La Crosse encephalitis
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Synonyms and Keywords: LACV; LaCrosse virus; LAC encephalitis; Lax encephalitis
La Crosse encephalitis is a mild infection of the central nervous system. La Crosse encephalitis virus belongs to the Group V negative-sense ssRNA family of viruses. La Crosse encephalitis virus is also known as an arbovirus. La Crosse encephalitis virus is usually transmitted via mosquitos to the human host. The amplification pattern of La Crosse encephalitis virus has been extensively studied. La Crosse encephalitis virus is contracted by the bite of an infected mosquito, primarily Aedes triseriatus. La Crosse encephalitis virus must be differentiated from other diseases that cause fever, headache, seizures, and altered mental status. There are approximately 70-115 cases of La Crosse encephalitis virus per year in the United States, most commonly affecting infants and children between the ages of 6 months and 15 years old. La Crosse encephalitis virus usually clears in 1 to 2 weeks and rarely recurs. Less than 1% of cases result in mortality. The diagnostic method of choice for La Crosse encephalitis virus is laboratory testing. There is no treatment for La Crosse encephalitis virus; the mainstay of therapy is supportive care.
In 1963, the cause of La Crosse encephalitis was discovered near La Crosse, Wisconsin by the Hooper Foundation. La Crosse encephalitis was first discovered within the brain during the autopsy of a 4 year old boy who died from encephalitis. Upon further microscopic histopathological analysis by Whitman and Shope, it was confirmed that the La Crosse encephalitis virus was genetically related to the California encephalitis virus.
La Crosse encephalitis may be classified according to neuroinvasiveness into 2 groups: neuroinvasive and non-neuroinvasive. La Crosse encephalitis virus belongs to the Group V negative-sense ssRNA virus within the Bunyaviridae family of viruses, and the genus Orthobunyavirus. La Crosse encephalitis virus is also known as an arbovirus, or an arthropod-borne virus. The La Crosse virus is the principal member of the California encephalitis serogroup, which contains genetically similar viruses such as California encephalitis virus.
La Crosse encephalitis virus is usually transmitted via mosquitos to the human host. La Crosse encephalitis virus contains negative-sense viral RNA; this RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation. La Crosse encephalitis virus is made up of an enveloped virion with a spherical capsid. The envelope contains G1 glycoproteins. Neutralizing antibodies against these proteins block fusion of the virus with host cells and inhibit hemagglutination. The virus genome is over 12,000 nucleotides in length, approximately 90-100 nm in diameter, and consists of three segments of various sized single-stranded RNA (negative sense and ambi-sense).
La Crosse encephalitis virus is contracted by the bite of an infected mosquito, primarily Aedes triseriatus. The virus is maintained and amplified in Aedes triseriatus populations through transovarial and venereal transmission. The virus overwinters in the mosquito egg. Amplification also occurs in chipmunks and squirrels, upon which mosquitos feed. Humans are dead-end hosts for the virus, meaning there is an insufficient amount of La Crosse encephalitis virus in the blood stream to infect a mosquito. Subsequently, the disease cannot be spread to other humans. The incubation period is 5-15 days.
La Crosse encephalitis virus is transmitted in the following pattern:
- Virus attaches to host receptors though Gn-Gc glycoprotein dimer, and is endocytosed into vesicles in the host cell.
- Fusion of virus membrane with the vesicle membrane; ribonucleocapsid segments are released in the cytoplasm.
- Transcription occurs; viral mRNAs are capped in the cytoplasm.
- Replication presumably begins when sufficient nucleoprotein is present to encapsidate neo-synthetized antigenomes and genomes.
- The ribonucleocapsids buds at Golgi apparatus, releasing the virion by exocytosis.
La Crosse encephalitis virus causes La Crosse encephalitis.
Differentiating La Crosse encephalitis from Other Disases
|Meningitis||Classic triad of fever, nuchal rigidity, and altered mental status||Photophobia, phonophobia, rash associated with meningococcemia, concomitant sinusitis or otitis, swelling of the fontanelle in infants (0-6 months)|
|Brain abscess||Fever, headache, hemiparesis||Varies depending on the location of the abscess; clinically, visual disturbance including papilledema, decreased sensation; on imaging, a lesion demonstrates both ring enhancement and central restricted diffusion|
|Demyelinating diseases||Ataxia, lethargy||Multiple sclerosis: clinically, nystagmus, internuclear ophthalmoplegia, Lhermitte's sign; on imaging, well-demarcated ovoid lesions with possible T1 hypointensities (“black holes”)|
|Substance abuse||Tremor, headache, altered mental status||Varies depending on type of substance: prior history, drug-seeking behavior, attention-seeking behavior, paranoia, sudden panic, anxiety, hallucinations|
|Electrolyte disturbance||Fatigue, headache, nausea||Varies depending on deficient ions; clinically, edema, constipation, hallucinations; on EKG, abnormalities in T wave, P wave, QRS complex; possible presentations include arrhythmia, dehydration, renal failure|
|Stroke||Ataxia, aphasia, dizziness||Varies depending on classification of stroke; presents with positional vertigo, high blood pressure, extremity weakness|
|Intracranial hemorrhage||Headache, coma, dizziness||Lobar hemorrhage, numbness, tingling, hypertension, hemorrhagic diathesis|
|Trauma||Headache, altered mental status||Amnesia, loss of consciousness, dizziness, concussion, contusion|
Epidemiology and Demographics
There are approximately 80-100 cases of La Crosse encephalitis per year in the United States. There is significant under-diagnosis and under-reporting of less severe cases of La Crosse encephalitis.
La Crosse encephalitis commonly affects individuals between 6 months old and 15 years of age. Adults comprise the most under-diagnosed group.
The majority of La Crosse encephalitis cases are reported in the summer months between July and September, and peaks in August.
The majority of La Crosse encephalitis cases are reported in the Midwestern United States, especially those living in rural and suburban settings surrounded by deciduous forests. Historically, most cases of La Crosse encephalitis occur in the Midwest states (Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio). Recently, more cases are being reported from states in the Southeast United States (Virginia, Virginia, North Carolina, Alabama and Mississippi).
Maps regarding geographic distribution of La Crosse encephalitis cases can be found here.
Common risk factors in the development of La Crosse encephalitis virus include:
- Young age
- Residing or working in rural and suburban settings
- Mosquito contact
- Summer season
- Outdoor activities such as camping or hunting
Natural History, Complications, and Prognosis
Common complications of La Crosse encephalitis virus include:
Neuroinvasive vs non-neuroinvasive La Crosse encephalitis can be differentiated based on both clinical and laboratory findings. These include:
|La Crosse Encephalitis Subtype||Clinical Presentation||Laboratory Findings|
History and Symptoms
Common physical examination findings of La Crosse encephalitis include:
The diagnostic method of choice for La Crosse encephalitis is laboratory testing. Laboratory findings consistent with the diagnosis of La Crosse encephalitis include:
- Serologic cross-reactivity
- Presence of IgM antibodies
- Persistence of IgG and neutralizing antibodies
- Confirmation of arboviral-specific neutralizing antibodies in enzyme linked immunosorbent assay (ELISA)
- Mildly elevated white blood cell count
- Normal glucose levels
There are no CT findings associated with La Crosse encephalitis.
On EEG, La Crosse encephalitis virus is characterized by periodic lateralizing epilepitoform discharges. However, results on imaging are not sufficient evidence to warrant La Crosse encephalitis virus diagnosis. In rare cases, EEG findings may resemble herpes simplex encephalitis.
There is no treatment for La Crosse encephalitis; the mainstay of therapy is supportive care. Because supportive care is the only treatment for La Crosse encephalitis, physicians often do not request the tests required to specifically identify the La Crosse encephalitis virus. These cases may be reported as aseptic meningitis or viral encephalitis of unknown etiology.
Surgical intervention is not recommended for the management of La Crosse encephalitis.
There are no available vaccines against La Crosse encephalitis virus. Primary prevention strategies include:
- Removal of standing water
- Screens on doors and windows
- When outdoors, wearing:
- Insect repellent containing DEET
- Long sleeves, pants; tucking in pants into high socks
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