What To Know About West Nile As Officials Warn Against Deadly Disease At Start Of Mosquito Season

Culex mosquitoes are known vectors for the West Nile virus, which regularly rears its head in the U.S.

Health officials across the continental U.S. are starting to warn about the annual return of West Nile virus, a potentially lethal human disease without treatments or vaccines that is rearing its head earlier than usual as the changing climate makes the environment more hospitable for the mosquitoes that spread it.

KEY FACTS

The West Nile virus is primarily transmitted through the bites of infected mosquitoes and is the leading cause of mosquito-borne disease in the continental United States, according to the Centers for Disease Control and Prevention, infecting between roughly 1,000 and 10,00 people each year for the last 20 years.

The virus naturally circulates among birds and mosquitoes but sometimes infects other mammals, notably humans and horses, who can get sick but act as dead-end hosts for the pathogen because they cannot pass it back to mosquitoes biting them.

The vast majority of people infected with West Nile — eight out of every 10 people — do not develop any symptoms, but the 20% of people that do will develop West Nile virus disease or West Nile fever, which includes a fever alongside an array of other symptoms like headache, joint pains, nausea, diarrhea, vomiting, fatigue or a rash.

Most people with febrile illness will recover from West Nile completely, though fatigue and weakness may last for weeks or months, but a small number of people who are infected will develop serious and potentially fatal illness affecting the central nervous system with possible symptoms including confusion, muscle weakness, inflammation of the brain (encephalitis) or membranes surrounding the brain and spinal cord (meningitis), seizures, paralysis and coma.

Approximately one in 150 infections leads to serious illness, though the risk is greater for people over the age of 60 years old (around 1 in 50 infected people) and for those with certain medical conditions like cancer, diabetes, high blood pressure and with compromised immune systems.

Roughly one in 10 people who develop severe West Nile affecting the central nervous system will die from the disease and for those who don’t, recovery take weeks or months and many will experience long-lasting neurological symptoms from the infection such as memory loss, depression, hearing loss, muscle weakness and motor issues like difficulty walking. 

WHAT HAPPENS IF YOU’RE INFECTED AND HOW DO YOU PREVENT WEST NILE?

West Nile is typically diagnosed by a number of tests a clinician may order if disease is suspected based on symptoms and likely exposure to the kinds of mosquitoes that carry the virus. Such tests could include blood tests that check for West Nile virus antibodies or a lumbar puncture (spinal tap) to test cerebrospinal fluid. There are no specific treatments for West Nile in humans and in most cases doctors will provide supportive therapy to treat specific symptoms, such as pain relief, IV fluids, ventilators and hospital admission for intensive care. According to the CDC, a variety of drugs have been tested for use against West Nile but none have demonstrated specific benefit so far. Similarly, there are no vaccines to prevent West Nile in humans — there are several licensed for use in horses — and clinical trials have not progressed to late stages in humans. Previous infection is believed to confer lifelong immunity against getting West Nile disease again, according to the CDC, which says the best way of preventing West Nile is to avoid getting bitten by an infected mosquito. The agency recommends insect repellant, wearing long-sleeve shirts and pants and taking steps to control mosquitoes indoors and outdoors through means like water management and biological controls as the best ways of preventing mosquito bites.

TANGENT

No human-to-human transmission of West Nile following casual contact has been documented, according to the World Health Organization. However, the health agency said rare infections have been recorded between humans during exceptionally close contact such as mother-to-child transmission during pregnancy, delivery or breastfeeding, and through blood transfusions and organ transplants. Such cases can be minimized through public health messaging and implementing restrictions on blood and organ donation during outbreaks in affected areas, the WHO said.

NEWS PEG

West Nile is found worldwide, including the U.S. It was first isolated from a woman in Uganda’s West Nile district in 1937 and mosquitoes carrying it were first found in the U.S. in the late 90s, when a strain circulating in Tunisia and Israel was imported to New York, sparking what the WHO describes as a “large and dramatic outbreak” that spread across continental parts of the country. Since being introduced to the U.S., the WHO said the West Nile virus has become “widely established from Canada to Venezuela,” adding to its original range across Africa, parts of Europe, the Middle East, West Asia and Australia. West Nile is most common in the U.S. during periods of high activity for the Culex species of mosquito largely responsible for transmitting the virus, which is around summer and fall, and health agencies across the country have recently started issuing their annual warnings for the disease and the mosquitoes that carry it. Such warnings have been happening weeks earlier than usual on account of the milder winter and spring being more favorable for mosquitoes, bringing an early mosquito season.

NEWS PEG

On Tuesday, the Illinois Department of Public Health said the virus had been detected in 13 counties including Woodford, Fulton and LaSalle according to testing in birds and mosquitoes, though no human cases have been reported yet. In early June, Los Angeles County also reported its first detection of mosquitoes carrying the virus for the year, which was also earlier than typical. Public health official Steve Vetrone said it should serve as a “critical reminder for all residents to take preventative actions.”

BIG NUMBER

7. That’s how many West Nile virus disease cases have been reported in humans across the U.S. this year, according to data from the CDC. These were reported in six different states — Michigan, Maryland, Tennessee, Kansas (2 cases), Arkansas and Arizona — and five infections resulted in neuroinvasive disease.

WHAT TO WATCH FOR

This year’s figures come from the very start of the mosquito season and numbers can be expected to climb significantly as the year goes on. Last year, the CDC says there were more than 2,500 human disease cases of West Nile, including 1,840 hospitalizations and 182 deaths. More than half of cases were reported in August, with the rest largely reported in September and July. In the future, experts fear human-caused climate change could worsen the spread of West Nile and a host of other mosquito-borne illnesses by creating ideal conditions for the mosquitoes that spread it.