West nile virus epidemiology and demographics

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Epidemiology

The human case-fatality rate in the U.S. has been 7% overall, and among patients with neuroinvasive WNV disease, 10%.

In general, the WNV transmission season in the U.S. is longer than that for other domestic arboviruses and requires longer periods of ecologic and human surveillance.

1. Northeastern and Midwestern U.S.

In the northeastern states in 2001-2002, human illness onset occurred as early as early July and as late as mid-November. During these same years, avian cases occurred as early as the first week of April and as late as the second week of December. Active ecological surveillance and enhanced passive surveillance for human cases should begin in early spring and continue through the fall until mosquito activity ceases because of cold weather. Surveillance in urban and suburban areas should be emphasized.

2. Southern U.S.

In 2001-2002, WNV circulated throughout the year, especially in the Gulf states. Although, in 2001-2002, human illness onset was reported as early as mid-May and June and as late as mid-December, equine and avian infections were reported in all 22 months of the year. Active ecologic surveillance and enhanced passive surveillance for human cases should be conducted year round in these areas.

3. Western U.S.

In 2002, WNV activity was first reported among humans and animals in Rocky Mountain states and among animals in Pacific coast states. These events occurred relatively late in the year (mid-August). Predicting the temporal characteristics of future WNV transmission seasons based on these limited reports is not possible. Despite this limitation, active ecological surveillance and enhanced passive surveillance for human cases beginning in early spring and continuing through the fall until mosquito activity ceases because of cold weather should be encouraged.

4. Other Areas of the Western Hemisphere

In 2002, Canada experienced a WNV epidemic in Ontario and Quebec provinces and an equine/avian epizootic that extended from the maritime provinces to Saskatchewan. Recent serologic evidence supports the conclusion that WNV has now reached Central America. Further spread to South America by migratory birds seems inevitable, if this has not already occurred. Development of surveillance systems capable of detecting WNV activity should be encouraged in the Caribbean and Central and South America. WNV surveillance should be integrated with dengue surveillance in these areas, and with yellow fever surveillance in areas where urban or peri-urban transmission of this virus occurs.

Demographics

2007 West Nile Virus Activity in the United States; reported to CDC as of August 7, 2007

Map shows the distribution of avian, animal, or mosquito infection occurring during 2007 with number of human cases if any, by state. If West Nile virus infection is reported to CDC from any area of a state, that entire state is shaded.

Data table:

Avian, animal or mosquito WNV infections have been reported to CDC ArboNET from the following states in 2007: Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Illinois, Indiana, Kansas, Louisiana, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Jersey, New Mexico, New York, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Puerto Rico, South Carolina, South Dakota, Tennessee, Texas, Utah, Virginia, Wisconsin, and Wyoming.

Human cases have been reported in Alabama, Arizona, Arkansas, California, Colorado, Georgia, Idaho, Illinois, Iowa, Kansas, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, North Dakota, Ohio, Pennsylvania, South Dakota, Texas, Utah, Virginia, and Wyoming.[2] [3]

West Nile virus has been described in Africa, Europe, the Middle East, west and central Asia, Oceania (subtype Kunjin), and most recently, North America.

Recent outbreaks of West Nile virus encephalitis in humans have occurred in Algeria (1994), Romania (1996 to 1997), the Czech Republic (1997), Congo (1998), Russia (1999), the United States (1999 to 2003), Canada (1999–2003), and Israel (2000).

Epizootics of disease in horses occurred in Morocco (1996), Italy (1998), the United States (1999 to 2001), and France (2000). In 2003, West Nile virus spread among horses in Mexico.

Recent outbreaks

United States: From 1999 through 2001, the CDC confirmed 149 cases of human West Nile virus infection, including 18 deaths. In 2002, a total of 4,156 cases were reported, including 284 fatalities. 13 cases in 2002 were contracted through blood transfusion. The cost of West Nile-related health care in 2002 was estimated at $200 million. The first human West Nile disease in 2003 occurred in June and one West Nile-infected blood transfusion was also identified that month. In the 2003 outbreak, 9,862 cases and 264 deaths were reported by the CDC. At least 30% of those cases were considered severe involving meningitis or encephalitis. In 2004, there were only 2,539 reported cases and 100 deaths. In 2005, there was a slight increase in the number of cases, with 3,000 cases and 119 deaths reported. 2006 saw another increase, with 4,261 cases and 174 deaths.

West Nile Virus Cases in the United States

See also Progress of the West Nile virus in the United States

Canada: One human death occurred in 1999. In 2002, ten human deaths out of 416 confirmed and probable cases were reported by Canadian health officials. In 2003, 14 deaths and 1,494 confirmed and probable cases were reported. Cases were reported in 2003 in Nova Scotia, Quebec, Ontario, Manitoba, Saskatchewan, Alberta, British Columbia, and the Yukon. In 2004, only 26 cases were reported and two deaths; however, 2005 saw 239 cases and 12 deaths. By October 28, 2006, 127 cases and no deaths had been reported. One case was asymptomatic and only discovered through a blood donation. Currently in 2007, 445 Manitobans have confirmed cases of West Nile and two people have died with a third uncomfirmed but suspected. 17 people have either tested positive or are suspected of having the virus in Saskatchewan, and only one person has tested positive in Alberta.

Saskatchewan has reported 826 cases of West Nile plus three deaths.

Israel: In 2000, the CDC found that there were 417 confirmed cases with 326 hospitalizations. 33 of these people died. The main clinical presentations were encephalitis (57.9%), febrile disease (24.4%), and meningitis (15.9%).

Romania: In 1996-1997 about 500 cases occurred in Romania with a fatality rate of nearly 10%.

2011 Cases

2011 Map

2011 Neuroinvasive diseases

References

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