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West Nile Virus Fact Sheet

West Nile Virus in Humans at CECDP

Synopsis of West Nile Virus

West Nile virus is an arbovirus (arthropod-borne virus) that causes encephalitis.  It is a single-stranded RNA virus of the genus Flavivirus.  The West Nile virus is transmitted via the blood by blood-feeding insects (mosquitoes).  Most cases of West Nile virus have been seen in wild birds, horses, and humans.  It can also affect many other species of wild or domesticated animals.

According to a brochure from the American Veterinary Medical Association (AVMA), listed below, mosquitoes draw the virus from infected birds and they then transmit it to other animals when they bite them. Ticks have also been found that are infected with the West Nile virus in Asia and Africa, but there are not any verified reports of them spreading the virus; their role in West Nile virus has not been determined. West Nile viral encephalitis (West Nile fever) develops when the virus multiplies and crosses the blood-brain barrier. It is not transmitted through inter-personal or animal contact.

West Nile virus was first identified in the West Nile district of Uganda in 1937, and since then has been found in other African locations, Eastern Europe, West Asia, and the Middle East. The first outbreak of West Nile virus in the Western Hemisphere occurred in the summer of 1999; the strain found in the United States (U.S.) closely resembles one found in the Mediterranean ad Middle East.

West Nile fever is both endemic and epidemic. In the Nile delta in Egypt, the disease is endemic, and primarily affects children. It occurs in epidemic form in Israel, and occurs clinically in large quantities of individuals. West Nile fever occurs sporadically in South Africa with small epidemic outbreaks occurring regularly over the summer.

According to a brochure from the AVMA, the risk of becoming ill from a single mosquito bite is extremely low.  In areas where mosquitoes carry the virus, less than 1% of them are actually infected.  Even if they are infected, less than 1% of people bitten and infected by them become severely ill.

Description:
  • Mosquito-borne virus
  • First diagnosed in the West Nile District of Uganda in Africa in 1937
  • Primarily infects and multiplies in birds (primary reservoir): corvids (crows, blue jays, and ravens) are most susceptible to disease
  • Corvids are the focus of surveillance efforts
  • When the level of virus transmission among birds and mosquitoes is high, horses and humans (and other mammals) can become incidentally infected
  • Horses and humans are considered dead-end hosts because they do not produce enough virus in their blood to infect biting mosquitoes
  • Most commonly documented in wild birds, humans, and horses, but a few cases have also occurred in other species (sheep, bats, squirrels, chipmunks, raccoons, skunks, rabbits, cats, and dogs)
  • Distribution: Egypt, Asia, Israel, South Africa, parts of Europe and Australia, Canada, Mexico, and the United States
  • The 1st known case occurred in the U.S. in 1999 in New York; the strain that has appeared in the U.S. is most closely related to a strain found in Israel in 1998
  • 15,257 confirmed cases in horses during 2002 in 43 states; approximately 33% of affected horses either die or are euthanized due to the severity of their condition
  • Horses are highly susceptible to WNV infection and comprise more than 99% of veterinary mammalian cases
Transmission (Animals):
  • Common sources of exposure for animals:
    • Birds: mosquitoes (enzootic cycle)
    • Bird-to-bird transmission (oral and cloacal secretions containing water and food)
    • Mammals: “bridge vector” mosquito (takes virus out of enzootic cycle)
Transmission (Humans):
  • Mosquitoes
  • Blood transfusions
  • Organ transplantation
  • Transplacental transfer
  • Breast feeding

Acha Pedro N., Szyfres Boris. Zoonoses and Communicable Diseases Common to Man and Animals: Chlamydioses, Rickettsioses, and Viroses. 3 vols. Vol. 2. Washington D.C: Pan American Health Organization, 2003. American Veterinary Medical Association, "What You Should Know About West Nile Virus." AVMA, 2006.

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Clinical Signs and Symptoms
  • Susceptibility
    • Risk Factors
      • Time of Year
      • Geographic Region
      • Amount of Time Spent Outside
    • At Risk Individuals
      • Adults over 50 years old
      • Individuals with a weakened immune system
  • Communicability
    • Transmission Routes
      • Bite from infected mosquito (mosquito bites infected bird and becomes infected » infected mosquito bites human and human becomes infected)
      • No person-to-person contact transmission
    • At Risk Individuals
      • Adults over 50 years old
      • Individuals with a weakened immune system
Signs and Symptoms (Animals):
  • Incubation: 3 to 14 days
  • Corvids (crows, blue jays, ravens, etc.)
    • Most die within 3 weeks of infection
    • Clinical signs prior to death:
      • Uncoordinated walking
      • Weakness
      • Lethargy
      • Tremors
      • Abnormal head posture
  • Horses
    • Can range from mild signs to fatal illness
    • Primarily neurological
      • Weakness
      • Ataxia
      • Somnolence
      • Dullness
      • Muscle twitching
      • Listlessness
      • Facial paralysis
      • Inability to rise
      • Encephalitis
      • Mortality rate for symptomatic horses is 30 to 40 percent
  • Dogs and cats
    • Are relatively resistant to WNV
    • Mostly asymptomatic
    • The CDC has only documented one dog and one cat death in the U.S. attributable to WNV infection
    • One dog in Michigan developed intermittent seizures and rear limb weakness but has since recovered completely
  • Differential diagnoses
    • Rabies
    • Eastern Equine Encephalitis
    • Moldy corn poisoning
    • Equine Protozoal Myelitis
Signs and Symptoms (Humans):
  • Incubation: 3 to 14 days
  • Less than 1% of people who are bitten become infected and severely ill
  • Signs and Symptoms in Humans
    • Asymptomatic in most
    • Mild illness with flu-like symptoms develops in approximately 20%
      • Fever
      • Fatigue
      • Headache
      • Body aches
      • Swollen lymph nodes
      • Body rash
    • Serious disease affects 1 in 150 of people infected with WNV, particularly in the elderly
      • Serious neurological damage
      • Encephalitis
      • Meningitis
      • Meningeoencephalitis
      • Headaches
      • High fever
      • Stiff neck
      • Disorientation
      • Stupor
      • Tremors
      • Seizures or convulsions
      • Paralysis
      • Muscle weakness
      • Loss of consciousness
      • Death
  • Human case-fatality rate has been about 7% overall
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Diagnosis and Treatment
Diagnosis (Animals):
  • Corvids, either dead or those exhibiting signs characteristic of WNV, are collected by the health departments or DNR wildlife offices
    • Laboratory samples:
      • Oral swabs from Corvids
      • Blood or cerebrospinal fluid sample from horses
      • Tissue samples (kidney, heart, and brain) from non-corvid birds and mammals
    • Laboratory Tests:
      • Blood test for horses, must be done within 7-10 days of becoming ill
      • No live-animal tests for WNV in animals other than horses
      • Corvid samples are submitted to Michigan State University’s Diagnostic Center which uses a commercial kit: VecTest West Nile Virus Antigen Assay (rapid immunochromatographic assay) to test for the presence of the WNV antigen
      • Non-corvid birds and mammals are collected and submitted to the DNR for a complete necropsy. They collect tissue samples which are then submitted to MSU’s Diagnostic Center where they are analyzed using immunohistochemistry (stained for the WNV antigen)
Treatment (Animals):
  • No drug available currently to kill WNV
  • Supportive care
    • Anti-inflammatory medications
    • Intravenous fluid administration
    • Parenteral nutrition
    • Physical support
  • No available vaccine for mammals (other than horses) and birds
  • Vaccine for horses is available
    • West Nile Innovator developed by Fort Dodge and licensed for use in 2003
    • RecombiTEK developed by Merial and released in 2004
Diagnosis (Humans):

Diagnosos is based on factors including:

  • History
    • Exposure
    • Local presence of West Nile Virus
    • Travel locations
  • Clinical Signs
  • Diagnostic test results
    • Detection of West Nile Virus specific antibody using NAC-ELISA
      • Blood (serum)
      • CSF
Differential Diagnosis (Humans):

Differential Diagnosos for West Nile Virus include:

  • Enterovirus
  • St. Louis encephalitis
  • Western Equine encephalitis
  • Herpes simplex
  • Mycoplasma
  • Coccidiodomycosis
  • Tuberculosis
  • Other bacterial encephalitis
Clinical Diagnosis (Humans):

Diagnosis is based on high clinical suspicion and results of specific laboratory tests.

  • West Nile Virus should be considered in patients with unexplained encephalitis and meningitis. (CDC)
  • Laboratory tests include
    • Laboratory samples:
      • Blood serum
      • Cerebrospinal fluid (preferred specimen)
    • Laboratory Tests:
      • Blood sample submitted to the Michigan Department of Community Health, Bureau of Laboratories
      • Arbovirus serology panel detects WNV, SLE, LC, and EEE
      • Measures antibodies (IgM antibodies) produced during the early stage of infection with WNV
Treatment (Humans):
  • Supportive care
    • Lowering fever
    • Avoiding dehydration
    • Decreasing brain swelling
    • Dealing with loss of automatic breathing activity
  • Because West Nile infections are caused by a virus, antibiotics are not an effective treatment.
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Economic Consequences and Disease Eradication
Economic Consequences:
  • Environmental costs: Costs associated with slaughter and disposal of infected/dead animals
  • Public health: Costs associated with the care and treatment of patients
Disease Eradication Efforts:
  • Avian morbidity/mortality surveillance appears to be the most sensitive early detection system for WNV activity
  • Public awareness: public involvement in reporting dead bird sightings
    • In 2001, a toll-free hot line was established for citizens to report dead crows
  • Equine health is an important economic issue: veterinarians, veterinary service societies/agencies, and state agriculture departments are essential elements for any surveillance activities involving equine WNV disease
  • Basic approach:
    • Surveillance
    • Source reduction: Control of watery breeding sites for mosquitoes is the single best way to prevent transmission of this disease
    • Sanitation: by-products of human activities have been a major contributor to the creation of mosquito breeding habitats
    • Water management
    • Chemical control: Use of insecticides
      • Immature(larvicide) or adult stage (adulticide) of the mosquito life cycle
      • Mosquito repellents for use on horses
      • Mosquito foggers for use in barns and stables
      • Veterinarian-approved repellent for dogs
    • Resistance management
    • Biological control: Use of biological organisms, or their by-products, to control pests (larvivorous fish, predaceous fish, predaceous mosquito, predaceous copepods, parasitic nematodes, and fungus)
    • Personal protection: reduce time outdoors, especially at dusk, during mosquito seasons and wear light weight long sleeves and long pants if you are outdoors as well as applying insect repellent on exposed skin or clothing that contains the active ingredient, DEET
  • Agencies involved in eradication efforts:
    • CDC Division of Vector-borne Infectious Diseases
    • The American Control Association
    • The National Pesticide Information Center
    • State and Local Health Departments
WNV and Bioterrorism risks:

According to the CDC, West Nile Virus is considered a Category B bioterrorism agent.
This is due to:

  • Its moderate ease of dissemination
  • The fact that it results in low mortality rates
  • The fact that it requires enhanced diagnostic and surveillance capabilities
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Outbreaks
Recent outbreaks (Animals):

2008-Avian, animal, and mosquito infections were reported to the CDC from Alabama, Arkansas, Arizona, California, Colorado, Connecticut, District of Columbia, Delaware, Florida, Georgia, Iowa, Idaho, Illinois, Indiana, Kentucky, Louisiana, Massachusetts, Maryland, Michigan, Minnesota, Missouri, Mississippi, Montana, North Dakota, Nebraska, New Hampshire, New Jersey, New Mexico, Nevada, New York, Ohio, Oklahoma, Oregon, Pennsylvania, Puerto Rico, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Virginia, Vermont, Washington, Wisconsin, West Virginia, and Wyoming.

2007- Avian, animal, and mosquito infections were reported to the CDC from Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Puerto Rico, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, and Wyoming.

2006- Avian, animal, and mosquito infections were reported to the CDC from Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, and Wyoming.

Outbreaks (Humans):

2005: 25 human cases and one death (in Missouri) reported thus far to the CDC from AZ (3), CA (2), CO (7), GA (1), IN (1), KS (1), MO (1), NM (2), OH (1), SD (5), and TX (1)

2004: 2,539 human cases and 100 deaths reported to the CDC from AL (16), AZ (391; 16 deaths), AR (28), CA (779; 28 deaths), CO (291; 4 deaths), CT (1), DC (2), FL (41; 2 deaths), GA (21, 1 death), ID (3), IL (60; 4 deaths), IN (13; 1 death), IA (23; 2 deaths), KS (43; 2 deaths), KY (7), LA (109; 7 deaths), MD (16), MI (34; 2 deaths), MN (34; 2 deaths), MS (51; 4 deaths), MO (36; 2 deaths), MT (6), NE (53), NV (44), NJ (1), NM (88; 4 deaths), NY (10), NC (3), ND (20; 2 deaths), OH (12; 2 deaths), OK (22; 3 deaths), OR (3), PA (15; 2 deaths), SC (2), SD (51; 1 death), TN (14), TX (176; 8 deaths), UT (11), VA (5; 1 death), WI (12; 2 deaths), WY (10)

2003: human cases reported to the CDC from AL (37), AR (25), AZ (13), CA (3), CO (2,947), CT (17), DC (3), DE (17), FL (94), GA (50), IA (147), ID (1), IL (54), IN (47), KS (91), KY (14), LA (124), MA (17), MD (73), MI (19), MN (148), MO (64), MS (87), MT (222), NC (24), ND (617), NE (1,942), NH (3), NJ (34), NV (2), NY (71), OH (108), OK (79), PA (237), RI (7), SC (6), SD (1,039), TN (26), UT (1), VA (26), WI (17), WV (2), WY (375)

2002: WNV activity identified in 44 states and the District of Columbia; 4,156 human cases reported and 284 deaths; 16,741 dead birds, 6,604 infected mosquito pools, and 14,571 equine cases

2001: Human cases were found in AL, GA, FL, and LA

2000: Human cases were found in PA and MA

1999: The first human cases occurred in NY, NJ, DE, CT, and RI

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Sources and Related Articles
Sources:
  1. Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention
    http://www.cdc.gov/nczved/dfbmd/
  2. Animal and Plant Health Inspection Services, United States Department of Agriculture
    http://www.aphis.usda.gov/
  3. Center for Food Security and Public Health at Iowa State University
    http://www.cfsph.iastate.edu/
  4. CDC. "Fact Sheet: West Nile Virus Infection: Information for Clinicians."
    http://www.cdc.gov/ncidod/dvbid/westnile/resources/fact_sheet_clinician.htm
  5. Arizona Department of Health Services: Vector-Borne & Zoonotic Disease. "Clinician Fact Sheet: West Nile Virus."
    http://www.westnileaz.com/pdf/wnv_clinician_fact_sheet.pdf
  6. CDC Division of Vector-Borne Diseases. "West Nile Virus."
    http://www.cdc.gov/ncidod/dvbid/westnile/
  7. Baylor College of Medicine: Department of Molecular Virology and Microbiology. "Potential Bioterrorism Agents."
    http://www.bcm.edu/molvir/eidbt/eidbt-mvm-pbt.htm
  8. Mayo Clinic staff. "West Nile Virus." Mayo Foundation for Medical Education and Research.
    http://www.mayoclinic.com/print/west-nile-virus/DS00438/METHOD=print&DSECTION=all
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