically affect one’s quality of life and productivity at work. Recent studies indicate that IBS patients incur an average annual medical cost of $5,049 and $406 in out-pocket expenses. In addition, a study of workers afflicted with IBS found that they reported a 34.6% loss in productivity, corresponding to 13.8 hours lost for a 40-hour work week.
Reiter’s Syndrome, which is increasingly referred to as reactive arthritis, is an uncommon but debilitating result of a Salmonella infection. Reiter’s Syndrome cannot be diagnosed by any single laboratory test; instead, physicians must assess a constellation of symptoms, often seemingly unrelated, to arrive at the diagnosis.
The American Rheumatism Association advocates using the following paradigm to evaluate possible cases of reactive arthritis: the patient experiences an episode of peripheral arthritis that lasts more than one month and is accompanied by either urethritis (inflammation of the urethra, often evidenced by painful urination) or cervicitis (inflammation of the cervix), or both urethritis and cervicitis. An alternate set of criteria supports a diagnosis of Reiter’s Syndrome when the patient exhibits arthritis, conjunctivitis (eye inflammation and irritation), and urethritis; however, only about one-third of patients with Reiter’s Syndrome show this complete triad of symptoms (Wu and Schwartz, 2008).
There is a broad range in the symptoms from Reiter’s Syndrome. While most symptoms commonly appear within four weeks of the initial Salmonella infection, in some patients certain symptoms may manifest years after the initial infection. Generalized symptoms – including fever, fatigue, and weight loss – are often accompanied by urogenital, ophthalmologic, rheumatologic, dermatologic, and/or visceral manifestations of the disease.
Most medical literature suggests that many patients recover within a year. Unfortunately, for some patients, the condition can become permanent. One recent study confirmed that nearly 50% of patients continued to have symptoms more than a year after onset.
Every person suffering from Reiter’s Syndrome exhibits rheumatologic symptoms; typically, the arthritis is polyarticular (affecting more than one joint) and asymmetric (not the same on both sides of the body; for instance, affects the right elbow but not the left). During the initial episode, the joints most commonly affected are the knees, ankles, and feet; this often causes pain and swelling. According to one study, 49% of patients experience back pain at the onset of Reiter’s Syndrome (Wu and Schwartz, 2008). Such pain likely results from spondylitis, which is inflammation of the vertebrae in the spinal column. Wrists, fingers, and other joints can be affected, although this occurs less frequently. Symptoms of Reiter’s Syndrome are also found in the urogenital tract – which includes the prostate, urethra, and penis in men, and fallopian tubes, uterus, and vagina in women – with 90% of patients experiencing urethritis/cervicitis (Wu and Schwartz, 2008; Callen and Mahl, 1992). The severity and duration of urethritis varies, and it may even be asymptomatic (lack of noticeable symptoms). Men may notice an increase in urinating frequency, 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.
Patients with Reiter’s Syndrome commonly develop enthesopathy, i.e. inflammation where the ligament and tendon attach to the bone. Studies have shown that 40% of patients with Reiter’s Syndrome experience heel pain at the onset of the disease; this pain results from inflammation of the Achilles tendon, and is recognized as enthesitis (Wu and Schwartz, 2008; Amor, 1998).
The involvement of the eye in Reiter’s Syndrome is most commonly manifested as conjunctivitis, the inflammation of the mucus membrane that covers the eyeball; studies estimate that anywhere between 30 and 60% of patients suffering from reactive arthritis also experience conjunctivitis. Uveitis (inflammation of the inner eye) occurs in between 12 and 37% of cases, and patients experience keratitis (inflammation of the cornea) in 4% of reactive arthritis cases (Wu and Schwartz, 2008). These conditions can cause redness of the eyes, eye pain and irritation and blurred vision. If left untreated, uveitis can result in blindness.
Although there is no cure for Reiter’s Syndrome, there are available treatments for its symptoms. Doctors typically employ one or more of the following treatments for Reiter’s patients: bed rest, exercise, non-steroidal anti-inflammatory drugs, corticosteroid injections, topical corticosteroids; antibiotics; and immunosuppressive medicines.
In past Salmonella outbreaks, persistent reactive arthritis has been reported in up to 30% of the confirmed cases; the classic triad of Reiter’s Syndrome symptoms – arthritis, conjunctivitis, and urethritis – occurred in approximately 2% of patients (Craig and Zich, 2010).
Who is at risk for developing a Salmonella infection?
Persons who are most at-risk for developing Salmonella infection include:
What can I do to prevent a Salmonella infection?
To prevent a Salmonella infection, you should do the following:
References (In Order of Appearance)
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