The West Nile virus has caused outbreaks of West Nile fever in countries on a number of different continents since its discovery in 1937 in Uganda.
The West Nile virus has caused outbreaks of West Nile fever in countries on a number of different continents since its discovery in 1937 in Uganda.
The first cases of West Nile disease in the United States occurred in 1999 in New York. In the first three years only a small number of human cases were diagnosed, all in the Eastern or Southern U.S. (149 cases and 19 deaths, 1999–2001). In 2002, the disease exploded to most of the lower 48 states, causing 4,156 diagnosed cases of which 284 died. The disease continued to spread, reaching all but three states by 2006. The first case detected in Maine occurred in 2012. [1] The single cases detected in Hawaii in 2014 and in Alaska in 2018 were likely imported. [2] [3]
In the period from 1999 to 2018, the United States had 2,330 total deaths out of 50,830 diagnosed cases, giving a case fatality rate of 5%. [1] The worst years were 2003 for the most cases with 9,862 and 2012 for the most deaths with 286. [1] Most cases are thought to go undiagnosed. The WHO claims that about 80% of cases are asymptomatic. [4] Half of all cases (48.5%) diagnosed in the U.S. lead to neuroinvasive symptoms. Over 90% of deaths occurred in cases with neuroinvasive disease. [1] After several cases of West Nile disease transmitted by blood transfusion, the U.S. began testing all blood donations and dozens of presumed cases are found each year. [5] Some West Nile cases resulted from organ transplants and some but not all organs/donors are tested. [6]
As of 2020, the disease outbreak is considered ongoing and essentially endemic. As well as humans, countless birds and over 27,000 horses have been infected, providing additional reservoirs for the virus. [7]
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. In 2007, 445 Manitobans had confirmed cases of WNV and two people died with a third unconfirmed but suspected. [8] Summer of 2012 saw widespread viral penetration in Canada, coast to coast. [9] 17 people have either tested positive or are suspected of having the virus in Saskatchewan, and only one person has tested positive in Alberta. [10] Saskatchewan has reported 826 cases of WNV plus three deaths. [11] The spread of West Nile Virus infected mosquitoes to British Columbia for the first time was reported in 2009 [12]
| Year | Infections | Deaths |
|---|---|---|
| 2010 | 332 | ≥38 [14] [15] |
| 2011 | 125 | ≥9 [16] |
| 2012 | 267 | ≥3 [17] |
| 2013 | 273 | ≥1 [19] |
| 2014 | 75 | |
| 2015 | 122 | 2 |
| 2016 | 226 | 27 |
| 2017 | 201 | 25 |
| 2018 | 1549 | 166 |
| 2019 | 425 | 52 |
| 2020 | 316 | 37 |
| 2021 | 139 | 9 |
| 2022 | 394 | 21 |
| 2023 | 707 | 67 |
Lassa fever, also known as Lassa hemorrhagic fever, is a type of viral hemorrhagic fever caused by the Lassa virus. Many of those infected by the virus do not develop symptoms. When symptoms occur they typically include fever, weakness, headaches, vomiting, and muscle pains. Less commonly there may be bleeding from the mouth or gastrointestinal tract. The risk of death once infected is about one percent and frequently occurs within two weeks of the onset of symptoms. Of those who survive, about a quarter have hearing loss, which improves within three months in about half of these cases.
West Nile fever is an infection by the West Nile virus, which is typically spread by mosquitoes. In about 80% of infections people have few or no symptoms. About 20% of people develop a fever, headache, vomiting, or a rash. In less than 1% of people, encephalitis or meningitis occurs, with associated neck stiffness, confusion, or seizures. Recovery may take weeks to months. The risk of death among those in whom the nervous system is affected is about 10 percent.
The West Nile virus quickly spread across the United States after the first reported cases in Queens, New York, in 1999. The virus is believed to have entered in an infected bird or mosquito, although there is no clear evidence. The disease spread quickly through infected birds. Mosquitoes spread the disease to mammals. It was mainly noted in horses but also appeared in a number of other species. The first human cases usually followed within three months of the first appearance of infected birds in the area except where cold weather interrupted the mosquito vectors. Since the virus has become widely established in the U.S., an average of 130 deaths a year occurred.
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