Encephalitis: Definition and Etiology

00:02 Continuing our discussion of central nervous system infections, we come to encephalitis.

00:10 And encephalitis would be defined as the inflammation of the brain parenchyma, causing some kind of neurologic dysfunction.

00:22 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.

00:41 It’s got a variety of causes.

00:43 The majority are caused by viruses, but bacteria also account for about a third.

00:51 And that includes the organisms Borrelia burgdorferi, the cause of Lyme disease; Treponema pallidum, syphilis; and mycobacterium species.

01:03 Fungi also can cause meningoencephalitis, particularly Cryptococcus neoformans, prions and there are a variety of miscellaneous causes of infection of the brain parenchyma.

01:22 Encephalitis occurs either sporadically or in epidemic form.

01:30 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.

02:18 Measles virus is an unusual cause of sporadic encephalitis because measles vaccine is given widely in developed countries, so we don't see much of this anymore, but it is present in developing countries.

02:28 It can cause two types of encephalitis: the Acute Dissiminated Encephalomyelitis (ADEM) and also the Subacute Sclerosis Panencephalitis (SSPE), which takes years to develop and it is very rare.

02:42 And then a curious one, lymphocytic choriomeningitis virus, among handlers of laboratory animals, mice and rats.

02:53 The ones that are associated with epidemics are usually the vector-borne encephalitides.

03:00 Most are spread by mosquitoes, but some are spread by ticks.

03:08 The ones that are in the toga virus family are the ones that have equine in their name, Eastern equine, Western equine, Venezuelan equine encephalitis.

03:25 The flaviviruses are notorious for causing West Nile encephalitis, and Japanese encephalitis.

03:39 And of those three, Japanese encephalitis has the most serious sequelae.

03:48 The bunyaviruses are categorized in the California encephalitis group and include La Crosse virus, California encephalitis virus, and Jamestown Canyon virus.

04:01 The one that’s caused by the real viruses is Colorado tick fever virus.