So far, this has been the worst year on record for West Nile virus infections, says the Centers for Disease Control and Prevention (CDC). 48 states have reported infections in humans, birds and/or mosquitoes. 1,993 people have been confirmed nationwide with WNV (West Nile virus) infection, of whom 87 have died. 54% (1,069) were classified as neuroinvasive disease and 924 as non-neuroinvasive disease. In neuroinvasive disease, the patient went on to develop encephalitis or meningitis.

Up to September 4th, more cases have been reported this year than ever before, says the CDC. West Nile virus first infected humans in the USA in 1999.

Six states make up over 70% of all cases – Michigan, Louisiana, Oklahoma, Mississippi, South Dakota and Texas. Nearly 45% of all reported cases in the USA come from Texas.

Below is a breakdown by states of West Nile virus human infections reported to ArboNET, as of September 4th, 2012 (Source: CDC):
  • Alabama
    Total cases 16. Deaths 1
  • Arizona
    Total cases 25. Deaths 1
  • Arkansas
    Total cases 31. Deaths 3
  • California
    Total cases 55. Deaths 2
  • Colorado
    Total cases 23. Deaths 0
  • Connecticut
    Total cases 6. Deaths 0
  • Delaware
    Total cases 1. Deaths 0
  • District of Columbia
    Total cases 1. Deaths 1
  • Florida
    Total cases 19. Deaths 0
  • Georgia
    Total cases 22. Deaths 3
  • Idaho
    Total cases 4. Deaths 0
  • Illinois
    Total cases 48. Deaths 2
  • Indiana
    Total cases 26. Deaths 3
  • Iowa
    Total cases 7. Deaths 0
  • Kansas
    Total cases 24. Deaths 1
  • Kentucky
    Total cases 3. Deaths 0
  • Louisiana
    Total cases 73. Deaths 6
  • Maryland
    Total cases 14. Deaths 1
  • Massachusetts
    Total cases 4. Deaths 0
  • Michigan
    Total cases 95. Deaths 5
  • Minnesota
    Total cases 41. Deaths 1
  • Mississippi
    Total cases 113. Deaths 4
  • Missouri
    Total cases 6. Deaths 1
  • Montana
    Total cases 1. Deaths 0
  • Nebraska
    Total cases 29. Deaths 1
  • Nevada
    Total cases 0. Deaths 0
  • New Hampshire
    Total cases 1. Deaths 0
  • New Jersey
    Total cases 8. Deaths 0
  • New Mexico
    Total cases 9. Deaths 0
  • New York
    Total cases 26. Deaths 3
  • North Carolina
    Total cases 4. Deaths 2
  • North Dakota
    Total cases 35. Deaths 0
  • Ohio
    Total cases 53. Deaths 1
  • Oklahoma
    Total cases 113. Deaths 7
  • Oregon
    Total cases 1. Deaths 0
  • Pennsylvania
    Total cases 14. Deaths 1
  • South Carolina
    Total cases 8. Deaths 0
  • South Dakota
    Total cases 119. Deaths 2
  • Tennessee
    Total cases 10. Deaths 0
  • Texas
    Total cases 888. Deaths 35
  • Utah
    Total cases 2. Deaths 0
  • Virginia
    Total cases 5. Deaths 0
  • West Virginia
    Total cases 2. Deaths 0
  • Wisconsin
    Total cases 6. Deaths 0
  • Wyoming
    Total cases 2. Deaths 0
  • Total Nationwide
    Total cases 1,992. Deaths 87

Below are some common questions regarding West Nile virus:
  • What is West Nile virus? It is a flavivirus which is common in the Middle East, West Asia and Africa, and is closely related to St. Louis encephalitis virus, which exists in the USA. West Nile virus can infect horses, bats, cats, chipmunks, skunks, squirrels, domestic rabbits, other mammals including humans, birds, and mosquitoes.

  • What are West Nile encephalitis, West Nile meningitis and “neuroinvasive disease” and West Nile fever? When West Nile virus affects the person’s nervous system, it is called neuroinvasive disease. Neuroinvasive disease types include:

    – West Nile encephalitis (inflammation of the brain)
    – West Nile meningitis (inflammation around the brain and the spinal cord)
    – West Nile meningoencephalitis (inflammation of the brain and the membrane around the brain)

    West Nile Fever occurs when an infected person develops fever, aches, a skin rash, fatigue and headache. Usually illness lasts for just a few days, but can sometimes persist for weeks, even among healthy people.

  • When was the USA first affected by West Nile virus? Experts are not sure. The first reported case of a human infection was in 1999.

  • West Nile virus was found in “overwintering” mosquitoes in New York City in 2000. What does “overwintering” mean? The Culex species of mosquito that can carry West Nile virus survived the winter in New York in the adult stage. The virus survived inside those overwintering mosquitoes that year.

  • Is West Nile virus’ presence in North America temporary? – No, it has become permanently established in the Western Hemisphere.

  • Is West Nile virus human infection seasonal? – It depends on where you live. In temperate parts of the world, human infections generally occur in the late summer and early autumn, while in southern climates people can become infected throughout the year.

  • What is the death rate among people with severe illness caused by West Nile virus? According to the CDC, case-fatality rates range from 3% to 15%. Highest rates are found in elderly patients with severe illness. The CDC emphasizes that less than 1% of infected people develop severe illness. In the majority of human infections, there are no symptoms.
West Nile Virus can persist in humans – in some susceptible people, West Nile virus may persist in the kidneys. Much more rarely, it may persist in the brain, spinal fluid and blood, especially among those with compromised immune systems. According to the CDC, West Nile virus will only persist in people who had encephalitis during their initial illness.

Humans become infected with West Nile virus through the bite of an infected mosquito. Mosquitoes bite infected birds and become infected themselves. Mosquitoes cannot become infected, experts say, when they bite other types of infected animals, because their viral load is lower birds’. After biting an infected bird, the virus circulates in the mosquito’s blood for a few days, and then settles in the insect’s salivary glands. When the mosquito bites an animal, bird or human, the virus can be injected into the host, where it might multiply and cause illness.

People may also become infected through blood transfusions and transplanted organs, however, these routes are rare. Even rarer is mother-to-child transmission through breast milk.

Written by Christian Nordqvist