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Infective Endocarditis - Pathogens

Infective Endocarditis - Pathogens

To review the pathohistolgoical context, let's take a quick look at an endocarditis vegetation. We show the heart and valves, which are inflamed, and a vegetation in the mitral valve. As we discuss elsewhere, these vegetations can lead to emboli, even stroke.

Bacterial Pathogens

Bacterial pathogens account for the majority of infective endocarditis cases – approximately 98%.

Gram Positive Bacteria - 80% of cases

Staphylococcus is a major cause of both health care and community acquired endocarditis. Staphylococcus is a normal inhabitant of the human nares, pharynx, and skin.

S. aureus is the most common and virulent cause of IE; it causes acute, flu-like symptoms, and antibiotic resistant strains are increasingly common, even outside of hospital settings. "Aureus" means "golden"; on blood agar plates, S. aureus colonies produce a golden color. It occurs in "grape-like" clusters. It is coagulase-positive, which means that it produces enzymes that promote blood clotting.

Coagulase-negative strains (CoNS) of Staphylococcus contribute to the normal flora of the skin and mucosal membranes; two strains relevant to IE are:

S. epidermis, found on the skin, is specifically associated with prosthetic valve infective endocarditis and health-care associated IE. S. lugdunesis infection is rare, but aggressive with a high mortality rate.

Streptococcal strains Viridans group, which is a normal component of the flora of the oropharynx, urogenital and gastrointestinal systems. Specifically, S. salivarius, S. mitis, and S. sanguinis are associated with endocarditis (be aware of intertextual variation regarding the exact species);

Viridans group streptococci comprise the second most common cause of IE, but, unlike S. aureus, are associated with subacute infection.

S. pneumoniae, which is associated with prosthetic valve IE; alcoholism is a risk factor for this type of infection (some include S. pneumoniae in the Viridans group). S. gallolyticus (formerly S. bovis*) can cause subacute IE; this pathogen is commonly found in the gastrointestinal tract, and is associated with increased risk of colon cancer.

Enterococci

E. faecailis and E. faecium comprise the third most common cause of IE; they are part of the normal flora of the colon, and cause subacute IE. Use of broad-spectrum antibiotics increases the risk of Enterococci infection, and hospital-associated infections are on the rise.

Other Tropheryma whipplei, which is the causative agent of Whipple's disease; this pathogen should be considered when culture-negative endocarditis is suspected. Erysipelothrix rhusiopathiae is an example of a zoonotic pathogen; it tends to affect the aortic valve, and is associated with a high mortality rate. Species of Corynebacterium tend to infect prosthetic devices.

Gram-negative Bacteria Strains - account for 1-10%

Gram-negative bacterial causes of endocarditis are categorized as HACEK or non-HACEK.

HACEK strains: These tend to have low virulence, and are associated with subacute cases; they are characterized by Osler's nodes (tender, painful nodes on the tips of the fingers or toes).

Research suggests that HACEK infection is more common in younger individuals, particularly males, and those with mechanical heart valves or diabetes; there is evidence that stroke risk is increased with HACEK infections.

Haemophilus species are the HACEK strains most likely to cause IE; they tend to affect the aortic and mitral valves, specifically.

Aggregatibacter species are slower to grow, and tend to appear in individuals with underlying valve damage.

Cardiobaceterium hominis tends to affect those with underlying heart disease, and appears on the mitral and aortic valves.

Eikenella corrodens, a strain associated with intravenous drug use and/or pre-existing valve disease.

Kingella kingae, which is associated with the aortic and mitral valves can progress rapidly.

Non-HACEK strains:

These bacteria are rarely the cause of endocarditis; but, when they are, tend to be associated with health care settings and individuals with implanted devices.

Their rarity can lead to delayed diagnosis, and, consequently, increased risk of complications such as embolization.

Key Strains: Bartonella species, particularly B. quintana and B. henselae, tend to affect the aortic valve; they produce subacute infection, and should be a consideration where culture-negative endocarditis is suspected.

Coxiella burnetti-induced endocarditis is a complication of Q fever; it is a zoonotic infection spread via spores, and should be considered in cases of culture-negative endocarditis.

Enterobacteriaceae species infection is rare, but very severe; infection occurs in immunocompromised individuals and those with valvular heart disease.

Pseudomonas aeruginosa is associated with severe infection in immunocompromised hosts, and is resistant to antibiotics; not surprisingly, then, it is associated with a high mortality rate.

Fungal Pathogens

Fungal pathogens are rare causes of endocarditis; they account for approximately 2 percent of IE cases.

Fungal pathogens are opportunistic, and form large, warty vegetations; infections are associated with a high mortality rate.

Two key species are: Candida, particularly C. albicans, is a yeast that tends to infect cardiac devices, and is associated with intravenous drug use; complications include loss of vision and cutaneous nodules. Aspergillus is a ubiquitous mold; infection is associated with hemorrhagic black skin lesions, vascular invasion, and tissue necrosis.

Diagnosis

Modified Duke criteria

Clinical diagnosis of IE requires one of the following: 2 major criteria or 1 major and 3 minor criteria or 5 minor criteria

Pathological criteria: Evidence of micro-organisms in a vegetation, in a vegetative embolus, and/or within an intracardiac abscess. Clinical criteria are distinguished as major or minor.

Major criteria include: Positive blood cultures of a characteristic pathogen or consistently positive for a lesser-common pathogen. Echocardiographic evidence of vegetative masses or abscesses.

Minor criteria include: Predisposing heart condition or intravenous drug use Fever Vascular phenomena (for example, Janeway's lesions, which are small nodular lesions on the palms of the hands or soles of the feet) Immunological phenomena (for example, Osler's nodes, glomerulonephritis, or Roth spots) Microbiologic evidence that does not meet Major criteria standards (for example, a single positive culture for an uncommonly associated organism) Echocardiographic evidence that is consistent with, but not diagnostic of, endocarditis (for example, worsening of a heart murmur).

Images

Vegetation histology (Jamie Donnell, MD). S. aureus (Wikipedia; Credit: NIAID/RML) Streptococci (Wikipedia; CDC). E. faecalis (Wikipedia; CDC/ Pete Wardell). C. albicans (CDC/ Maxine Jalbert, Dr. Leo Kaufman). Aspergillus (Wikipedia; Author: Medmyco).

Infective Endocarditis - Pathogens

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