Salmonella are a group of closely related rod-shaped, Gram-stained negative bacteria that have flagella (tail-like structures used for movement). Salmonellatypes 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 has changed in recent decades. Currently, many investigators consider the over 2,500 serovars 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, 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. Salmonella were first isolated from infected pigs in 1885 by Theobald Smith and were named after his lab director, D. E. Salmon.
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, the 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, or 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. Unfortunately, the above symptoms can occur with many types of infectious organisms such as Shigella, Staphylococcus, Campylobacter, some additional bacteria, viruses, and protozoa.
Typhoid fever occurs when some of the Salmonella organisms (often identified as Salmonella typhi or 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 of typhoid 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 feveris also referred to as enteric fever.
Paratyphoid fever, like typhoid fever, is also termed enteric fever. Paratyphoid 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), gastroenteritis in conjunction with herpes labialis and gastroenteritis (B), rarely, with septicemia and abscesses (C). S. paratyphi is the organism that causes this disease.
Usually, Salmonella spp. are ingested. It is commonly accepted that at between 1 million to 1 billion bacteria are needed to cause infection although some investigators suggest some people may be infected by far fewer bacteria. Nevertheless, most data suggest food, water, or other sources of contamination contain large amounts of bacteria.
Although human stomach acid can reduce and sometimes eliminate Salmonellaspp., occasionally some bacteria get through to the intestine and then attach and penetrate the cells. Toxins produced by the bacteria (enterotoxin and cytotoxin) can damage and kill the cells that line the intestines, which results in intestinal fluid loss (diarrhea).
Some Salmonella can survive in cells of the immune system and can reach the bloodstream, causing blood infection (bacteremia). Other Salmonella spp. can enter the gallbladder, leaving the affected patient a chronic carrier of the organisms.
Salmonella can then be shed with the bile from the gallbladder into the feces and then may infect other people. Perhaps the most famous such carrier, the first to be identified in 1907, was a cook named Mary Mallon, also known as "Typhoid Mary". She was suspected of infecting hundreds of individuals.
Treatment for enteritis or food poisoning is controversial. Some doctors recommend no antibiotics since the disease is self-limited, while others suggest using antibiotics such as ciprofloxacin (Cipro, Cipro XR, Proquin XR) for 10-14 days. Some medical investigators believe antibiotics prolong the carrier state.
However, patients identified as immunosuppressed (for example, patients with AIDS or undergoing cancer chemotherapy, infants under 2 months of age, or the elderly) should receive antibiotics; they may need to be hospitalized and have some of their care managed by an infectious disease consultant. In addition, pregnancy often predisposes the mother to get all types of food poisoning, including Salmonella.
Pregnant females should take care to wash and cook foods well before eating. They should contact their OB/GYN doctor if they begin to have any signs of dehydration, especially if they develop a fever above 101 F.
Supportive therapy for both enteritis and enteric fevers consists mainly of preventing dehydration and electrolyte abnormalities (for example, abnormal levels of potassium and sodium ions) with fluids containing electrolytes (for example, IV fluids or oral fluids like sports drinks).
Carriers of Salmonella are considered to be infected even though they may show no symptoms. Carriers can infect other people and need to be cured of the carrier state. About 85% of carriers can be cured by a combination of surgery to remove their gallbladder and antibiotic treatments.
How can Salmonella infection be prevented?
Cleanliness is a key to prevention. Hand washing with soap and hot water, especially after handling eggs, poultry, and raw meat is likely to reduce the chance for infections. The use of antibacterial soaps has been recommended by some investigators.
By using chlorine-treated drinking water, washed produce, and by not ingesting undercooked foods such as eggs, meat, or other food, people can also reduce the chance of exposure to Salmonella. Avoiding direct contact with animal carriers of Salmonella (for example, turtles, snakes, pigs) also may prevent the disease.
Public-health authorities that enforce restaurant cleanliness and employee hand washing have helped in general prevention. Human carriers of Salmonella should never work in the food-handling service industry and ideally should undergo gallbladder removal and antibiotic therapy for an attempt for a cure of the carrier state.
Public-health authorities also ask for product recalls when products are contaminated with Salmonella or other contaminating organisms or toxins. In 2009, there was a recall for peanut-containing foods (for example, peanut butter, cookies, crackers).
The Westco Fruit and Nut Co., Inc., provided peanut-based paste that was reported to contain Salmonella and was used to make many food products. Eventually, about 3,800 products were recalled. Similar recalls have occurred for pistachio nuts from a California provider in March 2009 and for tomatoes in 2008, both contaminated with Salmonella. Recent recalls for contaminated beef in July and August 2009 contained Salmonella with multiple drug resistance.
The huge egg recall in September 2010 may have been prevented if the several egg-farming companies had practiced good sanitation and provided sanitary conditions for the hens to be housed. Recalls increase the safety of the population from exposure to microbial and toxic food contaminants.
Although some Salmonella vaccines are available for poultry and animals, human vaccines are available only for typhoid fever. However, the CDC does not recommend that everyone get vaccinated for typhoid fever; they recommend that only those people going to developing countries where typhoid fever is endemic (for example, regions in Africa, Asia, and Latin America) should receive the vaccine.
The typhoid fever vaccine is available in an oral (Ty21) and injectable form (ViCPS). People planning to request these vaccines should notify their doctors well in advance (about eight to 10 weeks) before they need the vaccine as it may not be readily available and need to be administered about two weeks before travel. Researchers are attempting to develop other vaccines for all types of Salmonella infections.
The animal vaccine is being used in Europe to make hens less likely to transmit disease to their eggs and to other hens. However, those European egg producers who use the vaccine on hens are cautioned to also maintain sanitary conditions and good housing for the poultry since the vaccine is not 100% protective.
The FDA does not require U.S. egg producers to vaccinate their hens; however, the 2010 egg recall has caused the agency to reexamine this policy and the FDA may require vaccination of hens in the future.