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Investigation of large-scale mortality in nestling straw-necked ibis (Threskiornis spinicollis) with emphasis on giardiasis

McRoberts, Karen Michele (2000) Investigation of large-scale mortality in nestling straw-necked ibis (Threskiornis spinicollis) with emphasis on giardiasis. PhD thesis, Murdoch University.

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The straw-necked ibis (Threskiornis spinicollis) is a large wading bird of the family Plataleidae, subfamily Threskiomithidae, endemic to Australia and Papua New Guinea. It breeds in large, noisy colonies, often numbering in thousands to tens-of-thousands of breeding pairs. This thesis was undertaken to investigate sickness and mortality occurring in ibis nestlings at two colonies in Western Australia (Capel, approximately 300 km south of Perth, and Cataby, 160 km north of Perth).

A multi-disciplinary approach was taken. A detailed post-mortem and clinical investigation was conducted (gross pathology, bacteriology, toxicology for selected pesticides and heavy metals, parasitology, haematology, biochemistry, and limited virology using scanning electron microscopy). As a result of these investigations, Giardia, a flagellate protozoan known to cause disease in many vertebrate species, was the only consistent pathogen recognised. Toxicological studies were negative, and haematological and biochemical results inconclusive. As such, cohorts of nestlings were monitored over time for prevalence of Giardia infection, weight gain, and survival. Histological studies and statistical analyses were used to determine whether or not Giardia was responsible for sickness and death in the ibis nestlings. The progress of infected birds removed from the wild and held in captivity was also monitored. As a first step in determining if infection might be a threat to other avian species, Giardia prevalence in other wild bird species was investigated. In an effort to better understand the nature of isolates obtained, they were characterised using scanning electron microscopy and multi-locus enzyme electrophoresis, then compared with avian isolates described by other researchers.

A detailed clinical and post-mortem investigation was conducted during the 1993 breeding season. Giardia trophozoites were detected histologically in the duodenums of seven of 28 birds examined, but there was no histological evidence of pathogenicity. Twenty-six of the 28 birds had trophozoites in scrapings of duodenal contents. Blood smears from nine of 25 birds examined exhibited numerous immature heterophils, a condition that may have been indicative of serious disease; however, no cause could be found and the condition was not associated with Giardia infection or ill health. Further, Giardia infection was not associated with weight loss or death.

During the 1993 breeding season, the mortality rate of birds wing-tagged at approximately two to three weeks of age and monitored to fledging (seven to nine weeks of age) was found to be 48% at Capel and 83% at Cataby. During three breeding seasons (1990 to 1993), the overall prevalence of Giardia infection of young ibis at both sites was 74%. Prevalence was significantly higher at Cataby than at Capel (χ21= 5.76, P< 0.05), and was significantly higher at Capel in 1992 than in 1991 (χ21= 6.67, P < 0.05), although these results may have been influenced by the different ages at which birds were sampled. The proportion of infected birds at Capel increased from 1991 to 1993 (χ21= 4.72, P < 0.05). This work indicated that Giardia infection is probably endemic in these populations and that prevalence varies between sites and seasons.

Sick birds removed from the wild thrived in captivity and were successfully released. Treatment with dimetradazole appeared to clear the birds of giardial infection, with no cysts being evident in faeces later than one week after treatment.

The survey conducted for the presence of Giardia included species of birds nesting with infected ibis, and other species collected near Perth. One-hundred and forty-eight individual birds, representing 39 species, were examined. Nine birds. representing two orders and four species, were positive for Giardia'. one rufous night heron (Nycticorax calendonicus) (n = 1); four great egrets (Egretta alba) (n = 9); three sacred ibis (Threskiornis aethiopica) (n = 7); and one Pacific black duck (Anas superciliosa) (n = 12). All positive samples came from living birds at the Capel site. with the exception of one from a sacred ibis collected in suburban Perth. These results suggest that Giardia infection is not widespread among bird taxa in Western Australia; but that ciconiiforms are principal carriers of Giardia in wild bird populations, as appears to be the case in North American situations. The survey results also suggest that infection with Giardia among wild birds in south-western Australia may be confined to particular wetlands, and that ardeids such as the great egret (Egretta alba) nesting near infected straw-necked ibis may also have high infection rates.

Attempts were made to adapt a commercially-available enzyme-linked immunosorbent assay (ELISA), used for the detection of Giardia antigen in human faeces, for antigen detection in ibis faeces. The ELISA achieved a 100% specificity. but sensitivity was poor and was thought most likely to be a reflection of a low concentration of antigen in the ibis faecal samples.

Scanning electron microscopy (SEM) and multi-locus enzyme electrophoresis (MLEE) performed on the ibis isolates showed them to have identical enzyme profiles and morphology, suggesting that one strain was infecting birds at both colonies. Comparison with G. ardeae, isolated in the U.S.A. from the great blue heron, Ardea herodias, (Erlandsen et al., 1990), revealed distinct enzyme profiles and morphology, although the ibis strain shared some bands and most morphological characteristics with G. ardeae. Comparison with isolates of Giardia obtained from humans (G. duodenalis) revealed a very different enzyme profile and morphology. This suggested that the ibis-derived isolates may have no zoonotic potential; although humans can be infected with genetically different strains, isolates similar to G. ardeae have not been recovered from humans.

It was concluded that one strain of Giardia was infecting birds at both sites. that it was probably closely related to G. ardeae, and was not likely to possess zoonotic potential. Giardia was highly prevalent in ibis nestlings, but, as a sole factor, did not appear to be causing the deaths observed. However, because it was the only potential pathogen consistently identified, the role of Giardia infection in conjunction with some other factor or factors should be further investigated.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Veterinary and Biomedical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Thompson, Andrew, Halse, Stuart and Robertson, Ian
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