No human in North or South America had ever suffered from it before. The United States is home to many viruses that make people sick. Some are old and some are new. When the first humans made their way into the Western Hemisphere some fifteen thousand years ago, they brought a number of viruses with them.
Human papillomavirus, for example, retains traces of its ancient emigration. The strains of the virus found in Native Americans are more closely related to each other than they are to HPV strains in other parts of the world. Europeans brought viruses causing diseases such as influenza and smallpox that wiped out most Native Americans.
In later centuries, still more viruses arrived. It had only been six decades since West Nile virus was discovered anywhere on the planet. In , a woman in the West Nile district of Uganda came to a hospital with a mysterious fever, and her doctors isolated a new virus from her blood.
Over the next few decades, scientists found the same virus in many patients in the Near East, Asia, and Australia. But they also discovered that West Nile virus did not depend on humans for its survival. Researchers detected the virus in many species in birds, where it could multiply to far higher numbers. At first it was not clear how the virus could move from human to human, from bird to bird, or from bird to human.
That mystery was solved when scientists found the virus in a very different kind of animal: mosquitoes. As the mosquito drinks, it squirts saliva into the wound.
Along with the saliva comes the West Nile virus. Virus-laden immune cells crawl into the lymph nodes, where they release their passengers, leading to the infection of more immune cells. From the lymph nodes, infected immune cells spread into the bloodstream and organs such as the spleen and kidneys. It takes just a few days for the viruses in a mosquito bite to multiply into billions inside a bird. Despite their huge numbers, West Nile viruses cannot escape a bird on their own.
They need a vector. An mosquito must bite the infected bird, drawing up some of its virus-laden blood. Once in the mosquito, the viruses invade the cells of its midgut. Vector-borne viruses like West Nile virus require a special versatility to complete their life cycle. Mosquitoes and birds are profoundly different kinds of hosts, with different body temperatures, different immune systems, and different anatomies. West Nile virus has to be able to thrive in both environments to complete its life cycle.
Thanks to their versatility, vector-borne viruses also pose special challenges to doctors and public health workers who want to stop their spread. Mosquitoes, in effect, give the viruses wings. Studies on the genes of West Nile virus suggest that it first evolved in Africa.
As birds migrated from Africa to other continents in the Old World, they spread the virus to new bird species. Along the way, West Nile virus infected humans. Severe CNS disease continued to be predominantly seen in older individuals, but more cases of milder febrile illness in younger patients were detected, possibly as a result of enhanced surveillance efforts. The factors contributing to the magnitude of the epidemic remain unclear, but it is interesting to note that in several areas of the country, climatic and geographic factors during the Spring and Summer of were very similar to those in , when a large epidemic of the related flavivirus, St.
Louis encephalitis virus, occurred in the US The particular factors and an understanding of how they may have contributed to or facilitated these large arboviral outbreaks require further elucidation. The reason for the increase in frequency and severity of outbreaks of WNV since remains unclear.
The movement of the virus into areas with large immunologically naive populations, with an age structure including many elderly and immunocompromised individuals, may account in part for this observation 1 ; however, a more virulent strain of the virus has been suggested as well The future epidemiology of WNV in North America is uncertain, and the historic pattern of the virus provides little guidance as to its potential course in the US.
Whether continued infection among the population will lead to a decline in susceptible avian and human hosts, with a subsequent decline in the number of cases, remains to be seen.
Following the large outbreak in , Romania continued to experience cases during following years, although at greatly diminished rates, and seroprevalence rates among avians appeared to remain high Comparison with the epidemiologic patterns of other related flaviviruses may be illustrative: St.
Louis encephalitis tends to occur sporadically in various regions of the US, with the appearance of occasional larger clusters and, rarely, large geographically dispersed epidemics On the other hand, Japanese encephalitis tends to be a hyperendemic disease in areas of Southeast Asia, where symptomatic illness predominates in serologically naive children The fact that WNV illness in the US seems to predominate in adults and the elderly, with children less frequently developing symptomatic illness, may suggest that a substantial immune population will develop over time; however, the future pattern cannot be predicted.
During the period of WNV transmission in North America, arbovirus surveillance capacity has been increased substantially; however, the complex epidemiology of the illness and the difficulties associated with serologic testing for the virus continue to present challenges for surveillance and prevention measures.
Efforts to control and reduce vector populations, reinforcement of public health messages of personal protection from mosquitoes, and vigilant surveillance and public awareness campaigns are likely to remain the cornerstones of the public health response to WNV.
Through continued surveillance and further study, it is hoped that the remaining questions regarding the epidemiologic and clinical features of WNV may be answered.
National Center for Biotechnology Information , U. Journal List Ochsner J v. Ochsner J. James J. Sejvar , MD. Author information Copyright and License information Disclaimer.
This article has been cited by other articles in PMC. West Nile virus. Lancet Infect Dis. A neurotropic virus isolated from the blood of a native of Uganda.
Am J Trop Med. West Nile in the Mediterranean basin: — Ann NY Acad Sci. Isolation of West Nile virus in Israel. J Infect Dis. A study of the ecology of West Nile virus in Egypt. Transmission of West Nile virus by infected Aedes albopictus. Proc Soc Exp Biol Med. Induced virus infections in man by the Egypt isolates of West Nile virus. Am J Trop Med Hyg. Several epidemics had been documented prior to , some involving hundreds to thousands of cases in mostly rural populations, but only a few cases of severe neurological disease had been reported.
The occurrence between and of three major epidemics, in southern Romania, the Volga delta in southern Russia, and the northeastern United States, involving hundreds of cases of severe neurological disease and fatal infections was totally unexpected.
These were the first epidemics reported in large urban populations. Mild disease, generally called West Nile fever, may cause some or all of the following symptoms:.
More severe forms of disease are called West Nile encephalitis or West Nile meningitis, depending on what part of the body is affected. The following symptoms can occur, and need prompt attention:. Signs of West Nile virus infection are similar to those of other viral infections. There may be no specific findings on a physical examination.
About one half of people with West Nile virus infection may have a rash. Because this illness is not caused by bacteria, antibiotics do not treat West Nile virus infection. Supportive care may help decrease the risk of developing complications in severe illness. For those with severe infection, the outlook is more uncertain. West Nile encephalitis or meningitis may lead to brain damage and death. One in ten people with brain inflammation do not survive. Call your health care provider if you have symptoms of West Nile virus infection, particularly if you may have had contact with mosquitoes.
If you are very sick, go to an emergency room.
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