Rabies is one of the oldest known diseases, having a history of more than 4000 years. A disease affecting the Central Nervous System, rabies is almost invariably fatal once manifested
(Bhosale et al., 2022). It has been recorded in all the continents except Antarctica although the vast majority of the fatalities are from Asia and Africa (
WHO, 2023). Considered as a neglected tropical disease (
WHO, 2018), it is responsible for the death of more than 60,000 people worldwide annually (
WHO, 2013).
India is endemic for rabies bearing an annual burden of 35 per cent of the world’s human mortality most of which is canine-mediated (
WHO, 2018;
Bharathy and Gunaseelan, 2016). Except for RABV, no other lyssaviruses have been reported from India (
WHO, 2013).
Rabies is caused by a bullet-shaped RNA virus called Lyssavirus belonging to the family Rhabdoviridae (Gr. Rhabdos-rod) and genus Lyssavirus (Gr. Lyssa- madness), affecting all warm-blooded vertebrates, including man. For almost two-millennia, rabies was considered to be transmitted almost exclusively by the bite of rabid dogs
(McColl et al., 2000). However, today it is well established that a number of animals like wolves, foxes, coyotes, jackals, cats, bobcats, lions, mongooses, skunks, badgers, monkeys including bats are equally capable of transmitting the deadly disease (
Menezes, 2008). Although rare, airborne and transplantation transmission of rabies has also been documented. About seventeen species of Lyssaviruses have been isolated from various species of mammals worldwide out of which fifteen species have been reported in bats (
WHO, 2018). Except for rabies virus (RABV), no other lyssaviruses have been reported from India (
WHO, 2013).
In India, atleast 131 species of bats have been documented so far
(Saikia et al., 2022), out of which the North-eastern region of India boasts the presence of over 80 species. Among these, thirty-three species of bats have been recorded so far from Assam
(Boro et al., 2018; Saikia, 2023).
Bats are an indispensable component of ecology and human economics. They are an important agent of plant pollination especially in the tropical and arid regions. Bats may be the primary pollinators for over 1000 species of plants across the tropics many of which are of commercial significance lik mango, guava, banana etc
(Raghuram et al., 2011; Tremlet et al., 2019). One of the most important ecosystem services of insectivorous bats is the control of herbivorous arthropods including pest insects
(Kunz et al., 2011; Ghanem and Voigt, 2012).
Contrary to the positive roles played by bats in maintaining the health of ecosystem and human economics, they have their fair share of burdens too. Bats are recognized to be a natural reservoir of a large variety of zoonotic viruses, which can potentially cross species barriers to infect humans and other domestic or wild animals (
Wang and Cowled, 2015). In numerous communites across Asia and Africa bats have also been frequently hunted and exploited for bush meat, traditional medicine and souvenir. This increased contact with wildlife also enhances the odds of interspecific transmission and bat associated spill over.
To date, more than 80 virus species of different groups have been isolated or detected
(Callisher et al., 2006). Despite being a carrier with a very high load of various pathogens bats are rarely seen to suffer clinical diseases owing to a unique immune system namely pathogen sensing pattern recognition receptors (PRRs) by expressing anti-viral pro-inflammatory cytokines (
Janeway and Medzhitov, 2022). Their cells have been found to contain a number of modifications that allow for effective antiviral immune responses. The evolution of flight in bats appears to have selected for a special set of antiviral immune responses by species-specific adaptor proteins expressing antiviral and pro-inflammatory cytokines that control virus spread, while limiting self-destructive inflammatory responses
(Banerjee et al., 2020). Such modifications of their immune system possibly allow bats to serve as reservoirs of some of the hazardous viruses, including the
Lyssavirus.
Modern diagnostic techniques like Direct Fluorescent Antibody Assay (DFA), Direct Rapid Immunohistochemical Test (dRIT), Lateral Flow Assay (LFA) and One-step PCR are efficient tests for the detection of Lyssavirus. The World Organisation for Animal Health (WOAH) recommends DFA as the gold standard test for diagnosis of rabies. One-step PCR is utilized to detect the partial N gene sequence which is common for all Lyssaviruses identified so far (
WOAH, 2019).
Till date, fifteen of the seventeen recognised species of lyssaviruses have been isolated from different bat species worldwide (
WHO, 2018). In the recent years, isolation of Lyssavirus or detection of rabies neutralising antibodies from bats have been occasionally reported from Asia suggesting that bats are regularly exposed to sylvatic lyssaviral infections. Circulation of Lyssaviruses in bats of Asia have been confirmed by isolation of Aravan, Khujand, Irkut and Gannoruwa bat lyssaviruses
(Arguin et al., 2002; Jiang et al., 2010; Liu et al., 2012; Gunawardena et al., 2016). From South-east Asia, neutralising antibodies against lyssaviruses were detected in the bat samples from Philippines
(Arguin et al., 2002), Cambodia
(Reynes et al., 2004), Bangladesh
(Kuzmin et al., 2006) and Northern Vietnam
(Nguyen et al., 2014).
Mani et al., (2017) conducted surveillance for rabies virus (RABV) infection in bats in Nagaland, northeastern India, at sites with intense human-bat interfaces during traditional bat harvests. Brain tissues and sera from bats were tested for evidence of infection due to RABV. None of the bat brains tested (n = 164) were positive for viral antigen or viral RNA. However, they detected rabies neutralizing antibodies in 4/78 (5·1%) bat sera tested, suggesting prior exposure to RABV or related lyssaviruses.
For the first time in the South-east Asia
Pal et al., (1980) reported an active infection of frugivorous flying fox (
Pteropus poliocephalus) with a virus belonging to the Rhabdovirus group-a bat virus. Negri body-like structures were demonstrated by Sellar’s stain and direct immunofluorescence in the brain and salivary gland of the dead bat. The virus was isolated after intracerebral inoculation of homogenate of bat brain, salivary gland or brown fat separately in new-born mice.
Although no bat lyssaviruses have been reported from India so far, this can also be attributed to a lack of systematic surveillance within its geographic boundary.
Assam, a north-eastern land-locked state of India, is well known for its rich biodiversity and unique cultural heritage. However, its fast dwindling forest cover is a cause of concern as it has resulted in more frequent man-animal conflicts
(Das et al., 2012). In interiors of Assam certain tribal communities traditionally go bat hunting for consumption as a delicacy and folklore medicine. This has also resulted in habitat destruction, reduction of foraging and roosting grounds for bats and also facilitated increased human and domesticated animal contact with wildlife. This is an issue of One-health concern because increasing interactions between the two ecological components with result in spill-over events and flaring up new pandemics
(Phelps et al., 2019). Moreover, rabies in livestock with no dog/animal bite history has been reported while bat exposure has yet to be investigated as mentioned by
Dutta and Isloor, 2019. Under such circumstances it becomes increasingly important to carry out surveillance in areas with intense human-wildlife interactions, through hunting or clearing of forests.
Considering the rise in reports of bat lyssaviral infections from other parts of the globe and absence of proper surveillance on bat rabies in this part of the country, an attempt to study the prevalence of bat lyssavirus in Assam was undertaken for the first time.
