Microbial Diseases of the Nervous System

Structure and Function of the Nervous System

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS), each with distinct anatomical and functional roles. Understanding these structures is essential for recognizing how infections affect the nervous system.

• Central Nervous System (CNS): Composed of the brain and spinal cord, protected by the skull and backbone.

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

• Meninges: Three protective membranes (dura mater, arachnoid mater, pia mater) surround the CNS. Cerebrospinal fluid (CSF) circulates in the subarachnoid space between the arachnoid and pia mater.

• Blood–Brain Barrier: Restricts entry of many substances, including antibiotics, into the brain.

• Routes of Infection: Microorganisms can enter the CNS via trauma, peripheral nerves, bloodstream, or lymphatic system.

• Meningitis: Inflammation of the meninges.

• Encephalitis: Inflammation of the brain.

Bacterial Diseases of the Nervous System

Bacterial Meningitis

Bacterial meningitis is a serious infection of the meninges, often caused by specific pathogens. Early diagnosis and treatment are critical due to high mortality rates.

• Major Causes: Haemophilus influenzae, Streptococcus pneumoniae, Neisseria meningitidis.

• Other Causes: Nearly 50 other opportunistic bacteria can cause meningitis.

• Haemophilus influenzae: Normal throat microbiota; requires blood factors for growth. Type b (Hib) is most common in children under 4 years. Hib-conjugated vaccine targets capsular polysaccharide antigen.

• Neisseria meningitidis: Found in healthy carriers' throats; transmitted by aerosols or direct contact. Accesses meninges via bloodstream. Vaccines: MenACYW (capsule polysaccharides), MenB (surface proteins).

• Streptococcus pneumoniae: Common in nasopharynx; young children most susceptible. High mortality if untreated. Conjugated vaccine available.

• Listeria monocytogenes: Causes meningitis in newborns, immunosuppressed, pregnant women, cancer patients. Acquired via contaminated food; can cross placenta causing abortion/stillbirth.

• Diagnosis: CSF sampling for pathogen identification.

• Treatment: Antibiotics; effectiveness may be limited by blood–brain barrier.

Tetanus

Tetanus is a neurological disease caused by a potent exotoxin produced by Clostridium tetani. Prevention is primarily through vaccination.

• Causative Agent: Clostridium tetani produces tetanospasmin neurotoxin.

• Symptoms: Muscle spasms, lockjaw, respiratory muscle paralysis, death.

• Transmission: Entry via wounds.

• Prevention: DTaP immunization (vaccine targets toxin, not bacterium).

• Treatment: Booster (tetanus toxoid), tetanus immune globulin, debridement, antibiotics.

Botulism

Botulism is caused by a neurotoxin from Clostridium botulinum and can result in flaccid paralysis. Proper food handling and preparation are key preventive measures.

• Causative Agent: Clostridium botulinum produces botulinum toxin.

• Symptoms: Blurred vision, progressive flaccid paralysis, respiratory/cardiac failure.

• Transmission: Ingestion of toxin-contaminated food; wound and infant botulism also occur.

• Prevention: Proper canning, nitrites in foods, boiling destroys toxin ( for 5 min).

• Diagnosis: Mouse inoculation with antitoxin.

• Treatment: Antitoxin administration.

Leprosy (Hansen’s Disease)

Leprosy is a chronic disease caused by Mycobacterium leprae or M. lepromatosis, affecting skin and nerves. It is not highly contagious and requires prolonged contact for transmission.

• Causative Agents: Mycobacterium leprae, M. lepromatosis.

• Symptoms: Tuberculoid form (loss of sensation, nodules); lepromatous form (disseminated nodules, tissue necrosis).

• Transmission: Prolonged contact with exudates.

• Diagnosis: Acid-fast rods in skin biopsy.

• Research: Cultured in armadillos and mouse footpads.

• Complications: Secondary infections (e.g., tuberculosis).

Viral Diseases of the Nervous System

Poliomyelitis

Poliomyelitis is a viral disease that can cause paralysis. Vaccination has greatly reduced its incidence worldwide.

• Causative Agent: Poliovirus.

• Symptoms: Sore throat, nausea, paralysis (rare).

• Transmission: Ingestion of water contaminated with feces.

• Diagnosis: Isolation from feces and throat secretions.

• Vaccines: Salk (IPV, inactivated), Sabin (OPV, live attenuated).

• Elimination: Polio is a candidate for eradication.

Rabies

Rabies is a fatal encephalitis caused by rabies virus, typically transmitted by animal bites. Postexposure prophylaxis is effective if administered promptly.

• Causative Agent: Rabies virus (Lyssavirus).

• Symptoms: Spasms, brain/spinal cord damage, death.

• Transmission: Animal bites, skin invasion.

• Reservoirs: Skunks, bats, foxes, raccoons, domestic animals.

• Diagnosis: DFA tests of saliva, serum, CSF, brain smears.

• Treatment: Postexposure: rabies immune globulin (RIG) + vaccine; preexposure: vaccination.

Zika Virus Disease

Zika virus disease is generally mild in adults but can cause severe CNS birth defects in fetuses, including microcephaly.

• Causative Agent: Zika virus.

• Symptoms: Mild in adults; CNS birth defects in fetuses.

• Transmission: Mosquito bites.

Arboviral Encephalitis

Arboviral encephalitis is transmitted by mosquitoes and can cause severe inflammation of the brain. Prevention focuses on vector control.

• Causative Agents: Various arboviruses.

• Transmission: Mosquitoes.

• Symptoms: Encephalitis, neurological impairment.

• Prevention: Vector control, public health interventions during outbreaks.

Fungal Disease of the Nervous System

Cryptococcosis (Cryptococcus neoformans Meningitis)

Cryptococcosis is a fungal infection of the CNS, particularly affecting immunosuppressed individuals.

• Causative Agent: Cryptococcus neoformans (encapsulated yeast-like fungus).

• Reservoir: Dried pigeon or chicken droppings.

• Symptoms: Begins as lung infection, may spread to brain/meninges.

• Diagnosis: Latex agglutination tests for cryptococcal antigens in serum or CSF.

• Susceptibility: Immunosuppressed individuals.

Protozoan Diseases of the Nervous System

African Trypanosomiasis (Sleeping Sickness)

African trypanosomiasis is a protozoan disease transmitted by the tsetse fly, affecting the nervous system and causing lethargy and coma.

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

• Vector: Tsetse fly.

• Symptoms: Lethargy, coma.

• Vaccine Challenge: Protozoan changes surface antigens.

Amebic Meningoencephalitis

Amebic meningoencephalitis is a rare but often fatal protozoan infection of the CNS.

• Causative Agents: Naegleria fowleri (acute, fatal), Acanthamoeba spp., Balamuthia mandrillaris (chronic).

• Symptoms: Encephalitis, rapid progression, high mortality.

Nervous System Diseases Caused by Prions

Prion Diseases

Prion diseases are slow-progressing, transmissible encephalopathies caused by self-replicating proteins without nucleic acids.

• Prions: Infectious proteins, no detectable nucleic acid.

• Diseases: Classic Creutzfeldt-Jakob disease (inherited or transmitted), kuru (New Guinea, cannibalism), variant Creutzfeldt-Jakob disease (from bovine spongiform encephalopathy).

• Transmission: Contaminated tissues, instruments, ingestion.

• Sterilization: Special recommendations for surgical instruments when prion contamination is possible.

Summary Table: Major Microbial Diseases of the Nervous System

Additional info: This guide expands on brief points with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.