Continuing our discussion of central
nervous system infections,
we come to encephalitis.
And encephalitis would be defined as the
inflammation of the brain parenchyma,
causing some kind of neurologic dysfunction.
In terms of the epidemiology,
the incidence is fairly low,
2.2 per 100,000 persons,
but it tends to be most frequent in
individuals less than a year old
and more than 65 years old.
It’s got a variety of causes.
The majority are caused by viruses,
but bacteria also account
for about a third.
And that includes the organisms
Borrelia burgdorferi, the cause of Lyme disease;
Treponema pallidum, syphilis;
and mycobacterium species.
Fungi also can cause meningoencephalitis,
particularly Cryptococcus neoformans,
and there are a variety of miscellaneous
causes of infection of the brain parenchyma.
Encephalitis occurs either
sporadically or in epidemic form.
And the causes of sporadic encephalitis would be the –
number one would be herpes simplex
virus, types one and two –
and in the US,
HSV-1 is the most common cause, not HSV-2 –
human herpes virus type VI;
cytomegalovirus, especially in immunocompromised individuals;
Epstein-Barr virus, the cause of infectious mononucleosis,
may cause encephalitis;
the enteroviruses, particularly the Coxsackie viruses;
and, of course, the most feared and
dreaded cause of encephalitis would be rabies.
Measles virus is an unusual
cause of sporadic encephalitis
because measles vaccine is given widely in developing countries,
so we don't see much of this anymore,
but it is present in developing countries.
And then a curious one,
lymphocytic choriomeningitis virus,
among handlers of laboratory animals,
mice and rats.
The ones that are associated with epidemics
are usually the vector-borne encephalitides.
Most are spread by mosquitoes,
but some are spread by ticks.
The ones that are in the toga virus family
are the ones that have equine in their name,
Venezuelan equine encephalitis.
The flaviviruses are notorious for
causing West Nile encephalitis,
St. Louis encephalitis,
and Japanese encephalitis.
And of those three,
Japanese encephalitis has
the most serious sequelae.
The bunyaviruses are categorized in
the California encephalitis group
and include La Crosse virus,
California encephalitis virus,
and Jamestown Canyon virus.
The one that’s caused by the
real viruses is Colorado tick fever virus.