Spectrum of Imaging Findings in Salmonella Infections
Abstract
OBJECTIVE. Radiologic findings in Salmonella infections are not well described. In most patients, Salmonella infections produce mild and self-limiting clinical manifestations and therefore are treated empirically with antibiotics. Radiologic investigations are usually performed for patients with severe clinical manifestations or complications and for patients with unusual findings.
CONCLUSION. This pictorial essay illustrates various imaging findings in culture-proven cases of Salmonella infection, described broadly as common and uncommon manifestations.
Salmonella species are gram-negative facultative intracellular anaerobes that can cause a broad spectrum of clinical manifestations [1]. The clinical spectrum of infection ranges from gastroenteritis, enteric fever (caused by typhoid and paratyphoid serotypes), Salmonella bacteremia, and localized infections to a convalescent lifetime carrier state [2]. The manifestation of disease is dependent on the serotype and various host factors. More than 2500 serovars of genusSalmonella have been described [3], some of which, such as S. typhi, are restricted only to human hosts [1].
Salmonella infection most commonly begins with ingestion of bacteria in contaminated food and water. Once a person is infected, the organism can spread from person to person via the fecal-oral route [4]. The bacterium proliferates in the intestine and can then penetrate the lymphoid tissue of the gastrointestinal tract, usually in the distal ileal loops and terminal ileum, leading to hematologic dissemination and transmission of the organism to various organ systems [5].
Salmonella infection can be endemic, particularly in developing countries. However, an increased incidence of this infection is emerging in developed countries secondary to the prevalence of immunocompromised conditions with the ongoing surge of HIV infection and the evolution of antibiotic resistance. This pictorial essay thus aims to illustrate the various clinical manifestations ofSalmonella infection, taking into account the changing global environment.
Common Manifestations
Gastrointestinal
A large inoculum of Salmonella species is needed to overcome stomach acidity and compete with normal intestinal flora for an established infection of the intestine. The bacterium then proliferates in the small intestine and invades enterocytes in the distal ileum and colon. Elaboration of several toxins by the bacterium contributes to the dysfunction of the intestinal cells [4].
Small-intestine infection may be seen as symmetric and homogeneous thickening of the ileal wall, which may be focal or diffuse on CT (Fig. 1). A feathery pattern of mucosal thickening may also be seen on ultrasound (Fig. 2). Sometimes colonic involvement can be seen in the absence of ileal involvement and may be patchy or continuous (Fig. 3). Salmonella enteritis may even simulate pseudomembranous colitis, with toxic megacolon as a known complication [6]. Enlarged mesenteric nodes may be seen adjacent to the involved segment of intestine (Fig. 4).
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Hepatobiliary and Splenic
The gallbladder and spleen are common sites of intraabdominal Salmonella infections, and the manifestations can range from nonspecific organomegaly (Fig. 6) to an abscess formation (Fig. 7). Rarely, the spleen may rupture secondary to splenomegaly after trivial trauma.
Acute acalculous cholecystitis is a recognized manifestation of Salmonella infection [8]. Gallbladder wall thickening, distention, and pericholecystic fluid can be seen on both ultrasound and CT (Figs. 8A and 8B). Perforation and empyema of the gallbladder and Salmonella cholangitis are also recognized but rare entities [9].
Mesenteric and Peritoneal
Ascites, localized or generalized mesenteric stranding, thickening, and adenopathy are frequent manifestations of Salmonella infection [10]. Salmonella infections have a predilection for the gastrointestinal tract. Involvement of the terminal ileum or the proximal colon with mesenteric lymphadenopathy may be specific imaging findings.
Uncommon Manifestations
Genitourinary
The manifestations of Salmonella infection in the genitourinary tract are nonspecific and do not differ clinically from the manifestations of urinary tract infections secondary to other Enterobacteriaceae [11]. They range from asymptomatic bacteriuria to cystitis, pyelonephritis, and renal abscess formation (Figs. 9A and 9B). Salmonella infection in a preexisting hydrocele, ovarian cyst, and even epididymoorchitis have been reported [12].
Pulmonary and Cardiac
Vascular
Arterial—Although arterial infection due to Salmonella bacteria is unusual, it remains one of the most common causes of infective aneurysms [14]. Most infections occur in preexisting atherosclerotic foci or in an aneurysm, and the risk of infective aneurysms secondary to Salmonella infection is thereby significantly increased in patients older than 50 years.
The most common site of infection is the abdominal aorta. CT findings may reveal a periaortic gas collection, an interrupted ring of aortic wall, or a rapidly enlarging saccular structure arising from the aortic wall (Figs. 11A and 11B). These aneurysms are at risk of causing life-threatening rupture and hemorrhage (Figs.12A and 12B).
CNS
Salmonella meningitis occurs most commonly in infants [15]. Contrast-enhanced MRI is useful in suspected cases because it reveals meningeal enhancement. Encephalitis in the form of increased signal on FLAIR and T2-weighted sequences and abnormal enhancement may be seen (Figs. 14A, 14B, and 14C). Complications such as hydrocephalus, ventriculitis, and cerebral abscesses can then follow in untreated cases.
Musculoskeletal
The skeletal and soft-tissue infections due to Salmonella infection occur mostly in patients with preexisting diseases. The association between sickle cell disease and Salmonella osteomyelitis is well known. Other manifestations include polymyositis, septic arthritis, periosteitis, and abscess formation. MRI plays an important role in the imaging of musculoskeletal complications (Figs. 15A and15B).
Soft Tissues
Soft-tissue infections are often indolent with a paucity of systemic symptoms. The imaging features are no different from those of abscesses due to any other cause and may not be suspected until culture of a surgically obtained specimen is performed. Superficial abscesses are quite common, with abscesses in the parotid, breast, pancreas, and thyroid having been reported in the literature [14].
Conclusion
A wide spectrum of radiologic manifestations due to Salmonella infection may be encountered, especially in endemic areas and immunocompromised patients. However, the imaging findings in Salmonella infection are not unique and can mimic other infective diseases. Knowledge of radiologic manifestations is important to aid in early diagnosis and timely initiation of appropriate management. In our experience, in the appropriate clinical setting, radiologic findings of thickened terminal ileum or proximal colon with mesenteric lymphadenopathy are specific for Salmonella infection.
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