Impending rupture of mycotic aortic aneurysm infected with Streptococcus equi subspecies zooepidemicus

Abstract

Streptococcus equi subspecies zooepidemicus is a beta-haemolytic, group C streptococcal bacterium. Although it is an opportunistic pathogen commonly found in horses, transmission to human can lead to severe infections. Here, we present a patient with S. equi subspecies zooepidemicus bacteraemia and consequent development of mycotic aneurysms.

Background

Streptococcus equi is a group C beta-haemolytic zoonotic pathogen that rarely leads to human infections.¹ It is divided into three subspecies including S. equi subspecies zooepidemicus, S. equi subspecies ruminatorum and S. equi subspecies equi.^2–4 The most common pathogen seen in human is S. equi subspecies zooepidemicus.⁴ Human infections with S. equi subspecies zooepidemicus can manifest as glomerulonephritis, rheumatic fever, purulent arthritis, bacteraemia and meningitis.^5–7 However, reported cases of mycotic aneurysm associated with S. equi subspecies zooepidemicus infection are scarce.

Case presentation

The patient is a 79-year-old man who presented to the emergency department (ED) with several days of diffuse abdominal pain and constipation. A week prior to presentation, he received an intra-articular facet injection of L3/4, L4/5, L5/6 bilaterally for chronic back pain. His medical comorbidities include insulin-dependent type 2 diabetes (haemoglobin A1c 7.9%), chronic low back pain due to multilevel lumbar spinal stenosis, 3.9 cm abdominal aorta aneurysm (AAA), hyperlipidaemia and myasthenia gravis. Patient was afebrile and vitals were within normal limits. Examination was remarkable for distended abdomen, with normal active bowel sounds, no rebound or guarding.

Investigations

Labs were notable for leucocytosis at 15.6×10⁹/L, creatinine 1.3 mg/dL, urinalysis was negative for nitrite and leucocyte esterase. Erythrocyte sedimentation rate (ESR) was 93 mm/hour and C-reactive protein (CRP) >270 mg/L. Chest X-ray was negative. CT of the abdomen and pelvis on presentation showed a 4.3 cm infrarenal AAA (4 mm growth compared with 6 months ago) without evidence of rupture, but was concerning for impending rupture versus aortitis. To better visualise the aortic involvement, a positron emission tomography scan was performed, and revealed a 4.7 cm focally inflamed and partially thrombosed infrarenal AAA with fluoro deoxyglucose uptake, suggestive of aortitis (figure 1).

Figure 1.

Positron emission tomography scan showing distal infrarenal abdominal aorta aneurysm with moderate periaortic inflammatory change.

Both infectious and autoimmune aetiologies of aortitis were evaluated. Initial microbiology workup was negative. Patient developed a fever on hospital day 3, repeated blood cultures were obtained and were positive for S. equi subspecies zooepidemicus. Blood mycology culture was negative. Hepatitis panel (hepatitis C antibody, hepatitis B core antibody and surface antigen), tuberculosis and Treponema pallidum were all negative. A series of autoimmune workup was done as well and unrevealing. Transthoracic and transoesophageal echocardiogram did not reveal any valvular vegetation and aortic root was normal in size.

Differential diagnosis

Aortitis refers to inflammation of aortic wall which can be due to infectious or inflammatory aetiologies. In our patient, suspicion for infectious aortitis was heightened due to recent spinal facet injections and underlying insulin-dependent type 2 diabetes. Infection of the aorta leading to aneurysm is most commonly due to haematogenous seeding, but direct bacterial inoculation, septic embolism and contiguous infection have also been identified.⁸

Evaluations for both infectious and autoimmune causes of aortitis were undertaken in this case. Autoimmune workup was unrevealing.

Treatment

Patient was started on ceftriaxone due to penicillin allergy and subsequently improved clinically. Repeated blood cultures 2 days after initiation of antibiotic therapy were negative. Patient was discharged home with intravenous ceftriaxone 2 g per day with an anticipated course of 6 weeks of total duration. After 2 weeks of antibiotic therapy, both ESR and CRP were improving (ESR was 41 mm/hour and CRP was 15 mg/L) and patient was continuing to do well clinically.

Outcome and follow-up

Five weeks after initial presentation, patient returned to the ED with increasing abdominal pain. Repeated CT showed evolving thrombosed saccular abdominal aneurysm, now at 5.9 cm (figure 2). This was a 1.2 cm increase in size in 5 weeks. There was also interval resolution of previously demonstrated inflammatory changes surrounding the infrarenal aorta. Vascular surgery was consulted and patient underwent abdominal endovascular aneurysm repair (EVAR) due to concern for a contained rupture of AAA. Patient tolerated the EVAR surgery well. Follow-up CT scan showed significant decrease in the size of AAA (figure 3). Patient was transitioned to intravenous clindamycin (at 900 mg every 8 hours) during the third week of antibiotic course due to suspected ceftriaxone-induced myelosuppression. Repeated blood cultures after antibiotic course were negative. Patient was transitioned to oral clindamycin (at 450 mg every 8 hours) for suppression since endovascular graft was placed in the presumably infected area. Patient remained on oral clindamycin for 3 months and was doing well at his 6-month and 12-month follow-ups.

Figure 2.

CT abdomen/pelvis prior to endovascular aneurysm repair. Red arrow indicating evolving thrombosed saccular abdominal aorta aneurysm at 5.9 cm.

Figure 3.

CT abdomen/pelvis after endovascular aneurysm repair showing decreased size of aneurysm sac.

Discussion

In 1885, Osler described the first case of mycotic aneurysm: an aneurysm that is associated with endocarditis.⁹ Mycotic aneurysms, or an infected aneurysm, can be caused by trauma, infection from a contiguous site, haematogenous seeding of pre-existing aneurysm secondary to bacteraemia or embolisation into vasa vasorum secondary to endocarditis. Infections only account for up to 2% of AAAs¹⁰ but carry a surprisingly high mortality rate of 90%.¹¹ The most common bacteria associated with infected AAA are Salmonella sp and Staphylococcus sp due to their affinity for the arterial wall, whereas infections with streptococcal species are rare.^11–13 Patient may be culture negative in up to 20% of the cases.¹⁴

S. equi subspecies zooepidemicus is an opportunistic organism found in domesticated animals.^{2 15} The most common route of zoonotic transmission includes consumption of unpasteurised dairy products and direct animal contact.⁶ Studies show that up to 70% of patients affected by S. equi subspecies zooepidemicus have serious underlying disease such as malignancy, cardiovascular diseases, diabetes mellitus and immunosuppression.¹⁵ Sharing about 80% sequence homology with S. pyogenes, the presentation of S. equi subspecies zooepidemicus infection can be similar to that of S. pyogenes.³ Reported cases of S. equi subspecies zooepidemicus infection in human range from skin and soft tissues infection to meningitis. However, there have only been few reported cases of S. equi subspecies zooepidemicus bacteraemia leading to mycotic aneurysm with overall unfavourable prognosis.

Review of the literature showed less than 10 cases of S. equi subspecies zooepidemicus bacteraemia associated with mycotic aneurysm. Edwards et al described 11 patients with milk-borne S. equi subspecies zooepidemicus infection, one of which developed mycotic aneurysm in a femoral artery vein graft. Patient was treated with surgery and antibiotics.¹⁶ Altreuther et al reported two patients with S. equi subspecies zooepidemicus bacteraemia who responded well to surgical fixation and antibiotic administration.¹⁷ Yuen et al described two cases of aortic mycotic aneurysm among 11 patients with S. equi subspecies zooepidemicus bacteraemia. Both patients received antibiotics and surgery, however, only one patient survived.¹⁸ Trell et al reported one case of S. equi subspecies zooepidemicus aortitis. Patient was treated with 150 days of clindamycin and lifelong amoxicillin.¹⁹ Finally, Madani et al reported one case of mycotic abdominal aortic and iliac aneurysm in setting of S. equi subspecies zooepidemicus bacteraemia. Patient was treated with prolonged antibiotic course and surgical debridement. Patient was infection-free 1 year after discharge.¹

Infected aortic aneurysms are often a diagnostic challenge since patients present with vague symptoms such as abdominal pain and mild leucocytosis, as in our case. Management of infected aneurysms can be a challenge as well since there are no randomised trials to provide guidance. Mortality related to infected aortic aneurysm is lower when medical and surgical therapies are combined (38% vs 96%) compared with medical treatment alone.²⁰ The optimal duration of antibiotic therapy is uncertain and depends on multiple factors including location of infection, specific bacteria and its resistance pattern. Suppressive antibiotics may be considered for patients with prosthetic graft material in situ during active infections such as our patient. Surgical therapies are recommended but carry high mortality especially in patients with multiple comorbidities, like our patient. Due to its rarity, there is no consensus on the rate of growth or change of morphology in mycotic aneurysm that determines optimal timing for surgical intervention. Furthermore, rapid expansion of mycotic aortic aneurysm has been described.^21–23 More importantly, aortic infection can rapidly progress into aneurysm leading to aortic rupture and death.²¹ Our case demonstrates the rapid growth in mycotic aneurysm seen by serial CT scan and highlights the importance of prompt diagnosis and treatment with intravenous antibiotics and surgical intervention.

Learning points.

• Streptococcus equi subspecies zooepidemicus is a zoonotic pathogen that can cause serious human illnesses.

• Vascular compromise, especially aortic aneurysm rupture, is a rare but fatal complication of infective aortitis.

• Early detection and surgical management of mycotic aneurysms are essential to prevent life-threatening complications.

• Patients with mycotic aneurysm should be monitored closely for evidence of rupture.