Microbiology: A Systems

Approach, 2nd ed.

Chapter 19: Infectious Diseases

Affecting the Nervous System

19.1 The Nervous System and Its

Defenses

• Two component parts to the nervous system– CNS

– PNS

• Three important functions– Sensory

– Integrative

– Motor

• Brain and spinal cord: made up of neurons, both surrounded by bone, encased with meninges

• PNS: cranial and spinal nerves

Defenses of the Nervous System

• Mainly structural

• Bony casings

• Cushion of CSF

• Blood-brain barrier

• Immunologically privileged site

Figure 19.1

Figure 19.2

19.2 Normal Biota of the Nervous

System

• No normal biota

• Any microorganisms in the PNS or CNS is a

deviation from the healthy state

19.3 Nervous System Diseases Caused

by Microorganisms

• Inflammation of the meninges

• Many different microorganisms can cause an

infection

• More serious forms caused by bacteria

• If it is suspected, lumbar puncture is

performed to obtain CSF

• Typical symptoms: headache, painful or stiff

neck, fever, and usually an increased number of white blood cells in the CSF

Neisseria meningitides

• Gram-negative diplococcic lined up side by

side

• Commonly known as meningococcus

• Often associated with epidemic forms of

meningitis

• Causes the most serious form of acute

meningitis

Figure 19.3

Figure 19.4

Figure 19.5

Streptococcus pneumonia

• Referred to as the pneumococcus

• Most frequent cause of community-acquired

meningitis

• Very severe

• Does not cause the petechiae associated with

meningococcal meningitis- useful

diagnostically

• Small gram-positive flattened coccus that appears in end-to-end pairs

Haemophilus influenza

• Tiny gram-negative pleomorphic rods

• Sensitive to drying, temperature extremes,

and disinfectants

• Causes severe meningitis

• Symptoms: fever, stiff neck, vomiting, and

neurological impairment

Listeria monocytogenes

• Gram-positive

• Ranges in morphology from coccobacilli to long filaments in palisades formation

• Resistant to cold, heat, salt, pH extremes, and bile

• In normal adults- mild infection with nonspecific symptoms of fever, diarrhea, and sore throat

• In elderly or immunocompromised patients, fetuses, or neonates- affects the brain and meninges and results in septicemia

Figure 19.6

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Cryptococcus neoformans

• Fungus

• More chronic form of meningitis

• More gradual onset of symptoms

• Sometime classified as a meningoencephalitis

• Headache- most common symptom; also

nausea and stiff neck

• Spherical to ovoid shape and a large capsule

Figure 19.7

Figure 19.8

Coccidioides immitis

• At 25°C forms a moist white to brown colony

with abundant, branching, septate hyphae

• Hyphae fragment into arthroconidia at

maturity

• Usually begins with pulmonary infection

Figure 19.9

Figure 19.10

Viruses

• Aseptic meningitis

• Majority of cases occur in children

• 90% caused by enteroviruses

• Generally milder than bacterial or fungal

meningitis

Neonatal Meningitis

• Almost always a result of infection transmitted

by the mother, either in utero or during

passage through the birth canal

• Two most common causes

– Streptococcus agalactiae

• Group B strep

– Escherichia coli

Meningoencephalitis

• Encephalitis: inflammation of the brain

• Two microorganisms cause

meningoencephalitis (both amoebas)

– Naegleria fowleri

– Acanthamoeba

Naegleria fowleri

• Small, flask-shaped amoeba

• Forms a rounded, thick-walled, uninucleate cyst

• Infection begins when amoebas are forced into human nasal passages as a result of swimming, diving, or other aquatic activities

• Amoeba burrows in to the nasal mucosa, multiplies, and migrates into the brain and

surrounding structure

• Primary amoebic meningoencephalitis (PAM)

Figure 19.11

Acanthamoeba

• Large, amoeboid trophozoite with spiny

pseudopods and a double-walled cyst

• Invades broken skin, the conjunctiva, and

occasionally the lungs and urogenital epithelia

• Granulomatous amoebic meningoencephalitis

(GAM)

Acute Encephalitis

• Encephalitis can present as acute or subacute

• Always a serious condition

• Acute: almost always caused by viral infection

• Signs and symptoms vary but may include

behavior changes, confusion, decreased

consciousness, seizures

Arborviruses

• Borne by insects; most feed on the blood of

hosts

• Common outcome: acute fever, often

accompanied by rash

Western Equine Encephalitis (WEE)

• Appears first in horses then in humans

• Carried by mosquito

• Extremely dangerous to infants and small

children

Eastern Equine Encephalitis (EEE)

• Eastern coast of North American and Canada

• Usually appears first in horses and caged birds

• Very high case fatality rate

California Encephalitis

• May be caused by two different viral strains:

California strain and the LaCrosse strain

• Children living in rural areas: primary target

group

St. Louis Encephalitis (SLE)

• May be most common of all American viral

encephalitides

• Epidemics in the US occur most often in the

Midwest and South

West Nile Encephalitis

• Increasing in numbers in the United States

Herpes Simplex Virus

• Can cause encephalitis in newborns born to

HSV-positive mothers

• Prognosis is poor

JC Virus

• Infection is common

• In patients with immune dysfunction, cause

progressive multifocal leukoencephalopathy

(PML)- uncommon but generally fatal

Subacute Encephalitis

• Symptoms take longer to show up and are less

striking

• Most common cause: Toxoplasma

Toxoplasma gondii

• Flagellated parasite

• Most cases go unnoticed

• In the fetus and immunodeficient people,

severe and often fatal

• Asymptomatic or marked by mild symptoms

such as sore throat, lymph node enlargement,

and low-grade fever

Figure 19.13

Measles Virus: Subacute Sclerosing

Panencephalitis (SSPE)

• Occurs years after an initial measles episode

• Seems to be caused by direct viral invasion of

neural tissue

Prions

• Transmissible spongiform encephalopathies

(TSEs): neurodegenerative diseases with long

incubation periods but rapid progression once

they begin

• Human TSEs

– Creutzfeldt-Jakob disease (CJD)

– Gerstmann-Strussler-Scheinker disease

– Fatal familial insomnia

Figure 19.14

Rabies• Slow, progressive zoonotic disease

• Characterized by fatal encephalitis

• Average incubation time: 1-2 months or more

• Prodromal phase begins with fever, nausea, vomiting, headache, fatigue, and other nonspecific symptoms

• Furious rabies– Periods of agitation, disorientation, seizures, and twitching

– Spasms in the neck and pharyngeal muscles lead to hydrophobia

• Dumb rabies

– Patient is not hyperactive but is paralyzed, disoriented and stuporous

• Both forms progress to the coma phase, resulting in death

Figure 19.15

Figure 19.16

Poliomyelitis• Acute enteroviral infection of the spinal cord

• Can cause neuromuscular paralysis

• Often affects small children

• Most infections are contained as short-term, mild viremia

• Some develop mild nonspecific symptoms of fever, headache, nausea, sore throat, and myalgia

• Then spreads along specific pathways in the spinal cord and brain

• Neurotropic: the virus infiltrates the motor neurons of the anterior horn of the spinal cord

• Nonparalytic: invasion but not destruction of nervous tissue

• Paralytic: various degrees of flaccid paralysis

• Rare cases: bulbar poliomyelitis

Figure 19.17

Figure 19.18

Tetanus

• Also known as lockjaw

• Clostridium tetani

• Gram-positive, spore-forming rod

• Releases a powerful neurotoxin, tetanospasmin, that binds to target sites on peripheral motor neurons, spinal cord and brain, and in the sympathetic nervous system

• Toxin blocks the inhibition of muscle contraction

• Results in spastic paralysis

• First symptoms : clenching of the jaw, followed in succession by extreme arching of the back, flexion of the arms, and extension of the legs

• Risus sardonicus

Figure 19.20

Figure 19.21

Figure 19.22

Botulism

• Intoxication associated with eating poorly

preserved foods

• Can also occur as a true infection

• Three major forms

– Food-borne botulism

• Ingestion of preformed toxin

– Infant botulism

• Entrance of botulinum toxin into the bloodstream

– Wound botulism

• Entrance of botulinum toxin into the bloodstream

• Symptoms: double vision, difficulty in

swallowing, dizziness; later symptoms include

descending muscular paralysis and respiratory

compromise

• Clostridium botulinum

– Spore forming anaerobe

– Releases an exotoxin

Figure 19.23

African Sleeping Sickness

• Trypanosoma brucei

• Also called trypanosomiasis

• Affects the lymphatics and areas surrounding blood vessels

• Usually a long asymptomatic period precedes onset of symptoms

• Symptoms include intermittent fever, enlarged spleen, swollen lymph nodes, and joint pain

• Central nervous system is affected with personality and behavioral changes that progress to lassitude and sleep disturbances

Figure 19.24

Figure 19.25

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