EPIDEMIOLOGY
Rats acquire third-stage larvae by ingesting infected intermediate hosts, which in Australia are many species of native and introduced terrestrial gastropods (snails and slugs) (Chan et al.
2015). Larvae penetrate the stomach, enter the hepatic portal and mesenteric lymphatic systems and are carried to the heart and lungs. They enter alveoli, invade the pulmonary veins, are returned to the left heart and distributed around the body by the arterial circulation. Larvae reach the CNS, predominantly the cerebrum and cerebellum, grow and moult twice in the parenchyma before the young adults invade the subarachnoid space of the brain. After ~2 wk they invade the cerebral vein and move to the heart and pulmonary arteries where they mature. Eggs are carried in the blood to the lungs where they embryonate. First-stage larvae move up the bronchi and trachea, are swallowed, pass out in the faeces, are ingested by intermediate hosts and develop to third-stage infective larvae. A range of other animals (planarians, prawns, crabs, frogs and lizards) may serve as paratenic hosts in which infective larvae reside but undergo no further development. Humans and other animals are accidental hosts and infection may occur through unintentional or intentional ingestion of intermediate or paratenic hosts (Fig. 24.1) (Barrett 2004; Spratt 2015).It has also been demonstrated that the infective third- stage larvae can emerge spontaneously from gastropods and remain viable for at least 72 hr in the mucus trail of slugs and snails (Chan et al. 2015; Spratt 2015). Humans and other animals can be infected by ingestion of larvae in mucus on vegetables, other plants or processed animal feed (e.g. pellets
Table 24.1. Australian native mammals reported with neural angiostrongyliasis
Species
Grey-headed flying-fox (Pteropus poliocephalus)1~5
Black flying-fox (P.
alecto)2~4Little red flying-fox (P. scapulatus)2
Greater bilby (Macrotis lagotis)67
Rufous bettong (Aepyprymnus rufescens)8
Purple-necked rock-wallaby (Petrogale purpureicollis)9
Parma wallaby (Notamacropus parma)1 Parma/tammar wallaby hybrid (N. parma/eugenii)1 Tammar wallaby (N. eugenii)12 Swamp wallaby (Wallabia bicolor)13 Agile wallaby (N. agilis)14
Red-necked wallaby (N. rufogriseus)15-17
Western grey kangaroo (Macropus fuliginosus)18,19
Common wallaroo (Osphranter robustus)20
Southern hairy-nosed wombat (Lasiorhinus latifrons)21,22
Eastern ring-tailed possum (Pseudocheirus peregrinus)23~25 Common brush-tailed possum (Trichosurus vulpecula)26~30 Tasmanian devil (Sarcophilus harrisii)31 Water rat (Hydromys chrysogaster)32~34 Platypus (Ornithorhynchus anatinus)35,36
1Gordon 1992; 2Reddacliff etal. 1999; 3Barrett etal. 2002; 4Barrett 2004; 5ARWH 2018 case numbers: 5715.1, 9253.1, 9848.1; 6ARWH 2018 case numbers: 4686.1, 7854.1; 7Vere Nicholson pers. comm.; 8Higgins etal. 1997; 9ARWH 2018 case number: 2225.1; 10ARWH 2018 case number: 6858.1; 11ARWH 2018 case number: 2434.1; 12ARWH 2018 case number: 3330.1 (pathology suggestive of NA but not confirmed); 13ARWH 2018 case number: 6912.1; 14UQ 80/980, UQ 80/1014 (Veterinary Pathology Department, University of Qld); 15McKenzie et al. 1978; 16Miller 1992; 17UQ 88/298 (Veterinary Pathology Department, University of Qld); 18L Vogelnest, unpublished 2003; 19ARWH 2018 case number: 3715.1; 20UQ 78/910 (Veterinary Pathology Department, University of Qld); 21Martin and Vogelnest 2004; 22ARWH 2018 case number: 3778.1 (pathology suggestive of NA but not confirmed); 23Prociv pers.
obs. in Spratt 2015; 24Johnson and Hemsley 2008; 25UQ 84/385, UQ 82/876 (Veterinary Pathology Department, University of Qld); 26ARWH 2018 case numbers: 630.1, 5811.1, 5847.1; 27Spratt, 2001 pers. obs.; 28Prociv etal. 2000; 29Ma etal. 2013; 30UQ 96/418 (Veterinary Pathology Department, University of Qld); 31UQ 79/1685 (Veterinary Pathology Department, University of Qld); 32Shamsi etal. 2019; 33Australia Zoo Wildlife Hospital records; 34C Whitten pers. comm.; 35J Whinfield pers. comm.; 36Australia Zoo Wildlife Hospital records.and kibble) and do not need to ingest the intermediate host. This is considered a likely mechanism of transmission to native Australian mammals in managed care.
In Australian wildlife, NA has primarily been a disease affecting native mammals in managed care, free-ranging flying-foxes (Pteropus spp.) and a few other native mammal species and various bird species both managed and free-ranging (Spratt 2015) (see Table 24.1 for mammal species). Disease emergence in Australian wildlife appears to have followed the southern extension of the range of the parasite from Qld. The first cases were reported from the
Fig. 24.1. Life cycle of Angiostrongylus cantonensis, illustrating potential pathways of infection for Australian mammals. Illustration: Angie Jarman
Brisbane area in the late 1970s to early 1980s (McKenzie et al. 1978; Veterinary Pathology Department, University of Qld archives). No evidence of the disease was detected in histological surveys of neurological disease in tawny frogmouths (Podargus strigoides) or common brush-tailed possums (Trichosurus vulpecula) in the Sydney region before the early 1990s (Hartley 1993).
Seasonality in the incidence of cases of NA in native Australian mammals has not been observed. However, in dogs and tawny frogmouths there is a higher incidence in autumn and winter (Lunn et al. 2012; Ma et al. 2013). Presumably the cool damp nights at this time of year favour the emergence of terrestrial gastropods to seek food (Aghazadeh et al. 2015a).
3.