Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

West Nile Symptoms Often Go Undetected

Dear Doctor: Amid all the talk about the Zika virus, I thought we'd stopped worrying about West Nile. Now I see that it's back -- or never left. What are the risks of this? Are they greater than Zika?

Dear Reader: You are correct; West Nile virus continues to be a threat in the United States. Forty-eight states and the District of Columbia have reported cases of West Nile; overall, the U.S. reported 43,937 cases of the virus and 1,911 related deaths between 1999 and 2015. Because many people have only mild symptoms and don't seek medical attention, the actual number of cases is undoubtedly much higher.

The species of West Nile virus in the U.S., which was first detected in New York City in 1999, is believed to have originated in the Middle East. It's transmitted by mosquitos, which starts the disease cycle by infecting birds. There, the virus multiplies within its avian hosts. When another mosquito draws blood from an infected bird, the mosquito then can infect another bird, or a human. Because transmission relies on mosquitos, most cases of West Nile virus occur in the summer and early fall months, when the insects are most plentiful. Humans rarely transfer the virus among themselves; when they do, it's because infected mothers pass the virus to their child in the womb.

The majority of people with West Nile virus infection don't have symptoms; in fact, symptoms are seen in only 20 to 40 percent of cases. Further, early symptoms (fever, muscle aches, headache, fatigue and rash) are not much different from those of many other viruses, so the true cause may go undiagnosed. These symptoms usually last from three to 10 days, but some people report fatigue, muscle aches and difficulty concentrating for up to 30 days or longer after contracting West Nile virus.

In less than 1 percent of people, West Nile virus invades the nervous system. There it can cause inflammation of the brain and surrounding tissues, leading to confusion and even coma or death. The virus can also enter the peripheral nerves, causing muscle paralysis; sometimes, this paralysis involves the respiratory muscles, leaving some people unable to breathe on their own. The death rate when West Nile virus invades the nervous system is 10 percent. Those at higher risk of death from West Nile virus include older adults; people with diabetes, heart disease or chronic hepatitis C; people with a depressed immune system; and people who abuse alcohol.

Patients who survive West Nile virus' assault on the nervous system can have prolonged symptoms. One-third of those with paralysis related to the virus will fail to improve, and if the brain is affected, people can experience significant difficulties with brain function even after a year. One study found that 40 percent of patients reported fatigue and weakness up to eight years after the infection.

As for treatment, anti-viral medications appear to have some effect against West Nile, especially when the drugs are used early in the disease. However, no large human studies have assessed their efficacy. The best way to avoid West Nile virus is by draining areas of standing water or using mosquito repellants.

While other viruses, such as Zika, are understandably worrisome, don't ignore the possibility of infection with West Nile. In the continental United States, Zika transmission is much rarer than West Nile transmission.

(Send your questions to, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)