West Nile Virus, Emergency Medicine

Basics

Description

Infectious agent is an arbovirus, an RNA member of the Flaviviridae family. ‚

Etiology

Vector-borne virus
Transmitted by infected mosquitoes in late summer/early fall
Wild birds are primary reservoir hosts; humans are infected by cross-feeding mosquitoes.
Introduced to Western Hemisphere in 1999; became more widespread owing to vector of Culex mosquito and is now endemic in North America
Infection after blood transfusion and solid-organ transplant can occur.
There are case reports of occupational exposure and infection of lab workers via percutaneous inoculation.
Following recovery, immunity is considered lifelong. Reoccurrence is rare
The 2011 outbreak had a mortality rate of 4 " “5%. Cases were reported in 48 states.

Infection via transplacental transmission and breast-feeding has been reported. ‚

Diagnosis

Signs and Symptoms

Variable severity of illness:
- 80% asymptomatic
- 20% mild symptoms, flu-like illness
- ¢ ˆ ¼1/150 with CNS involvement (encephalitis, meningitis)
Incubation period is usually 2 " “6 days but can be up to 14 days in average patient and up to 21 days in immunocompromised patient.
Symptoms have a sudden onset and last <1 wk with mild infection.
Mortality rate in severe cases is estimated at 7%.
Severity of illness is related to degree of CNS invasion by virus. Risk is enhanced with increased age and immunosuppression
Immunocompromised patients have prolonged viremia, delayed development of antibody, and increased likelihood of severe disease.
Persistent symptoms of fatigue, memory impairment, weakness, and headache have been reported to last for 1 " “2 mo

Patients >60 yr, if infected, are at higher risk for developing more severe disease and neurologic consequences.
Advanced age is the most important risk factor for death.

History

General:
- Fever
- Malaise
- Anorexia
- Headache
- Acute phase resolves within several days but fatigue and weakness may persist for weeks
Neurologic:
- Altered mental status (change in level of consciousness, confusion, agitation, irritability)
- Severe, diffuse muscle weakness; may be asymmetric and involve the face
- Flaccid paralysis, which may resemble poliomyelitis-like syndrome, associated with anterior horn cell injury. Cranial nerve and bulbar abnormalities have been reported
- May resemble Guillain " “Barre syndrome
- Seizures
- Encephalitis more commonly reported in adults and meningitis in children
GI:
- Nausea, vomiting, diarrhea, anorexia
- Abdominal pain
Musculoskeletal:
- Myalgia
- Arthralgia
- Back pain
Respiratory:
- Cough
- Sore throat
Ophthalmologic:
- Photophobia
- Eye pain

Physical Exam

General:
- Temperature >38 ‚ °C (>100 ‚ °F)
- Transient maculopapular rash
- Rhabdomyolysis
Neurologic:
- Altered mental status
- Hyporeflexia, areflexia
- Ataxia
- Extrapyramidal signs
- Cranial nerve palsies, paresis
- Myoclonus
- Profound motor weakness
- Flaccid paralysis
GI:
- Hepatosplenomegaly, hepatitis, pancreatitis
Musculoskeletal:
- Nuchal rigidity
Hematologic:
- Lymphadenopathy
Dermatologic:
- Rash (maculopapular or morbilliform on neck, trunk, extremities) usually lasting <1 wk
Cardiovascular:
- Myocarditis (rare)
Ophthalmologic:
- Optic neuritis
- Vitritis
- Chorioretinitis

Essential Workup

Most sensitive screening test is serologic testing of CSF and serum for IgM antibody-capture enzyme-linked immunosorbent assay (MAC-ELISA) and culture.
Centers for Disease Control and Prevention (970-221-6400)
Can be detected during 1st 4 days of illness, nearly all tests are positive by day 7 " “8; may remain positive up to 1 yr after infection
Procedures for submitting samples vary by state.
Refer to local public health department for guidelines.

Diagnosis Tests & Interpretation

Lab

CSF:
- Pleocytosis with lymphocyte predominance
- Elevated protein
- Normal glucose
CBC:
- WBCs may be mildly elevated (50%) or normal.
- Leukopenia may be present (15%).
- Anemia can occur.
Chemistry:
- Hyponatremia sometimes seen:
  - Cause uncertain, possibly syndrome of inappropriate antidiuretic hormone (SIADH) when CNS involvement exists
- Pancreatitis (rare)
- Fulminant hepatitis (rare)

Imaging

CT head usually normal
MRI can be useful to identify CNS inflammation:
- 1/3 of patients show abnormality.
- Imaging findings generally nonspecific but may include enhancement of leptomeninges and/or periventricular white matter or can mimic demyelinating process.

Diagnostic Procedures/Surgery

Lumbar puncture
MAC-ELISA may be used on serum and CSF samples

Differential Diagnosis

Other causes of meningitis:
- Bacterial
- Viral
- Tuberculous
- Fungal
Other causes of viral encephalitis:
- Other arboviruses, especially St. Louis encephalitis virus
- Enterovirus, particularly in patients ≤16 yr of age
- Herpes simplex virus (HSV)
- Cytomegalovirus (CMV)
- Epstein " “Barr virus (EBV)
- Mumps virus
- Varicella zoster virus
- Rabies virus
Intracranial abscess
CNS vasculitis
Nonspecific viral syndrome
Gastroenteritis

Treatment

Initial Stabilization/Therapy

ABCs
Seizure precautions

Ed Treatment/Procedures

Supportive care
IV fluids for signs of dehydration
For signs of meningitis, administer antibiotics pending results of CSF.
Consider acyclovir if index of suspicion for the only treatable cause of viral encephalitis, HSV, is high.
Administer antipyretics and pain medications.
No known effective antiviral therapy or vaccine
No controlled studies proving effectiveness of interferonα-2b, ribavirin, corticosteroids, anticonvulsants, or osmotic agents

Follow-Up

Disposition

Admission Criteria

Neurologic symptoms
Dehydration
Concerning risk factors (advanced age, immunocompromise)

Discharge Criteria

No signs of CNS involvement (encephalitis, meningitis)
Able to tolerate oral solutions

Followup Recommendations

Neurologist to monitor for potential ongoing residual. ‚

Pearls and Pitfalls

Consider HSV in differential, since HSV is treatable. ‚

Additional Reading

Centers for Disease Control and Prevention (CDC): Interim guidelines for the evaluation of infants born to mothers infected with West Nile Virus during pregnancy. MMWR Morb Mortal Wkly Rep. 2004;53:154 " “157.
Hayes ‚ EB, O 'Leary ‚ DR. West Nile virus infection: A pediatric perspective. Pediatrics. 2004;113:1375 " “1381.
Loeb ‚ M, Hanna ‚ S, Nicolle ‚ L, et al. Prognosis after West Nile virus infection. Ann Intern Med. 2008;149:232 " “241.
Petersen ‚ LR, Marfin ‚ AA, Gubler ‚ DJ. West Nile virus. JAMA. 2003;290:524 " “528.
Peterson ‚ LR, Hayes ‚ EB: West Nile virus in the Americas. Med Clin North Am. 2008;92:1307 " “1322.
West Nile Virus: Information and Guidance for Clinicians. Available at http://www.cdc.gov/ncidod/dvbid/westnile/clinicians
Zak ‚ IT, Altinok ‚ D, Merline ‚ JR, et al. West Nile virus infection. AJR Am J Roentgenol. 2005;184(3):957 " “961.

Codes

ICD9

066.40 West Nile Fever, unspecified
066.41 West Nile Fever with encephalitis
066.42 West Nile Fever with other neurologic manifestation
066.49 West Nile Fever with other complications
066.4 West nile fever

ICD10

A92.30 West Nile virus infection, unspecified
A92.31 West Nile virus infection with encephalitis
A92.32 West Nile virus infection with oth neurologic manifestation
A92.39 West Nile virus infection with other complications
A92.3 West Nile virus infection

SNOMED

417093003 Disease due to West Nile virus (disorder)
392662004 West Nile encephalitis (disorder)
430397002 disorder of nervous system due to West Nile virus (disorder)
404233006 West Nile meningitis (disorder)
397420007 West Nile fever without encephalitis (disorder)

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