Which States Have Confirmed Cases, What Are the Symptoms?

West Nile virus was first confirmed in the United States in 1999, and in 2018, all but three of the lower 48 states have seen infections in either humans or animals.

The Centers for Disease Control and Prevention (CDC) shared a color-coded map that showed states that had human infections in dark green, states that had only nonhuman activity in light green and states with no activity in white. The only three states in the lower 48 to have no documented West Nile virus activity this year are Arkansas, Maine and Wyoming.

Human infections were found in 37 states and the District of Columbia. This includes presumptive blood donors, meaning people who showed no signs of symptoms when they donated blood but whose donation tested positive for the virus.

The states with the highest number of cases were identified by the CDC as:

  • South Dakota: 40 cases
  • Louisiana: 25 cases
  • Mississippi: 23 cases
  • California: 21 cases
  • Texas: 17 cases

South Dakota also had the highest number of deaths, with two people having lost their lives to West Nile virus.

Michigan, Montana and New Mexico have not had any confirmed cases, but at least one person in each state was identified as a presumptive viremic blood donor.  

A color-coded map of West Nile virus activity by state as of Tuesday. Centers for Disease Control and Prevention

Nevada, Idaho, Washington, West Virginia, New Hampshire, Vermont and Massachusetts have not had any confirmed cases of West Nile virus in human beings, but they have identified it in infected mosquitoes, birds, sentinel animals or veterinary animals.

States that have identified infected mosquitoes have issued warnings to residents to be cautious when they’re in a situation where they could be bitten.

From the first case in 1999 to 2016, California had the most cases of West Nile, with over 6,000 people becoming infected. Of the lower 48 states, Maine had the least. Alaska is the only state in the United States to have never had a resident become infected with West Nile. The deadliest year for West Nile was 2012, with 286 deaths, followed closely by 2002, when 284 people died.

West Nile virus is spread through mosquitos, and about 80 percent of people who get infected with West Nile virus never experience symptoms, according to the CDC. Symptoms of the virus include headache, body aches, joint pains, vomiting, diarrhea and rash. Of the 20 percent who develop these symptoms, most recover fully but can take weeks or months to stop feeling fatigued and weak.

Less than 1 percent of people who are infected with West Nile develop illnesses that can severely affect the central nervous system, including encephalitis or meningitis. The virus can cause serious complications for anyone, but it poses more of a risk to people over 60 years old. Effects to the central nervous system, including paralysis, can be permanent, and about 10 percent of people who develop a severe illness die from it.

west nile virus by age Average annual incidence of West Nile virus reported to the U.S. Centers for Disease Control and Prevention by age group from 1999 to 2016. Centers for Disease Control and Prevention

There is no specific vaccine or treatment for West Nile virus, but people will often take over-the-counter pain relievers to help combat fever and other symptoms. In severe cases, the person may be hospitalized.

Not every mosquito is carrying the virus, but the best way to prevent contracting West Nile virus is to reduce the likelihood of being bitten. When possible, cover up with long-sleeved shirts and pants to limit the amount of skin that’s exposed. Whenever outside, people can reduce their risk by using insect repellent.

To prevent mosquitoes from coming indoors, the CDC recommended that people run the air conditioning instead of opening windows, or install screens on windows and doors.

Since mosquitoes lay eggs in standing water, the CDC also advised people to empty and scrub any items that hold water, such as planters, flowerpot saucers and trash containers. Containers holding water should be covered with either a lid or mesh made of holes smaller than a mosquito, the CDC said.

Most cases of West Nile occur during summer and fall.