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West Nile Virus Study Notes

Definition of Basic Terms Relevant to the Study of West Nile Virus

  • Nervous system: Consists of the brain, spinal cord, and peripheral nerves.

  • Meninges: Tissue membranes that surround the brain and spinal cord.

  • Cerebrospinal fluid: Clear fluid that fills the spaces in and around the brain and spinal cord.

  • Neuroinvasive disease: Any disease that invades any part of the nervous system.

  • Encephalitis: Inflammation of the brain.

  • Myelitis: Inflammation of the spinal cord.

Background of West Nile Virus Outbreak in New York
Summer 1999, Queens, New York

  • June-August 1999: Numerous dead birds found.

    • Birds discovered in cages and the wild at both the Bronx Zoo and the Queens Zoo.

  • August 1999: 7 patients admitted to Flushing Queens Hospital with encephalitis symptoms; one patient died.

    • Symptoms included muscle weakness.

    • Deborah Asnis, M.D. reported cases to the New York City Department of Health (NYC DOH).

    • Epidemiologic investigators identified multiple mosquito breeding sites near patients’ residences.

The Epidemiologic Investigation: August-September 1999

  • Samples of cerebrospinal fluid (CSF) and blood sent to:

    • New York State Department of Health (NY State DOH).

    • CDC Division of Vector Borne Disease in Fort Collins, Colorado.

  • Over 400 samples of dead birds sent to the U.S. Geological Survey of National Wildlife Center.

  • Findings indicated the presence of a flavivirus, which is a genus of RNA viruses that includes:

    • Yellow fever.

    • Arthropod-borne viruses (e.g., dengue).

    • Encephalitis-causing viruses (e.g., Japanese encephalitis, St. Louis encephalitis).

Initial Findings of the Epidemiologic Investigation

  • NY State DOH and CDC found evidence of St. Louis encephalitis virus.

  • Increase in cases; total reached 40 with spread to Westchester County, NY.

  • NY State DOH launched a mosquito control program.

  • The FBI investigated potential terrorism connections due to unusual numbers of cases.

Problems with the Initial Conclusions

  • Historical context revealed only 9 cases of St. Louis encephalitis were documented in NY State.

  • St. Louis encephalitis was historically absent from New York City.

  • The virus was not involved in any national outbreak.

  • St. Louis encephalitis had never been documented in birds, and symptoms included muscle weakness not seen in St. Louis encephalitis cases.

  • PCR tests at both NY State DOH and CDC returned negative for St. Louis encephalitis.

Definitive Studies

  • Brain samples analyzed by Dr. Ian Lipkin at UC Irvine Medical School.

    • Genomic sequencing and PCR testing confirmed West Nile virus, first observed in North America during this outbreak.

  • The Pasteur Institute provided West Nile genome sequences to CDC and UC Irvine lab confirming the virus in both bird and human cases.

  • By the end of September 1999:

    • 15 out of 15,000 mosquito pools tested positive for West Nile virus.

    • Cases increased to 62 human cases, 7 fatalities, and approximately 17,000 dead birds identified (1/3 being crows).

    • Primary mosquito vectors were Culex species, with some Aedes identified.

Geographic Distribution of West Nile Virus
Prior to 1999

  • West Nile virus first reported in 1937 in a patient from Uganda.

  • Outbreaks recorded in Israel during the 1950s.

  • Geographical limitations included Africa, the Middle East, West Asia, and Mediterranean nations.

Since 1999

  • West Nile virus spread across:

    • All 48 continental U.S. States and Washington, D.C.

    • Canada, Mexico, Central America, South America, the Caribbean, and every continent except Antarctica.

Definition of Emerging Infectious Diseases (NIAID)

  • Emerging infectious diseases are defined as:

    • Newly appearing in a population (e.g., COVID-19).

    • Existing diseases rapidly increasing in incidence or geographic range (e.g., West Nile infection).

West Nile Viral Infection in Birds

  • Primary reservoir: Avian; virus detected in over 300 bird species.

  • Viral transmission cycle involves:

    • Migratory birds carrying the virus globally.

    • Bird-mosquito-bird cycle continues due to high virus titers in infected birds.

  • Symptoms in birds can vary from asymptomatic to severe neuroinvasive disease.

Introduction of West Nile Virus to New York City in 1999

  • Outbreak coincided with proximity to two international airports in Queens.

  • Infected humans are unlikely to be the virus source because of low titers and brief presence in blood relative to birds.

  • Testing revealed that 33% of 430 birds tested positive for the virus, indicating established bird-mosquito-bird cycle prior to human cases.

Current Epidemiology of West Nile Viral Infection

  • Most widely distributed of all arboviruses; largest cause of arboviral encephalitis.

  • Now endemic to North America; causes sporadic infections and larger epidemics:

    • 19 neuroinvasive cases reported in 2000.

    • 64 in 2001.

    • 2,946 in 2002.

    • 2,866 in 2003.

    • 2,873 in 2012.

    • 1,335 in 2024.

  • From 1999 to 2024: 60,992 cases reported to CDC; 31,806 neuroinvasive disease cases reported; 3,134 fatalities attributed to West Nile (2,992 with neuroinvasive disease).

  • Estimated infection rates suggest approximately 4.45-7.814 million infections in total from 1999 to 2024, calculated as only 1/140-1/256 individuals with West Nile develop neuroinvasive disease.

Epidemiology for 2025

  • As of September 16, 2025, there were 986 reported West Nile cases:

    • 630 neuroinvasive infections.

    • Cases reported across 40 states.

Seasonality and Transmission Factors of West Nile Infection

  • Spring emergence: Mosquitoes emerge; the bird-mosquito-bird amplification cycle peaks in early fall.

  • Human infections peak in late August to September due to the cycle.

  • Sporadic cases occur throughout the year in southern states.

  • Factors such as temperature and rainfall affect mosquito populations.

  • Reasons for fewer cases in Central/South America and the Caribbean remain unclear; possible protection from cross-exposure to other viruses.

Infection in Human Beings

  • Transmission avenues include:

    1. Bite from infected mosquito.

    2. Receipt of infected blood donation or organ transplant.

    3. Transplacental transmission (from mother to fetus).

    4. Infection through breast milk.

  • Virus entry through the skin, followed by migration to lymph nodes and potentially the bloodstream and nervous system.

  • Entry mechanism into the CNS: Not well understood.

Clinical Illness from West Nile Infection in Humans

  • Incubation period: 2-14 days.

  • Symptoms:

    • 80% of infections are asymptomatic.

    • 20% exhibit West Nile fever.

    • 1/150-1/250 may develop neuroinvasive disease.

    • Approximately 10% mortality rate in neuroinvasive cases.

West Nile Fever Symptoms

  • Commonly reported symptoms include:

    • Fever, headache, malaise, back pain, and myalgias (muscle aches).

    • Anorexia (loss of appetite).

    • Eye pain, nausea, vomiting, diarrhea can also be observed.

  • Duration: Acute symptoms last 3-10 days.

  • Rash:** Observed in 25-50% of cases, usually at the end of the febrile period, lasting one week.

  • Prognosis: Self-limited illness leads to life-long immunity. Individuals with rash exhibit a smaller likelihood of developing neuroinvasive disease.

West Nile Neuroinvasive Disease

  • Presenting symptoms: Fever with meningitis, encephalitis, myelitis, or a mixed pattern.

  • Risk factors for neuroinvasive disease:

    • Age: 30 times more likely in patients aged 80-90 compared to those under 10.

  • Meningitis:

    • Symptoms include fever, headache, stiff neck, photophobia.

    • More common in children.

  • Encephalitis:

    • More prevalent in older adults.

    • Symptoms include confusion, tremor, rigidity, coma, muscle weakness, paralysis, seizures, ataxia (incoordination), and facial paralysis.

    • Death rate of approximately 10% among those with neuroinvasive disease.

    • Survivors may experience lingering cognitive issues for over a year.

  • Myelitis:

    • Symptoms mimic poliomyelitis, affecting anterior horn cells leading to limb paralysis.

    • Partial respiratory failure may occur due to muscle innervation.

    • Prognosis: A third of affected individuals improve fully; a third partially; a third do not recover.

MRI Findings in West Nile Neuroinvasive Disease

  • Typically shows elevated CSF protein levels and moderate pleocytosis (increased white blood cell count).

    • Normal CSF cell count: 0-5 cells/mL.

    • Changes often noted in the deep structures of the brain may be seen, although MRI may also appear normal.

Diagnosis of West Nile Infection

  • Key diagnostic marker: Presence of IgM (immunoglobulin M) antibodies in serum and/or CSF.

  • Diagnostic methods used:

    • MAC-ELISA (IgM antibody capture enzyme-linked immunosorbent assay).

    • PCR testing for neuroinvasive disease, noting low sensitivity rates (15% for blood; 55% for CSF).

    • PRNT (plaque reduction neutralization test) used when cross-reactivity concerns arise due to prior flavivirus infections or vaccinations.

Differential Diagnosis for West Nile Infection

  • Must consider infections with other viruses that produce similar neurologic signs:

    • Possible conditions include:

    • St. Louis Encephalitis (SLE).

    • Japanese Encephalitis.

    • Dengue.

    • Herpes simplex virus.

    • Enteroviruses (summer prevalence).

    • Powassan virus.

    • Lyme disease.

    • Tick-borne illnesses (anaplasmosis, Rocky Mountain spotted fever).

    • Bacterial meningitis/brain abscess.

    • Guillain Barre syndrome.

  • Diagnosis aided by:

    • Temporal context (season and year).

    • Travel and exposure history of patients.

    • Vaccination history of patients.

    • Clinical examination results.

    • History of illnesses in contacts.

Treatment and Prevention of West Nile Infection

  • Treatment generally supportive (no specific antiviral therapy exists).

  • Prevention strategies involve:

    • Mosquito avoidance measures.

    • Public health campaigns for mosquito control.

Zika Virus Overview
Introduction: Summer 2015, Recife, Brazil

  • Surge in microcephaly cases noted among infants born to mothers experiencing mild illnesses with rash.

Environmental Context in Recife, Brazil

  • Features include many deep trenches for water collection, slow-moving water, large blue containers for water storage, and homes lacking screens.

  • Hot, humid conditions benefit mosquito populations.

Causes and Associated Flaviviruses

  • Flaviviruses include:

    • Yellow Fever.

    • Dengue.

    • Japanese Encephalitis.

    • West Nile Encephalitis.

    • St. Louis Encephalitis.

    • Zika.

    • Others.

    • Most flaviviruses are arthropod-borne, RNA viruses averaging 50 nm in diameter.

Emergence of Zika Virus

  • 1947-1953: Zika virus isolated in Uganda from a rhesus macaque during yellow fever studies; detected in Aedes africanus mosquitoes.

  • Prevalence: Antibodies found in 6.1% of Ugandan residents; additional seroprevalence studies across multiple regions.

  • 1952: First confirmed infections diagnosed in Uganda/Tanzania; subsequent years resulted in sporadic mild cases reported.

Zika Outbreak History

  • 2007: Yap State, Federated States of Micronesia reports 5,000 infections of a 6,700 population.

  • 2013-2014: French Polynesia outbreak involved 32,000 evaluations with an attack rate of 66%; 42 cases of Guillain-Barre syndrome observed.

  • 2015-2016: Local transmission reported in Brazil starting in May 2015, escalating with 1.3 million estimated cases and linked to more than 5,000 cases of microcephaly noted.

  • February 1, 2016: World Health Organization declared a Public Health Emergency of International Concern regarding Zika due to its association with birth defects.

Zika Virus Cases in the United States

  • Since January 2016: Notable cases linked to Zika-associated congenital issues reported; outbreak led to increased local transmission in U.S. territories.

  • Breakdown of cases as of November 9, 2016:

    • Travel-associated cases: 4,035.

    • Local cases: 1.

    • Sexually transmitted cases: 34.

    • Guillain-Barre cases: 13.

    • U.S. Territories had similar reporting with 31,093 local cases documented.

Zika Virus Transmission Cycle

  • Mosquito vectors responsible include:

    • Aedes aegypti: the yellow fever mosquito.

    • Aedes albopictus: the Asian tiger mosquito.

  • Other transmission methods:

    • Maternal transmission during pregnancy.

    • Transmission during delivery.

    • Sexual contact with an infected partner (both symptomatic and asymptomatic).

    • Blood transfusions from infected donors.

    • Rare cases documented via percutaneous needle stick injuries.

Clinical Features of Zika Virus Infection

  • Incubation period: 2-14 days.

  • Symptom profile includes:

    • 75-80% cases are asymptomatic.

    • Mild illness typically lasts 2-7 days.

    • Occasionally requires hospitalization.

  • Reported symptoms include:

    • Low-grade fever (65%).

    • Macular rash (90%).

    • Muscle aches (48%).

    • Joint pain (60%).

    • Headaches (45%).

    • Retro-orbital pain (39%).

    • Conjunctivitis (55%).

  • Complications from Zika Virus Infection:

    • Meningoencephalitis, myelitis, Guillain-Barre syndrome.

    • Fetal anomalies, including miscarriage, stillbirth, microcephaly, and developmental impairments.