25.2 Bacterial Infections of the Circulatory and Lymphatic Systems

Pathogenesis and Clinical Manifestations

Pathogenesis of Circulatory Bacterial Infections

Sepsis is a life-threatening condition where the body's response to infection spirals out of control. Bacterial toxins or components like lipopolysaccharide (LPS) trigger a systemic inflammatory response that damages the endothelium (blood vessel lining) and disrupts normal coagulation. The result is widespread inflammation and impaired blood flow to organs.

• Clinical signs: fever, tachycardia (rapid heart rate), tachypnea (rapid breathing), hypotension
• Severe cases progress to septic shock and multi-organ failure (kidneys, liver, lungs)
• Sepsis exists on a spectrum: bacteremia → sepsis → severe sepsis → septic shock

Endocarditis is an infection of the endocardium, the inner lining of the heart, most often affecting the heart valves. Bacteria like Staphylococcus aureus (acute) or Streptococcus viridans group (subacute) enter the bloodstream and attach to damaged or abnormal valve surfaces, forming clumps of bacteria, fibrin, and platelets called vegetations.

• Clinical signs: fever, new or changing heart murmur, embolic events
• Classic findings include Janeway lesions (painless red spots on palms/soles) and Osler nodes (painful nodules on fingers/toes)
• Vegetations can break off and embolize, causing strokes, splenic infarcts, or kidney damage
• Risk factors: IV drug use, prosthetic heart valves, pre-existing valve damage

Bacteremia refers to the presence of viable bacteria in the bloodstream. It can result from primary infections elsewhere in the body, surgical procedures, or IV drug use. Transient bacteremia can even occur after routine dental procedures.

• Clinical signs: fever, chills, malaise
• Bacteremia itself isn't always dangerous, but if the immune system can't clear the bacteria, it can progress to sepsis
• Outcome depends heavily on the causative organism and the patient's immune status

Host-Pathogen Interactions

Once bacteria enter the bloodstream, several factors determine whether infection takes hold or gets cleared.

• Hematogenous spread allows bacteria to disseminate from a local site to distant organs, seeding secondary infections (e.g., a skin infection leading to endocarditis or a brain abscess)
• Bacterial colonization and adhesion is the first step: bacteria must attach to host tissues before they can invade
• The vascular endothelium actively regulates inflammation and coagulation during infection. Damage to endothelial cells is a major driver of sepsis pathology
• Innate immunity (neutrophils, complement, macrophages) provides the first line of defense, while adaptive immunity (antibodies, T cells) mounts a targeted response
• Immunocompromised patients (HIV/AIDS, chemotherapy, organ transplant recipients) face much higher risk of severe or disseminated infections
• Antibiotic resistance complicates treatment significantly. Organisms like MRSA (methicillin-resistant S. aureus) can cause persistent or recurrent bloodstream infections that are difficult to clear

Zoonotic Bacterial Diseases

Zoonotic Diseases of the Circulatory System

Zoonotic infections are transmitted from animals to humans. Several important zoonoses target the circulatory and lymphatic systems, each with distinct vectors and clinical presentations.

Plague (Yersinia pestis)

Transmitted primarily through bites from infected fleas that feed on rodents (rats, prairie dogs, squirrels). Direct contact with infected animal tissues is another route.

Plague presents in three forms:

1. Bubonic (most common): fever, chills, weakness, and buboes, which are extremely swollen, tender lymph nodes typically in the groin, axilla, or neck
2. Pneumonic: fever, cough, chest pain, dyspnea (difficulty breathing). This form can spread person-to-person via respiratory droplets and is the most dangerous
3. Septicemic: bacteria overwhelm the bloodstream, causing fever, abdominal pain, shock, and disseminated intravascular coagulation (DIC), where clotting and bleeding occur simultaneously

Treatment: streptomycin or gentamicin (first-line), doxycycline as an alternative

Tularemia (Francisella tularensis)

This organism has multiple transmission routes, which makes it distinctive:

• Contact with infected animals (especially rabbits and hares)
• Tick or deer fly bites
• Inhalation of contaminated aerosols
• Ingestion of contaminated water or undercooked meat

Symptoms include fever, chills, headache, muscle aches, skin ulcers at the site of entry, and regional lymphadenopathy. The pneumonic form causes cough, chest pain, and dyspnea. F. tularensis is highly infectious, requiring as few as 10 organisms to cause disease via inhalation.

Treatment: streptomycin or gentamicin (first-line), doxycycline as an alternative

Lyme Disease (Borrelia burgdorferi)

Transmitted by the bite of infected Ixodes ticks (black-legged ticks, commonly called deer ticks). The tick typically needs to be attached for 36-48 hours before transmission occurs.

Lyme disease progresses through stages:

• Early localized: erythema migrans, the characteristic expanding "bull's-eye" rash at the bite site, along with fever, fatigue, headache, and muscle/joint pain
• Early disseminated (weeks to months): multiple erythema migrans lesions, facial nerve palsy (Bell's palsy), meningitis, cardiac involvement including heart block
• Late disseminated (months to years): Lyme arthritis (especially large joints like the knee), chronic neurologic symptoms

Treatment: doxycycline (first-line for adults), amoxicillin or cefuroxime (alternatives, preferred in children and pregnant patients)

Virulence Factors and Diagnostics

Virulence Factors in Lymphatic and Circulatory Infections

Bacteria that successfully infect the circulatory and lymphatic systems rely on specific virulence factors to establish infection, evade immunity, and cause damage.

• Adhesion molecules allow bacteria to attach to host cells and extracellular matrix components. Fibronectin-binding proteins and collagen-binding proteins are particularly important for organisms like S. aureus that colonize heart valves.
• Toxins damage host tissues and manipulate immune responses:
  • Endotoxin (LPS): a component of Gram-negative bacterial outer membranes that triggers massive inflammatory cytokine release. This is the primary driver of Gram-negative sepsis.
  • Exotoxins: secreted proteins like staphylococcal enterotoxins (which can act as superantigens) and hemolysins like alpha-toxin, which directly lyse host cells
• Capsules surround the bacterial cell and inhibit phagocytosis by preventing recognition by immune cells and blocking complement-mediated killing. Streptococcus pneumoniae is a classic example.
• Biofilm formation allows bacteria to create a protective extracellular matrix on surfaces like heart valves or catheters. Biofilms make infections extremely difficult to treat because antibiotics and immune cells can't penetrate effectively.

Diagnostic Approaches for Circulatory Infections

Blood cultures remain the gold standard for diagnosing bacteremia and sepsis.

1. Blood is drawn (ideally from two separate sites to reduce contamination risk) and inoculated into culture bottles
2. Automated systems like BACTEC or BacT/ALERT continuously monitor for bacterial growth by detecting $CO_2$ production
3. Positive cultures are Gram stained to provide a rapid preliminary identification
4. Biochemical tests (API strips, VITEK systems) or mass spectrometry (MALDI-TOF) then identify the specific organism
5. Antimicrobial susceptibility testing guides antibiotic selection

Serological tests detect antibodies against specific bacterial antigens in the patient's serum. For Lyme disease, a two-tier testing approach is standard: an initial ELISA screen followed by a confirmatory Western blot. Serology is less useful in early infection because antibodies take time to develop.

Molecular methods detect bacterial DNA or RNA directly from clinical samples.

• PCR (polymerase chain reaction) amplifies specific bacterial gene sequences, offering rapid and sensitive detection
• 16S rRNA sequencing identifies bacteria by targeting conserved ribosomal RNA genes, which is especially useful when cultures are negative but infection is suspected

Imaging techniques help visualize the infection and its complications.

• Echocardiography (transthoracic or transesophageal) is essential for detecting vegetations in suspected endocarditis. Transesophageal echo is more sensitive.
• CT scans identify deep-seated infections like abscesses (psoas abscess, epidural abscess, splenic abscess)