University of Tasmania
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The endoparasitic mite Sternostoma tracheacolum Lawrence (Ridnonyssidae) and the endangered Gouldian Finch erythrura gouldiae gould (estrildidae) : a parasite-host relationship

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posted on 2023-05-26, 02:34 authored by Bell, PJ
The Gouldian Finch Erythrura gouldiae (Gould) is an endemic species to the grassy woodlands of tropical northern Australia. In recent decades it has undergone a substantial decline, both in abundance and distribution. The cause of the decline remains unclear, however, habitat alteration from inappropriate fire regimes and or grazing by cattle, trapping for the avicultural market, disease and parasitism have all been suggested to be involved. In 1987, the respiratory system inhabiting mite Stemostoma tracheacolum Lawrence was discovered in wild populations of the Gouldian Finch. This mite is known to be pathogenic in captive birds, particularly the Gouldian Finch. The prevalence and intensity of infection in wild Gouldian Finches was thought to be high and its presence was found to be associated with respiratory disease (Tidemann et al. 1992). Unfortunately, little was known of the biology of S. tracheacolum or any of the respiratory system inhabiting mites of vertebrates. Furthermore, little was understood of the parasite-host relationship of these mite groups. In order to appreciate the potential role of S. tracheacolum in the decline of the Gouldian Finch the present study undertook to improve the knowledge of the biology of the parasite and the parasite-host relationship. The investigation involved studies of both naturally infected wild birds and experimentally infected captive reared birds. The life stages of S. tracheacolum from the wild Gouldian Finch are identified and described. The Morphology of specimens from the Gouldian Finch is compared with that of specimens described from other wild Australian host species and both captive and wild host species overseas. Significant morphological differences between specimens coming from psittaciform and passeriform hosts are described. The life history, patterns of infection and the mode of transmission of S. tracheacolum are described. The study indicates the following sequence: arrhenotoky, ovoviviparity, egg laying in the lung with subsequent development in the posterior airsacs and a rapid progress from the egg to the adult stage. The stage responsible for transmission is identified as the adult female (non-gravid, non-gorged). Observational data on behaviour and longevity of the infective stage indicates direct transmission via the nares of the host. S. tracheacolum infrapopulation biology is described in captive reared Gouldian Finches. Following initial infection there was an exponential growth rate and maximum infrapopulation size commonly exceeded 200 mites. In birds that survive these levels of infection there was a subsequent decline in the size of the infrapopulation until long term infections (i.e. up to two years duration) contain few mites. The mortality rate of mites increases with duration of infection and the transmission rate positively correlates with the size of the infrapopulation. Mite generations are continuous and overlapping with no evidence of synchronisation of reproduction or transmission with host biology. Gross pathology associated with S. tracheacolum infection is described, for individual birds killed at three monthly intervals, over 12 months following initial infection. Occlusion of respiratory passages (trachea, syrinx and primary bronchi) and or secondary pyrogenic infections were a common cause of death in heavily infected birds. Haematological values (Haemoglobin, Packed Cell Volume, Mean Corpuscular Value, Red Blood Cell count, White Blood Cell count, Thrombocyte count and differential leucocyte counts) of Gouldian Finches infected with S. tracheacolum (taken 3, 6, 9 and 12 months following experimental infection) and Canaries (taken 6 months following experimental infection) were not statistically dissimilar to those values obtained from non-infected birds. Cellular haematology is therefore considered to be of limited value in the study of S. tracheacolum infection. The clinical signs of S. tracheacolum infection are described over time for Gouldian Finches of known infection size and duration. Infected birds were found to throat clear, sneeze, wheeze(¬¨¬± clicking and gurgling), bill wipe and head shake more often and preen, vocalise, sing, and fly less often than uninfected birds. Infected birds were significantly less able to maintain flight than their uninfected counterparts. The severity of clinical signs was found to be related to the duration and the size of the infection. Some clinical signs are considered to enhance transmission while others have the potential to increase both direct and indirect mortality in wild host populations. A survey of the intra-nasal mite fauna of wild Gouldian Finches, Masked Finches Poephila personata, Pictorella Mannikins Heteromunia pectoralis, Long-tailed Finches P. acuticauda, Double-barred Finches Taeniopygia bichenovii, Zebra Finches T. guttata and Budgerigars Melopsittacus undulatus from the Northern Territory, Australia revealed twelve new host records for rhinonyssid and kytoditid mites. The prevalence of rhinonyssid infection in these host species ranged from 16.2 to 60.8%, with a corresponding range in the intensity of infection from 1.6 to 26.7 mites per host and a range in the mean parasite burden from 0.26 to 12.7 mites per individual bird. S. tracheacolum was found to infect the Gouldian Finch, Pictorella Mannikin, Masked Finch and Budgerigar at a prevalence of 47% (n = 19), 34% (n = 74), 1.4% (n = 85) and 16% (n = 86) and an intensity of27, 10, 1 and 1.6 mites per host for each species respectively. Both the prevalence and intensity of infection in the Gouldian Finch and the Pictorella Mannikin were significantly higher than that found in the Masked Finch and the Budgerigar. The prevalence and intensity of infection in the Gouldian Finch and the Pictorella Mannikin were not significantly different. The frequency distribution of mites S. tracheacolum in the Gouldian Finch sample was significantly different from the frequency distributions in the Budgerigar and the Masked Finch samples but not significantly different from that in the Pictorella Mannikin sample. Captive reared Gouldian Finches and Canaries were successfully infected with S. tracheacolum obtained from wild Gouldian Finches using an experimental infection technique. Captive reared Budgerigars and wild caught Masked Finches could not be infected by the same technique. The results suggest that Budgerigars are not susceptible (or are resistant) to S. tracheacolum mites coming from the wild Gouldian Finch, providing further evidence for sub-specific separation of the two forms. Examination of S. tracheacolum mites reared in captive Gouldian Finches and Canaries at 6 months and at 12 months following experimental infection indicated no significant change in external morphology. The data support the proposition that, in the short term, the host does not affect morphological expression in S. tracheacolum mites and that observed differences in mites from passerine and psittacid hosts are genetic rather than environmental. Ivermectin oral dosing is found to be an effective treatment for the control or complete elimination of S. tracheacolum infection in captive reared Gouldian Finches. Single dose treatment with ivermectin (i.e. Ivomec¬¨vÜ, 0.8 gil ivermectin) at rates as high as 3.5 J.l.Vg b.w. were administered to both captive reared and wild caught Gouldian Finches without obvious signs of toxicity. The mean percentage of mites killed by the drug increased with increase in the quantity of ivermectin in the oral dose. Complete elimination was achieved at doses above 2.0 J.l.Vg b.w. S. tracheacolum eggs can continue development within the carcase of ivermectin killed female mites and stages from larva to adult male have been found within the mother's carcase. The potential for reinfection of the host from mites that have developed in this manner is not known. The influence of S. tracheacolum on mortality and fecundity in the captive reared Gouldian Finch is examined. Groups of experimentally infected birds showed higher mortality rates than uninfected birds and infected birds layed significantly less eggs per number of surviving females over 200 days of infection (for birds initially infected at the beginning of the breeding period) than their uninfected counterparts. In view of the results of the present study, S. tracheacolum is considered to have the potential for significant impact on the population dynamics of the wild Gouldian Finch.


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