Mycotic abdominal aortic aneurysm: two cases caused by Salmonella enterica

Summary

A mycotic abdominal aortic aneurysm was detected on computed tomographic angiography of two male patients who were followed up after reporting symptoms of abdominal pain, malaise, and fever of unknown origin. One of the patients' aneurysm was repaired with a tubular graft and the other patient had endovascular aneurysm repair due to his high co-morbidity. From pre-operative cultures and a pathological examination of the surgical specimens, it was observed that the aneurysms had developed in the abdominal aorta due to Salmonella enterica, and broad-spectrum antibiotic therapy was started. We present these two cases of mycotic aneurysm due to Salmonella. The patients were discharged after the postoperative course of antibiotic treatments were completed.

Mycotic aneurysm is caused by infection of the arterial wall due to bacteria or fungi.^1,2 It is uncommon and treatment may be difficult.¹ A septic embolism secondary to infective endocarditis or haematogenous spread of an infection are often the cause.² No guidelines exist for the treatment of mycotic aneurysms and there is a high risk of mortality (10–40%) despite the development of new treatment modalities and technology.^1,3,4

Although many bacteria may cause its aetiology, Staphylococcus aureus (22–28%) and Salmonella spp. (15–17%) are the most common pathogens.^5-7 Vascular infection is one of the most important extra-intestinal involvements of Salmonella, and endovascular involvement usually causes abdominal aortic aneurysm.⁸ In this study, two successfully treated cases of mycotic aneurysm due to Salmonella enterica are presented.

Case report 1

A 64-year-old male patient had been followed up in the infection service for 15 days with symptoms of left lower-extremity pain, weakness and fever of unknown origin. The patient was transferred to our service because computed tomographic angiography (CTA) detected a saccular abdominal aortic aneurysm, compatible with a mycotic aneurysm, with necrotic tissue and an abscess in the peri-aortic area (Fig. 1A, B ). His history included type 2 diabetes mellitus (DM), hypertension, hyperlipidaemia and coronary artery disease.

Fig. 1.

CT angiography: saccular abdominal aortic aneurysm and contained rupture.

The patient’s vital signs were normal. On physical examination, no pathology was detected except for left costovertebral angle tenderness. Biochemistry tests of the infection markers showed the leukocyte count was 24.550 cells/mm³, neutrophil count was 21.630 cells/mm³ (88%), C-reactive protein (CRP) was 20 mg/dl, procalcitonin was 0.38 ng/ml, the erythrocyte sedimentation rate (ESR) was 69 mm/h, and there was severe anaemia (haemoglobin: 7.6 g/dl, haematocrit: 23%).

The patient was started on empirical antibiotics [intravenous (IV) piperacillin/tazobactam 4.5 g daily, IV vancomycin 1 g once a day] and aortobi-iliac bypass was performed with a dacron graft. Samples for microbiology were taken from the necrotic tissue remnants and abscess in the peri-aortic area. The patient was followed up in the intensive care unit for two days after the operation, and was then transferred to the ward.

Gram-negative stained bacteria grew on the pre-operative blood culture samples, and Salmonella enterica grew from the intra-operative samples. The treatment was changed to ceftriaxone 2 g IV, according to the antibiogram. Acute-phase reactant (APR) values regressed to normal values on the 17th day postoperatively, and the IV antibiotic treatment was completed in eight weeks. The patient was discharged with ciprofloxacin 2 × 500 mg/day/1 week and acetylsalicylic acid (ASA). In the first month, control CTA showed the aortobi-iliac graft to be patent and not infected (Fig. 2A, B).

Fig. 2.

Repeat CT angiography in the first month after discharge, showing patent aortobi-iliac graft.

Case report 2

A 70-year-old male patient was followed in the infectious diseases ward with symptoms of low-back pain, fatigue and fever of unknown origin. The patient was transferred to our service because of signs of inflammation in the peri-aortic area on CTA, blurred edges of the aortic wall, and there was an abdominal aortic aneurysm of 51 mm in its widest part (Fig. 3A, B), compatible with mycotic aneurysm.

Fig. 3.

CT angiography on admission. CTA shows abdominal aortic aneurysm and infected/inflamed aortic aneurysm.

He stated in his anamnesis that he had had symptoms of coldness, weakness and dry mouth after meals for two months, and that his symptoms had worsened recently. His history included type 2 DM, hypertension, hyperlipidaemia and coronary artery bypass. In the radiological imaging (abdominal ultrasonography and CTA), which was performed two months earlier, the abdominal aortic dimensions and appearance were normal.

The patient’s blood pressure was 120/80 mmHg, heart rate was 122 beats/min, oxygen saturation was 98%, and body temperature was 39°C. No abnormality was found in the physical examination except mild pre-tibial oedema (2+). In biochemistry tests, the leukocyte count was15.950 cells/mm³, neutrophil count was 12.990 cells/mm³ (82%), CRP was 236.76 mg/dl and the ESR was 65 mm/h. Other biochemical values were normal. Blood, sputum and urine cultures were obtained from the patient for the focus of infection. Empirical broad-spectrum antibiotics were started.

Since Salmonella enterica grew on the blood cultures of the patient, IV piperacillin/tazobactam + ciprofloxacin 4.5 g four times a day was started. He described severe abdominal pain and sudden-onset pain in the right leg during his follow up. On physical examination, the pulses of the right lower extremity were non-palpable. In repeat CTA, it was observed that the abdominal aortic aneurysm had progressed to 90 mm at its widest point. Signs of peri-aortic inflammation and blurred edges of the aortic wall suggested that the aneurysm was of mycotic origin (Fig. 4).

Fig. 4.

Abdominal aneurysm increasing from 51 to 63 mm one week after admission.

Emergency surgery was decided due to the rapid enlargement of the mycotic abdominal aortic aneurysm and extremity ischaemia, but endovascular repair (EVAR) was performed due to the patient’s multiple co-morbidities. APR values regressed to normal values on the eighth postoperative day, and IV antibiotic treatment was completed in eight weeks. The patient was discharged with ciprofloxacin 2 × 500 mg/day/1 week and ASA. A patent EVAR graft with no signs of infection or periaortic accumulation was seen in the control CTA during the first month (Fig. 5).

Fig. 5.

CT angiography in the first month after discharge, showing a patent EVAR graft.

Blood culture samples of both patients were collected in BACT/ALERT FA Plus blood culture bottles and incubated in BACT/ALERT 3D 120 Combo system. Bacterial identification was done with MALDI TOF-MS (Biomerieux, France).

Antibiotic susceptibility testing was done with VITEC 2 (Biomerieux, France) as a routine procedure in our institution.

Discussion

The first step of the aetiopathogenesis of mycotic aneurysms is micro-organism invasion into the aortic wall. Causes include septic embolism, bacterial invasion secondary to a traumatic injury, invasion from adjacent infected tissues or vegetation onto a pre-existing aneurysm or an atherosclerotic plaque. Other predisposing factors include intravascular drug use, immunosuppression due to medications or malignancies, and iatrogenic causes such as catheterisation of the vessels for diagnostic or therapeutic purposes.^9-14

The incidence of non-typhoidal Salmonella infection has been increasing worldwide recently. These bacteria are usually food-borne pathogens that cause gastroenteritis, bacteraemia and focal infections.¹⁵ Advanced age, diabetes, changes in endogenous intestinal flora, cancer, human immunodeficiency virus infection, auto-immune disorders and immunosuppression are the main risk factors for non-typhoid Salmonella infections and bacteraemia.¹⁶

One of the most important extra-intestinal system involvement sites of infection is the endovascular system, and endovascular Salmonella infections in the abdominal aorta usually advance into mycotic abdominal aortic aneurysms.⁸ As the walls of the aorta degenerate, secondary to a local infection, an aneurysm forms or expands if present, leading to rupture and death if not managed acutely.

In both of our patients, there was no sign of an overt or an occult source of infection. The patients did not have any echocardiographic evidence of cardiac vegetations that could cause a septic embolism. The patients had no history of trauma, intravenous drug abuse, immunosuppression or malignancy. Both had mycotic aneurysms due to Salmonella infection. Given the age of the patients, the most likely aetiology was presumed to be infection of a pre-existing atheromatous plaque. No endovascular leakage from the graft or any surrounding secretion was observed in the control CT angiograms performed in both patients after surgery and after discharge.

In the early stages of mycotic aneurysm, symptoms are usually absent or non-specific. To date, there are no established guidelines for treating mycotic aneurysms, but practical treatment is a combination of antibiotic therapy and surgery.³ Early detection of a mycotic aneurysm is extremely important, as early initiation of treatment improves long-term survival.¹⁷ Overall, the mortality rate of mycotic aneurysms is high (23–31%).18 Surgical options include open resection and endovascular repair; however, the optimal surgical treatment is controversial.

Open debridement of a mycotic aneurysm has the advantage of eliminating the infected tissue but is associated with a high risk of mortality (13.3–40%).19 Endovascular repair is considered a less invasive procedure with lower mortality rates, especially in high-risk patients, but the lack of adequate surgical debridement of the infected area is a major disadvantage. This can bring about long-term issues due to direct insertion of a graft into an infected area.^4,12

There are also no clear guidelines on the choice or duration of antibiotics. Therefore, each case should be evaluated individually based on the patient’s clinical course, co-morbidities, the microorganism involved, the persistence of the micro-organism, and when the graft material was placed based on the patient’s course. Although there is no clear algorithm, when we look at the literature, long-term antibiotic suppressive therapy is recommended after surgical treatment in order to prevent possible haemothegenous seeding.^1,20

In both our patients, there was no focus of infection that could cause bacteraemia. Antibiotherapy was started because of growth of Salmonella enterica in the blood cultures taken before and after the operation. The infection parameters of the patients, who consulted with infectious diseases, were followed up and antibiotherapy was completed in eight weeks.

Conclusion

Mycotic abdominal aortic aneurysms result in high mortality rates. There is no established algorithm for treatment, which should be designed separately for each patient and carried out with the participation of an infectious diseases specialist. Despite timely intervention, unexpected complications and sepsis may lead to mortality, therefore rigorous clinical management is essential.