Infected Aortic Aneurysm Caused by Streptococcus zooepidemicus: A Case Report and Literature Review

A 66-year-old Japanese male working at a stable developed abdominal pain and fever and was brought to the emergency department. The computed tomography scan revealed an aneurysm of approximately 70 mm in diameter, with an irregular border, at the infrarenal abdominal aorta. Emergency surgery was performed with a bifurcated Dacron graft. Streptococcus zooepidemicus was observed on the aneurysm wall. He was discharged in good condition and was prescribed oral amoxicillin/clavulanic acid for 4 months. He has remained well and did not develop inflammation. Evaluation of patient history and data, including the consumption of unpasteurized dairy food, occupation, and direct contact with animals, is important for an early diagnosis, a prompt surgery, and an appropriate antibiotic therapy.


Introduction
Streptococcus zooepidemicus is a Lancefield group C β-hemolytic streptococcus. 1) It is well known to cause respiratory tract and wound infections in young horses. 1) Although it rarely infects humans, it may opportunistically infect humans with underlying diseases. 1) It is mainly transmitted through close contact with animals and consumption of unpasteurized dairy products. 2) The possible effects of S. zooepidemicus infection in humans include nephritis, arthritis, meningitis, pneumonia, infected aortic aneurysm, infective endocarditis, and sepsis. 1) When an infected aortic aneurysm (AA) is caused by S. zooepidemicus, a poor prognosis is expected. 3) Its clinical course, prognosis, and standard treatment are still largely unknown due to only a few cases being reported in the literature. To understand the traits of infected AA caused by S. zooepidemicus, we reviewed the literature and collected 12 cases. Those cases confirm the value of taking a full history for early diagnosis, prompt surgery, and accurate antibiotic treatment.

Case Report
A 66-year-old Japanese man working at a stable was brought to the emergency department of a peripheral hospital due to complaints of abdominal pain, fever, and weight loss. He had already been followed by the same hospital for 2 months due to rib fracture caused by a fall. His past medical history was not significant in other respects. The computed tomography (CT) scan revealed an infrarenal abdominal AA of about 70-mm diameter. He was transferred to our department with a saline drip.
He was hemodynamically stable upon his arrival at our hospital, and his blood pressure was 112/73 mmHg; heart rate, 84 bpm; body temperature, 38.1°C; and SO 2 , 99% (room air). The blood test revealed mild anemia (Hb 9.8 g/dL), normal white blood cell count (7400/µL), and increased C-reactive protein (CRP) level (11.08 mg/dL). Physical examination revealed emaciation and a pulsatile mass in his lower abdomen. The contrast-enhanced CT scan revealed an aneurysm with an irregular border (Fig. 1).
Emergency surgery was performed through midline laparotomy, which exposed an aneurysm strictly adhering to the duodenum, inferior vena cava, and retroperitoneum ( Fig. 2A). The aneurysm was cautiously resected and replaced with a bifurcated Dacron graft (J-graft 18-11 mm, Japan Lifeline Co., Ltd., Tokyo, Japan) (Fig. 2B). The normal intimal wall existed only at the posterior aortic wall, which indicated pseudoaneurysm. A wide-ranging debridement of the infected tissue, including the aneurysm wall, was performed with copious irrigation. The operation time was 143 min. The amount of bleeding was 1296 mL. Four red blood cell units (560 mL) were transfused.
S. zooepidemicus was identified from the aneurysm wall extracted during the operation, which led to the definitive diagnosis of an infected abdominal AA. However, blood culture was negative. The strain proved vulnerable to all the major antibiotics in the antibiotic susceptibility testing. Intravenous administration of cefazolin was empirically started before the operation and continued for 3 weeks. The patient s body temperature fell into the normal range on post-operative day (POD) 8. His CRP level also became normal from POD 20. Intravenous cefazolin was replaced with oral amoxicillin/clavulanic acid and continued for 4 months until the patient chose to discontinue this prophylaxis. The CT scan revealed no inflammation tissue around the residual aortic wall, duodenum, and retroperitoneum. He was discharged in good condition on POD 23. No recurrence of inflammation was observed in the 6 months following the operation.

Discussion
S. zooepidemicus infection is common in animals, such as horses, cattle, sheep, and pigs, but rare in humans. 1) When humans are infected, they can develop diseases, such as bacteremia, endocarditis, and infected AA. 1) Several case reports on bacteremia or S. zooepidemicus outbreaks have been observed. However, there have only been a few cases associated with S. zooepidemicus infection leading to infected AA. Although it seems potentially lethal, little is known about its clinical course, prognosis, and optimal treatment. For further information, we searched Google Scholar for case reports using the terms S. zooepidemicus and aneurysm between 1980 and 2020. This yielded 162 papers, among which were reports of 11 cases of infected AA caused by S. zooepidemicus, with information on underlying diseases, operations, and outcomes, as presented in the Table 1. [2][3][4][5][6][7][8] All patients were male, with a mean age of 66 (SD: ±7.2) years. The overall mortality was 42% (5 out of 12 patients). Among those 12 patients, only 3 (25%) had no underlying disease, and none of those 3 died. Among the other 9 patients (75%) who had underlying diseases, 5 (56%) died.
The most common organism associated with infected AA is Staphylococcus aureus, which accounts for some 30% of cases, followed by Salmonella spp. 9) Infected AA is associated with high mortality rates: around 21%-30% with medical treatment, reported by Kim,9) and 13% with surgical treatment, reported by Yeager et al. 10) The overall mortality of infected AA caused by S. zooepidemicus is higher than that of infected AA caused by Staphylococcus aureus and Salmonella spp.
Patients infected with S. zooepidemicus are likely to have underlying medical conditions. 1) S. zooepidemicus is generally regarded as an opportunistic pathogen. 4) In this review, we found that patients with underlying diseases are more vulnerable to S. zooepidemicus, and so to infected AA.
Our patient had been working as a horse trainer and had no medical conditions. This is a remarkable case as S. zooepidemicus is considered to be an opportunistic pathogen. We suspect that the past medical history of his blunt trauma 2 months earlier was relevant. Infected AA has been attributed to trauma, infection from a contiguous site, blood-borne spread from a preexisting aneurysm due to bacteremia, or embolization in the vasa vasorum due    to endocarditis. 4) Our review found that 7 patients (cases 3-8, 10) who had underlying medical conditions had a history of undercooked pork or unpasteurized dairy food consumption or direct contact with horses. Moreover, one patient (case 9), who did not have underlying disease, was found to have a history of trauma and worked as a horse trainer. Considering both our case and this review, we suspect that even people working regularly with livestock are unlikely to suffer infection, unless they have underlying medical conditions and/or a triggering event, such as trauma. As previously described, evaluation of patient history and data, including consumption of unpasteurized dairy food, occupation, and extensive contact with animals, is important for an early diagnosis and prompt surgery. Considering the mortality rate of infected AA caused by S. zooepidemicus, early diagnosis and prompt surgery are important. Penicillins, cephalosporins, and gentamicin were commonly used in the cases of our review. In our present case, we administered intravenous cefazolin, followed by long-term suppression with oral amoxicillin/clavulanic acid based on antibiotic susceptibility testing. The duration of antibiotic treatment is still controversial, and we intended to continue oral amoxicillin/clavulanic acid for 6 months, following Yeager et al. 10) ; however, our patient insisted on discontinuing the oral amoxicillin/clavulanic acid prophylaxis after 4 months. Knowing the severity of harm potentially caused by S. zooepidemicus, long-term suppression with oral antibiotics for 6 months is probably advisable. 3,10) Our review revealed that the patients (cases 6, 10, 11) who received long-term oral antibiotic care had no relapses.

Conclusion
Here, we report a new case of a previously healthy horse trainer who suffered from an infected AA caused by S. zooepidemicus, probably triggered by trauma. We also summarize several cases reported earlier. The reviewed case information suggests that the prognosis of infected AA caused by S. zooepidemicus is relatively poor in comparison with infected AA in general. Evaluation of the history of the present illness and patient data, including consumption of unpasteurized dairy food, occupation, and direct contact with animals, is important for an early diagnosis, a prompt surgical treatment, and an appropriate antibiotic therapy.