European Journal of Vascular & Endovascular Surgery
Volume 36, Issue 3 , Pages 292-296, September 2008

Abdominal Aortic Aneurysm and the Impact of Infectious Burden

  • A. Nyberg

      Affiliations

    • Department of Laboratory Medicine, Sundsvall County Hospital, Sundsvall, Sweden
    • ,
  • E. Skagius

      Affiliations

    • Department of Surgery, Sundsvall County Hospital, Sundsvall, Sweden
    • ,
  • E. Englund

      Affiliations

    • Research and Development Center, Sundsvall County Hospital, Sundsvall, Sweden
    • ,
  • I. Nilsson

      Affiliations

    • Department of Laboratory Medicine, Division of Medical Microbiology, University of Lund, Sweden
    • ,
  • Å. Ljungh

      Affiliations

    • Department of Laboratory Medicine, Division of Medical Microbiology, University of Lund, Sweden
    • ,
  • A.E. Henriksson

      Affiliations

    • Department of Surgery, Sundsvall County Hospital, Sundsvall, Sweden
    • Corresponding Author InformationCorresponding author. Anders Henriksson, MD, PhD, Department of Surgery, Sundsvall County Hospital, SE-851 86 Sundsvall, Sweden. Tel. +46 60 181430; fax: +46 60 181439.

Received 15 January 2008; accepted 24 April 2008. published online 11 June 2008.

Article Outline

Abstract 

Objectives

Little is known about the biological processes causing aortic aneurysm rupture. Chronic Chlamydophila pneumoniae infection has been suggested as a possible contributing factor to the development and expansion of abdominal aortic aneurysm (AAA). The importance of infection in AAA may be related to the previous pathogen burden, that is, the number of significant titres of antibodies against infectious pathogens rather than to single infectious agents. The aim of this study was to examine the relationship between infectious burden and AAA rupture.

Methods

In a case-control study, 119 patients with abdominal aortic aneurysm and 36 matched controls without aneurysm were prospectively investigated for specific IgG class antibodies against C. pneumoniae, Helicobacter pylori, Cytomegalovirus, and Herpes simplex virus.

Results

Patients with ruptured AAA have similar levels of pathogen burden as patients with nonruptured electively operated AAA, small AAA, and controls without aneurysm.

Conclusion

The present study fails to demonstrate a connection between infectious burden and abdominal aortic aneurysm rupture.

Keywords: Infectious burden, Chlamydophila pneumoniae, Aortic aneurysm, Abdominal, Rupture

 

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Introduction 

In a recent review it was stated that little is known about the biological processes causing abdominal aortic aneurysm (AAA) rupture.1 Extensive infiltration of T-cells, B-cells and macrophages are seen in biopsies from AAAs. The cause of the inflammatory process is unknown. A possible explanation for this inflammatory process is a chronic infection.

A microorganism which has received great attention as a possible contributing factor for the development and expansion of AAA is the widespread respiratory pathogen Chlamydophila (Chlamydia) pneumoniae. In a study by Karlsson et al. viable C. pneumoniae was detected in the wall of AAAs.2 However, it was concluded that the relevance of C. pneumoniae infection in AAA requires further studies to elucidate whether the infection contributes to aneurysm formation or whether the infection is secondary to a pre-existing aneurysm. An association exists between smoking and ethnicity with C. pneumoniae seropositivity.3, 4 These and other associations can confound the results of research on the epidemiology of C. pneumoniae infections. Furthermore, the lack of a reliable serologic marker for chronic or persistent C. pneumoniae infection may contribute to the different results reported.5 The importance of the single infectious agent C. pneumoniae in AAA has also been questioned since two antibiotic trials have been disappointing.6, 7

The gastric pathogen Helicobacter pylori has been implicated in the pathogenesis of atherosclerosis.8 H. pylori DNA has been detected in human carotid atherosclerotic plaques.9 H. pylori is categorized into two phenotypes on the basis of the presence or absence of the CagA gene (cytotoxin-associated gene A).10 It has been suggested that infection with CagA-positive H. pylori strain is more closely associated with peptic ulcer disease.10 Recent studies have shown increased risk of carotid atherosclerosis and coronary heart disease in subjects harbouring CagA bearing strains of H. pylori.11, 12 However, a causal relationship between H. pylori and atherosclerosis is not confirmed in other studies.13

An association of viral infection with atherosclerosis was first reported by Fabricant et al., when healthy chickens inoculated with an avian herpesvirus were found to develop arterial disease.14 Cytomegalovirus (CMV) has been implicated in the pathogenesis of atherosclerosis.15, 16, 17 As suggested for atherosclerosis the AAA risk of infection may be related to the pathogen burden, that is, the number of significant titres of antibodies against different infectious pathogens rather than to single infectious agents.18, 19

We hypothesised that if infectious burden was important in the pathogenesis of AAA, the highest number of seropositivities to infectious pathogens ought to be in patient with ruptured aortic aneurysm and the lowest in a control group without aneurysm if other risk factors are eliminated. Thus, in the present study patients with ruptured and nonruptured AAA and matched controls were tested for specific IgG class antibodies against Chlamydophila pneumoniae, Helicobacter pylori, Cytomegalovirus, and Herpes simplex 1 virus. The selected infectious pathogens were chosen according to previous studies on atherosclerosis.18, 20, 21

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Material and Methods 

The study was performed in accordance with the principles of the Declaration of Helsinki and was approved by the regional ethics committee. Patients and control subjects gave their approval by written informed consent. Controls and patients were of the same ethnic origin and had similar socio-economic backgrounds.

AAA patients 

Prospectively, we studied 119 patients with infrarenal AAA treated at Sundsvall County Hospital between January 2002 and March 2005. Patients with coexisting clinical signs of acute infection, or malignant disease were excluded.

Forty-one patients with ruptured infrarenal AAA (rAAA) were included. All patients had a retroperitoneal hematoma confirmed at operation and underwent a temporary initial supraceliac aortic clamping, which is the standard approach in this institution. No blood transfusions were given before laparotomy and in accordance with Crawford, no significant attempt was made to blood volume resuscitation until the time of operation.22

Thirty-eight patients with an elective operation for nonruptured infrarenal AAA (eAAA), with an aneurysm diameter of at least 5.0cm, were also included. All patients underwent conventional open AAA repair through a standard midline laparotomy incision and transperitoneal approach with infrarenal graft repair.

Forty patients with asymptomatic infrarenal aortic aneurysm on surveillance with an aneurysm diameter smaller than 5.0cm (small AAA) were also included.

Controls 

The control group was selected in accordance with the guidelines given by Grimes and Schulz.23 A control group of thirty-six volunteers with normal infrarenal aortic diameter were matched to the AAA patients according to age, gender and smoking habits. Smoking was defined as current smoking at the time of inclusion.

Imaging 

The largest aortic diameter was measured using the outermost ultrasonography reflection with the transducer parallel to the longitudinal axis of the infrarenal aorta. Normal diameter was defined as maximum infrarenal aortic diameter <3.0cm.24

Blood sampling 

Peripheral venous blood samples were taken from controls and each patient preoperatively, and before any blood products had been transfused. Samples were frozen in plastic tubes as serum and stored at −70°C until analysis. All tests were performed and interpreted in a blinded fashion by the same technician, according to the manufacturers' instructions.

Chlamydophila pneumoniae serology 

Sera were tested for IgG antibodies against C. pneumoniae using a commercial EIA kit (Euroimmun, Lübeck, Germany). It is a semi-quantitative assay and the antigen used in the EIA is SDS-extracted outer membrane protein (OMP) of elementary bodies of C. pneumoniae strain CDC/CWL-026.7 As no international reference serum exists for antibodies against C. pneumoniae, the calibration is performed in relative units (RU). The manufacturer recommends interpreting results as positive for RU values ≥1.1.

Helicobacter pylori serology 

The H. pylori EIA was performed as previously described.25 The EIA test results are expressed as corrected mean absorbance values (A 405nm) in percent of a reference standard (human γ-globulin, Pharmacia & UpJohn, Stockholm, Sweden) (relative antibody activity, RAA). For H. pylori the cut-off values are established based on EIA results obtained with sera from patients with positive and negative gastric cultures, healthy blood donors and children. A RAA value of >35 was regarded as positive.

Cytomegalovirus serology 

Sera were tested for IgG antibodies against CMV using a commercial EIA kit (Euroimmun, Lübeck, Germany). It is a semi-quantitative assay and the antigen used in an inactivated cell lysate of MRC-5 cells infected with the AD169 strain of cytomegalovirus. The manufacturer recommends interpreting results as positive for RU values ≥0.8.

Herpes simplex 1 virus serology 

Sera were tested for IgG antibodies against Herpes simplex 1 virus (HSV-1) using a commercial EIA kit (Euroimmun, Lübeck, Germany). The antigen used in an inactivated cell lysate of MRC-5 cells infected with the AD169 strain of cytomegalovirus. The calibration is performed in relative units (RU). The manufacturer recommends interpreting results as positive for RU values ≥0.8.

Statistical analysis 

In this study we made the assumption that a difference of one unit of seropositivity between rAAA and eAAA would be clinical useful in the rupture risk stratification. With a standard deviation of one unit, a power equal to 90%, and a significance level of 5%, we need at least 27 subjects in each group. Data are presented as median (interquartile range) or the number (percentage) of patients/controls. Differences in findings between study groups were assessed by Chi-square tests or Kruskal-Wallis tests as appropriate for categorical variables and by Mann-Whitney tests for continuous variables. A p-value<0.05 was considered significant. All analyses were carried out using SPSS® statistical software 14.0 for Windows™ (SPSS, Chicago, Illinois, USA).

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Results 

There were no significant difference between all AAA patients and controls according to age, gender and current smoking habits (Table 1). The median (interquartile range) aneurysm diameter were: rAAA 7.0 (6.0–8.0)cm, eAAA 5.9 (5.2–7.0)cm, and small AAA 4.0 (3.5–4.3)cm. There was no significant difference in number of seropositivity between the three patient groups and controls as shown in Fig. 1 (p-value=0.062, Kruskal-Wallis test). The controls show the lowest incidence of seropositivity for all four pathogens. However, the highest incidence of seropositivity for all pathogens was not in the rAAA patients but in the eAAA patients.

Table 1. Demographics and sero-positivity of AAA patients and controls
AAA patients (n=119)Controls(n=36)p-value
Age (years)72 (66–78)72 (67–78)0.958
Sex (male)95 (80%)30 (83%)0.644
Smoking51 (43%)15 (42%)0.899

Serostatusa
None/one positive4 (3%)2 (6%)0.916
Two positive26 (22%)11 (30%)0.395
Three positive44 (37%)13 (36%)0.925
Four positive45 (38%)10 (28%)0.366

The figures indicate median (interquartile range) or the number (percentage) of patients/controls, and the intergroup p-value.

aSeropositive to Chlamydophila pneumoniae, Helicobacter pylori, Cytomegalovirus, and/or Herpes simplex 1 virus.

  • View full-size image.
  • Figure 1 

    Number of seropositivities in subgroups with abdominal aortic aneurysm and controls. Number of positive sera against 1, 2, 3 or 4 of the tested microorganisms, Chlamydophila pneumoniae, Helicobacter pylori, Cytomegalovirus and Herpes simplex 1 virus.

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Discussion 

The pathogenesis of AAA is not completely explained by known risk factors for AAA (e.g., smoking, gender, age, etc.). Therefore, the study of AAA has taken new directions, one of the most intriguing of these being the infective theory of AAA that has been addressed in recent years. Several studies have suggested a role of C. pneumoniae infection in the pathogenesis of AAA, and viable C. pneumoniae have been detected in the wall of AAAs.2, 6, 7, 26 However, previous studies varied greatly in terms of population, criteria for defining AAA, size of AAA, and adjustment for confounding variables such as age, gender and smoking. This makes them prone to bias. The association between C. pneumoniae infection and the development of atherosclerosis and arterial aneurysms remains controversial and unresolved.26, 27, 28

In the present study we investigated the possible connection between infectious burden and AAA with special emphasis on the identification of infectious burden as a risk factor useful in rupture risk stratification.

The present study demonstrated a high prevalence of seropositivity of C. pneumoniae, H. pylori, CMV, and HSV-1 in patients with AAA. However, the seroprevalence for these infectious pathogens was similar as in controls without aneurysm matched for age, gender, and smoking habits. Furthermore, there was no significant difference in infectious burden between patients with ruptured, electively operated or small AAAs.

In summary, the present study fails to demonstrate a connection between infectious burden (defined as seropositivity to Chlamydophila pneumoniae, Helicobacter pylori, Cytomegalovirus, and Herpes simples 1 virus) and AAA rupture.

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Acknowledgements 

This study was supported by grants from the Medical Faculty, Lund University, the Swedish Research Council, Emil Andersson Foundation for Medical Research, and Jonas and Ketty Åkerberg Foundation.

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References 

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PII: S1078-5884(08)00241-4

doi:10.1016/j.ejvs.2008.04.017

European Journal of Vascular & Endovascular Surgery
Volume 36, Issue 3 , Pages 292-296, September 2008

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