INFECTIOUS DISEASE
1.1 Retrovirus
A novel retrovirus (MbRV) isolated from the grassland melomys (Melomys burtoni), shows astonishing genomic similarities to sequences of gibbon ape leukaemia virus (GALV) and to a lesser extent koala retrovirus (KoRV) (Simmons et al.
2014). The concentrated nucleotide sequence of MbRV shares 93% identity with the sequence from GALV and 83% identity with KoRV (Simmons et al. 2014). MbRV is an endogenous retrovirus and its presence is of little consequence in melomys species, but the genetic similarities it shares with KoRV is of interest in relation to the origin of these viruses in koalas. The geographical ranges of the grassland melomys and of the koala partially overlap and thus a species jump from MbRV from rodents to koalas is potentially possible. Interestingly, the genus Melomys is only found east of Wallace’s Line and so they cannot be implicated in the emergence of GALV in gibbons on mainland Asia (Simmons et al. 2014). Further research into the origin of these related leukaemia viruses identified a new virus that shared 98% nucleotide identity with woolly monkey virus (WMV) and 96.7% with MbRV in a new M. burtoni subspecies from Indonesia (Alfano et al. 2016). Based on this discovery it is hypothesised that M. burtoni, and potentially related lineages with an Australo-Papuan distribution, may have played a key role in cross-species transmission of leukaemia viruses to other taxa (Alfano et al. 2016).1.2 Tyzzer's disease
Tyzzer’s disease (Clostridium piliforme) was associated with mortalities in a managed colony of spinifex hoppingmice (Notomys alexis) (Stannard et al. 2017). Of the mice that died, only two individuals exhibited signs of lethargy and diarrhoea before death. All other animals died without premonitory signs. At necropsy the most notable lesions were small cream-coloured foci on the surface of the livers of affected mice.
Tyzzer’s disease was confirmed by applying a silver stain to portions of liver in order to detect the characteristic bacilli-shaped bacteria (Stannard et al. 2017).1.3 Brucella
Seven novel Brucella strains were identified from samples collected from native rodents in 1964 during a survey of wild animals. The strains were initially reported to be Brucella suis biovar 3 based on biochemical testing. However, genetic sequencing has revealed that the samples represent a new species in the Brucella genus (Tiller et al. 2010). The native rodents infected included the fawnfooted melomys (M. cervinipes), the small climbing rat (M. lutillus) (now grassland melomys [Melomys burtoni]), and the giant white-tailed rat (Uromys caudimaculatus) (Cook et al. 1966). There was no indication that Brucella infection resulted in pathology in the infected rodents, but it highlights that native rodents may serve as an important reservoir for Brucella infection in Australia (Tiller et al. 2010) and care should be taken when handling them to prevent zoonosis.
1.4 Listeriosis
Infection with Listeria ivanovii has been identified as the cause of death in zoo-housed spinifex hopping-mice. Histologically, lesions typically consisted of suppurative inflammation of the brain and meninges. In some cases, there was evidence of concurrent bronchopneumonia, but whether or not this was related to the listeriosis or caused by environmental conditions was unclear. The source of infection in this case series was not identified (ARWH 2018 case nos 11020.1; 11020.2; 11033.1).
1.5 Cryptococcosis
Cryptococcosis (Cryptococcus gattii) was diagnosed in greater stick-nest rats (Leporillus conditor) and plains mice (Pseudomys australis) in a zoological population. Affected animals presented with a range of conditions, including sinusitis, pneumonia and pleuritis. Diagnosis was confirmed using a combination of cytology and LCAT (Johnson 2008; ARWH 2018). Some animals were found dead without premonitory signs and the diagnosis was made at necropsy.
Treatment with systemic antifungals (oral triazole in combination with amphotericin) has been attempted in individuals where a diagnosis was made early in the course of disease. However, the prognosis for recovery is poor (L Vogelnest pers. comm.).In addition to C. gatti infection, C. neoformans has been identified in greater stick-nest rats and spinifex hopping-mice (Plate 44.1). Lesions ranged from localised enteritis extending to peritonitis, and multi-organ infection. In all cases animals were found dead in their enclosures (ARWH 2018 case no 7041.1).
1.6 Capillaria
A group of spinifex hopping-mice were euthanased after a prolonged period of ill-thrift. On gross necropsy the animals were in poor body condition with unkempt hair coats. In all mice, the livers contained numerous nematodes, identified as Capillaria hepatica. Histologically, the hepatic parenchyma was effaced by large collections of bipolar plugged, eosinophilic Capillaria eggs surrounded by a mixed inflammatory response (Plate 44.2). Interspersed throughout the liver were large adult worms that elicited only a minor inflammatory reaction (ARWH 2018 case no 7303.1; 7303.2). The source of infection in these cases was not identified. Capillaria hepatica has an indirect life cycle and the adults are found within the liver parenchyma of rodents where they deposit their eggs. Eggs may be released into the environment by several processes, including death and decomposition of the host, predation and subsequent defecation of eggs by an intermediate host, or cannibalism (Ceruti et al. 2001). Once in the environment the eggs become embryonated and infective. After ingestion, the first-stage larvae hatch and make their way to the liver through the hepatic portal system, develop into the adult stage and die after laying eggs (Ceruti et al. 2001). Treatment of infected animals is difficult and it may be best to depopulate an infected colony and restock from a clean population (Baker 2007). Most importantly, C. hepatica is a potential zoonotic disease and so care should be taken when dealing with infected rodent colonies.
1.7 Other infectious diseases
An unknown Cryptosporidium spp. was identified from a free-ranging pale field rat (Rattus tunneyi) in the NT but its significance remains unknown (Reiss et al. 2015). Health assessments were conducted on 101 bush rats (Rattus fuscipes) during a translocation study. Blood was collected from anaesthetised rats for haematology, biochemistry and serology for Toxoplasma gondii and encephalomyocarditis virus (EMCV) (McDonogh et al. 2015). All animals had negative T. gondii titres, but some
individuals had elevated EMCV titres. In addition, ectoparasites and bush rat faeces were collected and tested by qPCR for Rickettsia sp. and Coxiella burnetii. A Rickettsia sp. was detected in both Ixodes tasmani ticks and trombiculid mites but not in faeces; C. burnetii was not detected in any samples (L Vogelnest unpublished).
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