Salmonella
Medical Author: Charles Davis, MD, PhD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
Is It Salmonella?
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
The pistachio nut recall in March 2009 is only one example of numerous product recalls in recent years due to fears of contracting Salmonella food poisoning. Similarly, this year products processed by the Peanut Corporation of America (PCA) were found to be the source of a Salmonella outbreak. In 2008, an outbreak arose from the consumption of certain jalapeño and serrano peppers from Mexico. As these and numerous other outbreaks illustrate, virtually any food can become contaminated with one of the many species of Salmonella.
Salmonella is a bacterial infection that is passed to humans from animals, including poultry, cattle, pigs, and domestic animals. Eating undercooked poultry and drinking unpasteurized milk are among the ways humans can acquire the infection. But increasing media coverage has focused on vegetable products as the source of many Salmonella outbreaks. When vegetables or fruits are the source of an outbreak, it means that these products have been handled unsafely, such as processing or preparation on surfaces that have become contaminated with animal feces or raw poultry. Another way for vegetables to become contaminated is by an infected food handler.
What is Salmonella?
Salmonella (S.) is the genus name for a large number (over 2,500) of types of
bacteria. Each type is distinctly identifiable by its specific protein
coating. The types are otherwise closely related. Salmonella bacteria are rod-shaped,
flagellated, Gram stain-negative, and are known to cause disease in humans,
animals, and birds (especially poultry) worldwide.
The terminology that identifies the particular protein coats, or serovars, is
not well settled, and what previously were thought to be various species of the
genus Salmonella are now thought to be serovars of only two species by many researchers, S. enterica
and S. bongori. However, these designations are not always accepted in the
scientific literature and so common serovars that have been named in the past
are still used (for example, S. typhi, S. typhimurium, S. enteritidis,
S. cholerasuis, S. saintpaul). The serovars are identified by the Kauffman-White classification
that uses two major types of antigens (somatic O and flagellar H) to distinguish
the over 2,500 types of Salmonella bacteria. Sometimes laboratories or other
reporting agencies identify isolates simply as Salmonella spp (species) and do
not identify the serovars.
Who discovered Salmonella?
The bacteria were first
isolated by Theobald Smith in 1885 from pigs. The genus name Salmonella was
derived from the last name of D.E. Salmon, who was Smith's director.
What are symptoms of Salmonella infections?
Salmonellosis (gastroenteritis characterized by nausea, vomiting, and
diarrhea) is the most common disease caused by the organisms. Abdominal cramping also may occur. Salmonellosis thus produces the symptoms that are commonly referred to as food poisoning. Although food poisoning is usually a mild disease, nausea, vomiting, and diarrhea can lead to dehydration and even death (about 500 per year in the U.S.). It is important to note that many other organisms (for example, viruses,
E. coli, Shigella) and toxins (for example, botulism, mushroom toxin, pesticides) can produce food poisoning symptoms. However, over 1.4 million cases of salmonellosis occur per year
in the U.S., and the rest of industrialized countries have similar high rates. Countries with poor sanitation have a much higher incidence of
salmonellosis.
Typhoid fever occurs when some of the Salmonella organisms (often identified as
S. typhi) are not killed by the normal human immune defenses (macrophage cells) after they enter the gastrointestinal tract. Salmonella then survive and grow in the human spleen, liver, and other organs
and may reach the blood (bacteremia). Salmonella can be shed from the
liver to the gallbladder, where they can continue to survive and be
secreted into the patient's feces for up to a year. Symptoms include
high fevers up to 104 F, sweating, inflammation of the stomach and
intestines, and diarrhea. Symptoms usually resolve, but many patients
become Salmonella carriers. Approximately half of patients develop
slow heartbeat (bradycardia), and about 30% of patients get flat,
slightly raised red or rose-colored spots on the chest and abdomen.
Typhoid fever is also referred to as enteric fever.
Paratyphoid fever, also termed enteric fever, has symptoms like typhoid, but
it's usually not as severe. Subtypes are A, B, and C and vary by having small changes in
symptoms, such as more rose spots (A), in conjunction with herpes labialis and
gastroenteritis (B), rarely, with septicemia and abscesses (C).
Next: How is Salmonella transmitted to humans? »
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