Australian Bat Lyssavirus is a member of the Lyssavirus genus and antigenically close to classical rabies virus. It is endemic in both insectivorous bats and megabats of Australia. A pteropid variant and an insectivorous variant of ABLV occur, but the occurrence of additional undetected variants is also possible. The prevalence in wild populations is estimated at less than one percent but in bats that are showing neurological signs a much higher prevalence occurs.
Bats infected with ABLV show a range of clinical signs reflecting nervous system pathology. Aggression, paresis, paralysis, seizures, depression and vocalization changes are the most commonly reported signs in infected bats. These signs are not specific to ABLV infection, with trauma, lead poisoning and angiostrongylosis infection being some of the more common alternate causes of nervous disorders in bats.
If alive, affected bats are located frequently either on the ground or caught in a fence or low in a tree and are unwilling or unable to fly. Human exposure usually occurs during attempts to rescue a bat when it is assumed to be injured rather than sick when people get scratched or bitten. Bats close to the ground are also easy for dogs and cats to access and they may get exposed to ABLV by being scratched or bitten by a bat during contact. In many cases of exposure in pets all that is found are remnants of a bat that the pet may have killed and partially eaten and any cuts or scratches on the pet may not be readily apparent.
Humans infected with ABLV may have an incubation period of weeks to years and the illness is invariably fatal. There have been three human cases in Australia to date and all have had a history of bites or scratches from bats and all three were unvaccinated.
During 2013, two young horses in Queensland naturally infected with the insectivorous variant of ABLV developed a fatal illness and showed progressive gait and behavioral changes resulting in terminal recumbency and death after a short clinical course. Both horses were running in the same paddock and their illnesses developed four days apart. A microbat colony was located on the property and it is likely that the same microbat infected both horses.
There have been a number of ABLV seropositive dogs detected during investigation of pets with a history of bat contact. No clinical signs have been noted. Dogs and cats experimentally challenged with the ABLV pteropid variant showed mild, transient clinical signs, followed by complete recovery. Affected dogs had transient hind limb ataxia while affected cats showed mild behavioral changes (McColl et al 2005).
Overseas other lyssaviruses naturally infect and cause rabies-like illness in a broad range of domestic mammal species suggesting that occasional transmission of ABLV to other mammalian species is likely. However, endemnicity in these other species is unlikely as it would require adaptation to the new host species.
Testing of bats to determine their ABLV infection status is an important surveillance activity. The most common reason for testing is known or suspected animal or human contact with a bat. In 2015, there were 30 bats species sampled but positive bats were only detected among the megabats sampled (8/1%) with no positives in any of the tested insectivorous bats.
ABLV bat testing summary
|Year||Number tested||Number (%) infected|
Source: bat stats - www.wildlifehealthaustralia.com.au
Testing of non-bat species for ABLV status is reliant on autopsy samples (brain). Non-invasive sampling of suspect animals using hair and saliva are attractive options when positive a negative result is not evidence that the animal is not infected as virus levels in these sites is lower than seen in CNS samples.
Histology is not a reliable indicator of ABLV status as histological changes may be localised and subtle and missed in sections examined. Fluorescent antibody and polymerase chain reaction tests targeting the ABLV genome are the diagnostics of choice for detection of ABLV.
Serology has no role in the diagnosis of ABLV in individual animal as antibodies only appear after the onset of clinical signs but it can be useful to monitor population-level exposure.
Lyssavirus antibodies are rarely present in unvaccinated healthy animals and humans. When they are found their significance is difficult to interpret given the current knowledge of lyssavirus immunology. It is likely that antibodies signify prior exposure to virus and limited viral replication, probably in non-neural tissues. It is unlikely that antibodies consistently signify recovery from the clinical neurological disease typical of lyssavirus infection (AusVetplan ABLV RPB).
Sarah Britten (NSW DPI) and Karen Cox Whitten (WHA) assisted with data.