Giuseppe
Emeritus
[Source: European Centre for Disease Prevention and Control (ECDC), full PDF document: (LINK). Excerpts.]
TECHNICAL REPORT
West Nile virus risk assessment tool
This report of the European Centre for Disease Prevention and Control (ECDC) was coordinated by Zaida Herrador, Annick Lenglet, Wim Van Bortel and Herve Zeller.
Contributing authors
Anca Sirbu, Florin Popovici, Adriana Pistol, Roxana Serban, Daniela Pitigoi, Corina Posea, Cornelia Ceianu, Gabriela Nicolescu, Romeo Bellini, Denis Coulombier, Evelyn Depoortere, Dragoslav Domanovic, Zaida Herrador, Katrin Leitmeyer, Annick Lenglet, Laurence Marrama, Sybille Rehmet, Eve Robinson, Francisco Santos O?Connor, Wim Van Bortel, Eva Warns-Petit, and Herve Zeller
In 2009, the European Centre for Disease Prevention and Control (ECDC) hosted an expert consultation on West Nile virus (WNV) infection. At this meeting it was recommended that a tool be developed to assist Member States in assessing their risk of WNV. In response this tool was developed as part of a request for offer, in collaboration with the National Romanian Public Health Institute. The tool was adapted and fine-tuned based on the input from the expert consultation ?Risk assessment and outbreak mapping tools for West Nile virus infection in Europe? in November 2011 and from the request for offer ?Vector control approaches to prevent and control West Nile virus outbreak? delivered in 2012.
Suggested citation:
European Centre for Disease Prevention and Control. West Nile virus risk assessment tool Stockholm: ECDC; 2013. Stockholm, July 2013 ISBN 978-92-9193-482-9 - doi 10.2900/85718
? European Centre for Disease Prevention and Control, 2013 - Reproduction is authorised, provided the source is acknowledged
Executive summary
West Nile virus (WNV) is an emerging pathogen whose ecology and epidemiology extend across multiple interfaces including the viral pathogen, arthropod vectors, birds, domestic animals and human beings.
As the epidemiology and transmission cycle of WNV is complex, assessing the risk of WNV being transmitted to humans is not always straightforward. Therefore, the WNV risk assessment tool has been developed to provide operational guidance in support of the risk assessment process. The draft tool was reviewed during an expert consultation on ?Risk assessment and outbreak mapping tools for WNV infection in Europe? and adapted according to recommendations from the meeting. Information from the project ?Vector control approaches to prevent and control West Nile virus outbreak? was then added to complete the tool.
The tool specifically addresses the following two questions: How are geographically affected areas and areas at risk of WNV transmission defined and when is an alert for potential human WNV infection triggered using indicators from a range of different surveillance systems? The document is divided into three parts: key facts about WNV; description of surveillance systems used for WNV; and the risk assessment tool.
It is important to note that risk assessment is a continuous process. The WNV situation changes each year in Europe and will evolve further in the coming years. For this reason there is a need to reassess the risk on a regular basis in response to new evidence on the ecology of WNV in Europe as it emerges. Similarly, this risk assessment tool should be regularly revised and updated.
Background and objectives
West Nile virus (WNV) is an emerging pathogen whose ecology and epidemiology extend across multiple interfaces including the viral pathogen, arthropod vectors, birds, domestic animals and human beings [1, 2]. The significance of WNV for residents of European Union (EU) Member States has been repeatedly highlighted in recent years, with outbreaks in a number of European countries including Italy, Hungary, Romania and Greece.
As the epidemiology and transmission cycle of WNV is complex, assessing the risk of WNV being transmitted to humans is not always straightforward. Risk assessment for WNV transmission to humans at local, national and EU level is important because it facilitates timely implementation of preparedness activities and appropriate control measures, and expedites important decision-making in response to outbreaks and the targeting of resources.
The first ECDC expert consultation on WNV in April 2009 [3] established that an algorithm for risk assessment of WNV transmission to humans would be welcomed by both WNV-affected countries and those at risk of WNV. This WNV risk assessment tool has therefore been developed to provide operational guidance in support of the risk assessment process.
The draft tool was reviewed during an expert consultation on risk assessment and outbreak mapping tools for West Nile virus infection in Europe in November 2011 [4] and adapted according to the recommendations from the meeting. Information from the project ?Vector control approaches to prevent and control West Nile virus outbreak? was also added to complete the tool.
The tool specifically addresses the following questions:
The virus
WNV is a mosquito-transmitted enveloped RNA virus belonging to the Japanese encephalitis serocomplex (Flavivirus genus, Flaviviridae family), which contains medically important flaviviruses such as Japanese encephalitis virus in Asia, St Louis encephalitis virus in the Americas, Murray Valley virus in Australia, Usutu virus in Africa and Europe [5].
Phylogenetic analysis of complete viral genomes differentiates two distinct genetic lineages of WNV (lineage 1 and 2) diverging up to 29% at the nucleotide level. Viral strains responsible for outbreaks in Europe have belonged mainly to lineage 1 and shown a strong genetic similarity [5].
However, recent outbreaks in humans in southern Russia (2007 and 2010) and in Greece (2010?2012) were due to viruses from genetic lineage 2 [6-8].
Transmission dynamics of WNV in Europe
WNV is transmitted in a bird-mosquito cycle (see Figure 1), with birds as amplifying hosts, and mammals (primarily humans and horses) as dead-end hosts only. Transmission of WNV occurs when mosquitoes are active (i.e. between spring and autumn), but due to the amplification cycle in birds, most infections in humans and horses are usually observed between mid-July and October, peaking in September.
(?)
The WNV is introduced through migratory birds travelling from sub-Saharan Africa, North Africa or the Middle East or it can overwinter in local bird species or mosquitoes. Culex modestus, Culex pipiens, Coquillettidia richiardii and other mosquito species act as the vectors in the bird-mosquito-bird WNV transmission cycle. Culex pipiens and Culex modestus also act as bridge vectors, infecting equines and humans.
Mosquito vectors
Mosquitoes acquire infection by feeding on a viraemic bird. After entering through the gut wall into the haemolymph, the virus replicates in most of the internal tissues and eventually arrives in the salivary glands. This extrinsic incubation period in mosquitoes lasts 10?14 days depending on the temperature [9-11]. Once infected, the mosquito remains infectious throughout its lifespan, potentially transmitting the virus to every vertebrate on which it feeds. Among more than 15 potential vector species existing in European mosquito fauna, the principal vectors of WNV in Europe are of the Culex genus, especially Culex pipiens and Culex modestus species. Culex modestus is an important vector in deltaic and other wetland ecosystems and it was the vector responsible for the 1962 WNV outbreak in the Camargue in the south of France [12, 13]. Studies have shown it to be the most competent experimental vector, with a transmission rate of 51.5% [14]. Culex pipiens, which is a fairly ubiquitous species, has been incriminated as the main vector in recent outbreaks [15-18].
Birds
Bird species are the principal vertebrate host of WNV and act as viral amplifiers. The virus has been isolated from over 150 species of domestic and wild birds globally. The best reservoir species are passerines, including corvids, which develop high viraemic titres. In Europe, the virus has been isolated from several species of wild land and water birds.
The capacity of the WNV to induce an elevated and persistent viraemia in some species of birds would explain its ability to spread during migrations to new areas. Birds are also able to shed the virus at high titres through oral and cloacal secretions, and bird-to-bird transmission has been demonstrated. In Europe, avian infection is generally asymptomatic, probably reflecting a long co-evolution of virus and host in the Old World. In contrast to the United States (US), significant mortality related to WNV has not been seen in wild birds during the human outbreaks in Europe [5].
Equines
Equines, mainly horses, are infected through the bite of an infected mosquito. As horses have greater mosquito exposure than humans, horse infections often precede human infections. The infection is usually asymptomatic in horses and only a small percentage (approximately 10%) may show neurological signs [19]. Signs can range from mild ataxia to total recumbence. Some horses exhibit weakness, muscle fasciculation, and cranial nerve deficits. Fever is not always a recognised feature of the disease in horses [20].
The incubation period in horses is estimated to be 3?15 days and recovery is within 5?15 days. The mortality rate in horses with neurological symptoms may be as high as 38?57.1% [20]. Horses are not amplifying hosts as viraemia is low and transitory.
At EU level, over the last ten years Italy, France and Spain have reported WNV outbreaks in horses with no concurrent reported human cases [21]. However, other outbreaks in horses both in France (2003) and Italy (2009) have concurred with outbreaks in humans [22].
Humans
Similar to horses, the incubation period in humans is usually 3?15 days and most cases are asymptomatic. Viraemia occurs within 1?3 days and can last up to 11 days. In 15?20% of the cases, a mild flu-like illness is reported. These mild symptoms may last two to five days. A rash, typically maculopapular, may also be present in 25?50% of cases, and is less likely in neuroinvasive disease. In less than 1% of cases, neurological disease such as meningitis, meningoencephalitis, acute flaccid paralysis, or a mixed pattern of disease develops. Recovery from West Nile neuroinvasive disease (WNND) can be slow and longer term sequelae of weakness, myalgia and fatigue have been reported. WNF, in particular neuroinvasive disease, may also be associated with chronic kidney disease [23].
The mortality rate following WNND is approximately 10% and is generally associated with older age groups or comorbidities. During recent outbreaks, the case fatality rates among hospitalised patients have ranged from 4% in Romania (1996), 12% in New York (1999) and 14% in Israel (2000) to 17% in WNND cases in Greece (2010) [24].
Compared to adults, children infected with WNV have shorter hospitalisations and fewer neurological symptoms. They are more likely to have meningitis over encephalitis, have better neurological outcomes and lower mortality [25]. Transmission of WNV to humans is mainly through the bite of an affected mosquito. However, transmission is also possible through blood and blood components, tissues and cells, and organ transplants. Cases infected by these routes have been documented in both the US and Europe [26]. A single case of vertical transplacental mother-to-child transmission has been reported in the US [27]. In another case in the US, breastfeeding was considered the likely route of transmission to an infant [28]. Finally, WNV infection through occupational exposure has also been documented: in entomologists in the Camargue region of France collecting mosquitoes for surveillance [13]; a veterinary student in Gauteng, South Africa in 2009, diagnosed with WNV lineage 2 after performing an autopsy on a Welsh pony and two laboratory-acquired WNV infections reported in the US in 2002 after accidental percutaneous inoculation [29,30].
Environmental factors affecting transmission dynamics
WNV is a complex disease that is influenced by multiple environmental and climatic factors. WNV is frequently associated with river deltas and other wetland areas which serve as nesting sites for many migratory birds and breeding sites for ornithophilic mosquitoes. Besides natural habitats, there is a variety of artificial breeding sites in both rural and urban settlements.
These include stagnant and often dirty water in dishes, buckets, barrels and cans, flower pots, rain gutters, discarded tires and other containers that could collect water. In urban environments, infrastructure such as underground heating, sewage pipes, and basements liable to flooding can act as breeding and resting sites for the vectors, as was seen with outbreaks in Romania.
In a number of situations temperature has been cited as one of the important environmental variables modulating WNV activity in Europe as it affects both mosquito breeding and the external incubation of WNV [15].
The development of Culex pipiens larvae starts at 12?C and is optimal at 25?30?C. Transmission rates from mosquitoes are directly related to the temperature during the extrinsic incubation of the WNV [11, 31] and the optimum temperature for extrinsic incubation period depends on the mosquito species. Experiments have shown that extrinsic infection and transmission rates for WNV in Culex pipiens mosquitoes were highest in those that were maintained at a temperature of 30?C [9].
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TECHNICAL REPORT
West Nile virus risk assessment tool
This report of the European Centre for Disease Prevention and Control (ECDC) was coordinated by Zaida Herrador, Annick Lenglet, Wim Van Bortel and Herve Zeller.
Contributing authors
Anca Sirbu, Florin Popovici, Adriana Pistol, Roxana Serban, Daniela Pitigoi, Corina Posea, Cornelia Ceianu, Gabriela Nicolescu, Romeo Bellini, Denis Coulombier, Evelyn Depoortere, Dragoslav Domanovic, Zaida Herrador, Katrin Leitmeyer, Annick Lenglet, Laurence Marrama, Sybille Rehmet, Eve Robinson, Francisco Santos O?Connor, Wim Van Bortel, Eva Warns-Petit, and Herve Zeller
In 2009, the European Centre for Disease Prevention and Control (ECDC) hosted an expert consultation on West Nile virus (WNV) infection. At this meeting it was recommended that a tool be developed to assist Member States in assessing their risk of WNV. In response this tool was developed as part of a request for offer, in collaboration with the National Romanian Public Health Institute. The tool was adapted and fine-tuned based on the input from the expert consultation ?Risk assessment and outbreak mapping tools for West Nile virus infection in Europe? in November 2011 and from the request for offer ?Vector control approaches to prevent and control West Nile virus outbreak? delivered in 2012.
Suggested citation:
European Centre for Disease Prevention and Control. West Nile virus risk assessment tool Stockholm: ECDC; 2013. Stockholm, July 2013 ISBN 978-92-9193-482-9 - doi 10.2900/85718
? European Centre for Disease Prevention and Control, 2013 - Reproduction is authorised, provided the source is acknowledged
Executive summary
West Nile virus (WNV) is an emerging pathogen whose ecology and epidemiology extend across multiple interfaces including the viral pathogen, arthropod vectors, birds, domestic animals and human beings.
As the epidemiology and transmission cycle of WNV is complex, assessing the risk of WNV being transmitted to humans is not always straightforward. Therefore, the WNV risk assessment tool has been developed to provide operational guidance in support of the risk assessment process. The draft tool was reviewed during an expert consultation on ?Risk assessment and outbreak mapping tools for WNV infection in Europe? and adapted according to recommendations from the meeting. Information from the project ?Vector control approaches to prevent and control West Nile virus outbreak? was then added to complete the tool.
The tool specifically addresses the following two questions: How are geographically affected areas and areas at risk of WNV transmission defined and when is an alert for potential human WNV infection triggered using indicators from a range of different surveillance systems? The document is divided into three parts: key facts about WNV; description of surveillance systems used for WNV; and the risk assessment tool.
It is important to note that risk assessment is a continuous process. The WNV situation changes each year in Europe and will evolve further in the coming years. For this reason there is a need to reassess the risk on a regular basis in response to new evidence on the ecology of WNV in Europe as it emerges. Similarly, this risk assessment tool should be regularly revised and updated.
Background and objectives
West Nile virus (WNV) is an emerging pathogen whose ecology and epidemiology extend across multiple interfaces including the viral pathogen, arthropod vectors, birds, domestic animals and human beings [1, 2]. The significance of WNV for residents of European Union (EU) Member States has been repeatedly highlighted in recent years, with outbreaks in a number of European countries including Italy, Hungary, Romania and Greece.
As the epidemiology and transmission cycle of WNV is complex, assessing the risk of WNV being transmitted to humans is not always straightforward. Risk assessment for WNV transmission to humans at local, national and EU level is important because it facilitates timely implementation of preparedness activities and appropriate control measures, and expedites important decision-making in response to outbreaks and the targeting of resources.
The first ECDC expert consultation on WNV in April 2009 [3] established that an algorithm for risk assessment of WNV transmission to humans would be welcomed by both WNV-affected countries and those at risk of WNV. This WNV risk assessment tool has therefore been developed to provide operational guidance in support of the risk assessment process.
The draft tool was reviewed during an expert consultation on risk assessment and outbreak mapping tools for West Nile virus infection in Europe in November 2011 [4] and adapted according to the recommendations from the meeting. Information from the project ?Vector control approaches to prevent and control West Nile virus outbreak? was also added to complete the tool.
The tool specifically addresses the following questions:
- How are geographically affected areas and areas at risk for WNV transmission defined?
- When is an alert for potential human WNV infection triggered using indicators from a range of different surveillance systems?
- Key facts about WNV
- Description of surveillance systems used for WNV
- The risk assessment tool.
The virus
WNV is a mosquito-transmitted enveloped RNA virus belonging to the Japanese encephalitis serocomplex (Flavivirus genus, Flaviviridae family), which contains medically important flaviviruses such as Japanese encephalitis virus in Asia, St Louis encephalitis virus in the Americas, Murray Valley virus in Australia, Usutu virus in Africa and Europe [5].
Phylogenetic analysis of complete viral genomes differentiates two distinct genetic lineages of WNV (lineage 1 and 2) diverging up to 29% at the nucleotide level. Viral strains responsible for outbreaks in Europe have belonged mainly to lineage 1 and shown a strong genetic similarity [5].
However, recent outbreaks in humans in southern Russia (2007 and 2010) and in Greece (2010?2012) were due to viruses from genetic lineage 2 [6-8].
Transmission dynamics of WNV in Europe
WNV is transmitted in a bird-mosquito cycle (see Figure 1), with birds as amplifying hosts, and mammals (primarily humans and horses) as dead-end hosts only. Transmission of WNV occurs when mosquitoes are active (i.e. between spring and autumn), but due to the amplification cycle in birds, most infections in humans and horses are usually observed between mid-July and October, peaking in September.
(?)
The WNV is introduced through migratory birds travelling from sub-Saharan Africa, North Africa or the Middle East or it can overwinter in local bird species or mosquitoes. Culex modestus, Culex pipiens, Coquillettidia richiardii and other mosquito species act as the vectors in the bird-mosquito-bird WNV transmission cycle. Culex pipiens and Culex modestus also act as bridge vectors, infecting equines and humans.
Mosquito vectors
Mosquitoes acquire infection by feeding on a viraemic bird. After entering through the gut wall into the haemolymph, the virus replicates in most of the internal tissues and eventually arrives in the salivary glands. This extrinsic incubation period in mosquitoes lasts 10?14 days depending on the temperature [9-11]. Once infected, the mosquito remains infectious throughout its lifespan, potentially transmitting the virus to every vertebrate on which it feeds. Among more than 15 potential vector species existing in European mosquito fauna, the principal vectors of WNV in Europe are of the Culex genus, especially Culex pipiens and Culex modestus species. Culex modestus is an important vector in deltaic and other wetland ecosystems and it was the vector responsible for the 1962 WNV outbreak in the Camargue in the south of France [12, 13]. Studies have shown it to be the most competent experimental vector, with a transmission rate of 51.5% [14]. Culex pipiens, which is a fairly ubiquitous species, has been incriminated as the main vector in recent outbreaks [15-18].
Birds
Bird species are the principal vertebrate host of WNV and act as viral amplifiers. The virus has been isolated from over 150 species of domestic and wild birds globally. The best reservoir species are passerines, including corvids, which develop high viraemic titres. In Europe, the virus has been isolated from several species of wild land and water birds.
The capacity of the WNV to induce an elevated and persistent viraemia in some species of birds would explain its ability to spread during migrations to new areas. Birds are also able to shed the virus at high titres through oral and cloacal secretions, and bird-to-bird transmission has been demonstrated. In Europe, avian infection is generally asymptomatic, probably reflecting a long co-evolution of virus and host in the Old World. In contrast to the United States (US), significant mortality related to WNV has not been seen in wild birds during the human outbreaks in Europe [5].
Equines
Equines, mainly horses, are infected through the bite of an infected mosquito. As horses have greater mosquito exposure than humans, horse infections often precede human infections. The infection is usually asymptomatic in horses and only a small percentage (approximately 10%) may show neurological signs [19]. Signs can range from mild ataxia to total recumbence. Some horses exhibit weakness, muscle fasciculation, and cranial nerve deficits. Fever is not always a recognised feature of the disease in horses [20].
The incubation period in horses is estimated to be 3?15 days and recovery is within 5?15 days. The mortality rate in horses with neurological symptoms may be as high as 38?57.1% [20]. Horses are not amplifying hosts as viraemia is low and transitory.
At EU level, over the last ten years Italy, France and Spain have reported WNV outbreaks in horses with no concurrent reported human cases [21]. However, other outbreaks in horses both in France (2003) and Italy (2009) have concurred with outbreaks in humans [22].
Humans
Similar to horses, the incubation period in humans is usually 3?15 days and most cases are asymptomatic. Viraemia occurs within 1?3 days and can last up to 11 days. In 15?20% of the cases, a mild flu-like illness is reported. These mild symptoms may last two to five days. A rash, typically maculopapular, may also be present in 25?50% of cases, and is less likely in neuroinvasive disease. In less than 1% of cases, neurological disease such as meningitis, meningoencephalitis, acute flaccid paralysis, or a mixed pattern of disease develops. Recovery from West Nile neuroinvasive disease (WNND) can be slow and longer term sequelae of weakness, myalgia and fatigue have been reported. WNF, in particular neuroinvasive disease, may also be associated with chronic kidney disease [23].
The mortality rate following WNND is approximately 10% and is generally associated with older age groups or comorbidities. During recent outbreaks, the case fatality rates among hospitalised patients have ranged from 4% in Romania (1996), 12% in New York (1999) and 14% in Israel (2000) to 17% in WNND cases in Greece (2010) [24].
Compared to adults, children infected with WNV have shorter hospitalisations and fewer neurological symptoms. They are more likely to have meningitis over encephalitis, have better neurological outcomes and lower mortality [25]. Transmission of WNV to humans is mainly through the bite of an affected mosquito. However, transmission is also possible through blood and blood components, tissues and cells, and organ transplants. Cases infected by these routes have been documented in both the US and Europe [26]. A single case of vertical transplacental mother-to-child transmission has been reported in the US [27]. In another case in the US, breastfeeding was considered the likely route of transmission to an infant [28]. Finally, WNV infection through occupational exposure has also been documented: in entomologists in the Camargue region of France collecting mosquitoes for surveillance [13]; a veterinary student in Gauteng, South Africa in 2009, diagnosed with WNV lineage 2 after performing an autopsy on a Welsh pony and two laboratory-acquired WNV infections reported in the US in 2002 after accidental percutaneous inoculation [29,30].
Environmental factors affecting transmission dynamics
WNV is a complex disease that is influenced by multiple environmental and climatic factors. WNV is frequently associated with river deltas and other wetland areas which serve as nesting sites for many migratory birds and breeding sites for ornithophilic mosquitoes. Besides natural habitats, there is a variety of artificial breeding sites in both rural and urban settlements.
These include stagnant and often dirty water in dishes, buckets, barrels and cans, flower pots, rain gutters, discarded tires and other containers that could collect water. In urban environments, infrastructure such as underground heating, sewage pipes, and basements liable to flooding can act as breeding and resting sites for the vectors, as was seen with outbreaks in Romania.
In a number of situations temperature has been cited as one of the important environmental variables modulating WNV activity in Europe as it affects both mosquito breeding and the external incubation of WNV [15].
The development of Culex pipiens larvae starts at 12?C and is optimal at 25?30?C. Transmission rates from mosquitoes are directly related to the temperature during the extrinsic incubation of the WNV [11, 31] and the optimum temperature for extrinsic incubation period depends on the mosquito species. Experiments have shown that extrinsic infection and transmission rates for WNV in Culex pipiens mosquitoes were highest in those that were maintained at a temperature of 30?C [9].
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