The West Nile virus - come to stay?

West Nile virus (WNV), belonging to the Flaviviridae family, is an important zoonotic agent that is nowadays distributed almost worldwide. WNV is transmitted by bloodsucking mosquitoes, primarily mosquitoes of the genus Culex, circulates in a bird-mosquito-bird cycle and thus belongs to the arbo-viruses (abbreviation for "arthropod-borne") [1].

In Europe, WNV infections first occurred in the early 1960s, primarily in horses in the Camargue region of southern France, but were not detected there again for decades thereafter. It was not until the early 1990s that annually recurrent infections occurred in humans, horses and birds throughout southern and south-eastern Europe (e.g. France, Italy, Turkey, Romania and Russia) [2] and it can be assumed that such infections have been endemic in these countries ever since. Since 2003, WNV cases have also been detected for the first time in Hungary and later in Spain, Austria, Greece, Serbia, Croatia and Slovenia. In the last 5 years, the number of cases increased significantly across Europe and the virus spread further and further north, so that also in the Czech Republic and Austria, clustered cases of disease in birds, horses and humans have been reported since 2017/2018 (see also the annual data collection at the European Centre for Disease Prevention and Control https://ecdc.europa.eu/en/west-nile-fever)[3].

In most bird species, infection with WNV usually remains asymptomatic. Birds are the virus reservoir and serve as multiplication hosts due to the very high viral loads coupled with viraemia that occur in them. However, certain bird species are very susceptible to WNV, so that massive epidemics with fatalities can occur. In particular, passerine birds (Passeriformes), including frequently the corvids, but also various raptor, hawk and owl species are highly susceptible to WNV infection and often die peracutely [4].

Humans and horses can become infected with WNV and contract the infection, but epidemiologically they are considered dead-end hosts, i.e. they do not excrete any pathogens and (unlike birds) are not a source of infection for mosquitoes. In humans, 80% of infections are asymptomatic, while about 20% show symptoms of illness such as fever and flu-like symptoms. This mild form of progression is called the classic "West Nile fever". In less than one percent of infected persons, severe neuroinvasive forms with meningoencephalitis occur, which can lead to permanent neurological damage and can even be fatal in 5-10% of neurologically ill persons. This mainly affects older people and patients with a cardiovascular disease or immunosuppression [5].

The majority of horses infected with WNV do not develop any clinical symptoms similar to humans. However, about 8% of infected horses develop marked central nervous deficits due to meningitis or encephalitis. The clinically noticeable central nervous disorders include stumbling, paralysis of the hindquarters, ataxia, general weakness, muscle tremors and paralysis up to the point where the animals are immobilised. The affected horses rarely show febrile general illnesses. Horses with clinical neurological signs can survive the infection, but this form of the disease is associated with a high mortality rate of about 30% to 50%. Surviving horses often retain neurological damage for life [6].

Before 2018, autochthonous WNV infections did not occur in Germany, although suitable vectors (mosquitoes) and hosts (wild birds) were naturally present and WNV had already been circulating in immediate neighbouring countries (especially in southern and south-eastern Europe) for many years. It was therefore only a matter of time before WNV made its way to Germany. The Usutu virus (USUV), also belonging to the flavivirus family, also known as the so-called "blackbird virus", which is very closely related to WNV, already showed a good 10 years ago that previously non-existent arboviruses can spread in our geographical latitudes in a short time. USUV uses an almost identical multiplication cycle between native mosquitoes as vectors and wild birds as virus reservoirs [7]. It was first detected in a mosquito pool in Weinheim (Baden-Württemberg) in 2010, led to a regionally limited mass mortality of blackbirds there a year later, and has since established itself initially in southwestern Germany through annually recurring epidemics among wild birds [8]. However, since 2016, USUV began to spread further northwards [9, 10]. Interestingly, at least four different USUV strains have been introduced into Germany over the years. In summer/autumn 2018, there was a pronounced USUV epidemic among wild and zoo birds, as well as a nationwide spread of this arbovirus within Germany for the first time [11]. The strong spread was presumably favoured by favourable climatic conditions, as an above-average warm summer with very long-lasting heat periods and a preceding wet spring greatly favoured virus multiplication in mosquitoes.

As a result, the geographical spread of not only USUV but also WNV within the European Union was strongly accelerated in the 2018 season [12]. At the end of August 2018, a WNV-infected zoo bird was found for the first time in Halle (Saale) [13] and then by the end of the year a further 11 cases were found in domestic wild and aviary birds. Furthermore, WNV infections were detected in two clinically ill horses in the eastern part of Germany. The question of whether the virus would overwinter in native mosquitoes remained exciting. According to current data, this was the case, as cases of infection again occurred in birds as early as the beginning of July 2019. By early November 2019, a much larger wave of disease followed, with 76 infections in birds and 36 detections in horses confirmed by the Friedrich Loeffler Institute's National Reference Laboratory for WNV infections. The WNV hotspots for 2019 were again in Saxony, Saxony-Anhalt, Berlin and Brandenburg. For the first time, there was also detection in a wild bird in Hamburg and in a horse in Thuringia [ 14].

In view of this fulminant occurrence, it was not surprising that the first autochthonous infection was also detected in a patient in the Leipzig area by the Bernhard Nocht Institute for Tropical Medicine (BNITM) in Hamburg (in its function as National Reference Centre for Tropical Infectious Agents in Humans) at the end of September 2019. By the end of 2019, three further human cases of the disease had been confirmed in this region, as well as one case in Berlin [15]. Also in 2019, WNV was detected in mosquitoes in the Berlin area for the first time [16]. In 2020, there was also a significant wave of disease in the previously affected areas. There were 64 WNV infections diagnosed in zoo and wild birds and 22 diseases in horses in the previous endemic areas with a further geographical spread in the federal states of Brandenburg and Thuringia, as well as for the first time in the district of Helmstedt in Lower Saxony [17]. In humans, a total of 22 autochthonous infections were detected in 2020, all of which were localised in the previously affected areas, and in addition to asymptomatic blood donors, diseases with severe neurological courses occurred in some cases, as well as one death for the first time [18, 19]. WNV-infected mosquitoes were also detected again in 2020.

Initial molecular biological investigations of the WNV isolates from 2018 showed that the virus entered was a lineage 2 WNV, which revealed a close relationship to an isolate from a mosquito caught in the Czech Republic in 2013 and an isolate from a blood donor in Austria in 2015 [13]. A detailed phylogenetic study of the 2018 and 2019 isolates revealed that there must have been multiple WNV entries into Germany. Strains of a specific phylogenetic clade (so-called "Eastern German WNV clade") dominated in 2018 and 2019, which in all likelihood can be attributed to a single entry event into Germany [14]. These WNV strains encode a mutation associated with a characteristic amino acid exchange, which probably leads to a higher viral fitness, thus inferring the strong geographical spread. Phylogenetic analyses further demonstrate that WNV survived the winter of 2018/19 unscathed. In addition, exceptionally high temperatures in 2018/2019 enabled a low extrinsic incubation period (EIP), i.e. faster temporal replication of the virus in the mosquito, which drove the emergence of epizootics [14]. Phylogenetic studies of avian, equine and mosquito isolates for 2020 are currently underway. For 9 human isolates from the Leipzig area, it has already been shown that these in turn are assigned to WNV lineage 2 and here specifically to the "Eastern German WNV clade", which indicates an endemic seasonal circulation of these viruses in this region [19].

No vaccines are currently available to combat the disease in humans, but various vaccine candidates are in development and some have already been tested in clinical trials [20]. Recombinant vaccines based on other flaviviruses such as the yellow fever virus are particularly promising. These show the formation of neutralising antibodies already after the first vaccine dose in both younger and older test persons. However, due to the only sporadic occurrence of WNV disease in humans, there are problems initiating large phase III studies on the efficacy of these vaccines, so that currently none of these vaccine candidates have reached the third phase of clinical trials. In contrast, some effective WNV vaccines are available to veterinary medicine. In Germany, three vaccines are currently licensed for horses. The Standing Committee on Vaccination in Veterinary Medicine (StIKo Vet) already recommended vaccination of horses temporarily or permanently located in a WNV-affected area in Germany in October 2018 [21]. It is recommended to repeat the vaccination annually before the mosquito season in spring in order to achieve the highest antibody titres during the period of most probable virus transmission by mosquitoes. Furthermore, at the beginning of 2019, the StIKo Vet reiterated its recommendation to vaccinate horses in already affected areas, as well as all horses that are only briefly taken there for equestrian events during the mosquito season. The StIKo Vet continues to maintain this recommendation for the 2021 season [22].

In order to better assess the risk with regard to infections in humans and animals as well as the spread of WNV and USUV, a wild bird monitoring project has been carried out at the Institute for New and Novel Animal Pathogens (INNT) of the FLI for many years. In cooperation with various partners, blood samples and carcasses of wild, aviary and zoo birds are examined for the presence of arboviruses. Volunteer ornithologists and staff from the State Veterinary Investigation Offices, the bird clinics at the veterinary faculties, the BNITM, the Aktionsgemeinschaft zur Bekämpfung der Stechmückenplage (KABS), the Naturschutzbund (NABU) as well as several veterinary bird practices and wild bird capture stations support this wild bird monitoring project, which is unique in Germany, by generating samples for it. For the past 10 years, this wild bird network has made it possible to track the exact regional spread of USUV and contributed significantly to the detection of WNV introduction and its spread [11, 13, 14, 23]. In the future, it should also serve to reveal new WNV hot spots regionally and more quickly through the increased involvement of zoo birds, in order to better assess the risk potential for humans in certain areas.

The further spread of WNV within Germany cannot be predicted at present. However, further illnesses in humans and animals are to be expected. If conditions remain favourable for the virus - i.e. susceptible birds as reservoir hosts, indigenous mosquitoes as competent vectors and temperatures favourable for the multiplication cycle in the mosquito season and subsequent hibernation (mild winters with warm, humid springs) - then an establishment of WNV in the affected regions and further regional spread within Germany cannot be ruled out in the coming years. Experience with WNV in southern and south-eastern European countries supports these assumptions. Therefore, awareness of the importance of WNV and USUV as emerging pathogens in Germany should be raised among veterinarians and human health professionals.

Authors

Dr. Ute Ziegler
Institut für neue und neuartige Tierseuchenerreger
Friedrich-Loeffler-Institut
Südufer 10
17493 Greifswald-Insel Riems
Email: ute.ziegler@fli.de

Dr. Michael Sieg
Institut für Virologie
Veterinärmedizinische Fakultät der Universität Leipzig
An den Tierkliniken 29
04103 Leipzig
Email: michael.sieg@uni-leipzig.de

Prof. Dr. Martin H. Groschup
Institut für neue und neuartige Tierseuchenerreger
Friedrich-Loeffler-Institut
Südufer 10
17493 Greifswald-Insel Riems
Email: martin.groschup@fli.de

Univ.-Prof. Dr. Dr. Thomas Vahlenkamp
Institut für Virologie
Veterinärmedizinische Fakultät der Universität Leipzig
An den Tierkliniken 29
04103 Leipzig
Email: thomas.vahlenkamp@uni-leipzig.de

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