Eighteen West Nile Virus (WNV) human infections have now been diagnosed in seven states, according to the Centers for Disease Control and Prevention (CDC). Twelve of those cases are severe types of the virus, known as neuroinvasive disease, West Nile poliomyelitis or West Nile encephalitis or meningitis. A total of 23 states have reported finding the virus in birds and mosquitoes, as well as humans.
In 2013 there were 2,469 cases of WNV reported, 1,267 of which were neuroinvasive. 379 cases were in California, 322 in Colorado. There were 119 deaths in the year. California is leading the tally again this year, with 9 total cases reported as of July 8.
Commonly found in Europe, Africa, North America, West Asia and the Middle East, WNV travels through a cycle of transmission between mosquitoes and birds. Other mammals, such as humans and horses, can be infected. The first human case was isolated in 1937 in a woman in the West Nile district of Uganda. In 1953 it was found in birds in the Nile delta region. Human West Nile Virus infections have been reported for over 50 years in many countries. It was first introduced into the U.S. in 1999.
The largest outbreaks of the virus tend to occur near the major migratory routes for birds. The four major North American flyways, or bird migration routs, are the Pacific, the Central, the Mississippi and the Atlantic. Migration is generally thought of as north-south, with heavier traffic following coasts, mountain ranges and major rivers.
Human infections of WNV are now reported in Arizona, California, Mississippi, Missouri, South Dakota, Tennessee and Texas. One death has been reported. Transmission of West Nile Virus to humans is most often as a result of bites from mosquitoes, which contract the virus after feeding on infected birds. The virus eventually gets into the mosquito’s salivary glands, where it can be injected into humans and animals. No human-to-human transmission has been reported through casual contact, although there have been a small number of infections through blood transfusions, organ transplant and breast milk.
Approximately 80 percent of people infected with WNV show no symptoms. Three of the 18 West Nile Virus infections discovered so far in the seven states were identified in people with no symptoms of disease who donated blood, which tested positive when screened for the presence of the disease.
Twenty percent of people infected with WNV go on to develop West Nile Fever, experiencing fever, fatigue, body aches, nausea and headache. About 1 percent of infected people develop West Nile Encephalitis, a serious disease that infects the brain and has a mortality rate of 4-15 percent. Symptoms of West Nile Encephalitis, or neuroinvasive disease, are similar but also include neck stiffness, disorientation, tremors, coma, convulsions and paralysis. People over age 50 are at the highest risk of becoming severely ill. Three of the current seven cases are the neuroinvasive form of the disease.
West Nile Virus also infects other mammals, such as horses, which, like humans, are considered “dead-end” hosts, meaning they do not spread the disease. WNV outbreaks in animals usually happen before human cases begin to appear, so monitoring cases in birds and hoses can provide early warnings to public health officials. Vaccines are available for horses, but there is no immunization available for humans.
In 2013 the most common cause of neuroinvasive disease in the U.S. was WNV. The CDC says data for the number of patients infected depends on clinicians identifying and reporting the disease. Extrapolating from the 2013 reported cases they say that as many as 38,000-88,500 nonneuroinvasive cases of West Nile Virus may have occurred in 2013.
By Beth A. Balen