The biggest West Nile virus outbreak in the United States occurred right here in Arizona in the summer of 2021. Pima, Pinal and Maricopa counties reported 1,693 cases, and we do not know how many mild cases were not identified or reported. The United States total reported cases for the same time period was 2,911.
West Nile is a viral illness that is spread by mosquitoes and is the leading cause of mosquito-borne disease in the United States. It is also commonly found in Europe, Africa, the Middle East and West Asia. It affects humans, horses and birds and causes mild to very severe illness. The virus is most often seen in late summer to early fall in temperate climate zones but can occur year round in warmer southern climates. Our wetter-than-average 2021 monsoon season contributed to the large outbreak due to creating more standing water and, thus, breeding grounds for mosquitoes.
West Nile virus is zoonotic, meaning it can be transmitted from animals to people. It was first detected in the United States in 1999, and the most common theory is that it arrived via an infected bird. It is thought to have come from Central Africa during the 1990s and has become endemic throughout much of the world since then.
West Nile is common in some birds, such as sparrows and robins. They become carriers without being sick. Other birds such as jays and crows often die from the infection. This virus is spread through the bite of an infected female mosquito, which previously bit an infected bird. That mosquito can infect several people and other birds as well, increasing the circulating virus in a given region. Health departments often test dead birds for the virus to determine the risk of the virus in human communities.
West Nile virus symptoms are similar to those of the flu. Headache, fever, fatigue, nausea, muscle or joint aches, malaise and sometimes a stiff neck are common and usually mild, lasting only a few days. Some people might develop a generalized, non-itchy rash on the torso, arms and legs (but not hands and feet), but this could pass within a day and is usually accompanied by a fever. Most people will not need to seek medical care for mild illness.
Most of the reported cases are for the more serious, even fatal, illness that occurs in approximately 1 person in 150. The virus can invade the nervous system, resulting in meningitis, an inflammation of the protective membranes covering the spinal cord and brain, and encephalitis, a swelling or inflammation of the brain. This is called the neuroinvasive form of the disease and can cause long-term physical and mental disabilities. Muscle weakness, memory loss, cognitive compromise and increased difficulties managing activities of daily living can result from neuroinvasive West Nile virus illness.
Older adults and those with underlying chronic health issues or an impaired immune system are most at risk. Of those 1,693 reported cases in the Arizona outbreak, 1,117 had neuroinvasive disease and 121 people died (as of March 28, 2022). Though very rare, the virus can spread through a blood transfusion, organ transplantation, and a mother’s placenta to her unborn baby. Transmission has occurred through skin and mucosal surfaces in laboratory workers in occupational settings, though this also is rare.
Since 1999, more than 55,000 cases of West Nile virus infection have been reported in the United States, according to the Centers for Disease Control and Prevention (CDC). This includes 2,700 cases of neuroinvasive disease and 2,600 deaths. The good news is that once infected, most people will develop a lifelong immunity to this disease.
There is no vaccine or antiviral medication to combat West Nile virus. The only preventive measure is to avoid mosquito-prone areas and wear protective clothing and insect repellent. It is best to avoid gardening or hiking at dawn or dusk when mosquitoes are most active. Remove standing water where mosquitoes can breed. Community mosquito control and eradication measures are moderately effective.
The New England Journal of Medicine, May 4, 2023, has an interesting opinion article by Carolyn Gould, M.D., et al., calling for the development of a vaccine.
“Our experience over the past two decades has demonstrated that current prevention strategies are not enough to reduce the ongoing disease burden. West Nile Virus vaccination would be more effective in preventing disease and related deaths,” she wrote. J. Erin Staples, M.D., Ph.D., an epidemiologist at the CDC, agrees.
She stated, “Our current prevention messages, using insect repellent and wearing long-sleeved shirts and long pants, are not really enough to reduce the ongoing burden. A West Nile vaccine is really what we think would be most effective at preventing West Nile related disease and deaths”.
She suggested that targeting people in high-risk areas with a vaccine (as opposed to a national vaccination campaign) could reduce deaths by 30%. There have been vaccine trials that show promise, but we are a long way from development and distribution. Even if we do find effective treatment, awareness and prevention remain our best resources.