eMedicine Specialties > Emergency Medicine > Infectious Diseases

Salmonella Infection

Author: Michael D Owens, DO, FACEP, FAAEM, Clinical Faculty, Emergency Medicine Residency, Naval Medical Center Portsmouth; Consulting Staff, Department of Emergency Medicine, Chesapeake Emergency Physicians, Inc, Chesapeake Regional Medical Center
Coauthor(s): Dirk A Warren, MD, Emergency Medicine Resident, Naval Medical Center Portsmouth
Contributor Information and Disclosures

Updated: Apr 3, 2009

Introduction

Background

Salmonella are gram-negative facultative intracellular anaerobes causing a wide spectrum of disease. This spectrum can range from a gastroenteritis, enteric fever (caused by typhoid and paratyphoid serotypes), bacteremia, focal infections, to a convalescent lifetime carrier state. The type of infection depends on the serotype of Salmonella and host factors. It maintains a broad host range, and for unknown reasons, results in different diseases in different hosts.

Although the taxonomy of Salmonella can be confusing, all Salmonella serotypes are members of a single species, Salmonella enterica. More than 2500 serovars^1,2 have been described of which humans are almost exclusively infected by Salmonella enterica subsp enterica serotypes typhi, typhimurium, and choleraesuis worldwide.³  In the United States, Salmonella enteritidis (17%), Salmonella typhimurium (16%), Salmonella newport (10%), and Salmonella javiana (5%) account for nearly one half of the human isolates.⁴

Salmonella has a widespread distribution in the environment, and certain host factors make humans particularly susceptible to infection. Its increasing antimicrobial resistance, prevalence, virulence and adaptability, are a challenge worldwide.

Pathophysiology

Salmonella infection most commonly begins with ingestion of bacteria in contaminated food or water. However, direct contact with animal and human carriers has also been implicated. Reptile and amphibian carriers are the most commonly recognized sources of direct contact.⁵ Studies involving healthy human volunteers required a median dose of 1 million bacteria to produce disease. However, point outbreaks suggest as few as 200 bacteria may produce nontyphoid gastroenteritis.⁶

Once the bacteria survive the acidic stomach, it colonizes the intestine and translocates across the intestinal epithelium via 3 routes: (1) invasion of the enterocytes, (2) invasion of epithelial cells called M cells, and (3) through dendritic cells that intercalate epithelial cells.² Interaction with the epithelium and resident cells promote a proinflammatory response to include cytokines, chemokines, neutrophils, macrophages, dendritic cells, and T and B cells. This inflammatory host response can actually benefit the intestinal pathogens and contribute to the nature and severity of the infection by establishing a competitive advantage against the native flora.²

After crossing this epithelial layer, the bacteria replicate in macrophages in Peyer’s patches, mesenteric lymph nodes, and the spleen. Once colonized, the bacteria may then potentially disseminate to the lungs, gallbladder, kidneys, or central nervous system. The nontyphoid species of Salmonella tend to produce a more localized response because they are felt to lack the human-specific virulence factors. However, the typhi serotype can develop the more invasive disease resulting in bacteremia. The severity of disease is related to the serotype, number of organisms, and host factors.

Eggs and poultry are the most common sources of infection.^7,5 Ingestion of contaminated water, milk, milk products, beef, fruit, vegetables, and dairy products are also common sources. Potential sources of infection for infants with Salmonella are exposure to reptiles, riding in a shopping cart next to meat or poultry, or consuming liquid infant formula.⁸ Recent outbreaks have been associated with contaminated frozen potpies, puffed vegetable snacks, and exposure to turtles.⁴  More recently, multistate human outbreaks have been seen in contaminated dry dog food and peanut butter products.^9,10

Reservoirs of the bacteria include humans, poultry, swine, cattle, rodents, and pets such as iguanas, tortoises, turtles, terrapins, chicks, dogs, and cats. Up to 90% of reptiles and amphibians harbor Salmonella in their gastrointestinal tracts, and 6% of nontyphoid disease is related to direct contact with these animals.⁵

Fecal-oral transmission from person to person in areas with poor sanitation and contaminated or nonchlorinated water is the route for enteric or typhoid fever. Humans are the only known carriers of Salmonella typhi.⁵

Individual susceptibility to Salmonella infection increases with extremes of age, immunodeficiency states, prior antibiotic use, neoplastic disease, achlorhydria or antacid use, recent bowel surgery, and malnutrition.

Frequency

United States

Prevalence estimates vary secondary to inconsistent diagnosis and reporting techniques. It is estimated that only 3% of Salmonella infections are laboratory confirmed and reported to the Centers for Disease Control and Prevention (CDC).¹⁰ However, an estimated 1.4 million people in the United States are infected with nontyphoid Salmonella annually. The incidence of nontyphoid disease in the United States has been stable since 2004 but has decreased approximately 8% from 1996-1998 levels.⁴ The true burden of nontyphoid Salmonella in the United States is calculated to be 520 cases per 100,000 compared with 13.4 cases per 100,000 of laboratory-confirmed cases annually, taking into account approximately 38.6 cases of nontyphoid Salmonella for each culture confirmed case.^11,12 The reported 2007 incidence is 14.9 cases per 100,000.⁴  

Additionally, an estimated 500 people are infected with typhoid Salmonella annually.⁵ Most cases of documented typhoid disease are related to foreign travel to developing nations such as India (30%), Pakistan (13%), Mexico (12%), Bangladesh (8%), Philippines (8%), and Haiti (5%).⁵

International

Fully industrialized nations report frequencies of gastroenteritis similar to that of the United States. However, worldwide estimates of nontyphoid Salmonella range from 200 million to 1.3 billion, with an estimated death toll of 3 million each year.¹  

The serovars responsible for typhoid or enteric fever, typhi and paratyphi, that cause systemic illness lead to 16-20 million cases and 200,000 deaths worldwide.^13,14 Compared with tourists, travelers visiting friends or relatives in developing nations exhibit a much higher incidence of typhoid or enteric fever.⁵

Mortality/Morbidity

Twenty percent of patients require hospitalization, with an estimated death rate of 0.6%.¹² Infection with drug-resistant nontyphoid Salmonella and Salmonella typhi increase the likelihood of hospitalization and death.¹²

Invasive nontyphoid Salmonella infection occurs in about 5% of cases in Israel¹² and is responsible for 400-600 deaths in the United States each year⁵ . Mortality for nontyphoid Salmonell a is reported to be as high as 60% in African patients with HIV.¹³ Mycotic abdominal aortic aneurysms are more common in immunocompromised and HIV patients.

Treated typhoid cases have a 2% mortality rate with a 15% relapse rate.³ A significant number of typhoid patients become chronic asymptomatic carriers and can shed high numbers of bacteria in the stool for a lifetime without obvious symptoms.¹³ Depending on the serotype, roughly 1% of adults and 5% of children excrete organisms for greater than a year.¹⁵

Age

Attack rates are highest in persons younger than 20 years or older than 70 years. The highest rate is found in infants (130 isolates per 100,000).

Neonates are at a greater risk to fecal-oral transmission secondary to relative decreased stomach acidity and buffering of ingested breast milk and formula.

Elderly persons are at a relative greater risk to infection secondary to chronic underlying illness and weakened immunity. Nursing home residents have a particularly higher risk.

Clinical

History

• Obtain a dietary history. Inquire about potential restaurant sources, food preparation techniques, and exposure to potentially contaminated or nonchlorinated water sources.
• Obtain a travel history. Typhoid fever is increasingly associated with international travel to developing nations.
• Determine if other patient contacts have similar illnesses, food ingestions, or animal contacts.
• Salmonella syndromes can be divided into gastroenteritis, enteric fever, bacteremia, localized infection, and a chronic carrier state.
• Gastroenteritis
  • Incidence is highest during May through October in temperate climates.⁶
  • The incubation period is from 8-48 hours after the ingestion of contaminated food or water.
  • Symptoms are acute onset of fever and chills, nausea and vomiting, abdominal cramping, and diarrhea.
  • If a fever is present, it generally abides in 72 hours.
  • Diarrhea is usually self-limited, lasting 3-7 days and may be grossly bloody. Diarrhea lasting more than 10 days suggests another diagnosis.⁶   
• Enteric (typhoid) fever
  • The incubation period of enteric (typhoid) fever is 5-21 days.
  • Transmission is generally from contaminated water or animal products or contact with an infected person or carrier.
  • The initial prodrome lasting 7-10 days includes headache, cough, diaphoresis, anorexia, weakness, sore throat, malaise, abdominal pain, and constipation or “pea soup” diarrhea.
    • Abdominal pain is present in 20-40% of patients.⁶
    • Constipation is found in 10-38% of patients.⁶
  • These prodromal symptoms typically plateau as the fever increases in a stepwise fashion peaking in the second week of illness.
  • After the prodrome, splenomegaly, abdominal distention and pain, relative bradycardia, rash, meningismus, and mental confusion may occur. It may disseminate to lungs, gallbladder, kidneys, or CNS.
  • Untreated patients experience either complications or resolution by the fourth week.
    • Intestinal perforation occurs in 3-10% of patients.⁶
    • Other complications include endocarditis, pericarditis, pneumonitis, orchitis, and focal abscess.
• Bacteremia
  • Bacteremia typically occurs in immunocompromised patients.
  • Prolonged or recurrent fevers may occur.
  • It may include focal infections.
  • Mycotic abdominal aortic aneurysm may occur.  
•  Localized infection
  • Localized infection occurs in 5-10% of persons with bacteremia.⁶
  • The endocardium, arteries, CNS (more commonly infants), lungs, bones, joints, muscles, soft tissues, reticuloendothelial system, kidneys, and genital regions have all been documented sites of extraintestinal infection. 
• Chronic carrier state
  • Chronic carrier state is defined as Salmonella in the stool or urine for greater than 1 year.
  • A chronic carrier state occurs in 0.2-0.6% of patients with nontyphoid Salmonella.⁶
  • A chronic carrier state occurs in 1-4% of patients with untreated typhoid Salmonella.

Physical

Physical findings can vary depending on the clinical syndrome, serotype, and patient’s immune status. However, the physical findings in gastroenteritis, enteric (typhoid) fever, and bacteremia frequently overlap.

• Gastroenteritis 
  • Fevers 38-39°C are common.
  • Physical signs of dehydration may be found.
  • Stool examination can be negative to grossly bloody.
  • Diffuse nonfocal abdominal tenderness is commonly present.
  • In rare cases, Salmonella infection mimics inflammatory bowel disease or pseudoappendicitis.⁶
• Enteric or typhoid fever
  • A stepwise increase in temperature that plateaus in the second week at 39-40°C may be noted.
  • Cervical adenopathy may occur.
  • Relative bradycardia occurs in fewer than 50% of cases.
  • Abdominal examination may reveal distention with pain on deep palpation.
  • Hepatosplenomegaly is found in 50% of patients.⁶
  • A rose spot rash that typically occurs in the second week of disease is seen in 30% of patients. This rash is described as a faint salmon-colored 2-3 mm papule lesion located primarily on the trunk that fades with pressure.
  • Findings of meningismus may appear after the early prodrome.
• Bacteremia
  • Bacteremia is usually associated with a prolonged or recurrent fever.
  • Generally, it is associated with a localized infection.
  • It may be a part of a mixed Salmonella infection.¹⁶
• Chronic carrier state is asymptomatic.

Causes

Currently, more than 2500 serotypes of Salmonella enterica have been identified.^1,2 Although clinical manifestations of each overlap, typhi and paratyphi tend to cause enteric or typhoid fever and the more invasive form of the disease, whereas most others cause a self-limited form of gastroenteritis.

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