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West Nile Virus

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At a glance
  • Reporting time frame: 4 calendar days
  • Individual cases need follow-up? Yes
  • Timeline for patient interview: 0-3 days
  • Responsibility for investigation: Local public health agency
  • CDPHE Program: Zoonoses and One Health
  • Mode(s) of transmission: Mainly mosquito-borne. Rarely by blood transfusion, organ transplant, or vertically from pregnant person to child during pregnancy, delivery, or breastfeeding.
  • Incubation period: 2-6 days (range from 2 to 14 days)
  • Infectious period: not applicable
  • Treatment: none
  • Prophylaxis: none
  • Exclusion criteria: none
  • Additional: Blood products are actively screened for West Nile virus. Positive tests are reported to CDPHE.

Contents

Reporting criteria

What and how to report to the Colorado Department of Public Health and Environment (CDPHE) or local public health agency

  • Report any positive laboratory results of West Nile virus that incidence recent infection, including IgM, RT-PCR, and NAAT, in blood or cerebrospinal fluid (CSF).
  • Cases should be reported in EpiTrax under the diagnosis West Nile virus, non-neuroinvasive. CDPHE will change the diagnosis to West Nile virus, neuroinvasive if indicated when the investigation is complete.

Purpose of surveillance and reporting

  • To initiate mosquito surveillance and control measures to prevent additional cases. 

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The disease and its epidemiology

Etiologic agent

West Nile virus (WNV) is an arthropod-borne virus (arbovirus) most commonly spread by infected mosquitoes. WNV can cause many symptoms such as a febrile illness, encephalitis (inflammation of the brain), meningitis (inflammation of the lining of the brain and spinal cord), and meningoencephalitis.

WNV transmission has been documented in Europe and the Middle East, Africa, India, parts of Asia and Australia. It was first detected in North America in 1999 and has since spread across the continental United States and Canada. 

Clinical description

Most (~80%) West Nile virus infections are asymptomatic. Clinical disease ranges from a mild febrile illness to severe encephalitis which can result in death. For the purposes of surveillance and reporting, WNV cases are categorized into two primary groups: neuroinvasive and non-neuroinvasive disease. 

Reservoirs

In Colorado, West Nile virus is transmitted to people by Culex species mosquitoes, most commonly Culex tarsalis or Culex pipiens. These are medium-sized mosquitoes that typically feed around dusk and dawn.

Modes of transmission

West Nile virus is transmitted when an infected mosquito bites a host. Humans and horses are dead-end hosts, which means that an infected human or horse cannot transmit WNV to uninfected mosquitoes. Person-to-person transmission does not occur except possibly through blood or organ donation. West Nile virus is transmitted in Colorado whenever mosquitoes are active; cases have been acquired as early as May and as late as December. The majority of human cases are acquired when mosquito abundance is highest, primarily from August through September. 

In a very small number of cases, West Nile virus has been spread through blood transfusions, organ transplants, and from mother to baby during pregnancy, delivery, or breastfeeding. Blood collection agencies in the United States actively screen donations for WNV.

Incubation period

The incubation period is usually 2-6 days, but can range from 2 to 14 days.

Infectious period

Person-to-person transmission of West Nile virus has not been reported.

Epidemiology

CDPHE publishes historic and current West Nile virus case data. 
National data can be found on CDC's data and maps for West Nile webpage.

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Case definition 

Clinical description

Neuroinvasive disease 

West Nile virus can cause neuroinvasive disease such as aseptic meningitis, encephalitis, or acute flaccid paralysis (AFP). These illnesses, which occur in a small proportion of infected people, are usually characterized by acute onset of fever with headache, myalgia, stiff neck, altered mental status, seizures, limb weakness, or cerebrospinal fluid (CSF) pleocytosis. AFP may result from anterior (‘polio’) myelitis, peripheral neuritis or post-infectious peripheral demyelinating neuropathy (i.e., Guillain-Barré syndrome). Less common neurological manifestations, such as cranial nerve palsies, also occur.

Non-neuroinvasive disease

West Nile virus can cause an acute systemic febrile illness (e.g., West Nile fever) that may include headache, myalgias, arthralgias, rash, or gastrointestinal symptoms. Rarely, myocarditis, pancreatitis, hepatitis, or ocular manifestations such as chorioretinitis and iridocyclitis can occur.

West Nile virus disease should be considered in any person with a febrile or acute neurologic illness who has had recent exposure to mosquitoes, a transfusion of blood or blood products, or organ transplants, especially during June through September in areas where virus activity has been reported. The diagnosis should also be considered in any infant born to a mother infected with West Nile virus during pregnancy or while breastfeeding.

Clinical criteria for diagnosis

Neuroinvasive disease

  • Meningitis, encephalitis, acute flaccid paralysis, or other acute signs of central or peripheral neurologic dysfunction, as documented by a physician; and
  • Absence of a more likely clinical explanation. Other clinically compatible symptoms include headache, myalgia, rash, arthralgia, vertigo, vomiting, paresis, and/or nuchal rigidity.

Non-neuroinvasive disease

  • Fever (chills) as reported by the patient or a health-care provider; and
  • Absence of neuroinvasive disease; and
  • Absence of a more likely clinical explanation. 

Laboratory criteria for diagnosis

  • Isolation of virus from, or demonstration of specific viral antigen or nucleic acid in, tissue, blood, CSF, or other body fluid; or
  • Four-fold or greater change in virus-specific quantitative antibody titers in paired sera; or
  • Virus-specific IgM antibodies in serum with confirmatory virus-specific neutralizing antibodies in the same or a later specimen; or
  • Virus-specific IgM antibodies in CSF or serum.

Case classification

Neuroinvasive disease 

  • Confirmed
    • A case that meets the above clinical criteria for neuroinvasive disease and one of more of the following laboratory criteria for a confirmed case:
      • Isolation of virus from, or demonstration of specific viral antigen or nucleic acid in, tissue, blood, CSF, or other body fluid; or
      • Four-fold or greater change in virus-specific quantitative antibody titers in paired sera; or
      • Virus-specific IgM antibodies in serum with confirmatory virus-specific neutralizing antibodies in the same or a later specimen; or
      • Virus-specific IgM antibodies in CSF, with or without a reported pleocytosis, and a negative result for other IgM antibodies in CSF for arboviruses endemic to the region where exposure occurred.
  • Probable
    • A case that meets the above clinical criteria for neuroinvasive disease and the following laboratory criteria:
      • Virus-specific IgM antibodies in CSF or serum but with no other testing

Non-neuroinvasive disease 

  • Confirmed
    • A case that meets the above clinical criteria for non-neuroinvasive disease and one or more of the following laboratory criteria for a confirmed case:
      • Isolation of virus from, or demonstration of specific viral antigen o nucleic acid in, tissue, blood, or other body fluid, excluding CSF; or
      • Four-fold or greater change in virus-specific quantitative antibody titers in paired sera; or
      • Virus-specific IgM antibodies in serum with confirmatory virus-specific neutralizing antibodies in the same or a later specimen.
  • Probable
    • A case that meets the above clinical criteria for non-neuroinvasive disease and laboratory criteria for a probable case:
      • Virus-specific IgM antibodies in serum but with no other testing

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Laboratory testing

Commercial laboratory testing for West Nile virus does not require confirmatory testing at CDPHE. If there is a concern that a commercial IgM may be a false positive (incompatible clinical illness, onset during off-season, result attributed to past illness, other flaviviruses active in the area, etc.), contact the Zoonoses and One Health team (cdphe_zoonoses@state.co.us) about additional testing.

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Case investigation

Interview all cases of West Nile virus infection to determine:

  • Clinical syndrome (asymptomatic blood donor, febrile illness, meningitis, etc.)
  • Illness onset date (or date of donation if case is an asymptomatic blood donor)
  • If illness was fatal, collect the date of death and notify CDPHE immediately within one working day.
  • Did the case donate or receive an organ transplant, or receive a blood transfusion within four weeks of symptom onset? If so, notify CDPHE immediately.
  • If a case was identified through blood donation screening, interview the case within four days of report and again 14 days after donation to determine if they developed symptoms.
  • If the case is pregnant, collect the expected delivery date and notify CDPHE immediately.
  • If the case is an infant or toddler, are they fed breast milk?

Forms 

All information required for case verification and counting must be entered into the CMR in EpiTrax. CDPHE’s WNV case investigation form can be used to collect data prior to entering into EpiTrax. 

Identify and evaluate contacts (if applicable, as not all pathogens are communicable)

West Nile virus is not generally transmitted person to person.

Reported incidence is higher than usual/outbreak suspected

Colorado historically has a very high incidence of West Nile virus. If local capacity for investigation is met, contact your Field Epidemiologist.

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Disease control measures

Treatment

There is no specific treatment for arboviral infections.

Prophylaxis

There are no human West Nile vaccines currently available.

Education

The most effective way to prevent West Nile virus disease is to prevent mosquito bites by:

  • Using insect repellents when you go outdoors. Choose an EPA-registered product and follow the label directions.
  • Wearing long sleeves and pants at dusk and dawn when the mosquitoes that transmit West Nile virus are more active.
  • Installing and repairing screens on windows and doors, or using air conditioning if available.
  • Emptying standing water from containers around your home including flower pots, gutters, buckets, pool covers, pet water dishes, discarded tires, and birdbaths to prevent mosquitoes breeding near your home. 

Environmental measures

Local public health agencies and CDPHE have conducted West Nile virus mosquito surveillance since 2001. Components of these programs may include mosquito trapping and pathogen testing, and equine case reporting. Questions regarding CDPHE’s mosquito surveillance program and data should be directed to the Zoonoses and One Health team at cdphe_zoonoses@state.co.us.

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References

Important telephone and fax numbers

CDPHE Communicable Disease Branch

  • Phone: 303-692-2700 or 800-866-2759
  • Fax: 303-782-0338
  • After hours: 303-370-9395

Zoonoses and One Health Program: cdphe_zoonoses@state.co.us

State Public Health Laboratory, Clinical Microbiology: 

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