West Nile Virus

West Nile Virus (WNV)

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Changed on: 13.09.2019
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The West Nile Virus is transmitted by mosquitoes and is the pathogen that causes West Nile Fever. Isolated in Uganda for the first time in 1937, it could only be found in Africa and parts of Asia and Southern Europe up to 1999. Today, it is found across America and increasingly in Central Europe. Birds (over 300 bird species) are the natural reservoir of the West Nile virus. Humans and other mammals, especially horses, can also catch it. The first West Nile virus infection of the genotype lineage 2 in birds of prey was first detected in Austria in August 2008.

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The West Nile Virus is transmitted by mosquitoes and is the pathogen that causes West Nile Fever. Isolated in Uganda for the first time in 1937, it could only be found in Africa and parts of Asia and Southern Europe up to 1999. Today, it is found across America and increasingly in Central Europe. Birds (over 300 bird species) are the natural reservoir of the West Nile virus. Humans and other mammals, especially horses, can also catch it. The first West Nile virus infection of the genotype lineage 2 in birds of prey was first detected in Austria in August 2008.

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Das West Nil Virus wird, wie z. B. auch die Erreger von Malaria, Gelbfieber oder Dengue-Fieber, durch Stechmücken (Gelsen) übertragen. Die Bedeutung des Klimawandels als Ursache für das Vordringen von "fremden" Stechmückenarten in die nördliche Hemnisphäre wird zwar diskutiert, als gesichert gilt jedoch, dass auch unsere "einheimischen" Stechmücken-Arten zur Verbreitung des West Nil Virus beitragen.

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Das West Nil Virus wird, wie z. B. auch die Erreger von Malaria, Gelbfieber oder Dengue-Fieber, durch Stechmücken (Gelsen) übertragen. Die Bedeutung des Klimawandels als Ursache für das Vordringen von "fremden" Stechmückenarten in die nördliche Hemnisphäre wird zwar diskutiert, als gesichert gilt jedoch, dass auch unsere "einheimischen" Stechmücken-Arten zur Verbreitung des West Nil Virus beitragen.

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Spread

Erkrankungsrisiko in Österreich gering

Das Risiko, sich in Österreich anzustecken und an West Nil Fieber zu erkranken, ist derzeit noch sehr gering. Da die Übertragung durch Stechmücken erfolgt, treten West Nil Virus Infektionen in den Sommermonaten auf. In Österreich wurden zwischen 2009 und 2018 insgesamt 45 im Inland erworbene West Nil Virus-Fälle bestätigt; davon 27 im Jahr 2018, wovon 21 Fälle auf Infektionen in Österreich zurückgehen. Die wahrscheinlichen Ansteckungsorte sind in Wien, Niederösterreich und im Burgenland zu finden.

Im Jahr 2019 wurden bisher vier West Nil Virus-Fälle beim Menschen gemeldet. Es gab bislang keinen Todesfall beim Menschen. Bei zwei Pferden aus Wien und Niederösterreich wurde eine Infektion mit West Nil Virus festgestellt.

Number of West Nile virus detections in Austria, 2010-2018

[Translate to English:] WNV-Nachweise bei: 2010 2011 2012 2013 2014 2015 2016 2017 2018
Mensch 1 0 0 0 2 8 6 7 27
Pferd 0 0 0 0 0 0 2 3 3
Vögel 0 0 0 1 1 2 0 12 4
Stechmücken-Pools 0 1 1 0 2 3 2 0 0

Spread

The West Nile virus (WNV) is found in both tropical and climatically moderate regions. The pathogen hibernates predominantly in bloodsucking mosquitoes, but can also survive for months inside the internal organs of some bird species and be carried over long distances by migrating birds (Aspöck 2002).

WNV was already isolated in 1937 from the blood of a woman suffering from a fever in the West Nile District (hence, the name!) of Uganda and detected subsequently in many parts of Africa and Asia, as well as in South-Eastern Europe and even in Central Europe.

Examinations for the whole of Africa found high contamination rates in people in Sudan (1939: 46.4%) and Egypt (1950: 90 % of individuals above 40 years showed WNV antibodies). WNV (Lineage 1) was diagnosed in an outbreak in Israel as the cause of severe meningoencephalitis in older patients in 1957. Epidemic clusters of encephalitis caused by the West Nile virus were documented in Algeria (1994), the Democratic Republic of Congo (1998), Russia and North America (1999) and Israel (1998, 2000; Lineage 1) in the past.

The West Nile virus was first documented in the Western Hemisphere in 1999 with cases of encephalitis in humans, dogs, cats and horses. The appearance of the West Nile virus (Lineage 1) in North America in 1999 put the issue into the media (Aspöck 2002). The virus outbreak started in the area of New York City in the USA. Its first signs were birds (mainly corvids) that dropped dead from the trees in Central Park (Calisher 2000, Steele et al. 2000). Soon, older people living in the area were also infected.

Since then, the virus has spread across the North American continent. It was isolated in the USA, Canada, Mexico, the Caribbean, as well as in Central and South America.  The presence of dead birds, in particular corvids, played a role in both the American and Israeli outbreaks. The fact that the virus occurs in some European regions – presumably more or less regularly via migrating birds – has been known for over 30 years.

Cases of infections in birds, horses and even humans in the EU region have been reported since 1996. There were two infections in humans in Romania, two in Italy and 14 in Hungary between August and September 2008 (Source: ECDC – see literature list). In 2009, there were seven cases reported in Hungary, 14 in Italy and two in Romania. One year later, in 2010, Greece reported 260 cases of human infections, Hungary 10, Italy one, Romania 41 and Spain two. The increase in human infections in Europe between 2008 and 2010 can also be ascribed to increases in examinations of humans.

Outbreaks in animals were documented in Romania in 1996/97 for the first time and also in Italy in 1998. Cases of infected animals in Romania and Russia were published between 2005 and 2008, in France between 2005 and 2007, and the Czech Republic and Hungary in 1997 and 2008 (Source: OIE). An outbreak of the disease in geese and birds of prey was also documented in Hungary between 2003 and 2004 (Bakonyi et al. 2006). Several outbreaks in Italy’s animal population (horses, birds) were discovered in 2008 and 2009. A WNV infection in 14 horses was confirmed in Greece in 2010 and the virus was detected in the serum of 18 sheep and 15 birds in Romania. The virus has affected mostly birds and horses in Europe.

Requirements for WNV presence in Europe:

Imported by migrating birds or mosquitoes

Existence of appropriate mosquito species

Relatively high density of the relevant mosquito species 

Appropriately high temperatures: while the virus titre inside the insect increases slowly at 18 °C, the virus multiplies rapidly at 30 °C and has a high probability of transmission

Presence of bird species with high virus levels

Spread of these bird species in the course of migration activities.

Transmission

Transmission

The West Nile virus is transmitted by an unusually large number of mosquito species for Flaviviridae. A vast number of different mosquito species (43 species in North America alone) are possible vectors, in particular the genii Culex, Aedes and Ochlerotatus (also see Linke 2007). Within the genus Culex, these are most notably Culex quinquefasciatus, C. molestus, C. pipiens, C. restuans, C. salinarius and C. tarsalis (Hayes et al. 2005, Trevejo & Eidson 2008). Culex pipiens, as a parasite in birds and humans, is one of the possible vectors in the transmission chain from animals to humans or from birds to mammals. The virus can survive the winter inside Culex and start its reproduction cycle inside the mosquito the following year in spring (Bugbee & Forte 2004).

Mosquitoes of the Culex genus spend the winter in sheltered spaces, such as basements, stables, tunnels and caves. WNV is transferred via a bite 10-15 days after its incubation inside the mosquito.

The largest vertebrae reservoir (also see Linke 2007) are birds, in particular migrating birds (primarily Passeriformes, such as corvids and passerines). Over 150 bird species, various species of mammals (apes and monkeys, horses, ruminants – cattle, goats, sheep, red deer, camelidae, buffalos - , boars, dogs, wolves, foxes, bears, cats, bats, skunks, squirrels, hares and other rodents), reptiles (alligators, snakes), amphibians (Ranidae) and humans can get WNV infections. Psittaciformes, galliformes (chickens, quails) and columbiformes (rock doves) are unsuitable hosts for WNV given their subpar viraemic properties as hosts (short viraemia phase with low virus concentration levels; Trevejo & Eidson 2008).

Mammals, reptiles and amphibians have no significant role in the spread of the disease as they are dead-end hosts (bad hosts similar to psitacciformes).

The virus was first described in horses in France and Egypt in 1960. Humans and horses can get infections, but are of no importance in maintaining the transmission cycle. The direct transmission from humans to animals is unlikely. Cats and dogs only show mild symptoms when infected (Source: CDC). There are no known cases of direct transmissions from dogs or cats to humans. Human-to-human transmission is very rare. Transmission via blood transfusions, organ transplants, intrauterine exposure and nursing is possible (CDC 2002a-d, 2003).

Virus transmission is also thought to be possible via direct contact. The importance of oral or faecal transmission is unknown. Ingesting the virus via aerosols is also a subject of discussion (Trevejo & Eidson 2008).

Symptom

Symptoms

80 % of infections are asymptomatic

20 % of patients show influenza-like symptoms with sudden, high fever (= mild West Nile fever), muscle pain, headaches, gastrointestinal symptoms, possible cold symptoms, swollen lymph nodes and rashes. The incubation period is 2-8 days. The most prominent symptoms will subside within 7-10 days. 

West Nile meningitis or encephalitis may follow in individual cases (0.7 %). Neuroinvasive disorders with disorientation, coordination disorders, swallowing problems, extreme fatigue and dizziness combined with altered behaviour and mental states are all typical signs. These are followed by hepatitis, myocarditis, nephritis, pancreatitis and splenomegaly, coupled with a long period of recovery (Perelman & Stern 1974, Mathiot et al. 1990, Omalu et al. 2003). Patients with compromised immune systems and individuals aged over 50 have a higher risk of developing the severe form of the disease.

There have only been a few larger local outbreaks, in addition to the sporadic infections described in Southern, Central and Eastern Europe, such as in Bucharest (393 infections, 17 fatalities) in 1996, in Volgograd (826 infections, 40 fatalities) in 1999 and in Northern Greece (292 infections and 35 fatalities) in 2010. Such outbreaks are irregular, geographically and temporary limited phenomena, the occurrence of which cannot be foreseen at the current level of knowledge (ECDC Factsheet for Health Professionals).

Commissioned by the Federal Ministry of Health, the National Reference Centre at the Virology Department at the Medical University Vienna has found evidence of sporadic West Nile virus infections in Austria (two in 2009, one in 2010, none in 2011), as part of an examination of archived samples of patients suffering from diseases of the central nervous system. Two cases “imported” from Serbia were documented in 2012. There were no cases of human West Nile fever in Austria in 2011 and 2013. While there is no evidence of frequent cases of the West Nile virus in Austria, the possibility of an infection should be considered, in particular, in older individuals who have returned from a visit to a country with confirmed WNV cases during the warmer time of the year.

A total of 31 people were infected in Hungary in 2013 and a further 24 in Romania and 86 in Greece. There were 69 reported cases of West Nile fever in Northern Italy in 2013, 16 in Croatia and 302 in Serbia. While there is no evidence of frequent cases of the West Nile virus in Austria, the possibility of an infection should be considered, in particular at warmer times of the year. In places where West Nile fever outbreaks have occurred abroad, it has been observed that individuals over 50 years and people who spend large amounts of time outdoors suffered above average infection levels.

The Federal Ministry of Health has established a programme to monitor the West Nile situation in Austria in both the human and veterinary fields, given the fact that the regions affected may expand geographically.

Symptoms in Animals

Birds show few or no clinical symptoms. Occasionally, a disorder of the central nervous system and its accompanying symptoms can be diagnosed: lethargy, loss of coordination, ataxia, depression, wry neck, opisthotonos and bleeding in the beak and cloaca area could be seen. Young birds (1-11 days old chicks, goslings) showed a considerably more severe form of viraemia, as opposed to older animals. The mortality rate is particularly high in crows and other corvids. Myocarditis and encephalitis were diagnosed in dead birds. Artificially infected chickens and turkeys showed no symptoms. Symptoms in geese were a loss of appetite, loss of weight, depression, myocarditis and neurological disorders. It was often observed that the birds made a rhythmic movement of the head to the side. The infection rate in birds can be 10 – 53 % in endemic regions.

The main symptoms found in horses are fever and neurological symptoms, such as diffuse encephalitis and encephalomyelitis in combination with ataxia and pareses. This is often followed by coma and death. The morbidity rate in horses is relatively high. About 20 -43 % of infected horses show neurological symptoms. Approximately 25 – 45 % of infected animals may die.

The disease usually subsides without any symptoms in cats and dogs.

Sheep suffer from ataxia and fever, as well as behavioural disorders (the grinding of teeth, depression), in addition to neurologic disorders. Mother sheep can have miscarriages or stillbirths. Death can occur soon after birth in young lambs. Infected mother sheep may also die.

Prevention

The treatment of West Nile fever in humans focuses solely on the symptoms -- i.e. there is no medication that affects the virus directly. In animals, the treatment is focused on the symptoms and limiting consequences, too.

The only available vaccine is for horses. There is no vaccine for humans that can be used for prevention. Avoiding mosquito bites is the easiest way to prevent infections (Hayes & Gubler 2006). Wearing bright clothes with long sleeves, using insect repellents and avoiding stays in mosquito infested regions is recommended.

Literature

Trevejo R.T. & Eidson M. (2008): West Nile virus – Zoonosis Update. JAVMA 232(9):1302-1309.

Special Publications:

Aspöck H. (2002): Die West Nil Virus -  Epidemie in New York aus der Sicht des (reisenden und nicht reisenden) Mitteleuropäers. Entomol. Austriaca 5:5-6.

Bakonyi T. et al. (2005): Novel Flavivirus or New Lineage of West Nile virus, Central Europe. Emerging Infect. Dis. 11(2):225-231

Bakonyi T. et al. (2006): Lineage 1 and 2 Strains of Encephalitis West Nile virus, Central Europe. Emerging Infect. Dis. 12(4):618-623.

Bakonyi, T., Gould, E.A., Kolodziejek, J., Weissenböck, H., Nowotny, N., 2004. Complete genome analysis and molecular characterization of Usutu virus that emerged in Austria in 2001: comparison with the South African strain SAAR-1776 and other flaviviruses. Virology 328, 301–310.

Bugbee L.M. & Forte L.R. (2004): The discovery of West Nile virus in overwintering Culex pipiens (Diptera: Culicidae) mosquitoes in Lehigh County, Pennsylvania. J. Amer. Mosquito Control Association 20(3):326–327.

Calisher Ch. (2000): West Nile virus in the New World: appearance, persistence, and adaptation to a new econiche - an opportunity taken. Viral Immunol. 13(4):411–414.

CDC (2002a): Investigation of blood transfusion recipients with West Nile virus infections. MMWR, Wkly. Rep. 51 (36):823.

CDC (2002b): West Nile virus infection in organ donor and transplant recipients—Georgia and Florida, 2002. MMWR, Wkly. Rep. 51 (35):790.

CDC (2002c): Intrauterine West Nile virus infection—New York, 2002. MMWR, Wkly. Rep. 51 (50):1135–1136.

CDC (2002d): Possible West Nile virus transmission to an infant through breast-feeding--Michigan, 2002. MMWR, Wkly. Rep. 51 (39): 877–878.

CDC (2003): Detection of West Nile virus in blood donations--United States, 2003. MMWR, Wkly. Rep. 52 (32):769–772.

Fauquet C., Fauquet, C.M. & Mayo M.A. (2004): Eighth Report of the International Committee on Taxonomy of Viruses. Academic Pr Inc, London, San Diego

Hayes E.B. (2005): Virology, pathology, and clinical manifestations of West Nile virus disease. Emerging Infect. Dis. 11(8): 1174-1179.

Hayes E.B. & Gubler D.J. (2006): West Nile virus: epidemiology and clinical features of an emerging epidemic in the United States. Annu. Rev. Med. 57:181–194.

Hayes E.B. et al. (2005): Epidemiology and transmission dynamics of West Nile virus disease. Emerging Infect. Dis. 11(8):1167–1173.

Kecskeméti S. et al. (2007): Encephalitis due to West Nile virus in a sheep. Vet. Rec. 161:568-569.

Linke S. (2007): Die Prävalenz und Inzidenz von West Nil Virus in Deutschland. Diss Freie Universität Berlin (http://www.diss.fu-berlin.de/diss/receive/FUDISS_thesis_000000002910)

Marr J.S. & Calisher Ch. (2003): Alexander the Great and West Nile virus encephalitis. Emerging Infect. Dis. 9(12):1599-1603.

Mathiot C.C. et al. (1990): Comparative analysis of West Nile virus strains isolated from human and animal hosts using monoclonal antibodies and cDNA restriction digest profiles. Res. Virol. 141(5):533–543.

Olejnik E. (1952): Infectious adenitis transmitted by Culex molestus. Bull. Res. Counc. Isr. 2:210–211.

Omalu B.I. et al. (2003): Fatal fulminant pan-meningo-polioencephalitis due to West Nile virus. Brain Pathol. 13(4):465–472.

Pauli G. (2004): West Nile virus. Prevalence and significance as a zoonotic pathogen. Bundesges.blatt, Ges.forschung, Ges.schutz 47(7):653-660.

Perelman A. & Stern J. (1974): Acute pancreatitis in West Nile Fever. Am. J. Trop. Med. Med. Hyg. 23 (6):1150–1152.

Sejvar J.J. et al. (2003): Neurologic manifestations and outcome of West Nile virus infection. JAMA 290(4):511–515.

Smithburn K.C. & Jacobs H.R. (1942): Neutralization-tests against neurotropic viruses with sera collected in central Africa. J. Immunol. 44:923.

Steele K.E. et al. (2000): Pathology of fatal West Nile virus Infections in native and Exotic birds during the 1999 Outbreak in New York City. Vet. Pathol. 37:208-224.

Wünschmann A. et al. (2005): Pathologic and Immunhistochemical findings in goshawks (Accipiter gentilis) and Great Horned Owl (Bubo virginianus) naturally infected with West Nile virus. Avian Dis. 49:252-259.


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