Chp 19 Bacterial and Viral Diseases of the Brain and Meninges (See Chp 5 for parasitic/fungal/helminthic infections)


I.                Bacterial Meningitis - Meningitis is an inflammation of the meninges, the membranes that cover the brain and spinal cord


Meninges (spinal & cranial) – protective coverings around the brain and spinal cord.

a.     dura mater - "tough mother;" outermost layer of the meninges; tough, fibrous connective tissue; gives structural support.

b.    arachnoid mater- "spider layer;" middle layer of the meninges; weblike appearance; arachnoid villi capillaries function in the absorption of cerebral spinal fluid (CSF) (CSF is the fluid that circulates around the brain and spinal cord to provide nutrients and oxygen and to take waste products away).

d.    subarachnoid space - between arachnoid & pia mater; where cerebral spinal fluid circulates.

e.     pia mater - "delicate mother;" innermost layer of the meninges; transparent fibrous membrane that adheres to the surface of the spinal cord & brain; it contains numerous blood vessels.


The Disease: Meningitis causes necrosis, clogging of blood vessels, increased pressure within the skull from edema (swelling), decreased CSF circulation, impaired central nervous system function.  Early symptoms are headache, fever, and chills. Death occurs from shock and other serious complications.


A.   Meningococcal Meningitis – Neisseria meningitidis causes 2000-3000 cases per year in the US.  Mortality is 85% if untreated, but only 1% if treated; Organism colonizes nasopharynx, spread to blood, and to meninges; cocci can invade all parts of body and cause endotoxin shock and death within hours (Waterhouse-Frederichsen syndrome); vaccine is available, but is not very effective.


B.    Haemophilus Meningitis – Hameophilus influenzae type B (Hib) causes 2/3 of bacterial meningitis cases during the first year of life; children can be carriers; human are exposed to bacteria early in life and rapidly acquire immunity, so disease is rare in adults; almost always fatal if not treated (about 1/3 die even with treatment); leading cause of mental retardation in US and worldwide; Hib vaccine has drastically reduced incidence.   

C.    Streptococcus Meningitis – leading cause among adults; organism spread via the blood from lungs, sinuses, and ear infections.


D.   Can also be caused by E. coli and Staphylococcus species.



II.            Viral Diseases of the Brain and Meninges


A.      Viral Meningitis – unlike bacterial meningitis, which is always fatal if untreated, viral meningitis is usually self-limiting and nonfatal; can be caused by enteroviruses or mumps virus; cannot be treated with antibiotics.


B.       Encephalitis


The Disease – Causes an inflammation of the brain.  4 types (all caused by a different virus):  Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), Venezuelan Equine Encephalitis (VEE), and St. Louis encephalitis (SLE).  Named because they infect horses more often than humans. 


Transmission – usually from a mosquito to a bird, back to a mosquito, and to a horse, human, or other mammal.


Clinical Signs – Fever, headache; convulsions sometimes occur.  EEE is the most serious (causes sever necrotizing infection of the brain).  VEE resembles the flu in humans.  SLE occurs in late summer epidemics about every 10 years and is severe in elderly patients.


Treatment only alleviates symptoms.  Vaccines are available for horses, but are not used on humans for fear of inducing a virulent form of the disease. 

            C.       Rabies

Reservoirs – dogs, bats, foxes, coyotes, skunks, raccoons; bats can be asymptomatic carriers.


Transmission – bite (viruses are shed in saliva)


Diagnosis – IFAT (immunofluorescent antibody test) developed in 1958; animal is killed and its brain is examined for rabies antigens; prior to 1958, brain was examined for Negri bodies (inclusion bodies-clusters of viruses in neurons).


The Disease – virus replicates in injured tissues and then slowly migrates to nerves where it eventually reaches the central nervous system (brain/spinal cord); the length of time required for symptoms to appear is proportional to the distance between the wound and the brain and is affected by the accessibility of nerve fibers (think about a face bite vs. a foot bite).


Clinical Signs – headache, fever, nausea, partial paralysis near the bite site (persist for 2-10 days, then worsen); paralysis then becomes more general, throat muscles undergo painful spasms, confusion and hallucinations occur; 10-14 days after onset, patient goes into a coma and dies.


Treatment – Ordinarily there is sufficient time for the bitten individual to be vaccinated and to respond by making enough protective antibodies to prevent onset of the disease; once symptoms have occurred it is too late to vaccinate and death usually follows quickly.  No longer administer 20 abdominal injections/day to treat rabies! The current vaccine is given intramuscularly on days 0, 3, 7, 14, and 28.  Hyperimmune globulin (antibody) is also placed deep in the wound and infiltrated around the wound (remember passive immunization).

III.       Bacterial Nerve Diseases

A.     Hansen’s Disease – already discussed

B.      Tetanus – already discussed

C.      Botulism & Infant Botulism – already discussed


IV.           Prion Diseases of the Nervous System


Prion Defined:  infectious agent composed only of protein (no DNA or RNA!).


Includes Kuru and Creutzfeldt-Jakob Disease (CJD) of humans, mad cow disease, scrapie of sheep, chronic wasting disease of elk and mule deer.  These diseases are referred to collectively as transmissible spongiform encephalopathies because they give brain tissue a spongy appearance.  


Kuru (“Laughing Death”) – Spread through breaks in the skin; occurred mainly in New Guinean women.  Discovered that they prepared the bodies of the dead for cannibalistic consumption and smeared their own bodies with the raw flesh of the corpses.  Since cannibalistic rites have been stopped, the disease has disappeared. 


CJD – in most cases, no source of prions has been identified; there seems to be a genetic predisposition in some families.


Mad Cow Disease – reached a peak in Britain in the early 1990’s; spread through the practice of boiling down animal remains for livestock feed.  The US  has banned the import of British beef, cattle, and beef products. 


Prions have clearly been transmitted from one species to another in lab trials.


Could play a role in Alzheimer’s and Parkinson’s diseases.