Microbial Infections of the Nervous System: Chapter 22 Study Guide

Microbial Infections - Nervous System

Overview of the Nervous System

The nervous system is divided into the central and peripheral components, each with distinct structures and functions. Understanding these divisions is essential for recognizing how infections affect neurological health.

• Central Nervous System (CNS): Composed of the brain and spinal cord.

• Peripheral Nervous System (PNS): Consists of nerves branching from the CNS.

Blood-Brain Barrier

• Definition: A highly selective, tightly packed layer of endothelial cells, astrocytes, and pericytes that protects the brain from toxins, pathogens, and harmful substances in the blood.

• Function: Maintains CNS homeostasis and limits entry of microbes and drugs.

Meninges

• Function: Protective membranes surrounding the brain and spinal cord.

• Layers: Dura mater, arachnoid mater, pia mater.

Inflammatory Diseases of the Nervous System

• Meningitis: Inflammation of the meninges.

• Encephalitis: Inflammation of the brain.

• Key Difference: Meningitis affects protective membranes; encephalitis affects brain tissue.

Bacterial Meningitis

Bacterial meningitis is a life-threatening infection of the meninges, most commonly caused by three pathogens.

• Major Pathogens:

  • Streptococcus pneumoniae (pneumococcus)

  • Neisseria meningitidis (meningococcus)

  • Haemophilus influenzae

• Pathogenicity Factor: Polysaccharide capsule (protects against phagocytosis).

• Vaccines: Available for all three; have significantly reduced incidence of meningitis.

• Leading Cause: Streptococcus pneumoniae is currently the most common cause.

• Carriers: 40% of people carry Neisseria meningitidis; 70% carry Streptococcus pneumoniae in the nasopharynx without symptoms.

• Outbreaks: Often occur in crowded conditions (e.g., military barracks, college dorms).

Listeriosis

• Causative Agent: Listeria monocytogenes

• Characteristics: Psychrotroph (grows in cold, damp environments), can contaminate food.

• Susceptible Groups: Fetuses, newborns, pregnant women, immunocompromised, elderly.

• Significance: Can cause meningitis in newborns if contracted during pregnancy.

Tetanus

• Causative Agent: Clostridium tetani (gram-positive, endospore-forming, obligate anaerobe)

• Toxin: Tetanospasmin (causes muscle spasms and lockjaw)

• Transmission: Enters through deep puncture wounds, often from soil contaminated with animal waste.

• Prevention: Vaccination with tetanus toxoid (DTaP).

• Treatment: Debridement and vaccination.

Botulism

• Causative Agent: Clostridium botulinum (gram-positive, endospore-forming, obligate anaerobe)

• Toxins:

  • Type A: 60-70% fatality, heat-resistant, proteolytic

  • Type B: 25% fatality

  • Type E: Found in marine/lake sediments, less heat-resistant

• Infant Botulism: Caused by ingestion of spores (commonly from honey), which grow and release toxins in the immature digestive tract.

Leprosy (Hansen’s Disease)

• Causative Agents: Mycobacterium leprae & M. lepromatosis (acid-fast rods, grow best at 30°C, generation time 12 days)

• Culturing: Requires living host cells; mouse footpad method used.

• Transmission: Prolonged respiratory exposure.

• Forms:

  • Tuberculoid (neural): Loss of sensation in skin areas.

  • Lepromatous (progressive): Disfiguring nodules, mucous membrane involvement.

• Treatment: Antibiotics (dapsone, rifampin, clofazimine) for 6-24 months.

Poliomyelitis (Polio)

• Causative Agent: Poliovirus

• Transmission: Fecal-oral route (contaminated water/food), saliva droplets.

• Sanitation Impact: Early exposure in poor sanitation areas led to mild cases; improved sanitation delayed exposure, increasing severity.

• Vaccines:

  • Salk: Inactivated, injectable, safe, prevents paralytic disease, weaker intestinal immunity.

  • Sabin: Oral, attenuated, strong intestinal immunity, rare risk of vaccine-derived polio.

Comparison of Salk and Sabin Vaccines

Attribute	Salk Vaccine	Sabin Vaccine
Type	Inactivated	Live attenuated
Administration	Injection	Oral drops
Immunity	Good for paralytic disease	Strong intestinal, lifelong
Transmission Prevention	Less effective	Effective
Risks	No vaccine-derived polio	Rare vaccine-associated polio

Rabies

• Causative Agent: Rabies virus

• Transmission (US): Bite from infected animals, mainly bats.

• Transmission (Worldwide): Bite from infected dogs.

• Types:

  • Furious (classical): Restlessness, excitability.

  • Paralytic (dumb): Minimal excitability, unaware of surroundings.

Vertical Transmission (Mother-to-Child)

• Definition: Disease passed from mother to baby during pregnancy, birth, or shortly after.

• Viral Infections: Zika virus, neonatal herpes, cytomegalovirus, rubella (can affect fetal development).

• Bacterial/Protozoan Pathogens: Treponema pallidum, Listeria monocytogenes, group B streptococcus, Elizabethkingia, Toxoplasma gondii.

• TORCH Screen: Panel testing for antibodies to Toxoplasmosis, Other (syphilis, chickenpox, HIV, measles, mumps, hepatitis B), Rubella, Cytomegalovirus, Herpes simplex virus.

Arboviruses

• Definition: Arthropod-borne viruses, mainly mosquito-borne.

• Common US Arbovirus: West Nile virus.

• Transmission Routes (Zika): Mosquito, vertical, sexual, blood transfusion, lab exposure.

• Congenital Zika Syndrome (CZS): May occur if infection during pregnancy.

• Control: Vector control, personal protection, vaccination.

African Trypanosomiasis (Sleeping Sickness)

• Causative Agents: Trypanosoma brucei gambiense, T. b. rhodesiense

• Vector: Tsetse fly (transmits via bites)

• Symptoms: Fever, headache, swollen lymph nodes, joint pain, neurological changes.

• Differences:

  • Gambiense: Slow, chronic, gradual CNS involvement.

  • Rhodesiense: Rapid, acute, early CNS involvement, more severe.

• Treatment: TBG: Eflornithine (crosses blood-brain barrier, blocks parasite enzyme); TBR: Melarsoprol.

Prion Diseases

• Definition: Prions are self-replicating infectious proteins.

• Diseases: Sheep scrapie (transmissible spongiform encephalopathy, TSE in sheep).

Additional info:

• Preventive strategies for vertical transmission include screening, vaccination, and avoiding exposure to known pathogens during pregnancy.