Hydatid infection in macropodids

Barnes, Tamsin (2007). Hydatid infection in macropodids PhD Thesis, School of Integrative Biology, The University of Queensland.

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Author Barnes, Tamsin
Thesis Title Hydatid infection in macropodids
School, Centre or Institute School of Integrative Biology
Institution The University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Glen Coleman
Collection year 2008
Language eng
Subjects L
Abstract/Summary Echinococcus granulosus is thought to have been introduced to Australia at the time of European settlement. Parasite transmission occurs in both a domestic cycle, between sheep and dogs, and a sylvatic cycle, in which many macropodid species and feral pigs serve as intermediate hosts and dingoes as definitive hosts. Infection with the larval stage of the parasite results in the development of hydatid cysts; in macropodids, cysts are usually found in the lungs. Fatalities, primarily due to pulmonary impairment, have been reported in some of the smaller threatened species, and this raises the question whether the parasite is contributing to the decline of these hosts. However, no extensive studies investigating the prevalence, risk factors and pathophysiology of infection in macropodids have been undertaken. The overall aim of this thesis was to determine the potential significance of hydatid infection to small macropodids. In doing so, the prevalence of hydatid infection in both commercially harvested kangaroos and a small, threatened rockwallaby species was estimated. Possible risk factors associated with infection were investigated and attempts made to develop an immunodiagnostic test. The pathology associated with natural infection in kangaroos was described and the pathophysiology following experimental infection of tammar wallabies monitored. The overall prevalence of hydatid infection in 2998 commercially harvested macropodids (predominantly eastern grey kangaroos), from 21 properties in south Queensland, was 2.4%. This survey revealed a marked clustering of hydatid infection by property, with prevalences varying from 0 – 12%. Multilevel models were used to investigate putative risk factors at both kangaroo and property levels. At the kangaroo level, females were twice as likely to be infected as males, but no property level risk factors were identified. A survey was undertaken to establish baseline health data for the threatened brush-tailed rock-wallaby. Thoracic radiography, validated as a diagnostic technique in experimentally infected tammar wallabies, was used to estimate the prevalence of hydatid infection in three wild colonies of this macropodid in south-east Queensland. The overall prevalence was 15.3%, with 20% of adult animals infected. This survey also determined the burdens/prevalence of various endo- and ectoparasites and their xi associations with time of year, age of the animal, its condition and blood values. The seroprevalence of antibodies to Toxoplasma gondii, another parasite that is potentially fatal to macropodids, was 4.7%. As a serological test would be useful as a rapid, convenient means of diagnosis of hydatid infection, attempts were made to modify serodiagnostic tests validated in humans for use in macropodids. Both crude hydatid cyst fluid (HCF) antigens and the recombinant protein, EpC1, were optimized using sera from post-mortem positive and negative animals. HCF antigens had a very low specificity (20.8%) and rEpC1 a very low sensitivity (41.2 – 47.9%), indicating that neither test was suitable for diagnosis of infection in individual macropodids. Post-mortem examinations of the commercially harvested macropodids revealed that all infected animals had cysts in the lung tissue, but two also had cysts in the pleural cavity and one animal had a liver cyst. The number of cysts in infected animals ranged from one to 17, with the majority of animals having one to three cysts. Estimated total cyst volume varied from 0.2 to 1,075cm3. In some animals, cysts resulted in a 55 – 80% loss of functional lung capacity that is likely to have impacted significantly upon respiratory function. The pathophysiology of infection following experimental dosing of captive tammar wallabies with doses of 1000, 2500 and 8000 E. granulosus eggs was monitored for up to 16 months using thoracic radiography and a final post-mortem examination. The infection rate was low (11/33, or 35.5%). The number of cysts per animal varied from one to ten, with the majority (36/40 or 90%) of cysts establishing in the lungs. Cyst development was more rapid than has been reported in sheep, and this pattern was also observed in the naturally infected brush-tailed rock-wallabies. Cyst growth resulted in loss of functional lung capacity, up to an estimated 28% within 14 months of infection. Complications associated with lung cyst development included fatal anaphylaxis, pneumothorax and atelectasis. Seven of the eleven infected tammars died or were euthanased as a result of infection during the experiment. The onset of cyst fertility occurred early, from nine months onwards, compared to previous observations in sheep (2 - 6 years), suggesting that the importance of the sylvatic cycle to the epidemiology of E. granulosus in Australia could be far greater than previously thought. The histology of cysts from experimentally infected tammar wallabies and sheep was compared. Cysts of similar age had a thicker germinal membrane with greater nuclear content and a thicker laminated layer in the tammars. The hostproduced adventitial layer differed; the layer of degenerate collagen prominent in ovine cysts was absent in those from the tammars. These features underlie the rapid cyst growth and early onset of fertility seen in the tammars. Hydatid infection is widespread in macropodids and may be prevalent in colonies of the small threatened species. This thesis has demonstrated that rapid cyst growth results in loss of lung capacity that may have adverse effects on survival within a year of infection. Other potentially serious consequences of infection are also seen. Thus, hydatid infection is an important consideration in the development of conservation management plans for threatened macropodids and further work is required to determine effective control measures.

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Created: Fri, 21 Nov 2008, 15:22:19 EST