Micro CH 17

Describe the nervous system's defenses and why CNS infections are difficult to treat.

• Structural defenses: Skull and vertebral column protect the brain and spinal cord; cerebrospinal fluid (CSF) cushions the CNS.

• Blood-Brain Barrier (BBB): Selective barrier that allows only certain molecules to enter the brain, preventing most microorganisms and many antibiotics from crossing.

• Immunologically privileged: The CNS mounts only a limited immune response because inflammation could permanently damage neurons.

• Microglia: Resident phagocytic cells (brain macrophages) that perform phagocytosis but are less active than macrophages elsewhere.

• Normal microbiota: The CNS and PNS have no normal microbiota. Finding microorganisms indicates infection. Herpes viruses may remain dormant in nerves but are not considered normal flora.

What is meningitis, what are its symptoms, and what organisms commonly cause it?

• Definition: Inflammation of the meninges surrounding the brain and spinal cord.

• General symptoms: Severe headache, stiff/painful neck, fever, nausea, vomiting, photophobia, increased lymphocytes in CSF, and sometimes skin rash.

• Three major bacterial causes:

  • Streptococcus pneumoniae

  • Neisseria meningitidis

  • Haemophilus influenzae type B (rare today because of vaccination)

• Common virulence factor: All have capsules that resist phagocytosis.

• Why dangerous: Bacterial meningitis progresses rapidly and may cause shock or death, especially from endotoxins of Gram-negative bacteria.

Describe Neisseria meningitidis, including characteristics, symptoms, transmission, treatment, and prevention.

• Gram-negative encapsulated diplococci (meningococcus).

• Causes epidemic meningitis and the most severe acute bacterial meningitis.

• Symptoms of meningococcemia: Sudden onset, fever >104°F (40°C), chills, sore throat, delirium, petechial/purpuric rash, shock, DIC, cardiac failure, adrenal gland damage, coma, and death within hours.

• Transmission: Respiratory droplets and close contact; survives poorly outside the body.

• High-risk groups: Families, daycare centers, college dormitories, military barracks.

• Treatment: Immediate IV Penicillin G plus supportive care for shock and DIC.

• Prevention: Meningococcal conjugate vaccine (11–12 years old); close contacts receive rifampin or tetracycline prophylaxis.

Compare cryptococcal meningitis and viral (aseptic) meningitis.

Cryptococcal Meningitis

• Caused by Cryptococcus neoformans (encapsulated yeast).

• Found in pigeon/bird droppings.

• Causes chronic meningitis with gradual onset.

• AIDS patients may develop rapid meningoencephalitis.

• High-risk: AIDS, diabetes, cancer, steroid therapy.

Viral (Aseptic) Meningitis

• No bacteria, fungi, or protozoa found in CSF.

• About 80% of meningitis cases are viral.

• 90% are caused by enteroviruses.

• Usually milder, resolves within 2 weeks, mortality <1%.

• Diagnosed after bacteria/fungi are excluded and confirmed with viral testing.

• Treatment is supportive.

Describe poliomyelitis, including disease progression, symptoms, vaccines, and prevention.

• Caused by Poliovirus.

• Mild disease: Fever, headache, sore throat, nausea, myalgia.

• Non-paralytic polio: Muscle pain, spasms, meningeal inflammation, hypersensitivity.

• Paralytic polio: Destroys motor neurons causing flaccid paralysis of the legs, abdomen, diaphragm, back, bladder, and respiratory muscles. Severe cases required an Iron Lung.

• Vaccines:

  • IPV (Jonas Salk, inactivated, cannot cause disease)

  • OPV (Albert Sabin, oral live vaccine, rare reversion to virulent form)

• WHO vaccination campaigns greatly reduced polio worldwide; India declared polio-free in 2014.

Describe botulism.

• Caused by Clostridium botulinum.

• Produces botulinum toxin, one of the strongest neurotoxins.

• Blocks acetylcholine release at neuromuscular junctions.

• Causes flaccid paralysis.

• Sources: Improperly canned foods, infant honey exposure, contaminated wounds.

• Symptoms: Blurred vision, difficulty swallowing/speaking, descending paralysis, respiratory failure.

• Treatment: Antitoxin and respiratory support.

Describe tetanus.

• Caused by Clostridium tetani.

• Endospores enter through puncture wounds or broken skin.

• Produces tetanospasmin, released when bacteria die.

• Blocks inhibitory neurotransmitters, causing continuous muscle contraction (spastic paralysis).

• Symptoms: Lockjaw (trismus), neck stiffness, fever, severe muscle spasms.

• Incubation: 5 days–15 weeks.

• Prevention: Tdap/Td vaccination.

• High risk: Unvaccinated individuals.

Describe rabies.

• Caused by Rabies virus.

• Zoonotic disease spread through bites or scratches from infected animals.

• Reservoirs include bats, raccoons, foxes, skunks, and unvaccinated dogs.

• Incubation usually 1–2 months but is shorter for bites closer to the brain (face/neck).

• Early symptoms: Fever, headache, nausea, vomiting, fatigue.

• Progresses to encephalitis, paralysis, hydrophobia, coma, and death.

• Once symptoms appear, rabies is almost always fatal.

• Treatment after exposure: Human Rabies Immune Globulin (HRIG) plus rabies vaccine.

• Prevention: Vaccinate dogs and cats and avoid wild animals.

What is encephalitis and what causes subacute encephalitis?

• Encephalitis: Inflammation of brain tissue.

• Subacute encephalitis: Develops slowly with less dramatic symptoms.

• Major causes:

  • Persistent measles virus (SSPE) occurring 7–15 years after infection.

  • Prions.

What are prions and how do they cause disease?

• Proteinaceous infectious particles containing no DNA or RNA.

• Cause Transmissible Spongiform Encephalopathies (TSEs).

• Misfolded PrP proteins convert normal PrP into abnormal forms, producing plaques and sponge-like degeneration of the brain.

• Resistant to heat, chemicals, radiation, and even prolonged autoclaving.

Describe Creutzfeldt-Jakob Disease (CJD).

• Human prion disease.

• Symptoms: Memory loss, behavioral changes, dementia, impaired senses, delirium, muscle contractions, progressive neurological decline.

• Usually fatal within one year.

• Variant CJD (vCJD) linked to eating beef contaminated with Mad Cow Disease (BSE).

• Great Britain experienced the largest outbreak.

• Can also be inherited or transmitted by contaminated surgical instruments.

Describe Acute Flaccid Myelitis (AFM).

• Rare disease associated with Enterovirus D68 (EV-D68).

• EV-D68 usually causes cold-like respiratory illness but may cause AFM in children.

• Symptoms: Sudden limb weakness, paralysis, difficulty swallowing, respiratory failure due to diaphragm paralysis.

• Treatment is supportive with intensive physical therapy.

• Less than 10% fully recover.

Bacterial meningitis

Cause: Bacteria

Hallmark Feature: Rapid, severe, medical emergency

Viral meningitis

Cause: Enteroviruses

Hallmark Feature: Mild, self-limiting

Cryptococcal meningitis

Cause: Fungus

Hallmark Feature: Chronic; common in AIDS

Polio

Cause: Poliovirus

Hallmark Feature: Flaccid paralysis from motor neuron destruction

Botulism

Cause: Botulinum toxin

Hallmark Feature: Flaccid paralysis from blocked ACh

Rabies

Cause: Rabies virus

Hallmark Feature: Fatal encephalitis after animal bite

Tetanus

Cause: Tetanospasmin

Hallmark Feature: Spastic paralysis and lockjaw

Prion disease (CJD)

Cause: Misfolded proteins

Hallmark Feature: Sponge-like brain degeneration

AFM

Cause: EV-D68

Hallmark Feature: Polio-like paralysis in children

Which vaccines prevent these nervous system diseases?

• Meningococcal conjugate vaccine: Prevents Neisseria meningitidis.

• Hib vaccine: Prevents Haemophilus influenzae type B meningitis.

• Polio vaccines: IPV (Salk) and OPV (Sabin).

• Tdap/Td: Prevents tetanus.

• Rabies vaccine + HRIG: Given after exposure.

• Vaccinating dogs and cats helps prevent rabies.

What are the highest-yield facts from Chapter 17?

• BBB blocks many pathogens and antibiotics.

• CNS has no normal microbiota.

• Capsules are the major virulence factor of bacterial meningitis pathogens.

• Neisseria meningitidis = Gram-negative diplococci, dorm outbreaks, petechial rash, DIC.

• Viral meningitis = 80% of cases, 90% due to enteroviruses.

• Polio and botulism = flaccid paralysis.

• Tetanus = spastic paralysis (lockjaw).

• Rabies is almost always fatal after symptoms appear.

• Prions contain no nucleic acid and resist standard sterilization.

• AFM is associated with EV-D68 and resembles polio.