Approach

Nontyphoidal salmonellosis most commonly presents as a self-limited gastroenteritis. The diagnosis is confirmed microbiologically with cultures or by detection of pathogen-specific nucleic acid.

History and physical examination

A detailed clinical and exposure history should be obtained from all patients presenting with diarrhea.[65]

An exposure history of ingestion of an item potentially contaminated with Salmonella should lead to the diagnosis of suspected salmonellosis. However, due to the wide range of foods that have been implicated, this may be of somewhat limited utility. Still, potential food exposures should be ascertained, as this information may have important public health and preventive implications.

Patients with nontyphoidal Salmonella gastroenteritis present with nausea, vomiting, and diarrhea. If a suspected contaminated food is recollected, the time from ingestion to the first symptoms is typically 6 to 48 hours. CDC: guide to confirming an etiology in foodborne disease outbreak Opens in new window Patients may also complain of fever, abdominal pain, chills, headache, and myalgias.[66] Examination may reveal a fever, mild abdominal tenderness, and hyperactive bowel sounds. Patients may also have signs of volume depletion, such as dry mucous membranes, decreased skin turgor, and orthostatic vital signs.

There are no pathognomonic signs or symptoms that can distinguish Salmonella gastroenteritis from other bacterial causes based on the history and physical exam alone.

Stool studies

The diagnosis of nontyphoidal Salmonella infection is established by isolating the organism from fresh stool samples sent for culture or by detection of specific DNA in feces. 

Stool collected by the patient is the conventional diagnostic sample. Cultures using rectal swabs are less sensitive and are not recommended (unless a stool culture cannot be obtained).[43] Microscopy may show leukocytes and occasionally the presence of red blood cells.

Stool samples are directly plated on enteric media (e.g., MacConkey agar) or more selective media for Salmonella (e.g., Hektoen agar). Highly selective media (e.g., selenite with brilliant green) are typically reserved for known carriers or during outbreak investigations.[44] Many laboratories use incubation in an enrichment broth such as Selenite-F to increase the sensitivity of the stool culture.

When to conduct stool tests

Guidelines recommend diagnostic studies if the patient has:

  • moderate to severe disease, and symptoms lasting more than 7 days[67]

  • diarrhea accompanied by fever, bloody or mucoid stools, severe abdominal cramping or tenderness, or signs of sepsis.[65]

Stool testing should also be performed in symptomatic patients who have clearly identified circumstances in which Salmonella infection is suspected:[65]

  • foodborne outbreaks in hotels, cruise ships, resorts, restaurants, institutions, and catered events

  • consumption of unpasteurized milk or dairy products

  • consumption of fruits or unpasteurized fruit juices; vegetables, leafy greens, and sprouts

  • consumption of undercooked eggs

  • swimming in, or drinking, untreated fresh water

  • recent antimicrobial therapy

  • travel to resource-challenged countries

  • contact with young poultry or reptiles

  • patient is an infant (birth to 3 months of age), or an adult >50 years or with a history of atherosclerosis

  • patient has an underlying immunocompromising condition

  • anal-genital, oral-anal, or digital-anal contact.

Multiplex polymerase chain reaction (PCR)

Salmonella may also be diagnosed from stool using a culture-independent diagnostic test (CIDT), in particular, multiplex PCR panels with primers for several common diarrheal pathogens. This technique is highly sensitive and specific.[68][69]

Multiplex PCR panels are increasingly used for the evaluation of patients with acute infective gastroenteritis, due to their superior sensitivity and shorter turnaround time.[70] Reflexive culture should be performed whenever Salmonella (or Shigella or Campylobacter) are detected, in order to provide isolates for susceptibility testing and strain typing.[71][72]

Chronic carrier state

Stool samples may be tested to diagnose a chronic carrier state. The diagnosis is established by positive stool culture or PCR assay for Salmonella at 12 months or more following the acute illness.[31]

Generally testing for a carrier state is not warranted, but should be based on local health department guidelines (e.g., may be required among certain occupations such as healthcare workers or food handlers).

Other investigations

Serology tests

The white blood count (WBC) count is typically normal in cases of Salmonella gastroenteritis, sometimes with a shift to immature forms, but occasionally, a mild leukocytosis is seen. However, a peripheral WBC count and differential and serologic assays are not recommended to establish the cause of acute infectious diarrhea.[65]

Blood tests for antibodies to O antigens, such as the Widal test, are nonspecific and are not recommended for the diagnosis of either nontyphoidal or typhoidal Salmonella infections.[34]

Blood cultures

Should be obtained from symptomatic patients:[65]

  • <3 months of age

  • with signs of septicemia, or when enteric fever is suspected

  • with systemic manifestations of infection

  • who are immunocompromised

  • with certain high-risk conditions such as hemolytic anemia

  • who traveled to, or have had contact with, travelers from enteric, fever–endemic areas with a febrile illness of unknown etiology.

Imaging

Imaging studies (e.g., ultrasonography, computed tomography, or magnetic resonance imaging) are not routine. However, they may be considered to detect aortitis, mycotic aneurysms, signs and symptoms of peritonitis, intra-abdominal free air, toxic megacolon, or extravascular foci of infection in older people with invasive Salmonella enterica infection if there is sustained fever or bacteremia, despite adequate antimicrobial therapy, or if the patient has underlying atherosclerosis, or has recent-onset chest, back, or abdominal pain.[65]

Polymerase chain reaction (PCR)-based fingerprinting

During outbreak investigations, advanced tests such as PCR-based fingerprinting and/or multilocus sequence typing were formerly used to determine the relatedness of the strains within a given serotype.[73][74]​ This was typically performed in public health or specialized research laboratories. PCR-based fingerprinting and multilocus sequence typing have largely been supplanted by whole genome sequencing, which can also be used to identify serotype in most instances.[75]

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