Diagnosis
Altered mental status and fever. Specific diagnosis depends on the organism.
Epidemiologic Risks
There are a host of reasons that people get encephalitis, depends on the cause.
Microbiology
Lots. The exposure history is everything in making the diagnosis.
Viral (NEJM Review): Astrovirus (in the immunocompromised), Dengue, Eastern equine (mosquito vector), Enterovirus (5% in California, PubMed) Epstein–Barr virus, Herpes, Hepatitis E, Influenza (more in children than adults), Japanese Encephalitis Virus (mosquito vector), La Crosse, Lymphocytic choriomeningitis (Hamster vector), Measles, Mumps, Murray Valley, Nipah, Parvovirus, Powassan, Rift Valley, Rubella, St. Louis (mosquito vector), Toscana (PubMed),Vaccinia, Varicella–zoster, Venezuelan equine (mosquito vector), West Nile (mosquito vector), Western equine (mosquito vector). And more.
Pseudorabies in pig handlers (PubMed).
St. Louis encephalitis virus was in St Louis in 1933. It faded, went to Argentina, then went to New Mexico and California by way of migrating birds (PubMed).
Every part of the world has its odd virus that can melt the brain, like the Variegated Squirrel (A pet in Germany, the squirrel comes from South America) related Bornavirus (Really I can't make that kind of stuff up).
In the US NE, there is a deer tick virus encephalitis (PubMed).
Beware. As the world warms the range of vectors and their pathogens are a-changing (Pubmed). So just because a pathogen wasn't in the area last year doesn't mean it hasn't arrived. Some things can't be stopped by a wall.
Non viral: Some important causes but by no means inclusive: Acanthamoeba (There was a case of fulminant Acanthamoeba castellanii encephalitis in a patient with CLL treated with ibrutinib (Pubmed).), Actinomycosis, Ballamuthia mandrillaris, Baylisascaris, Brucella, Cat–scratch disease, Chlamydia, Cryptococcus, Ehrlichia, Histoplasma, Legionella, Leptospirosis, Listeria, Lyme, Mycoplasma, Naegleria, Nocardia, Orientia tsutsugamushi (In India (PubMed), Plasmodium falciparum, Q fever, Relapsing fever, Rocky Mountain spotted fever, Syphilis (meningovascular), Toxoplasma, Trypanosomiasis, Tuberculosis, Typhus, Whipples disease. And more.
Many Rickettsia can cause an encephalitis (Review).
Transplanted organs have lead to West Nile virus, rabies virus, lymphocytic choriomeningitis virus, and Balamuthia mandrillaris amebae (PubMed).
In encephalitis with seizures, personality changes, autonomic dysfunction and speech problem, think of Anti-NMDAR, where antibodies directed against teratomas cross-react with limbic brain. "Anti-NMDAR encephalitis was identified 4 times as frequently as HSV-1, WNV, or VZV and was the leading entity identified (PubMed). Besides serology (blood and CSF), you can use cryo-auscultation. Put the patient in a cold room and listen for teeth chattering in the teratoma. Old joke. The syndrome can be triggered by HSV encephalitis.
From a 2006 review in CID (PubMed) "A confirmed or probable etiologic agent was identified for 16% of cases of encephalitis: 69% of these agents were viral; 20%, bacterial; 7%, prion; 3%, parasitic; and 1%, fungal. An additional 13% of cases had a possible etiology identified. Many of the agents classified as possible causes are suspected but have not yet been definitively demonstrated to cause encephalitis; these agents include M. pneumoniae (n=96 ), influenza p 96 virus ( n=22), adenovirus ( n=14), Chlamydia species ( n=10), and human metapneumovirus ( n=4). A noninfectious etiology was identified for 8% of cases, and no etiology was found for 63% of cases."
As part of IRIS, HIV patients can get a CD8 encephalitis (PubMed) that is treated with steroids.
Empiric Therapy
You always treat Herpes if you think it is a viral encephalitis as it is the one you can treat. Otherwise its support and specific treatment depending on what you suspect from the history.
Pearls
An important non-infectious disease cause, as if there is anything that is not infectious could possibly be important, is NMDA antibody disease, which causes maybe 1% of encephalitis in the young (PubMed)(Review).
And patients with seminoma can get a disease due to autoantibodies against Kelch-like Protein 11 (Pubmed).
Long term prognosis is poor, at least half have some kind of permanent impairment (PubMed).
Curious Cases
Relevant links to my Medscape blog
Miss the diagnosis once, shame on me, miss it twice, Fool me — you can't get fooled again.
Last Update: 01/30/20.