Cause:Several types of arboviruses: St. Louis (Ann IM 1969;71:681), eastern equine, West Nile (Ann IM 2004;140:545; Jama 2003;290:524; Ann IM 2002;137:173; Nejm 2001;344:1807), western equine, St. Louis encephalitis, and LaCrosse encephalitis
Single-stranded RNA flaviviruses, closely related to Japanese and Murray Valley encephalitis (rv of all—Nejm 2004;351:370)
Pathophys:Humans and horses don’t develop enough viremia to transmit these diseases. Two disease forms of WNV: WN fever (0.5% of all infections) and WN encephalitis (0.067%)
Culex mosquito and ticks from birds and invertebrates; rarely via organ transplant (Nejm 2003;348:2196). Incidence highest in small children and adults age >50 yr, except West Nile, which is a neuro-invasie disease mostly in older adults and the immunosuppressed; many inapparent cases, up to 99%; incidence up after rainy winter; equine types kill horses by 1000s and children age <10 yr. EEE transmitted by a different, cedar swamp mosquito than the West Nile virus.
Also transmissible by blood transfusions
Sx:
Vast majority of at least West Nile virus infections are asx.
Fever (90%), Guillaine-Barré type muscle weakness (56% w West Nile type, unlike others), severe headache (47%), nausea, and vomiting (52%)
Si:
Altered mental status (46-86%); frontal lobe si's (78%); stiff neck (71%); cranial nerve palsies, especially of III, IV, VI, VII (22%); Babinski’s sign (30%); seizures (10%)
WN virus: diffuse macular/papular rash; neurologic si (Jama 2003;290:511): meningitis, encephalitis, acute flaccid paralysis (polio-like), tremor, myoclonus, parkinsonism
WN virus only 3-6 d, rest 10-21 d; 10-36% mortality, higher if seize; 35-60% of survivors w neurologic morbidity; both predicted by CSF WBC counts and serum hyponatremia (E. equine type—Nejm 1997;336:1867). WN virus pts recover well except those w flaccid paralysis (Jama 2003;290:511)
SIADH. WNV neuro-invasive type has 10% mortality. EEE symptomatic pts have 50% sequellae unlike WNV.
r/o tick-borne Powassan encephalitis, which has similar clinical presentation to EEE (Mmwr 2001;50:761); yellow fever, dengue, zika virus (Nejm 2009;360:2536)
Lab:
Serol:ELISA for IgM antibodies; reverse transcriptase PCR identification of viral RNA in CSF
CSF:1-500 WBCs, 50% lymphs; protein 40-500+ mg %
Xray:MRI shows basal ganglia and thalamic focal lesions (Nejm 1997;336:1867)