Surveillance for European bat lyssavirus in Swiss bats

The genome sequence of Daubenton's bat, Myotis daubentonii (Kuhl, 1817)

We present a genome assembly from an individual male Myotis daubentonii (Daubenton's bat; Chordata; Mammalia; Chiroptera; Vespertilionidae). The genome sequence is 2,127.8 megabases in span. Most of the assembly is scaffolded into 23 chromosomal pseudomolecules, including the X and Y sex chromosomes. The mitochondrial genome has also been assembled and is 17.34 kilobases in length.

Phylogeographic Aspects of Bat Lyssaviruses in Europe: A Review

During the last few decades, bat lyssaviruses have become the topic of intensive molecular and epidemiological investigations. Since ancient times, rhabdoviruses have caused fatal encephalitis in humans which has led to research into effective strategies for their eradication. Modelling of potential future cross-species virus transmissions forms a substantial component of the recent infection biology of rabies. In this article, we summarise the available data on the phylogeography of both bats and lyssaviruses in Europe and the adjacent reg ions, especially in the contact zone between the Palearctic and Ethiopian realms. Within these zones, three bat families are present with high potential for cross-species transmission and the spread of lyssaviruses in Phylogroup II to Europe (part of the western Palearctic). The lack of effective therapies for rabies viruses in Phylogroup II and the most divergent lyssaviruses generates impetus for additional phylogenetic and virological research within this geographical region.

Retrospective Enhanced Bat Lyssavirus Surveillance in Germany between 2018-2020

Lyssaviruses are the causative agents for rabies, a zoonotic and fatal disease. Bats are the ancestral reservoir host for lyssaviruses, and at least three different lyssaviruses have been found in bats from Germany. Across Europe, novel lyssaviruses were identified in bats recently and occasional spillover infections in other mammals and human cases highlight their public health relevance. Here, we report the results from an enhanced passive bat rabies surveillance that encompasses samples without human contact that would not be tested under routine conditions. To this end, 1236 bat brain samples obtained between 2018 and 2020 were screened for lyssaviruses via several RT-qPCR assays. European bat lyssavirus type 1 (EBLV-1) was dominant, with 15 positives exclusively found in serotine bats (Eptesicus serotinus) from northern Germany. Additionally, when an archived set of bat samples that had tested negative for rabies by the FAT were screened in the process of assay validation, four samples tested EBLV-1 positive, including two detected in Pipistrellus pipistrellus. Subsequent phylogenetic analysis of 17 full genomes assigned all except one of these viruses to the A1 cluster of the EBLV-1a sub-lineage. Furthermore, we report here another Bokeloh bat lyssavirus (BBLV) infection in a Natterer's bat (Myotis nattereri) found in Lower Saxony, the tenth reported case of this novel bat lyssavirus.

Metagenomic analysis of fecal and tissue samples from 18 endemic bat species in Switzerland revealed a diverse virus composition including potentially zoonotic viruses

Many recent disease outbreaks in humans had a zoonotic virus etiology. Bats in particular have been recognized as reservoirs to a large variety of viruses with the potential to cross-species transmission. In order to assess the risk of bats in Switzerland for such transmissions, we determined the virome of tissue and fecal samples of 14 native and 4 migrating bat species. In total, sequences belonging to 39 different virus families, 16 of which are known to infect vertebrates, were detected. Contigs of coronaviruses, adenoviruses, hepeviruses, rotaviruses A and H, and parvoviruses with potential zoonotic risk were characterized in more detail. Most interestingly, in a ground stool sample of a Vespertilio murinus colony an almost complete genome of a Middle East respiratory syndrome-related coronavirus (MERS-CoV) was detected by Next generation sequencing and confirmed by PCR. In conclusion, bats in Switzerland naturally harbour many different viruses. Metagenomic analyses of non-invasive samples like ground stool may support effective surveillance and early detection of viral zoonoses.

Serological Survey of Lyssaviruses in Polish Bats in the Frame of Passive Rabies Surveillance Using an Enzyme-Linked Immunosorbent Assay

Background: Bats are known to host a number of nonpathogenic viruses, as well as highly pathogenic viruses causing fatal diseases like rabies. Serological surveys as part of active and passive bat rabies surveillance mainly use seroneutralization assays, demonstrating the presence of lyssavirus-specific antibodies in a variety of European bats, particularly against European bat lyssaviruses type 1 (EBLV-1). Here, we present the first serological survey in European bats of this kind during which European bats from Poland collected in the frame of passive rabies surveillance between 2012 and 2018, as well as Serotine bats (Eptesicus serotinus) and North American Big Brown bats (Eptesicus fuscus) from previous experimental studies, were tested using a commercial ELISA kit for the detection of anti-lyssavirus antibodies. Results: Lyssavirus-specific antibodies were detected in 35 (30.4%) out of 115 Polish bats of both sexes, representing nine out of 13 identified bat species endemic mainly to Central Southern Europe and Western Asia, i.e., Eptesicus serotinus, Nyctalus noctula, Myotis daubentonii, Plecotus auritus, Vespertillo murinus, Pipistrellus pipistrellus, Pipistrellus pipilstrellus/Pipistrellus pygmaeus, Myotis brandtii, and Barbastella barbastellus. Seroprevalence was highest in bat species of Nyctalus noctula, Eptesicus serotinus, Plecotus auritus, and Myotis daubentonii. More than 60% of the ELISA seropositive bats originated from the voivodeships of Silesia, Lower-Silesian, Warmian-Mazurian, and Mazowian. Rabies-specific antibodies were also found in Eptesicus fuscus bats from North America. Conclusions: The study demonstrates the principal application of the BioPro Rabies ELISA Ab Kit for the detection of anti-lyssavirus specific antibodies in body fluids and serum samples of bats. However, results may only be reliable for North American bats, whereas interpretation of results for European bats per se is difficult because proper validation of the test is hampered by the protected status of these species.

Molecular and serological survey of lyssaviruses in Croatian bat populations

BACKGROUND

Rabies is the only known zoonotic disease of bat origin in Europe. The disease is caused by species belonging to the genus Lyssavirus. Five Lyssavirus species, i.e., European bat lyssavirus (EBLV)-1, EBLV-2, Bokeloh bat lyssavirus, Lleida bat lyssavirus, and West Caucasian bat virus, have been identified in European bats. More recently, a proposed sixth species, Kotalahti bat lyssavirus, was detected. Thus, in this study, active surveillance was initiated in order to obtain insights into the prevalence of lyssaviruses in Croatian bat populations and to improve our understanding of the public health threat of infected bats.

RESULTS

In total, 455 bats were caught throughout Continental and Mediterranean Croatia. Antibodies were found in 20 of 350 bats (5.71%, 95% confidence interval 3.73-8.66). The majority of seropositive bats were found in Trbušnjak cave (Continental Croatia, Eastern part), and most seropositive bats belonged to Myotis myotis (13/20). All oropharyngeal swabs were negative for the presence of Lyssavirus.

CONCLUSIONS

The presence of lyssaviruses in bat populations was confirmed for the first time in Croatia and Southeastern Europe. The results of this study suggest the need for further comprehensive analyses of lyssaviruses in bats in this part of Europe.

Molecular Epidemiology and Evolution of European Bat Lyssavirus 2

Bat rabies cases in Europe are mainly attributed to two lyssaviruses, namely European Bat Lyssavirus 1 (EBLV-1) and European Bat Lyssavirus 2 (EBLV-2). Prior to the death of a bat worker in Finland in 1985, very few bat rabies cases were reported. Enhanced surveillance in the two subsequent years (1986-1987) identified 263 cases (more than a fifth of all reported cases to date). Between 1977 and 2016, 1183 cases of bat rabies were reported, with the vast majority (>97%) being attributed to EBLV-1. In contrast, there have been only 39 suspected cases of EBLV-2, of which 34 have been confirmed by virus typing and presently restricted to just two bat species; Myotis daubentonii and Myotis dasycneme. The limited number of EBLV-2 cases in Europe prompted the establishment of a network of European reference laboratories to collate all available viruses and data. Despite the relatively low number of EBLV-2 cases, a large amount of anomalous data has been published in the scientific literature, which we have here reviewed and clarified. In this review, 29 EBLV-2 full genome sequences have been analysed to further our understanding of the diversity and molecular evolution of EBLV-2 in Europe. Analysis of the 29 complete EBLV-2 genome sequences clearly corroborated geographical relationships with all EBLV-2 sequences clustering at the country level irrespective of the gene studied. Further geographical clustering was also observed at a local level. There are high levels of homogeneity within the EBLV-2 species with nucleotide identities ranging from 95.5-100% and amino acid identities between 98.7% and 100%, despite the widespread distribution of the isolates both geographically and chronologically. The mean substitution rate for EBLV-2 across the five concatenated genes was 1.65 × 10^-5, and evolutionary clock analysis confirms the slow evolution of EBLV-2 both between and within countries in Europe. This is further supported by the first detailed EBLV-2 intra-roost genomic analysis whereby a relatively high sequence homogeneity was found across the genomes of three EBLV-2 isolates obtained several years apart (2007, 2008, and 2014) from M. daubentonii at the same site (Stokesay Castle, Shropshire, UK).

Second case of European bat lyssavirus type 2 detected in a Daubenton's bat in Finland

European bat lyssavirus type 2 (EBLV-2) was detected in Finland in a Daubenton’s bat (Myotis daubentonii) found in the municipality of Inkoo (60°02′45″N, 024°00′20″E). The bat showed neurological signs and was later found dead. The laboratory analysis revealed the presence of lyssavirus, and the virus was characterized as EBLV-2. This isolation of EBLV-2 was the second time that the virus has been detected in a Daubenton’s bat in Finland. This provides additional proof that EBLV-2 is endemic in the Finnish Daubenton’s bat population.

First detection of European bat lyssavirus type 2 (EBLV-2) in Norway

BACKGROUND

In Europe, bat rabies is primarily attributed to European bat lyssavirus type 1 (EBLV-1) and European bat lyssavirus type 2 (EBLV-2) which are both strongly host-specific. Approximately thirty cases of infection with EBLV-2 in Daubenton's bats (Myotis daubentonii) and pond bats (M. dasycneme) have been reported. Two human cases of rabies caused by EBLV-2 have also been confirmed during the last thirty years, while natural spill-over to other non-flying mammals has never been reported. Rabies has never been diagnosed in mainland Norway previously.

CASE PRESENTATION

In late September 2015, a subadult male Daubenton's bat was found in a poor condition 800 m above sea level in the southern part of Norway. The bat was brought to the national Bat Care Centre where it eventually displayed signs of neurological disease and died after two days. EBLV-2 was detected in brain tissues by polymerase chain reaction (PCR) followed by sequencing of a part of the nucleoprotein gene, and lyssavirus was isolated in neuroblastoma cells.

CONCLUSIONS

The detection of EBLV-2 in a bat in Norway broadens the knowledge on the occurrence of this zoonotic agent. Since Norway is considered free of rabies, adequate information to the general public regarding the possibility of human cases of bat-associated rabies should be given. No extensive surveillance of lyssavirus infections in bats has been conducted in the country, and a passive surveillance network to assess rabies prevalence and bat epidemiology is highly desired.

Evolutionary trends of European bat lyssavirus type 2 including genetic characterization of Finnish strains of human and bat origin 24 years apart

Among other Lyssaviruses, Daubenton’s and pond-bat-related European bat lyssavirus type 2 (EBLV-2) can cause human rabies. To investigate the diversity and evolutionary trends of EBLV-2, complete genome sequences of two Finnish isolates were analysed. One originated from a human case in 1985, and the other originated from a bat in 2009. The overall nucleotide and deduced amino acid sequence identity of the two Finnish isolates were high, as well as the similarity to fully sequenced EBLV-2 strains originating from the UK and the Netherlands. In phylogenetic analysis, the EBLV-2 strains formed a monophyletic group that was separate from other bat-type lyssaviruses, with significant support. EBLV-2 shared the most recent common ancestry with Bokeloh bat lyssavirus (BBLV) and Khujan virus (KHUV). EBLV-2 showed limited diversity compared to RABV and appears to be well adapted to its host bat species. The slow tempo of viral evolution was evident in the estimations of divergence times for EBLV-2: the current diversity was estimated to have built up during the last 2000 years, and EBLV-2 diverged from KHUV about 8000 years ago. In a phylogenetic tree of partial N gene sequences, the Finnish EBLV-2 strains clustered with strains from Central Europe, supporting the hypothesis that EBLV-2 circulating in Finland might have a Central European origin. The Finnish EBLV-2 strains and a Swiss strain were estimated to have diverged from other EBLV-2 strains during the last 1000 years, and the two Finnish strains appear to have evolved from a common ancestor during the last 200 years.

Real-Time RT-PCR for the Detection of Lyssavirus Species

The causative agents of rabies are single-stranded, negative-sense RNA viruses in the genus Lyssavirus of Rhabdoviridae, consisting of twelve classified and three as yet unclassified species including classical rabies virus (RABV). Highly neurotropic RABV causes rapidly progressive encephalomyelitis with nearly invariable fatal outcome. Rapid and reliable diagnosis of rabies is highly relevant for public and veterinary health. Due to growing variety of the genus Lyssavirus observed, the development of suitable molecular assays for diagnosis and differentiation is challenging. This work focused on the establishment of a suitable real-time RT-PCR technique for rabies diagnosis as a complement to fluorescent antibody test and rabies tissue culture infection test as gold standard for diagnosis and confirmation. The real-time RT-PCR was adapted with the goal to detect the whole spectrum of lyssavirus species, for nine of which synthesized DNA fragments were used. For the detection of species, seven probes were developed. Serial dilutions of the rabies virus strain CVS-11 showed a 100-fold higher sensitivity of real-time PCR compared to heminested RT-PCR. Using a panel of thirty-one lyssaviruses representing four species, the suitability of the protocol could be shown. Phylogenetic analysis of the sequences obtained by heminested PCR allowed correct classification of all viruses used.

Enhanced passive bat rabies surveillance in indigenous bat species from Germany--a retrospective study

In Germany, rabies in bats is a notifiable zoonotic disease, which is caused by European bat lyssaviruses type 1 and 2 (EBLV-1 and 2), and the recently discovered new lyssavirus species Bokeloh bat lyssavirus (BBLV). As the understanding of bat rabies in insectivorous bat species is limited, in addition to routine bat rabies diagnosis, an enhanced passive surveillance study, i.e. the retrospective investigation of dead bats that had not been tested for rabies, was initiated in 1998 to study the distribution, abundance and epidemiology of lyssavirus infections in bats from Germany. A total number of 5478 individuals representing 21 bat species within two families were included in this study. The Noctule bat (Nyctalus noctula) and the Common pipistrelle (Pipistrellus pipistrellus) represented the most specimens submitted. Of all investigated bats, 1.17% tested positive for lyssaviruses using the fluorescent antibody test (FAT). The vast majority of positive cases was identified as EBLV-1, predominately associated with the Serotine bat (Eptesicus serotinus). However, rabies cases in other species, i.e. Nathusius' pipistrelle bat (Pipistrellus nathusii), P. pipistrellus and Brown long-eared bat (Plecotus auritus) were also characterized as EBLV-1. In contrast, EBLV-2 was isolated from three Daubenton's bats (Myotis daubentonii). These three cases contribute significantly to the understanding of EBLV-2 infections in Germany as only one case had been reported prior to this study. This enhanced passive surveillance indicated that besides known reservoir species, further bat species are affected by lyssavirus infections. Given the increasing diversity of lyssaviruses and bats as reservoir host species worldwide, lyssavirus positive specimens, i.e. both bat and virus need to be confirmed by molecular techniques.

Bat rabies surveillance in Finland

Background

In 1985, a bat researcher in Finland died of rabies encephalitis caused by European bat lyssavirus type 2 (EBLV-2), but an epidemiological study in 1986 did not reveal EBLV-infected bats. In 2009, an EBLV-2-positive Daubenton’s bat was detected. The EBLV-2 isolate from the human case in 1985 and the isolate from the bat in 2009 were genetically closely related. In order to assess the prevalence of EBLVs in Finnish bat populations and to gain a better understanding of the public health risk that EBLV-infected bats pose, a targeted active surveillance project was initiated.

Results

Altogether, 1156 bats of seven species were examined for lyssaviruses in Finland during a 28–year period (1985–2012), 898 in active surveillance and 258 in passive surveillance, with only one positive finding of EBLV-2 in a Daubenton’s bat in 2009. In 2010–2011, saliva samples from 774 bats of seven species were analyzed for EBLV viral RNA, and sera from 423 bats were analyzed for the presence of bat lyssavirus antibodies. Antibodies were detected in Daubenton’s bats in samples collected from two locations in 2010 and from one location in 2011. All seropositive locations are in close proximity to the place where the EBLV-2 positive Daubenton’s bat was found in 2009. In active surveillance, no EBLV viral RNA was detected.

Conclusions

These data suggest that EBLV-2 may circulate in Finland, even though the seroprevalence is low. Our results indicate that passive surveillance of dead or sick bats is a relevant means examine the occurrence of lyssavirus infection, but the number of bats submitted for laboratory analysis should be higher in order to obtain reliable information on the lyssavirus situation in the country.

Twenty years of active bat rabies surveillance in Germany: a detailed analysis and future perspectives

In Germany, active bat rabies surveillance was conducted between 1993 and 2012. A total of 4546 oropharyngeal swab samples from 18 bat species were screened for the presence of EBLV-1- , EBLV-2- and BBLV-specific RNA. Overall, 0·15% of oropharyngeal swab samples tested EBLV-1 positive, with the majority originating from Eptesicus serotinus. Interestingly, out of seven RT–PCR-positive oropharyngeal swabs subjected to virus isolation, viable virus was isolated from a single serotine bat (E. serotinus). Additionally, about 1226 blood samples were tested serologically, and varying virus neutralizing antibody titres were found in at least eight different bat species. The detection of viral RNA and seroconversion in repeatedly sampled serotine bats indicates long-term circulation of the virus in a particular bat colony. The limitations of random-based active bat rabies surveillance over passive bat rabies surveillance and its possible application of targeted approaches for future research activities on bat lyssavirus dynamics and maintenance are discussed.

Ecological factors associated with European bat lyssavirus seroprevalence in spanish bats

Bats have been proposed as major reservoirs for diverse emerging infectious viral diseases, with rabies being the best known in Europe. However, studies exploring the ecological interaction between lyssaviruses and their natural hosts are scarce. This study completes our active surveillance work on Spanish bat colonies that began in 1992. Herein, we analyzed ecological factors that might affect the infection dynamics observed in those colonies. Between 2001 and 2011, we collected and tested 2,393 blood samples and 45 dead bats from 25 localities and 20 bat species. The results for dead confirmed the presence of EBLV-1 RNA in six species analyzed (for the first time in Myotis capaccinii). Samples positive for European bat lyssavirus-1 (EBLV-1)-neutralizing antibodies were detected in 68% of the localities sampled and in 13 bat species, seven of which were found for the first time (even in Myotis daubentonii, a species to date always linked to EBLV-2). EBLV-1 seroprevalence (20.7%) ranged between 11.1 and 40.2% among bat species and seasonal variation was observed, with significantly higher antibody prevalence in summer (July). EBLV-1 seroprevalence was significantly associated with colony size and species richness. Higher seroprevalence percentages were found in large multispecific colonies, suggesting that intra- and interspecific contacts are major risk factors for EBLV-1 transmission in bat colonies. Although bat-roosting behavior strongly determines EBLV-1 variability, we also found some evidence that bat phylogeny might be involved in bat-species seroprevalence. The results of this study highlight the importance of life history and roost ecology in understanding EBLV-1-prevalence patterns in bat colonies and also provide useful information for public health officials.

Bat Rabies

The lyssaviruses are a diverse group of viruses capable of causing rabies, which is an invariably fatal encephalitic disease in both humans and animals. Currently, the lyssavirus genus consists of 12 species with 11 of these distinct species having been isolated from bats. The basis for the apparent geographical segregation of bat lyssavirus infection between the Old and New World is poorly understood. In the New World species of insectivorous, frugivorous, and hematophagous bats, all represent important reservoirs of rabies virus. In contrast, rabies virus has never been detected in Old World bat populations, despite being endemic in terrestrial mammals. Instead, both insectivorous and frugivorous bat species across the Old World appear to act as reservoirs for the non-rabies lyssaviruses. In this chapter, we describe the association of the different lyssaviruses with different bat species across the world, classifying bat species by their feeding behavior.

Bat rabies surveillance in Europe

Rabies is the oldest known zoonotic disease and was also the first recognized bat associated infection in humans. To date, four different lyssavirus species are the causative agents of rabies in European bats: the European Bat Lyssaviruses type 1 and 2 (EBLV-1, EBLV-2), the recently discovered putative new lyssavirus species Bokeloh Bat Lyssavirus (BBLV) and the West Caucasian Bat Virus (WCBV). Unlike in the new world, bat rabies cases in Europe are comparatively less frequent, possibly as a result of varying intensity of surveillance. Thus, the objective was to provide an assessment of the bat rabies surveillance data in Europe, taking both reported data to the WHO Rabies Bulletin Europe and published results into account. In Europe, 959 bat rabies cases were reported to the RBE in the time period 1977-2010 with the vast majority characterized as EBLV-1, frequently isolated in the Netherlands, North Germany, Denmark, Poland and also in parts of France and Spain. Most EBLV-2 isolates originated from the United Kingdom (UK) and the Netherlands, and EBLV-2 was also detected in Germany, Finland and Switzerland. Thus far, only one isolate of BBLV was found in Germany. Published passive bat rabies surveillance comprised testing of 28 of the 52 different European bat species for rabies. EBLV-1 was isolated exclusively from Serotine bats (Eptesicus serotinus and Eptesicus isabellinus), while EBLV-2 was detected in 14 Daubenton's bats (Myotis daubentonii) and 5 Pond bats (Myotis dasycneme). A virus from a single Natterer's bat (Myotis nattereri) was characterized as BBLV. During active surveillance, only oral swabs from 2 Daubenton's bats (EBLV-2) and from several Eptesicus bats (EBLV-1) yielded virus positive RNA. Virus neutralizing antibodies against lyssaviruses were detected in various European bat species from different countries, and its value and implications are discussed.

Molecular epidemiology of bat lyssaviruses in Europe

Bat rabies cases in Europe are principally attributed to two lyssaviruses, namely European bat lyssavirus type 1 (EBLV-1) and European bat lyssavirus type 2 (EBLV-2). Between 1977 and 2011, 961 cases of bat rabies were reported to Rabies Bulletin Europe, with the vast majority (>97%) being attributed to EBLV-1. There have been 25 suspected cases of EBLV-2, of which 22 have been confirmed. In addition, two single isolations of unique lyssaviruses from European insectivorous bats were reported in south-west Russia in 2002 (West Caucasian bat virus) and in Germany in 2010 (Bokeloh bat lyssavirus). In this review, we present phylogenetic analyses of the EBLV-1 and EBLV-2 using partial nucleoprotein (N) gene sequences. In particular, we have analysed all EBLV-2 cases for which viral sequences (N gene, 400 nucleotides) are available (n = 21). Oropharyngeal swabs collected from two healthy Myotis daubentonii during active surveillance programmes in Scotland and Switzerland also yielded viral RNA (EBLV-2). Despite the relatively low number of EBLV-2 cases, a surprisingly large amount of anomalous data has been published in the scientific literature and Genbank, which we have collated and clarified. For both viruses, geographical relationships are clearly defined on the phylogenetic analysis. Whilst there is no clear chronological clustering for either virus, there is some evidence for host specific relationships, particularly for EBLV-1 where more host variation has been observed. Further genomic regions must be studied, in particular for EBLV-1 isolates from Spain and the EBLV-2 isolates to provide support for the existence of sublineages.

Bat rabies, public health and European bat conservation

Most records of European bat lyssaviruses (EBLVs) are confined to three species - the serotine bat for EBLV1 (900 records) and Daubenton's bat and the pond bat for EBLV2 (25 records). High levels of seroprevalence, which may vary from year to year, are also recorded. All bat vectors of EBLVs are synanthropic, some exclusively so. Despite this, there have been only five cases of human rabies resulting from EBLV infection in the 590 million people of greater Europe during the last 35 years. These have triggered major programmes of surveillance in many European countries. The emphasis on active versus passive surveillance and the intensity with which they have been carried out has varied from country to country. Both involve cooperation between bat researchers, virologists and public health officials and the latter, in particular, engages amateur bat workers and members of the public. Bat NGOs throughout Europe have worked to persuade the public not to handle bats or to do so only with gloved hands and, in the case of bat workers, to receive pre-exposure immunization. They have also countered negative media coverage of bat rabies. Householders with bat roosts in their dwellings have in general been persuaded to retain their bats. Attempts have been made to persuade all European countries to establish comparable EBLV surveillance programmes. In the last 25 years, virologists, public health officials, bat biologists and conservationists, both amateur and professional have worked closely and collaboratively for the protection of the public and the conservation of bats, with little polarization of views.