Nervous System Infections: Bacterial, Fungal, Protozoal, and Prion Diseases

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Bacterial Nervous System Infections of the Peripheral Nervous System (PNS)

Overview of PNS Infections

The peripheral nervous system (PNS) rarely serves as a long-term host for bacteria, but when infection occurs, it can severely impair sensory and motor neuron function while the central nervous system (CNS) remains intact.

Key Point: PNS infections can result in muscle weakness or numbness, leading to symptoms such as inability to move away from harmful stimuli.
Example: Peripheral neuropathy can cause muscle weakness or numbness, affecting reflexes and sensation.

Diagram of peripheral neuropathy showing affected nerves and symptoms

Hansen’s Disease (Leprosy)

Etiology and Characteristics

Hansen’s Disease, also known as leprosy, is caused by Mycobacterium leprae, an acid-fast bacillus with an optimal growth temperature of 30–35°C. The disease manifests in two main forms: tuberculoid (paucibacillary) and lepromatous (multibacillary).

Key Point: Tuberculoid leprosy is less contagious, causes localized sensory loss, and is easier to treat. Lepromatous leprosy is more contagious, causes irreversible tissue damage, and can result in bone and cartilage decay.
Example: WHO estimated 212,000 global cases in 2015.

Microscopic image of Mycobacterium leprae Clinical images of tuberculoid and lepromatous leprosy

Transmission, Signs, and Symptoms

Leprosy transmission is not fully understood but may occur via the respiratory route. The disease does not spread easily among humans; armadillos are known natural carriers. Leprosy damages the skin, peripheral nerves, and mucosal tissues in the eyes and respiratory tract.

Key Point: Armadillos are a zoonotic reservoir for M. leprae due to their lower body temperature.

Armadillo, a natural carrier of leprosy Diagram of leprosy transmission pathways including humans, animals, and environment

Mechanism, Complications, and Treatment

M. leprae thrives in cooler body regions, explaining its confinement to the PNS. The form of leprosy developed depends on the host's immune response. Untreated lepromatous leprosy is often fatal. Treatment involves multidrug therapy.

Key Point: Early treatment is crucial to prevent irreversible damage and fatality.

Clinical images showing complications of leprosy

Bacterial Nervous System Infections: Neurotoxins

Exotoxins and Endotoxins

Certain bacteria cause neurological disease primarily through the secretion of potent neurotoxins. Exotoxins are secreted proteins, while endotoxins are components of the bacterial membrane released upon cell lysis.

Key Point: Neurotoxins are among the most dangerous toxins known to humans.

Comparison of exotoxins and endotoxins

Botulism

Etiology, Transmission, and Symptoms

Botulism is caused by Clostridium botulinum, a Gram-positive, rod-shaped anaerobe that forms endospores and produces extremely potent neurotoxins. Transmission occurs via contaminated soil, honey, and improperly canned foods. The toxin causes flaccid paralysis, leading to difficulty breathing and speaking.

Key Point: Botulinum toxin blocks acetylcholine release, preventing muscle contraction.
Example: Infant botulism can result from honey consumption; foodborne botulism is linked to canned vegetables.

Microscopic image of Clostridium botulinum Clinical image of botulism patient with flaccid paralysis

Mechanism of Disease

The botulinum toxin inhibits the release of acetylcholine at neuromuscular junctions, resulting in flaccid paralysis.

Diagram of botulinum toxin action at the neuromuscular junction

Prevention and Treatment

Key Point: Children under 1 year should not be fed honey; avoid improperly canned foods.
Treatment: Antibiotics and supportive care.

Tetanus

Etiology, Transmission, and Symptoms

Tetanus, or "lock jaw," is caused by Clostridium tetani, which produces the neurotoxin tetanospasmin. The bacterium is found in soil and infects through deep puncture wounds. Tetanus causes spastic paralysis, where muscles contract and cannot relax.

Key Point: Tetanospasmin blocks inhibitory neurotransmitter release, leading to continuous muscle contraction.
Example: Patients may be unable to open their mouths (trismus).

Foot about to step on a nail, illustrating tetanus transmission risk Clinical image of tetanus patient with muscle spasms

Mechanism of Disease

Tetanospasmin travels via peripheral motor neurons to the spinal cord, where it blocks inhibitory neurotransmitter release, resulting in spastic paralysis.

Diagram of tetanospasmin action at the synapse

Prevention and Treatment

Key Point: Vaccination (DTaP or Tdap) is highly effective in preventing tetanus.
Treatment: Supportive care, wound cleaning, and breathing assistance.

Comparison of Botulism and Tetanus Paralysis

Condition	Type of Paralysis	Mechanism
Botulism	Flaccid paralysis	Blocks acetylcholine release (no muscle contraction)
Tetanus	Spastic paralysis	Blocks inhibitory neurotransmitter release (no muscle relaxation)

Diagram comparing spastic and flaccid paralysis

Fungal Nervous System Infections

Overview

Fungal infections of the nervous system are rare in healthy individuals but more common in immunocompromised patients. Fungal spores are ubiquitous in the environment, making prevention difficult.

Key Point: Only a few fungi are primary pathogens for healthy hosts.

Fungal spores being released into the air Diagram of Aspergillus fumigatus spores entering the lungs

Cryptococcosis

Caused by Cryptococcus neoformans, a yeast-like fungus that forms resistant spores. It primarily affects immunocompromised individuals and is a leading cause of death in HIV/AIDS patients. Transmission occurs via inhalation of aerosolized bird droppings.

Key Point: The fungus can invade the CNS after infecting the lungs and macrophages.
Treatment: Amphotericin B and flucytosine for severe cases.

Protozoal Nervous System Infections

Overview

Protozoans are single-celled eukaryotic parasites with complex life cycles. Some, such as flagellated and amoeboid protozoans, can cause neurological symptoms in humans.

African Sleeping Sickness

Caused by Trypanosoma brucei, transmitted by the tsetse fly. Two subspecies cause disease: T. b. rhodesiense (acute, rapid) and T. b. gambiense (chronic, 98% of cases). The disease is fatal if untreated.

Key Point: The parasite crosses the blood-brain barrier, causing sleep disturbances and neurological decline.

Primary Amoebic Meningoencephalitis (PAM)

Caused by Naegleria fowleri, the "brain-eating amoeba." Infection is rare but almost always fatal, typically acquired by swimming in warm, stagnant water. The amoeba enters through the nasal passages and migrates to the brain.

Key Point: Symptoms progress rapidly from headache and fever to coma and death.

Granulomatous Amebic Encephalitis

Caused by Acanthamoeba spp. and Balamuthia mandrillaris. The disease has a longer incubation period (3–24 months) but is also almost always fatal.

Toxoplasmosis

Caused by Toxoplasma gondii, an apicomplexan protist. Cats are the definitive host. Transmission occurs via cat feces, contaminated water, undercooked meat, or congenitally. Most infections are asymptomatic, but immunocompromised individuals and fetuses are at risk for severe disease.

Key Point: In immunocompromised patients, cysts rupture and tachyzoites damage brain tissue, causing encephalitis.
Treatment: Pyrimethamine and sulfadiazine (do not eliminate all cysts).

Prion Nervous System Infections

Transmissible Spongiform Encephalopathy (TSE)

Prion diseases are caused by infectious proteins (prions) that induce abnormal folding of normal cellular proteins in the brain. Examples include Creutzfeldt-Jakob disease (CJD), bovine spongiform encephalopathy (BSE, "mad cow disease"), and chronic wasting disease (CWD).

Key Point: Prions cause a fatal cascade, leading to sponge-like brain tissue and neurological decline.
Transmission: vCJD is linked to consumption of contaminated beef; classic CJD is usually spontaneous.

Summary & Review

Understand the structure and defenses of the CNS and PNS.
Know the unique characteristics of each microorganism causing nervous system infections.
Review vocabulary such as meninges, cerebrospinal fluid, meningitis, encephalitis, flaccid and spastic paralysis, and zoonosis.