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THE HACEK GROUP

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HACEK organisms are a group of fastidious, slow-growing, gram-negative bacteria whose growth requires an atmosphere of carbon dioxide. Species belonging to this group include several Haemophilus species, Aggregatibacter (formerly Actinobacillus) species, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae. HACEK bacteria normally reside in the oral cavity and have been associated with local infections in the mouth. They are also known to cause severe systemic infections—most often bacterial endocarditis, which can develop on either native or prosthetic valves (Chap. 20).

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In large series, 0.8–6% of cases of infective endocarditis are attributable to HACEK organisms, most often Aggregatibacter species, Haemophilus species, and C. hominis. Invasive infection typically occurs in patients with a history of cardiac valvular disease, often in the setting of a recent dental procedure or nasopharyngeal infection. The aortic and mitral valves are most commonly affected. Compared with non-HACEK endocarditis, HACEK endocarditis occurs in younger patients and is more frequently associated with embolic, vascular, and immunologic manifestations but less commonly associated with congestive heart failure. The clinical course of HACEK endocarditis tends to be subacute, particularly with Aggregatibacter or Cardiobacterium. However, K. kingae endocarditis may have a more aggressive presentation. Systemic embolization is common. The overall prevalence of major emboli associated with HACEK endocarditis ranges from 28% to 71% in different series. On echocardiography, valvular vegetations are seen in up to 85% of patients. Aggregatibacter and Haemophilus species cause mitral valve vegetations most often; Cardiobacterium is associated with aortic valve vegetations. The microbiology laboratory should be alerted when a HACEK organism is being considered; however, most cultures that ultimately yield a HACEK organism become positive within the first week, especially with improved culture systems such as BACTEC. In addition, polymerase chain reaction (PCR) techniques (e.g., of cardiac valves) and other tools, such as matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectrometry performed directly on agar colonies, are facilitating the diagnosis of HACEK infections. Because of the organisms’ slow growth, antimicrobial testing may be difficult, and β-lactamase production may not be detected. Resistance is most commonly noted in Haemophilus and Aggregatibacter species. E-test methodology may increase the accuracy of susceptibility testing. The overall prognosis in HACEK endocarditis is excellent and significantly better than that in endocarditis caused by non-HACEK pathogens.

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Haemophilus species
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Haemophilus parainfluenzae is the most common species isolated from cases of HACEK endocarditis. Of patients with HACEK endocarditis due to Haemophilus species, 60% have been ill for <2 months before presentation, and 19–50% develop congestive heart failure. Mortality rates as high as 30–50% were reported in older series; however, more recent studies have documented mortality rates of <5%. H. parainfluenzae has been isolated from other infections, such as meningitis; brain, dental, pelvic, and liver abscess; pneumonia; urinary tract infection; and septicemia.

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Aggregatibacter species
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The species of Aggregatibacter that most frequently cause infective endocarditis are A. actinomycetemcomitans, A. (formerly Haemophilus) aphrophilus, and A. paraphrophilus. Aggregatibacter is associated with prosthetic valve endocarditis more often than are Haemophilus species. A. actinomycetemcomitans...

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