Introduction
Of the known species of Angiostrongylus, rat lungworms, two are known from Australia. Angiostrongylus mackerrasae Bhaibulaya, 1968 is an endemic species known only from the native rodents the bush rat Rattus fuscipes (Waterhouse) and the swamp rat Rattus lutreolus (Gray), and in mixed infections with Angiostrongylus cantonensis (Chen, 1935) in the introduced brown rat Rattus norvegicus (Berkenhout). By contrast, A. cantonensis was originally described from the pulmonary arteries and right ventricle of the cosmopolitan rodents the black rat Rattus rattus (Linnaeus) and the brown rat in China (Chen, Reference Chen1935). The colonization by R. rattus and R. norvegicus of new territories has resulted in the geographical spread of A. cantonensis, infections having been reported from other Rattus species, Bandicota indica (Bechstein), Melomys spp. and Suncus murinus (Linnaeus) as well as from humans, across the Asian, Pacific and Australian regions (Alicata, Reference Alicata1968; Anderson, Reference Anderson2000). Recently infections have also been reported from North and South America, where it is now considered to be endemic (Spratt, Reference Spratt2015). Consequently neuroangiostrongyliasis, caused by A. cantonensis, has become a globally distributed zoonotic human disease (Spratt, Reference Spratt2015). In Australia A. cantonensis has been reported from the introduced black and brown rats in Queensland and as far south as Sydney (Spratt, Reference Spratt2005; Stokes et al., Reference Stokes2007) as well as from a number of macropodid and possum species, flying foxes, tawny frogmouths and cockatoos (Spratt, Reference Spratt2005; Stokes et al., Reference Stokes2007). In south-eastern Queensland, where infection has occurred accidentally in humans (Spratt, Reference Spratt2005), a recent study of free-living populations of Rattus spp. demonstrated a high prevalence (16.5%) of infection with Angiostrongylus spp. in urban Brisbane and surrounds (Aghazadeh et al., Reference Aghazadeh2015).
Adults of A. cantonensis are normally found in the pulmonary arteries of the definitive host. Eggs are deposited in the lungs and the first stage larvae are expelled through the respiratory tract and excreted in faeces, from where they infect terrestrial, aquatic or amphibious gastropods. The larvae develop through to third stage and emerge spontaneously from the gastropod. Definitive or paratenic hosts become infected by consuming the third stage larvae directly or by eating infected gastropods. Although no further development occurs in paratenic hosts, such as frogs, toads, freshwater prawns, land crabs, lizards or planarians, they may provide a source of infective larvae for definitive or accidental hosts such as humans. From the stomach most larvae enter the hepatic portal and mesenteric lymphatic systems and are carried to the heart and lungs, from where they invade the pulmonary veins and are distributed around the body. Larvae that reach the central nervous system grow, moulting twice to fifth stage, in the neural parenchyma. Young adults move from the meninges into the subarachnoid space, where they remain for c. 2 weeks before entering the cerebral vein and returning to the right heart and pulmonary arteries, where they mature (Anderson, Reference Anderson2000; Spratt, Reference Spratt2015).
In this paper we report, for the first time, A. cantonensis infection in a water rat Hydromys chrysogaster Geoffroy, a relatively large rodent native to Australia and New Guinea. The water rat is one of the few Australian mammals that is adapted to aquatic life. An opportunistic hunter and scavenger, it is known to take large aquatic insects, fishes, crustaceans, mussels, frogs, lizards, water birds and small mammals (Olsen, Reference Olsen, Van Dyck and Strahan2008).
Materials and methods
A mature male water rat had been held in captivity in Caboolture, Queensland. After a short period of illness, which included disoriented behaviour and intermittent circling for 1 week prior to death, it was presented for necropsy examination by the owner.
Gross necropsy examination was carried out. Organs were examined for helminths with the aid of a dissecting microscope. A pair of nematodes were recovered from the brain, fixed in formalin and stored in 70% ethanol prior to clearing in lactophenol for examination as temporary wet mounts. Measurements were taken with the aid of an ocular micrometer and are presented in micrometers, unless otherwise stated. Identification of the nematodes was confirmed using Bhaibulaya (Reference Bhaibulaya1968). The specimens were deposited in the South Australian Museum (registration number: AHC48221). Samples of brain, lung, heart, liver, stomach, testis, kidney, spleen and intestinal tissue were fixed in formalin and prepared for histopathological examination following a routine histological tissue processing procedure. Samples were embedded in paraffin, sectioned at 4 μm, stained with haematoxylin and eosin, and mounted using standard protocols.
Results and discussion
Young adult nematodes, one male and one female (6.5 and 5.4 mm long, respectively), were recovered from the meninges. The worms conformed to the descriptions of Angiostrongylus spp. given by Bhaibulaya (Reference Bhaibulaya1968) and Mackerras and Sandars (Reference Mackerras and Sandars1954) (fig 1). The lengths of the spicule and vagina were 1150 and 150 μm, respectively, suggesting that the worms were A. cantonensis. There were no eggs in the female nematode. As far as we are aware this is the first report of A. cantonensis in H. chrysogaster.
Fig. 1. Angiostrongylus cantonensis found in the present study: (a) posterior end of a male; (b) female. Scale bar: 500 μm.
In their life-cycle study of A. mackerrasae as A. cantonensis (see Alicata, Reference Alicata1969) Mackerras and Sandars (Reference Mackerras and Sandars1954) gave lengths of 11 and 12 mm, respectively, for young adults that had penetrated the subarachnoid space of the host brain, suggesting that the worms found in the meninges in this study were not yet mature enough to migrate back to the lungs. In the definitive hosts, larvae of A. cantonensis reach the central nervous system 2–3 days post infection, undergo the fourth moult at 7–9 days post infection and then, as young adults, invade the subarachnoid space of the brain 12–14 days post infection (Anderson, Reference Anderson2000), which woud be congruent with the onset of the behavioural symptoms in the present study.
Presence of nematodes in the pulmonary artery and absence of eggs in the female (fig. 2b) suggest that the water rat examined in this study may have been infected long enough for young adults to start migration from the brain to the lungs, the water rat being a definitive host, but not long enough for eggs to be produced by females in the lungs.
Fig. 2. (a) Section of stomach with many granulomatous nodules present in the submucosa and serosal surfaces containing larval A. cantonensis. (b) Section of lung with nematodes (indicated by arrows) within the lumen of a medium-sized pulmonary artery associated with significant transmural inflammation. (c) Adult A. cantonensis present within the leptomeninges associated with a marked granulomatous inflammatory cellular response.
Histopathological examination revealed widespread inflammation throughout the brain, lung and gastrointestinal tissue. Throughout the leptomeninges there was a moderate diffuse infiltration of macrophages and occasional granulocytes. Foci of inflammatory cells were also present within the cerebral parenchyma, with spaces present likely to be migrating nematode tracts. Cross-sections of nematodes with prominent coelomyarian musculature and prominent lateral cords were present in the meninges. Similar cross-sections of nematodes were also present in the lung and muscularis mucosae of the stomach as well as the attached omentum and peritoneal membranes (fig 2). In the lung, they were within a larger pulmonary artery with significant transmural inflammation (arteritis).
Together with the neurological symtoms recorded before death, the histopathology and finding of adults of A. cantonensis supports a diagnosis that infection with A. cantonensis was the cause of death. It is known that hosts that show strong signs of angiostrongyliasis are indeed dead-end hosts, in which worms do not migrate from the central nervous system. It is also known that A. cantonensis has a relatively broad host-specificity, and the absence of the fully grown and fertilized adult parasite in H. chrysogaster, a closely related species to Rattus rattus, may simply be because H. chrysogaster was not infected long enough for fertilized adult nematodes to be found. Future studies of H. chrysogaster infected with A. cantonensis would shed some light on details of the life cycle of the parasite in this host.
Third stage larvae penetrating the gastrointestinal tract mucosa, where they may cause gastritis and peritonitis, has been reported in aberrant hosts (Spratt, Reference Spratt2015). Similarly, severe neurological clinical signs and pathology, characterized by an eosinophilic meningoencephalitis, as a consequence of infection, have frequently been reported in accidental hosts, including humans (Spratt, Reference Spratt2005). The presence of young adult nematodes, associated with diffuse granulomatous meningoencephalitis, has not, however, been commonly reported.
The infection of H. chrysogaster with the invasive lungworm A. cantonensis and not the endemic A. mackerrasae is a cause for concern. As discussed by Spratt (Reference Spratt2005), A. cantonensis may cause paralysis and paresis in a broad spectrum of marsupial and eutherian mammal hosts as well as in birds, including the tawny frogmouth Podargus strigoides. It is now known whether there is the potential for fatal neurostrongyliasis in a wide range of native Australian mammals and birds, some of which may be endangered species.
Acknowledgements
We wish to thank the staff of the Charles Sturt University Veterinary Diagnostic Laboratory for their contribution to the histopathology.
Financial support
Charles Sturt University Veterinary Diagnostic Laboratory provided support for this study.
Conflict of interest
None.
Ethical standards
All procedures followed the Veterinary Practitioners Code of Professional Conduct and the Australian code for the care and use of animals for scientific purposes 8th edition (2013).
