Microbial Diseases of the Nervous, Cardiovascular, Lymphatic, and Respiratory Systems

Chapter 22: Microbial Diseases of the Nervous System

Encephalitis in Humans

Encephalitis refers to the inflammation of the brain, often caused by viral infections. Many cases are due to arboviruses (arthropod-borne viruses) transmitted by mosquitoes or ticks.

• Arboviral Encephalitis: Includes viruses such as West Nile virus and Eastern Equine Encephalitis virus.

• Transmission: Primarily via mosquito or tick bites.

• Clinical Features: Can result in permanent brain damage, neurological deficits, or death.

• Prevention: Vector control and personal protective measures.

Treatment of Bacterial Encephalitis and Meningitis

Bacterial infections of the central nervous system are medical emergencies due to their rapid progression and high mortality.

• Empiric Therapy: Broad-spectrum, third-generation cephalosporins are commonly used before a specific diagnosis is made.

• Rationale: Early treatment is critical to improve outcomes.

General Symptoms of Bacterial Meningitis

Bacterial meningitis presents with a characteristic set of symptoms that may progress rapidly.

• Initial Symptoms: Fever, headache, stiff neck.

• Progression: Nausea, vomiting, convulsions, coma, and potentially death.

Key Microbes to Know

• Neisseria meningitidis: Aerobic, gram-negative diplococcus with a capsule. Causes meningococcal meningitis, often seen in crowded environments (e.g., dormitories). Notable for a non-blanching rash.

• Haemophilus influenzae: Gram-negative, aerobic bacterium, part of normal throat flora. Pathogenicity is mainly due to its type b capsule. Primarily affects young children.

• Mycobacterium leprae: Acid-fast rod causing leprosy (Hansen’s disease). Infects peripheral nerves and skin, leading to sensory loss (tuberculoid form) or disfiguring nodules (lepromatous form).

• Cryptococcus neoformans: Encapsulated soil fungus associated with bird droppings. Transmitted via inhalation; can cause meningitis in immunocompromised individuals.

• Naegleria fowleri: Free-living protozoan found in warm freshwater. Enters through the nasal mucosa, causing primary amebic meningoencephalitis (PAM), which is almost always fatal.

Rabies (Extra Credit)

Rabies is a fatal viral encephalitis caused by a bullet-shaped RNA virus, typically transmitted via animal bites.

• Pathogenesis: Virus replicates in muscle, then travels via peripheral nerves to the CNS.

• Symptoms: Hydrophobia (fear of water), muscle spasms, progressive neurological symptoms, and death.

Chapter 23: Microbial Diseases of the Cardiovascular and Lymphatic Systems

Epstein-Barr Virus (EBV) Infections

EBV is a herpesvirus that infects B lymphocytes and is associated with several diseases.

• Infectious Mononucleosis: Characterized by fever, sore throat, and splenomegaly.

• Burkitt’s Lymphoma: A childhood cancer common in Africa, linked to EBV infection.

• Other Associations: Possible links to multiple sclerosis and Hodgkin's lymphoma.

Characteristics of Bacillus anthracis

Bacillus anthracis is a gram-positive, endospore-forming aerobic rod found in soil, known for causing anthrax.

• Toxins: Produces edema and lethal toxins.

• Forms of Anthrax:

  • Cutaneous (most common)

  • Gastrointestinal

  • Inhalational (pulmonary, most fatal)

Plague (Yersinia pestis)

The plague is a severe zoonotic disease transmitted by fleas, with several clinical forms.

• Transmission: Rat flea bites; rodents are reservoirs.

• Forms:

  • Bubonic: Swollen lymph nodes (buboes), 50-75% mortality if untreated.

  • Septicemic: Bacteria in the bloodstream.

  • Pneumonic: Airborne transmission, nearly 100% mortality if untreated.

Rickettsial and Other Vector-Borne Diseases

Disease	Agent	Vector	Key Features
Epidemic Typhus	Rickettsia prowazekii	Body louse	High fever, rash
Endemic Murine Typhus	Rickettsia typhi	Rat flea	Rodent hosts
Rocky Mountain Spotted Fever	Rickettsia rickettsii	Tick	Measles-like rash on palms/soles
Relapsing Fever	Borrelia spp.	Soft tick	Recurring fever, jaundice

Gram-Negative Sepsis (Endotoxic Shock)

Gram-negative sepsis is a life-threatening condition caused by the release of endotoxins (lipopolysaccharide, LPS) from gram-negative bacteria.

• Common Agents: Klebsiella spp., Escherichia coli, Pseudomonas aeruginosa

• Pathogenesis: Endotoxins trigger a severe drop in blood pressure.

• Antibiotic Caution: Some antibiotics may worsen the condition by causing more endotoxin release as bacteria die.

Chapter 24: Microbial Diseases of the Respiratory System

Tuberculosis (Mycobacterium tuberculosis)

Tuberculosis is a chronic respiratory disease caused by an acid-fast bacterium with a lipid-rich cell wall, conferring resistance to drying and many drugs.

• Transmission: Inhalation of airborne droplets.

• Pathogenesis: Bacteria are walled off in tubercles in the lungs, which can rupture and cause active disease.

• Diagnosis: Acid-fast staining, chest X-ray, and tuberculin skin test.

Legionellosis (Legionella pneumophila)

Legionellosis is a severe pneumonia caused by a gram-negative rod that thrives in aquatic environments.

• Reservoirs: Water systems, cooling towers, humidifiers.

• Transmission: Inhalation of contaminated aerosols (not person-to-person).

• Symptoms: High fever, cough.

Normal Protection of the Lower Respiratory Tract

The lower respiratory tract is protected by several innate defense mechanisms.

• Ciliary Escalator: Moves trapped particles upward toward the throat for removal.

• Alveolar Macrophages: Engulf and destroy microbes in the alveoli.

• Respiratory Mucus: Traps dust and microorganisms.

Influenza Virus

The influenza virus is a segmented RNA virus responsible for seasonal flu epidemics and occasional pandemics.

• Genome: Eight RNA segments.

• Surface Proteins: Hemagglutinin (HA) for cell attachment; Neuraminidase (NA) for viral release.

• Antigenic Shift: Major genetic changes that can result in new pandemic strains due to lack of population immunity.

Group A Streptococci (GAS) Infections

Streptococcus pyogenes is the main GAS pathogen, causing a range of diseases.

• Strep Throat (Pharyngitis): Sore throat, fever.

• Scarlet Fever: Caused by erythrogenic toxin-producing strains; characterized by a red rash.

Other Notable Respiratory Pathogens (Extra Credit)

Microbe	Disease	Transmission	Key Features
Coxiella burnetii	Q fever	Unpasteurized milk, aerosols from farm animals	Fever, headache, pneumonia
Mycoplasma pneumoniae	"Walking pneumonia"	Person-to-person	Lacks cell wall, "fried-egg" colonies
Chlamydophila pneumoniae	Mild respiratory illness	Person-to-person	Common in young people
Mycobacterium (see above)	Tuberculosis	Inhalation	Acid-fast, chronic infection