Histopathology and immunohistochemistry of bats infected by Australian bat lyssavirus

Incursion of European Bat Lyssavirus 1 (EBLV-1) in Serotine Bats in the United Kingdom

Lyssaviruses are an important genus of zoonotic viruses which cause the disease rabies. The United Kingdom is free of classical rabies (RABV). However, bat rabies due to European bat lyssavirus 2 (EBLV-2), has been detected in Daubenton’s bats (Myotis daubentonii) in Great Britain since 1996, including a fatal human case in Scotland in 2002. Across Europe, European bat lyssavirus 1 (EBLV-1) is commonly associated with serotine bats (Eptesicus serotinus). Despite the presence of serotine bats across large parts of southern England, EBLV-1 had not previously been detected in this population. However, in 2018, EBLV-1 was detected through passive surveillance in a serotine bat from Dorset, England, using a combination of fluorescent antibody test, reverse transcription-PCR, Sanger sequencing and immunohistochemical analysis. Subsequent EBLV-1 positive serotine bats have been identified in South West England, again through passive surveillance, during 2018, 2019 and 2020. Here, we confirm details of seven cases of EBLV-1 and present similarities in genetic sequence indicating that emergence of EBLV-1 is likely to be recent, potentially associated with the natural movement of bats from the near continent

Experimental Lagos bat virus infection in straw-colored fruit bats: A suitable model for bat rabies in a natural reservoir species

Rabies is a fatal neurologic disease caused by lyssavirus infection. Bats are important natural reservoir hosts of various lyssaviruses that can be transmitted to people. The epidemiology and pathogenesis of rabies in bats are poorly understood, making it difficult to prevent zoonotic transmission. To further our understanding of lyssavirus pathogenesis in a natural bat host, an experimental model using straw-colored fruit bats (Eidolon helvum) and Lagos bat virus, an endemic lyssavirus in this species, was developed. To determine the lowest viral dose resulting in 100% productive infection, bats in five groups (four bats per group) were inoculated intramuscularly with one of five doses, ranging from 100.1 to 104.1 median tissue culture infectious dose (TCID50). More bats died due to the development of rabies after the middle dose (102.1 TCID50, 4/4 bats) than after lower (101.1, 2/4; 101.1, 2/4) or higher (103.1, 2/4; 104.1, 2/4) doses of virus. In the two highest dose groups, 4/8 bats developed rabies. Of those bats that remained healthy 3/4 bats seroconverted, suggesting that high antigen loads can trigger a strong immune response that abrogates a productive infection. In contrast, in the two lowest dose groups, 3/8 bats developed rabies, 1/8 remained healthy and seroconverted and 4/8 bats remained healthy and did not seroconvert, suggesting these doses are too low to reliably induce infection. The main lesion in all clinically affected bats was meningoencephalitis associated with lyssavirus-positive neurons. Lyssavirus antigen was detected in tongue epithelium (5/11 infected bats) rather than in salivary gland epithelium (0/11), suggesting viral excretion via the tongue. Thus, intramuscular inoculation of 102.1 TCID50 of Lagos bat virus into straw-colored fruit bats is a suitable model for lyssavirus associated bat rabies in a natural reservoir host, and can help with the investigation of lyssavirus infection dynamics in bats.

Emerging horizon for bat borne viral zoonoses

Bats are the only flying placental mammals that constitute the second largest order of mammals and present all around the world except in Arctic, Antarctica and a few oceanic islands. Sixty percent of emerging infectious diseases originating from animals are zoonotic and more than two-thirds of them originate in wildlife. Bats were evolved as a super-mammal for harboring many of the newly identified deadly diseases without any signs and lesions. Their unique ability to fly, particular diet, roosting behavior, long life span, ability to echolocate and critical susceptibility to pathogens make them suitable host to harbor numerous zoonotic pathogens like virus, bacteria and parasite. Many factors are responsible for the emergence of bat borne zoonoses but the most precipitating factor is human intrusions. Deforestation declined the natural habitat and forced the bats and other wild life to move out of their niche. These stressed bats, having lost foraging and behavioral pattern invade in proximity of human habitation. Either directly or indirectly they transmit the viruses to humans and animals. Development of fast detection modern techniques for viruses from the diseased and environmental samples and the lessons learned in the past helped in preventing the severity during the latest outbreaks.

Pathogenesis of bat rabies in a natural reservoir: Comparative susceptibility of the straw-colored fruit bat (Eidolon helvum) to three strains of Lagos bat virus

Rabies is a fatal neurologic disease caused by lyssavirus infection. People are infected through contact with infected animals. The relative increase of human rabies acquired from bats calls for a better understanding of lyssavirus infections in their natural hosts. So far, there is no experimental model that mimics natural lyssavirus infection in the reservoir bat species. Lagos bat virus is a lyssavirus that is endemic in straw-colored fruit bats (Eidolon helvum) in Africa. Here we compared the susceptibility of these bats to three strains of Lagos bat virus (from Senegal, Nigeria, and Ghana) by intracranial inoculation. To allow comparison between strains, we ensured the same titer of virus was inoculated in the same location of the brain of each bat. All bats (n = 3 per strain) were infected, and developed neurological signs, and fatal meningoencephalitis with lyssavirus antigen expression in neurons. There were three main differences among the groups. First, time to death was substantially shorter in the Senegal and Ghana groups (4 to 6 days) than in the Nigeria group (8 days). Second, each virus strain produced a distinct clinical syndrome. Third, the spread of virus to peripheral tissues, tested by hemi-nested reverse transcriptase PCR, was frequent (3 of 3 bats) and widespread (8 to 10 tissues positive of 11 tissues examined) in the Ghana group, was frequent and less widespread in the Senegal group (3/3 bats, 3 to 6 tissues positive), and was rare and restricted in the Nigeria group (1/3 bats, 2 tissues positive). Centrifugal spread of virus from brain to tissue of excretion in the oral cavity is required to enable lyssavirus transmission. Therefore, the Senegal and Ghana strains seem most suitable for further pathogenesis, and for transmission, studies in the straw-colored fruit bat.

Rabies is a neurologic disease that causes severe suffering and is almost always fatal. The disease is caused by infection with a virus of the genus Lyssavirus, of which 16 species are known. These viruses replicate in neurons, are excreted in the mouth, and are transmitted by bites. Dogs are the most important source of rabies for humans, but recently there is a relative increase in people contracting the disease from bats. To better understand the development of human rabies caused by these bat-acquired viruses, we need to study this disease in its bat host under controlled circumstances. To do so, we chose a naturally occurring lyssavirus–host combination: Lagos bat virus in straw-colored fruit bats. We compared three available strains of Lagos bat virus (all isolated from brains of this bat species) for their ability to mimic a natural infection. We used intracranial inoculation to ensure infection of the brain. All three strains infected brain neurons, resulting in fatal neurologic disease, however only two of the strains showed the ability to reach the site of excretion—the mouth—and were considered a suitable virus to use for further studies of this disease in bats.

Chiroptera

With over 1200 species identified, bats represent almost one quarter of the world’s mammals. Bats provide crucial environmental services, such as insect control and pollination, and inhabit a wide variety of ecological niches on all continents except Antarctica. Despite their ubiquity and ecological importance, relatively little has been published on diseases of bats, while much has been written on bats’ role as reservoirs in disease transmission. This chapter will focus on diseases and pathologic processes most commonly reported in captive and free-ranging bats. Unique anatomical and histological features and common infectious and non-infectious diseases will be discussed. As recognition of both the importance and vulnerability of bats grows, particularly following population declines in North America due to the introduction of the fungal disease white-nose syndrome, efforts should be made to better understand threats to the health of this unique group of mammals.

Comparative pathogenesis of rabies in bats and carnivores, and implications for spillover to humans

Bat-acquired rabies is becoming increasingly common, and its diagnosis could be missed partly because its clinical presentation differs from that of dog-acquired rabies. We reviewed the scientific literature to compare the pathogenesis of rabies in bats and carnivores-including dogs-and related this pathogenesis to differences in the clinical presentation of bat-acquired and dog-acquired rabies in human beings. For bat-acquired rabies, we found that the histological site of exposure is usually limited to the skin, the anatomical site of exposure is more commonly the face, and the virus might be more adapted for entry via the skin than for dog-acquired rabies. These factors could help to explain several differences in clinical presentation between individuals with bat-acquired and those with dog-acquired rabies. A better understanding of these differences should improve the recording of a patient's history, enable drawing up of a more sophisticated list of clinical characteristics, and therefore obtain an earlier diagnosis of rabies after contact with a bat or carnivore that has rabies.

Neurologic amebiasis caused by Balamuthia mandrillaris in an Indian flying fox (Pteropus giganteus)

A 4-5-month-old intact male Indian flying fox (Pteropus giganteus) was presented to the Baton Rouge Zoo's veterinary hospital with an acute onset of obtundation that was diagnosed with amebic encephalitis. Histologic examination revealed numerous amebic trophozoites within necrotic foci, affecting the occipital cerebrum and surrounding the mesencephalic aqueduct. The etiologic agent, Balamuthia mandrillaris, was determined by multiplex quantitative real-time polymerase chain reaction, immunohistochemistry, and indirect fluorescent antibody test. The current report documented a case of amebic encephalitis within the order Chiroptera.

Australian bat lyssavirus infection in two horses

In May 2013, the first cases of Australian bat lyssavirus infections in domestic animals were identified in Australia. Two horses (filly-H1 and gelding-H2) were infected with the Yellow-bellied sheathtail bat (YBST) variant of Australian bat lyssavirus (ABLV). The horses presented with neurological signs, pyrexia and progressing ataxia. Intra-cytoplasmic inclusion bodies (Negri bodies) were detected in some Purkinje neurons in haematoxylin and eosin (H&E) stained sections from the brain of one of the two infected horses (H2) by histological examination. A morphological diagnosis of sub-acute moderate non-suppurative, predominantly angiocentric, meningo-encephalomyelitis of viral aetiology was made. The presumptive diagnosis of ABLV infection was confirmed by the positive testing of the affected brain tissue from (H2) in a range of laboratory tests including fluorescent antibody test (FAT) and real-time PCR targeting the nucleocapsid (N) gene. Retrospective testing of the oral swab from (H1) in the real-time PCR also returned a positive result. The FAT and immunohistochemistry (IHC) revealed an abundance of ABLV antigen throughout the examined brain sections. ABLV was isolated from the brain (H2) and oral swab/saliva (H1) in the neuroblastoma cell line (MNA). Alignment of the genome sequence revealed a 97.7% identity with the YBST ABLV strain.

Differential chemokine responses in the murine brain following lyssavirus infection

The hallmark of lyssavirus infection is lethal encephalomyelitis. Previous studies have reported distinct lyssavirus isolate-related differences in severity of cellular recruitment into the encephalon in a murine model of infection following peripheral inoculation with rabies virus (RABV) and European bat lyssavirus (EBLV)-1 and -2. In order to understand the role of chemokines in this process, comparative studies of the chemokine pattern, distribution and production in response to infection with these lyssaviruses were undertaken. Expression of CCL2, CCL5 and CXCL10 was observed throughout the murine brain with a distinct caudal bias in distribution, similar to both inflammatory changes and virus antigen distribution. CCL2 immunolabelling was localized to neuronal and astroglial populations. CCL5 immunolabelling was only detected in the astroglia, while CXCL10 labelling, although present in the astroglia, was more prominent in neurons. Isolate-dependent differences in the amount of chemokine immunolabelling in specific brain regions and chemokine production by neurons in vitro were observed, with a greater expression of CCL5 in vivo and CXCL10 production in vitro after EBLV infection. Additionally, strong positive associations between chemokine immunolabelling and perivascular cuffing and, to a lesser extent, virus antigen score were also observed. These differences in chemokine expression may explain the variation in severity of encephalitic changes observed in animals infected with different lyssavirus isolates.

Pathobiology of rabies virus and the European bat lyssaviruses in experimentally infected mice

A comparison of the clinicopathology of European bat lyssavirus (EBLV) types-1 and -2 and of rabies virus was undertaken. Following inoculation of mice at a peripheral site with these viruses, clinical signs of rabies and distribution of virus antigen in the mouse brain were examined. The appearance of clinical signs of disease varied both within and across the different virus species, with variation in incubation periods and weight loss throughout disease progression. The distribution of viral antigen throughout the regions of the brain examined was similar for each of the isolates during the different stages of disease progression, suggesting that antigen distribution was not associated with clinical presentation. However, specific regions of the brain including the cerebellum, caudal medulla, hypothalamus and thalamus, showed notable differences in the proportion of virus antigen positive cells present in comparison to other brain regions suggesting that these areas are important in disease development irrespective of virus species.

European bat lyssaviruses: Distribution, prevalence and implications for conservation

Worldwide, there are more than 1100 species of the Order Chiroptera, 45 of which are present in Europe, and 16 in the UK. Bats are reservoirs of, or can be infected by, several viral diseases, including rabies virus strains (in the Lyssavirus genus). Within this genus are bat variants that have been recorded in Europe; European bat lyssavirus 1 (EBLV-1), European bat lyssavirus 2 (EBLV-2) and, four currently unclassified isolates. Since 1977, 783 cases of EBLVs (by isolation of viral RNA) have been recorded in Europe. EBLV-1 or EBLV-2 has been identified in 12 bat species, with over 95% of EBLV-1 infections identified in Eptesicus serotinus. EBLV-2 is associated with Myotis species (Myotis daubentonii and Myotis dasycneme). A programme of passive surveillance in the United Kingdom between 1987 and 2004 tested 4871 bats for lyssaviruses. Of these, four M. daubentonii (3.57% of submitted M. daubentonii) were positive for EBLV-2. Potential bias in the passive surveillance includes possible over-representation of synanthropic species and regional biases caused by varying bat submission numbers from different parts of the UK. In 2003, active surveillance in the UK began, and has detected an antibody prevalence level of 1-5% of EBLV-2 in M. daubentonii (n = 350), and one bat with antibodies to EBLV-1 in E. serotinus (n = 52). No cases of live lyssavirus infection or lyssavirus viral RNA have been detected through active surveillance. Further research and monitoring regarding prevalence, transmission, pathogenesis and immunity is required to ensure that integrated bat conservation continues throughout Europe, whilst enabling informed policy decision regarding both human and wildlife health issues.

Australian bat lyssavirus: a recently discovered new rhabdovirus

Australian bat lyssavirus (ABLV), first identified in 1996, has been associated with two human fatalities. ABLV is genetically and serologically distinct from, but is closely related to, classical rabies. It has a bullet-shaped morphology by electron microscopy. There are two strains of ABLV known: one circulates in frugivorous bats, sub-order Megachiroptera, and the other circulates in the smaller, mainly insectivorous bats, sub-order Microchiroptera. Each strain has been associated with one human fatality. Surveillance indicates infected bats are widespread at a low frequency on the Australian mainland. It is unclear how long ABLV has been present in Australia, although molecular clock studies suggest the two strains separated 950 or 1,700 years ago based on synonymous or non-synonymous nucleotide changes, respectively. Recent serological surveys suggest a closely related virus may exist in the Philippines. Due to demonstrated cross-protection in mice, rabies vaccine is used to prevent infection. Rabies post-exposure prophylaxis (PEP) protocols have been adopted for when a human is scratched or bitten by a suspect bat. A long-term commitment to public health programs that test bats that have been involved in scratch or bite incidents, followed by PEP if appropriate, will be necessary to minimise further human infection.

Pathogenesis studies with Australian bat lyssavirus in grey-headed flying foxes (Pteropus poliocephalus)

OBJECTIVE

To examine the susceptibility of the grey-headed flying fox (Pteropus poliocephalus) to Australian bat lyssavirus (ABL), and to provide preliminary observations on the pathogenesis of the disease in flying foxes.

PROCEDURE

Ten flying foxes were inoculated intramuscularly with ABL, and four with a bat-associated rabies virus. Inoculated animals were observed daily, and clinical samples collected every 9 to 14 days. Animals with abnormal clinical signs were euthanased, and samples collected for histological, serological, virological and immunohistological examinations. At 3 months post inoculation (PI), all survivors were euthanased, and each submitted to a similar examination.

RESULTS

Three ABL-inoculated flying foxes, and two rabies-inoculated animals developed abnormal clinical signs between 15 and 24 days PI. All three ABL-inoculated animals had histological lesions consistent with a lyssavirus infection, and lyssaviral antigen was identified in the central nervous system (CNS) of each. Virus was isolated from the brain of two affected animals. Of the rabies-inoculated flying foxes, both had histological lesions and viral antigen in the CNS. Virus was recovered from the brain of only one. None of the five affected flying foxes developed anti-lyssavirus antibodies, but, by 3 months PI, five of the seven ABL-inoculated survivors, and one of the two rabies virus-inoculated survivors, had seroconverted. The dynamics of the immune responses were quite variable.

CONCLUSIONS

The response of flying foxes to ABL, administered by a peripheral route of inoculation, was similar to that of bats inoculated peripherally with bat-derived rabies viruses.

Emerging viral diseases of Southeast Asia and the Western Pacific

Over the past 6 years, a number of zoonotic and vectorborne viral diseases have emerged in Southeast Asia and the Western Pacific. Vectorborne disease agents discussed in this article include Japanese encephalitis, Barmah Forest, Ross River, and Chikungunya viruses. However, most emerging viruses have been zoonotic, with fruit bats, including flying fox species as the probable wildlife hosts, and these will be discussed as well. The first of these disease agents to emerge was Hendra virus, formerly called equine morbillivirus. This was followed by outbreaks caused by a rabies-related virus, Australian bat lyssavirus, and a virus associated with porcine stillbirths and malformations, Menangle virus. Nipah virus caused an outbreak of fatal pneumonia in pigs and encephalitis in humans in the Malay Peninsula. Most recently, Tioman virus has been isolated from flying foxes, but it has not yet been associated with animal or human disease. Of nonzoonotic viruses, the most important regionally have been enterovirus 71 and HIV.