What is salmonellosis?
Salmonellosis is an infection caused by the Salmonella genus of the bacterial family Enterobacteriaceae. Acute gastroenteritis is the most common form of salmonellosis. Other clinical manifestations of salmonellosis include enteric fever and bacteremia. Hematogenous seeding of other organs may lead to secondary manifestations of salmonellosis, including endocarditis, pneumonia, pyelonephritis, septic arthritis, and osteomyelitis.
Salmonella bacteria live in the intestinal tracts of humans and of a variety of wild and domestic animals throughout the world. Poultry, cattle, dairy cows,pigs, sheep, goats, and other agricultural livestock commonly carry Salmonella. Pet birds, lizards, turtles, iguanas, dogs, and cats may also carry disease-causing Salmonella species. Bacteria pass in the feces of infected animals. Fecal contamination of food or water, unsanitary food-handling practices, and infection of egg-laying hens can lead to salmonellosis.
In the developing world, lack of sewage systems, inadequate water treatment, and inconsistent or inadequate sanitation practices may lead to Salmonella contamination of drinking water or agricultural fields. Most salmonellosis outbreaks involve acute gastroenteritis, although typhoid fever remains a significant health threat in impoverished parts of the world, including areas of Asia, Africa, Central America, and South America. S. enterica sub. enterica serovar Typhi (or S. typhi) and S. enterica sub. enterica serovar Paratyphi (or S. paratyphi), the causative agents of typhoid and paratyphoid fever, respectively, live exclusively in the human intestine. These illnesses pass from one person to another through human fecal contamination of foodstuffs. S. typhi carriers can cause community outbreaks, as in the infamous case of Typhoid Mary in New York City in the early twentieth century.
Infection of egg-laying hens is a common cause of salmonellosis outbreaks. The bacteria infect the ovaries of hens, contaminating the eggs before shell formation. Infected hens commonly appear healthy, complicating detection of Salmonella-contaminated eggs. Breaks in sanitation or hygiene protocols during food cultivation, processing, and packaging can also lead to salmonellosis outbreaks.
Ingestion of raw or undercooked eggs is a significant risk factor for eggborne salmonellosis. Unpasteurized milk can also transmit Salmonella. Inadequate handwashing and cross-contamination between meats and uncooked produce during food preparation can potentially lead to salmonellosis.
Because livestock and pets may carry Salmonella bacteria, the failure to wear gloves or thoroughly wash the hands after handling animals or their feces is a risk factor for salmonellosis. Cattle, dairy cows, poultry, turtles, lizards, and snakes are common sources of infection.
Young children, persons with human immunodeficiency virus infection or acquired immunodeficiency syndrome, organ transplant recipients, and those undergoing cancer treatments have an increased risk for salmonellosis because of their immature or weakened immune systems.
Salmonella gastroenteritis symptoms begin twelve to seventy-two hours after ingestion of water or food and include nausea, vomiting, diarrhea, and abdominal cramps, which may be accompanied by a fever of 100° to 102° Fahrenheit (38° to 39° Celsius), chills, and headache. Fever typically resolves within two to three days; gastrointestinal symptoms usually abate within seven days. Immunocompromised persons may develop severe symptoms, bacteremia, and dehydration.
Symptoms of typhoid and paratyphoid fever develop more gradually than those associated with salmonella gastroenteritis. Approximately six to thirty days after infection, a low-grade fever, headache, anorexia, fatigue, and abdominal pain develop, typically accompanied by constipation or diarrhea. Symptoms intensify over three to four days, with fevers typically reaching 101° to 104° F (38.5° to 40° C). Some persons will develop a maculopapular, rose-colored rash on the trunk. Although uncommon, persons with enteric fever may develop neuropsychiatric symptoms, including lethargy, confusion, frank delirium, seizures, and coma.
Routine screening is not conducted for salmonellosis, except in outbreak situations wherein carrier identification becomes critical. Isolation of Salmonella from body tissues, fluids, or excretions remains the cornerstone of diagnosis. For acute gastroenteritis, diagnosis is confirmed by isolation of Salmonella from the stool. In persons with bacteremia, isolation of Salmonella from the blood confirms the diagnosis.
Definitive diagnosis of enteric fever is often challenging because of the limited sensitivity of blood and stool cultures with this form of salmonellosis. Isolation of S. typhi or S. paratyphi from the blood, stool, bone marrow, intestinal secretions, urine, or secondary infection sites confirms the diagnosis; bone marrow cultures are the most sensitive. Serologic tests for antibodies to S. typhi antigens may also aid in the diagnosis of typhoid fever. Polymerase chain reaction testing for Salmonella is used in some locales.
In an otherwise healthy person, nontyphoidal, gastrointestinal salmonellosis is typically a self-limited illness, resolving spontaneously without antibiotic therapy. Oral or intravenous fluid replacement may be necessary for persons with severe symptoms. Antibiotic therapy is commonly prescribed for immunocompromised persons with gastrointestinal salmonellosis and for those who are otherwise at high risk for complications. Salmonella bacteremia and enteric fever are treated with antibiotic therapy, commonly a fluoroquinolone or a third-generation cephalosporin.
Proper food handling and handwashing are key to preventing gastrointestinal salmonellosis. One should wash hands with soap and running water before and after handling food, especially raw meats. Thoroughly cooking meat and eggs kills Salmonella contaminants that may be present. Handwashing after using the toilet, changing diapers, and handling animals also helps prevent salmonellosis.
The Centers for Disease Control and Prevention recommends typhoid immunization for travelers visiting areas with a high incidence of S. typhi. Oral and intramuscular vaccines effectively protect approximately 50 to 80 percent of persons immunized against typhoid fever.
Fischer Walker, Christa L., David Sack, and Robert E. Black. “Etiology of Diarrhea in Older Children, Adolescents, and Adults.” PLoS Neglected Tropical Diseases 4 (2010): e768. Systematic review of twenty-two studies from around the world, examining the causative agents of infectious diarrhea and demonstrating the ongoing, high incidence of salmonellosis in community settings.
Pegues, David A., and Samuel I. Miller. “ Salmonella Species, Including Salmonella typhi.” In Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, edited by Gerald L. Mandell, John F. Bennett, and Raphael Dolin. 7th ed. New York: Churchill Livingstone/Elsevier, 2010. Infectious disease text withreferenced discussion of salmonellosis epidemiology, microbiology, pathogenesis, treatment, and prevention.
Thaver, Durrane, et al. “A Comparison of Fluoroquinolones Versus Other Antibiotics for Treating Enteric Fever.” British Medical Journal 338 (2009): b1865.Meta-analysis of twenty randomized, controlled trials examining clinical and microbiological failures and relapse rates associated with antibiotics for the treatment of enteric fever.