Microbial Diseases of the Nervous System: Meningitis, Chickenpox/Shingles, Rabies, and Poliomyelitis

Microbial Diseases of the Nervous System

Overview

The nervous system is susceptible to a variety of microbial diseases, many of which can cause severe symptoms and long-term complications. The central nervous system (CNS) is protected by physical barriers and the blood-brain barrier, making infections rare but often serious. This guide covers major bacterial and viral diseases affecting the CNS, including meningitis, chickenpox/shingles, rabies, and poliomyelitis.

The Nervous System

Structure and Function

• Central Nervous System (CNS): Composed of the brain and spinal cord, acts as the master control center.

• Peripheral Nervous System (PNS): Nerves that transfer commands from the CNS to various body systems.

Protection of the CNS

• Bone: Brain encased within the skull; spinal cord protected by vertebrae.

• Meninges: Layers of tissue that surround the brain and spinal cord, absorb shock, and circulate cerebrospinal fluid (CSF).

• Blood-Brain Barrier: Tightly linked cells prevent large molecules and most pathogens from entering the CNS.

Portals of Infection

• CNS is an axenic environment (no normal microbiota).

• Pathogens may access the CNS via:

  • Breaks in bones/meninges

  • Medical procedures

  • Carried by blood/lymph and breach blood-brain barrier

  • Traveling in peripheral neurons

  • Circulation of CSF

Meningitis

Definition and General Symptoms

Meningitis is the inflammation of the meninges, the protective tissues covering the brain and spinal cord. Symptoms include headache, stiff neck, fever, altered mental state, and vomiting. Inflammation of cranial meninges produces these symptoms.

Bacterial vs. Viral Meningitis

Bacterial	Viral
Neisseria meningitidis Streptococcus pneumoniae	Picornaviridae (e.g., enteroviruses)

Bacterial Meningitis

• General Signs and Symptoms: Deafness, blindness, behavioral changes, coma, death. May occur within 12 hours. Sensitivity to light (photophobia), nausea, vomiting, confusion.

• Transmission: Droplet transmission, establishes infection in respiratory tissue, spreads via blood to meninges.

• Diagnosis:

  • Cloudy CSF (increased WBCs)

  • Decreased glucose in CSF

  • Spinal tap & Gram stain

  • Serological tests for antibodies

• Treatment: IV antibiotics

Bacterial Meningitis: Major Pathogens

• Neisseria meningitidis: Gram-negative diplococcus, non-motile. Only type that becomes epidemic. Mortality ~100% if untreated.

• Streptococcus pneumoniae: Gram-positive coccus, pairs or short chains. Leading cause in children & elderly.

Meningococcal Meningitis: Pathogenesis and Virulence Factors

• Polysaccharide capsule: Immune evasion, persistence in bloodstream.

• Fimbriae: Aid in attachment, required for infection.

• Lipo-oligosaccharide (LOS): Cell wall antigen, potent endotoxin, damages host tissue via 'blebbing'.

Meningococcal Meningitis: Signs and Symptoms

• Petechial rash: Not present in other types of meningitis.

• Rapid progression: Can kill within 6 hours.

• Milky white CSF: Dramatic increase in WBCs.

Prevention

• Vaccines available for both meningococcal and pneumococcal meningitis.

Viral Meningitis

• Most common form of meningitis.

• 90% of cases caused by enteroviruses (Picornaviridae).

• Symptoms similar to bacterial meningitis but generally less severe.

• No specific treatment; supportive care only.

• Prevention: Handwashing, avoiding crowded swimming pools, and minimizing contact with infected individuals.

Chickenpox and Shingles (Varicella-Zoster Virus, VZV)

Overview

• Varicella-Zoster Virus (VZV): Herpesvirus, enveloped DNA virus, fragile outside host.

• Transmission: Respiratory droplets (early infection), direct contact with fluid from lesions (late infection).

• Latency: Virus becomes latent in nerve ganglia as an episome and can reactivate in adults.

Signs and Symptoms

• Highly infectious.

• 2-3 weeks after infection: slight fever, characteristic skin lesions on back/trunk, then spread to face, neck, limbs.

• Lesions begin as macules, progress to papules, then thin-walled vesicles on red bases ('teardrops on rose petals').

• Vesicles turn cloudy, dry up, and crust over.

• Successive crops of lesions over 3-5 days.

• Not life-threatening, but may be dangerous for newborns.

Pathogenesis

• Spreads to liver, spleen, lymph nodes via blood/lymph.

• Second wave of viruses spreads throughout body and skin.

• Viruses shed before and during symptoms, primarily via respiratory droplets and lesion fluid.

• Virus becomes latent in nerve ganglia and can reactivate as shingles.

Shingles (Herpes Zoster)

• Reactivation of VZV in nerve ganglia (15-20% of infected individuals, usually after age 45).

• Symptoms: Intense pain (nerve cell destruction), skin rash (localized to one side of body, does not cross midline).

• Pain may persist for months to years after lesions heal (postherpetic neuralgia).

Epidemiology

• Chickenpox most common in children, more severe in adults.

• Adults have more developed immune system.

• Shingles can spread VZV to someone who has never had chickenpox, but chickenpox must occur first before shingles develops.

Prevention

• Attenuated vaccine (Varivax): Protects against chickenpox; given at 12-18 months and before school.

• Recombinant vaccine (Shingrix; RZV): Protects against shingles; 2 doses for those 50+ years, >90% efficacy.

Rabies

Overview

• Rabies virus: Lyssavirus, enveloped ssRNA virus, not stable outside host.

• Transmission: Saliva contact with broken skin; zoonotic (animal reservoirs).

• 99.9% fatality if neurological symptoms develop.

Signs and Symptoms

• Stage 1 (muscle cells): 2-10 days post infection; fever, headache, nausea, abnormal sensation around bite site (itching, burning, numbness).

• Stage 2 (CNS): Acute neurological symptoms; anxiety, agitation, dysphagia, paralysis, episodes of delirium, hydrophobia (17-80% of patients), coma preceding death (final 3-7 days).

Pathogenesis

• Virus replicates in muscle cells near wound.

• Travels along peripheral nerves to CNS by axonal transport (0.5-1.5 inches/day).

• Replicates in CNS, causes encephalitis and neurological symptoms.

• Virus travels back to peripheral nerves and enters salivary glands for dissemination.

Treatments and Prevention

• Postexposure prophylaxis (PEP): Preventative medical treatment given after exposure, within first few days.

• Active immunization: 4 doses of rabies vaccine.

• Passive immunization: Human rabies immunoglobulin.

• Rabies is one of the few infections treatable with active immunization after exposure.

Poliomyelitis

Overview

• Poliomyelitis was a major infectious disease in the US (1900-1955).

• Nerve damage results in paralysis.

Pathogenesis

• Poliovirus: Picornaviridae family, non-enveloped, very stable in environment.

• Transmission: Fecal-oral route (contaminated food and water).

• Infection presents with varying severity:

  • Asymptomatic (90-95%)

  • Minor polio (4-8%): non-specific symptoms

  • Nonparalytic polio (2%): meningitis, muscle spasms

  • Paralytic polio (<2%): flaccid paralysis

Treatment

• No specific treatment; only management of symptoms.

• Paralytic cases: 80% permanent paralysis, 10% fatal, 10% recover.

• Post-polio syndrome: Degeneration of muscles 30-40 years post infection due to aging of nerves damaged during original infection.

Prevention

• Oral Polio Vaccine (OPV): Live attenuated, induces humoral and cell-mediated immunity. Not approved for use in the US due to risk of reversion.

• Inactivated Polio Vaccine (IPV): Trivalent, induces immunity, used globally.

• Multiple immunizations required for prolonged immunity (3 doses in childhood plus booster at 4-6 years).

• Global eradication effort through vaccination; no animal reservoirs.

Summary Table: Major Microbial Diseases of the CNS

Disease	Pathogen	Transmission	Key Symptoms	Prevention
Bacterial Meningitis	Neisseria meningitidis, Streptococcus pneumoniae	Droplets	Headache, stiff neck, fever, photophobia, rapid progression	Vaccines
Viral Meningitis	Enteroviruses (Picornaviridae)	Fecal-oral, droplets	Similar to bacterial, less severe	Hygiene
Chickenpox/Shingles	Varicella-Zoster Virus	Respiratory droplets, direct contact	Fever, vesicular rash, pain (shingles)	Varivax, Shingrix vaccines
Rabies	Rabies virus (Lyssavirus)	Saliva, animal bite	Neurological symptoms, hydrophobia, coma	PEP, vaccine
Poliomyelitis	Poliovirus (Picornaviridae)	Fecal-oral	Paralysis, muscle weakness	OPV, IPV vaccines

Key Terms and Concepts

• Axenic: Free of microorganisms.

• Episome: Viral DNA that remains separate from host chromosome, allowing latency (e.g., herpesviruses).

• Postexposure prophylaxis (PEP): Medical treatment after exposure to prevent infection.

• Endotoxin: Toxin present inside bacterial cell, released when cell disintegrates (e.g., LOS in Neisseria meningitidis).

Relevant Equations and Immunology Concepts

• Vaccine Efficacy: $\text{Efficacy} = \frac{\text{Attack rate in unvaccinated} - \text{Attack rate in vaccinated}}{\text{Attack rate in unvaccinated}} \times 100\%$

• Humoral Immunity: $\text{Antibody production by B cells in response to antigen}$

• Cell-mediated Immunity: $\text{Activation of T cells in response to infected cells}$

Additional info:

• Some context and terminology expanded for clarity and completeness.

• Tables and diagrams described in text for accessibility.