Infectious Diseases Affecting the Nervous System: Microbiology Study Notes

CHAPTER 17: Infectious Diseases Affecting the Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for transmitting signals throughout the body. It is divided into the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which consists of nerves extending from the CNS to the rest of the body. The CNS is protected by bony structures, meninges, and cerebrospinal fluid (CSF), which provide both physical and immunological defenses.

• Central Nervous System (CNS): Brain and spinal cord, encased in bone and meninges.

• Peripheral Nervous System (PNS): Nerves that connect the CNS to limbs and organs.

• Cerebrospinal Fluid (CSF): Provides nutrition and cushioning for the brain and spinal cord.

• Normal Biota: There is no normal microbiota in the CNS or PNS; the presence of microorganisms indicates infection.

Defenses	Normal Biota
Bony structures, blood-brain barrier, microglial cells, and macrophages	None

Major Nervous System Infections

Microbial infections of the nervous system are serious and can be life-threatening. The most common conditions include meningitis, encephalitis, and meningoencephalitis.

• Meningitis: Inflammation of the meninges (protective membranes covering the brain and spinal cord).

• Encephalitis: Inflammation of the brain tissue itself.

• Meningoencephalitis: Inflammation of both the meninges and the brain.

Bacterial Meningitis

Etiology and Pathogenesis

Bacterial meningitis is a medical emergency, often caused by Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae, Listeria monocytogenes, and Group B streptococcus. Bacteria can enter the bloodstream, cross the blood-brain barrier, and infect the meninges, leading to inflammation and severe symptoms.

• Symptoms: Headache, stiff neck, fever, nausea, vomiting, photophobia (light sensitivity).

• Diagnosis: Lumbar puncture (spinal tap) to analyze CSF.

• Treatment: Immediate broad-spectrum antibiotics, adjusted after pathogen identification.

Neisseria meningitidis (Meningococcus)

Neisseria meningitidis is a Gram-negative diplococcus responsible for the most severe form of acute bacterial meningitis. It can cause rapid-onset disease with high mortality if untreated.

• Pathogenesis: Releases endotoxins causing vascular damage, hemorrhage, and petechiae (small red or purple spots) on the skin.

• Symptoms: High fever, chills, delirium, bleeding under the skin, shock, coma, and possible death within hours.

• Transmission: Spread via respiratory droplets, especially in close quarters (e.g., families, dormitories, military barracks).

• Prevention: Prophylactic antibiotics for close contacts; vaccination in high-risk groups.

Viral Meningitis (Aseptic Meningitis)

Etiology and Clinical Features

Viral (aseptic) meningitis is usually less severe than bacterial forms and is most often caused by enteroviruses. It is characterized by the absence of bacteria or fungi in the CSF.

• Symptoms: Similar to bacterial meningitis but generally milder; resolves within two weeks.

• Diagnosis: Exclusion of bacteria/fungi in CSF, confirmed by viral culture or antigen tests.

• Treatment: Supportive care; specific antiviral therapy is rarely needed.

Poliomyelitis

Etiology and Pathogenesis

Poliomyelitis is an acute viral infection of the spinal cord caused by poliovirus (genus Enterovirus). It can lead to neuromuscular paralysis and has been nearly eradicated due to global vaccination efforts.

• Transmission: Fecal-oral route via contaminated food, water, or objects; mechanical vectors.

• Symptoms: Fever, headache, nausea, sore throat, myalgia; can progress to paralysis.

• Paralytic Disease: Invasion of motor neurons causes flaccid paralysis.

• Prevention: Vaccination (IPV and OPV); IPV is used in the U.S.

• Treatment: Supportive care, respiratory support if needed, physical therapy.

Primary Amoebic Meningoencephalitis (PAM)

Naegleria fowleri

PAM is a rare but fatal infection caused by the amoeba Naegleria fowleri, typically acquired from swimming in warm, fresh water. The amoeba enters through the nasal passages and migrates to the brain, causing rapid and severe destruction of brain tissue.

• Symptoms: Hemorrhage, coma, death within a week.

• Treatment: Usually ineffective due to rapid progression.

Acute Encephalitis

Arboviruses and Herpes Simplex Virus

Acute encephalitis is most often caused by viruses, especially arboviruses (transmitted by mosquitoes) and herpes simplex virus (HSV). Arboviral encephalitis includes diseases such as West Nile virus infection.

• Transmission: Arthropod vectors (mosquitoes); HSV can be transmitted perinatally or by reactivation.

• Symptoms: Coma, convulsions, paralysis, memory deficits, and sometimes permanent brain damage.

• Treatment: Supportive; vector control for prevention.

Toxoplasmosis

Toxoplasma gondii

Toxoplasmosis is caused by the protozoan Toxoplasma gondii, with felines as the primary host. Most infections are asymptomatic, but immunocompromised individuals and fetuses are at risk for severe disease, including encephalitis and congenital defects.

• Transmission: Ingestion of undercooked meat or exposure to cat feces.

• Symptoms: Sore throat, lymphadenopathy, low-grade fever; severe cases involve brain and organ damage.

• Prevention: Proper meat cooking, hand hygiene after handling cat litter.

Rabies

Etiology, Transmission, and Prevention

Rabies is a slow, progressive zoonotic disease caused by the rabies virus. It is transmitted through bites or scratches from infected animals and is almost always fatal once symptoms appear.

• Reservoirs: Wild mammals such as canines, skunks, raccoons, bats.

• Symptoms: Fever, nausea, headache, progressing to neurological symptoms and death.

• Prevention: Post-exposure prophylaxis with rabies immune globulin and vaccine.

Tetanus

Clostridium tetani

Tetanus, or "lockjaw," is caused by Clostridium tetani, a spore-forming bacterium found in soil. The neurotoxin tetanospasmin causes uncontrollable muscle contractions, often leading to respiratory failure.

• Transmission: Spores enter through wounds, burns, or punctures.

• Symptoms: Jaw clenching, muscle stiffness, arching of the back, respiratory paralysis.

• Prevention: Vaccination series in infancy and booster shots.

Botulism

Clostridium botulinum

Botulism is a rare but serious illness caused by the neurotoxin produced by Clostridium botulinum. It can result from ingestion of contaminated food, wound infection, or in infants, colonization of the gut.

• Symptoms: Double vision, difficulty swallowing, dizziness, descending paralysis, respiratory failure.

• Transmission: Ingestion of improperly preserved foods, wound contamination, or infant colonization.

• Treatment: Administration of antitoxin, respiratory and cardiac support, hospitalization.

• Prevention: Proper food preservation techniques.