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

Microbial Diseases of the Nervous System

Overview

Microbial diseases affecting the nervous system can be caused by bacteria or viruses and often result in severe symptoms due to the sensitive nature of neural tissues. The central nervous system (CNS) is protected by physical barriers and the blood-brain barrier, making infections rare but potentially life-threatening.

• CNS: Brain and spinal cord, protected by bone and meninges.

• PNS: Nerves transferring commands from CNS to body systems.

• Axenic environment: CNS has no normal microbiota.

Central Nervous System: Protection and Portals of Infection

Protection Mechanisms

• Bone: Skull encases the brain; vertebrae protect the spinal cord.

• Meninges: Three layers (dura mater, arachnoid mater, pia mater) surround CNS, absorb shock.

• Cerebrospinal Fluid (CSF): Circulates around CNS, provides cushioning.

• Blood-Brain Barrier: Tight junctions between cells prevent passage of large molecules and pathogens.

Portals of Infection

• Breaks in bones or meninges

• Medical procedures

• Pathogens carried by blood or lymph

• Traveling in peripheral neurons

• Circulation of CSF

Meningitis

Definition and General Symptoms

Meningitis is the inflammation of the meninges, the protective membranes covering the brain and spinal cord. It can be caused by bacteria or viruses.

• Symptoms: Headache, stiff neck, fever, altered mental state, vomiting

• Inflamed cranial meninges produce pain and neurological symptoms

Bacterial Meningitis

• Common Pathogens: Neisseria meningitidis (Gram-negative diplococcus), Streptococcus pneumoniae (Gram-positive coccus)

• Other species: Haemophilus influenzae, Listeria monocytogenes, Streptococcus agalactiae

General Signs and Symptoms

• Deafness, blindness, behavioral changes, coma, or death

• Photophobia (sensitivity to light), nausea, vomiting, confusion

• Death may occur within 12 hours in severe cases

Transmission and Pathogenesis

• Droplet transmission: Infection starts in respiratory tissue, spreads via blood to meninges

• Infection of brain tissue causes encephalitis

Diagnosis and Treatment

• Diagnosis: Cloudy CSF (increased WBC), decreased glucose in CSF, spinal tap, Gram stain

• Treatment: IV antibiotics

Meningococcal Meningitis (Neisseria meningitidis)

• Gram-negative diplococcus, non-motile

• Polysaccharide capsule and fimbriae aid in immune evasion and attachment

• Lipo-oligosaccharide (LOS) endotoxin damages host tissue

• Only type of bacterial meningitis that becomes epidemic

• Rapid progression; can kill within 6 hours

• Petechial rash and milky white CSF are characteristic

Pneumococcal Meningitis (Streptococcus pneumoniae)

• Gram-positive coccus, often in pairs

• Leading cause in children under 5

• Virulence factors: Polysaccharide capsule, phosphocholine, secretory IgA protease, pneumolysin

Prevention

• Vaccines available for both meningococcal and pneumococcal meningitis

Viral Meningitis

• Most common form of meningitis

• Caused by Picornaviridae family (e.g., enteroviruses)

• Transmission: Fecal-oral route, contaminated food and water

• Symptoms similar to bacterial meningitis but generally less severe

• No specific treatment; supportive care only

Chickenpox and Shingles (Varicella-Zoster Virus, VZV)

Chickenpox

• Caused by Varicella-Zoster Virus (VZV), a herpesvirus (enveloped DNA virus)

• Transmission: Respiratory droplets, direct contact with fluid from lesions

• Highly infectious, especially in children

• Lesions begin as macules, progress to papules, then fluid-filled vesicles on red bases ("teardrops on rose petals")

• Lesions crust over; crops appear over 3-5 days

• Virus spreads to liver, spleen, lymph nodes via blood and lymph

• Virus becomes latent in nerve ganglia as an episome

Shingles

• Reactivation of latent VZV in nerve ganglia, usually in adults

• 15-20% of those who had chickenpox as children develop shingles

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

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

Relationship Between Chickenpox and Shingles

• Chickenpox always occurs first; shingles may develop later

• Shingles patient can spread VZV to someone who has never had chickenpox (causing chickenpox, not shingles)

• A person cannot get shingles from someone with chickenpox

Epidemiology

• Chickenpox more severe in adults due to immune response

• 4% develop a second case of shingles

Prevention

• Attenuated vaccine (Varivax) protects against chickenpox

• Recombinant vaccine (Shingrix; RZV) protects against shingles (2 doses, >90% efficacy)

Rabies

Overview

• Caused by Rabies virus (Lyssavirus), enveloped ssRNA virus

• Transmission: Saliva contact with broken skin (zoonotic, endemic in 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 at bite site

• Stage 2 (CNS): Acute neurological symptoms: anxiety, agitation, dysphagia, paralysis, delirium, hydrophobia, coma preceding death (final 3-7 days)

Pathogenesis

• Virus replicates in muscle cells at wound site

• 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

Treatment 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

Poliomyelitis

Overview

• Caused by Poliovirus (Picornaviridae family), non-enveloped, very stable in environment

• Transmission: Fecal-oral route, contaminated food and water

• Nerve damage results in paralysis

Pathogenesis

• Infection presents with varying severity:

  • Asymptomatic: 90-95% of infections, virus shed for up to 6 weeks

  • 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

• Inactivated Polio Vaccine (IPV): Trivalent, induces humoral immunity

• Multiple immunizations required for prolonged immunity

• Global eradication effort through vaccination; no animal reservoirs

Table: Bacterial vs. Viral Diseases of the CNS

Bacterial	Viral
Meningitis (Neisseria meningitidis, Streptococcus pneumoniae)	Meningitis (Picornaviridae)
	Poliomyelitis (Poliovirus, Picornaviridae)
	Rabies (Rabies virus)

Key Terms and Concepts

• Axenic: Free of microorganisms

• Episome: Viral DNA that remains separate from host chromosome, can reactivate

• Endotoxin: Toxic component of Gram-negative bacterial cell wall (e.g., LOS)

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

Relevant Equations

• Axonal transport rate (Rabies): $\text{Rate} = 0.5-1.5\ \text{inches/day}$

Summary

Microbial diseases of the nervous system are serious and often life-threatening. Understanding the pathogens, transmission routes, symptoms, and prevention strategies is essential for effective diagnosis and management. Vaccination remains a key tool in preventing many of these diseases.