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Health Care Provider Information

West Nile Virus

West Nile Virus

Clinical Features

Fast Facts:

  • Most patients infected are asymptomatic
  • Incubation period is typically 2-14 days
  • Following acute infection, neurological symptoms may persist and include:
    • Fatigue
    • Difficulty concentrating or memory impairment
    • Generalized weakness
    • Headache
    • Balance or gait disturbances

West Nile infection is usually self-limiting and is known as West Nile fever. The typical presentation for this patient includes an acute onset of headache, malaise, myalgia, decreased appetite, and a low-grade fever which typically lasts 3-10 days. Other symptoms may accompany the generalized symptoms and include ocular pain, sore throat, GI symptoms, and back pain. A rash may occur in about half of the patients with an acute West Nile infection. The rash typically lasts less than seven days in duration and is described as a morbilliform or maculopapular abruption on the trunk, back or arms of the patient. Some patients report pruritus along with the rash. The literature suggests that infection associated with skin manifestations is less likely to evolve into neuroinvasive disease. 

The primary risk factors associated with West Nile virus infection and mortality is linked to the possibility of neuroinvasive disease. The patient will present with fever and evidence of clinical manifestations suggestive of meningitis, encephalitis, or flaccid paralysis. Age is an important indicator for neuroinvasive disease and meningitis is more common in children while encephalitis is notable in older adults. The literature suggests that the mortality rate is linked to contributing risk factors such as male gender, comorbidities, immunosuppression, degree of neurovascular involvement and increasing age.

The clinician should be astute to findings of fever, headache with photophobia, as well as classic signs of meningeal involvement. The degree of encephalitis is varied with West Nile infection and may present as mild confusion, tremors, or parkinsonian features but may also quickly progress to coma or death. Patients that develop encephalitis associated with West Nile may report residual side effects associated with a decline in both functional or cognitive functions for many months following the acute infection.

Patients that present with asymmetric weakness of the limbs may or may not have symptoms associated with meningitis or encephalitis. Acute flaccid paralysis often improves to or near baseline for over half of the patients that develop this syndrome. Long-term complications from paralysis are possible and are linked to the morbidity and mortality of this infection. Proper diagnostic testing is recommended for any patient that presents with any of these neurological findings and is at risk from associated activities that are linked to vector-borne disease. 

Other associated manifestations include ocular complications which may be either persistent or permanent. Referral to ophthalmology is suggested for evaluation in any patient with abnormal ocular findings on exam or for visual disturbance symptoms reported by the patient. Literature review has identified additional rare complications associated with West Nile infection and include rhabdomyolysis, hepatic or pancreatic involvement, cardiac involvement (primarily myocarditis), diabetes insipidus, orchitis, and an association with fatal hemorrhagic fever with multi-organ failure. Other neurological findings include cranial nerve palsies, ataxia, and seizures. The clinician should consider West Nile testing in any patients with a new onset of any of these associated manifestations and referral to infectious disease is likely warranted. 


Risk Factors

  • Seasonal: (Late summer to early fall)
  • Travel 
  • Mosquito bites
  • Blood transfusion (Rare)
  • Transplacental (Rare)   

Development of neuroinvasive disease:

  • Age
  • Malignancy
  • Organ transplant (Rare)
  • Genetic Factors
  • Male Gender
  • Comorbidities
  • Diabetes
  • CVD
  • Hepatitis C
  • Alcohol abuse/misuse
  • Immunosuppression

Differential Diagnosis

Numerous viral infections can present similar to West Nile virus and can be linked to clinical manifestations of encephalitis or meningitis.

The clinician should consider varicella, herpes simplex type 1, Dengue, Powassan, St. Louis encephalitis, Japanese encephalitis, enteroviruses, Lyme disease, Rocky Mountain spotted fever, and bacterial meningitis as part of the differential diagnosis when evaluating for West Nile virus.

The key indicator is often linked to recent travel or can be linked to the geographic area in which the patient resides. Being informed to the epidemiological data of a particular area can guide the clinician with diagnosis and prevent a delay in treatment.


Routine lab tests cannot distinguish West Nile virus from other viral illnesses. Therefore, diagnosis is done primarily through IgM antibody findings in body fluid sampling. Antibody testing can be done on serum or CSF and is considered the gold standard for diagnosis.

  • Serum: MAC-ELISA
    • May repeat in 10 days if initial test is negative and suspicion remains high
  • CSF: Lumbar Puncture for those with neurological symptoms
  • PRNT: Done if suspected cross-reactivity (other vector-borne viruses)
  • IgG: Not recommended for acute diagnostic purposes due to increased number of false positives or prior infection with West Nile virus

Imaging is not typically used for diagnostic purposes because a CT scan or MRI scan do not typically show evidence of acute disease. Some of the literature denotes abnormal findings may be noted on a MRI scan but it is often late in the course of infection and should only be used for those with neuroinvasive disease. An EEG may show continuous slowing in either the frontal or temporal regions in those with meningitis or encephalitis but is not primarily disease specific and should be done in conjunction with antibody testing.

Indications for West Nile virus IgM testing:

  • Acute onset of fever and evidence of neuroinvasive disease during late summer to early fall
  • Evidence of West Nile infection in others in a similar region or linked to travel cases


Treatment should be focused on support for symptoms, pain control, and hydration for most patients. Patients with encephalitis will require inpatient monitoring for complications that include increased intracranial pressure and respiratory compromise. Those with flaccid paralysis could also require ventilation assistance due to acute respiratory failure which can develop rapidly and become life-threatening. 

The CDC offers an overview of the literature for West Nile virus therapy and can be accessed here.



To date, there is not a vaccine available for the prevention of West Nile virus. Prevention efforts are focused on mosquito-control programs, education regarding personal protection when outdoors, and screening on blood and tissue products prior to transfusion or transplant. The use of mosquito repellant, proper clothing, and limiting exposure to outdoor activities during the hours from dusk to dawn should be highlighted during patient education opportunities. Proper use of screens on windows and removal of standing water from outdoor spaces can help protect patients from infected mosquitos. Discussion about upcoming travel or recent travel should continue to be a topic during routine care to provide preventative conversation regarding the prevention of West Nile virus.


Centers for Disease Control and Prevention (CDC) (2018). West Nile virus: For Healthcare Providers. Retrieved  from

Petersen, L. R. (2018). Clinical manifestations and diagnosis of West Nile virus infection. UpToDate. Retrieved 7/28/2020

Petersen, L. R. (2018). Epidemiology and pathogenesis of West Nile virus infection. UpToDate. Retrieved 7/28/2020

Petersen, L. R. (2018). Treatment and prevention of West Nile virus infection. UpToDate. Retrieved 7/28/2020