CHAPTER
20
Microbial Diseases of the Nervous
System and Eyes
Chapter Outline
Structure of the Nervous System (pp. 596–598)
Structures of the Central Nervous System
Structures of the Peripheral Nervous System
Bacterial Diseases of the Nervous System (pp. 598–610)
Bacterial Meningitis
Hansen’s Disease (Leprosy)
Botulism
Tetanus
Viral Diseases of the Nervous System (pp. 610–618)
Viral Meningitis
Poliomyelitis
Rabies
Arboviral Encephalitis
Mycosis of the Nervous System (pp. 618–619)
Cryptococcal Meningitis
Protozoan Diseases of the Nervous System (pp. 619–621)
African Trypanosomiasis
Primary Amebic Meningoencephalopathy
Prion Disease (pp. 621–622)
Microbial Diseases of the Eyes (pp. 622–624)
Structure of the Eye
Trachoma
Other Microbial Diseases of the Eyes
Chapter 20 Microbial Diseases of the Nervous System and Eyes
Chapter Summary
Structure of the Nervous System (pp. 596–598)
The nervous system is divided into the central nervous system and the peripheral nervous
system.
Structures of the Central Nervous System
The central nervous system (CNS) is composed of the brain and spinal cord. The parts of the
brain are the cerebrum, which controls voluntary muscles, perception, and thinking; the
lower cerebellum, which controls involuntary body movement; and the brain stem, which con-
trols functions like breathing, heart rate, and blood pressure. The CNS is surrounded by the
Structures of the Peripheral Nervous System
The peripheral nervous system (PNS) is composed of nerves that transfer commands from the
CNS to muscles and glands throughout the body and transmit information to the CNS concern-
Cells of the Nervous System
The entire nervous system is composed of two basic types of cells: neurons and neuroglia. Neu-
roglia are supportive cells that provide scaffolding, insulation, nutrition, or defense. The cyto-
plasmic membrane of a neuron generates an electrical signal called an action potential or nerve
Portals of Infection of the Central Nervous System
The central nervous system is an axenic environment: it has no normal microbiota. Pathogens
may enter the CNS through breaks in the barriers such as bones and meninges or via axonal
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Bacterial Diseases of the Nervous System (pp. 598–610)
Bacteria and their toxins can affect neurons or the PNS, but bacterial meningitis is the most
common bacterial disease of the nervous system.
Bacterial Meningitis
Four strains are primarily responsible. The bacteria have fimbriae and a capsule as well as an
outer membrane of LOS; all of these serve as attachment factors. The capsule is also
antiphagocytic. The cells release LOS, which causes inflammation.
Streptococcus pneumoniae is a Gram-positive coccus, commonly called pneumococci. The
bacteria reside in the respiratory system and move to the meninges from there. Pneumococci
In most cases, bacteria spread to the meninges from infections of the lungs, sinuses, or
inner ear via the blood (bacteremia). Diagnosis is based on detecting bacteria in CSF and
serological assays. For all forms, quick IV antimicrobial therapy is vital. Vaccines are available
to prevent infection with N. meningitidis, S. pneumoniae, and H. influenzae b.
Hansen’s Disease (Leprosy)
Hansen’s disease (leprosy) is caused by infection with Mycobacterium leprae. Patients
with a strong T cell immune response develop a nonprogressive form of the disease called
tuberculoid leprosy, which is characterized by limited nerve damage and loss of sensation.
In contrast, patients with a weak T cell immune response develop lepromatous leprosy, in which
Chapter 20 Microbial Diseases of the Nervous System and Eyes
Botulism
Botulism is often not an infection, but instead an intoxication (poisoning) caused by a
neurotoxin of Clostridium botulinum that progressively paralyzes skeletal muscles. Food-borne
botulism results from consumption of botulism toxin in contaminated food, while infant botu-
lism and wound botulism infections result from endospore contamination. All three forms are
Tetanus
Tetanus is another disease caused by a Clostridium species, C. tetani. The diagnostic sign
of tetanus is severe muscle contraction, initially affecting the jaw and neck muscles (called lock-
jaw). This is caused by a potent neurotoxin called tetanospasmin, which blocks inhibitory neu-
Viral Diseases of the Nervous System (pp. 610–618)
Because viruses are smaller than cells, they can more readily cross the blood-brain barrier;
therefore, there are more viral infections of the nervous system than bacterial or fungal
infections.
Viral Meningitis
Viral meningitis (aseptic meningitis) is usually milder than bacterial or fungal meningitis.
Although it also causes fever, severe headache, stiff neck, and other symptoms associated with
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Poliomyelitis
Infection with poliovirus, another Enterovirus, causes any one of four forms of poliomyelitis (or
polio): (1) asymptomatic infection (almost 90%); (2) minor polio, which is characterized by
effective vaccines have resulted in a huge reduction of infections worldwide.
Rabies
Rabies is a degenerative neurologic disease characterized by pain or itching at the site of
infection, fever, headache, malaise, anorexia, hydrophobia (triggered by painful swallowing),
seizures, disorientation, hallucinations, paralysis, and death. Rabies virus is a negative, ssRNA
Arboviral Encephalitis
Arthropod-borne viruses, called arboviruses, are transmitted between hosts by blood-sucking
arthropods. Mosquitoes transmit various types of arboviral encephalitis, which affects birds,
Mycosis of the Nervous System (pp. 618–619)
Fungi rarely infect the central nervous system. Occasionally, mycoses (fungal diseases) spread
from the lungs via the blood to the meninges. Toxins from fungi can cause neurological dys-
function or hallucinations.
Chapter 20 Microbial Diseases of the Nervous System and Eyes
Cryptococcal Meningitis
Cryptococcal meningitis (cryptococcosis) is the leading form of fungal meningitis. The disease
manifests with the signs and symptoms common to other forms of meningitis. Cryptococcus
neoformans is a basidiomycete yeast, which is a saprobe of bird feces and eucalyptus sap. Hu-
Protozoan Diseases of the Nervous System (pp. 619–621)
Protozoan infections of the nervous system are relatively rare.
African Trypanosomiasis
African trypanosomiasis, also known as African sleeping sickness is characterized by three
clinical stages: (1) the bite wound becomes necrotic; (2) fever, swollen lymph nodes, and head-
ache; then (3) meningoencephalitis and extreme drowsiness, coma, and death. Also characteris-
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Primary Amebic Meningoencephalopathy
Two amoebae in the kingdom Euglenozoa, Acanthamoeba and Naegleria, cause primary ame-
bic meningoencephalopathy, a rare disease with symptoms similar to those of other forms of
meningitis and encephalitis. However, with primary amebic meningoencephalopathy, the symp-
Prion Disease (pp. 621–622)
A prion is an infectious protein. In sheep, prions cause a disease called scrapie. In cattle,
prions cause “mad cow disease.” Both diseases are classified as spongiform encephalopathies,
called such because they leave the infected animal’s brain riddled with holes, like
a sponge.
Variant Creutzfeldt-Jakob Disease (vCJD)
Creutzfeldt-Jakob disease is a naturally occurring dementia that strikes about one person in
a million at approximately 60 years of age. It is not contagious. In contrast, variant Creutz-
feldt-Jakob disease (vCJD) is contracted from eating the contaminated meat of
animals infected with prions. Medical procedures may also spread the disease. In vCJD, brain
Microbial Diseases of the Eyes (pp. 622–624)
The senses are considered parts of the nervous system. Vision is our primary sense; in fact, the
eyes can be considered extensions of the brain and dura mater.
Structure of the Eye
The eye is a hollow, roughly spherical ball. The fibrous tunic of the eye provides a tough
barrier against microbes. It consists of the sclera and cornea, the latter of which covers the front
Chapter 20 Microbial Diseases of the Nervous System and Eyes
Trachoma
Trachoma is the leading cause of nontraumatic blindness in humans. It develops in newborns
when Chlamydia trachomatis present in a woman’s reproductive tract infects the baby’s eyes
during birth. Infection can also occur in adults if the bacteria get into the eye. Bacterial infection
Other Microbial Diseases of the Eyes
Staphylococcus aureus infections of sebaceous glands near the eyes are called sties. Neisseria
gonorrhoeae infection of newborns’ eyes causes ophthalmia neonatorum, which is contracted in