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HISTORY:
- EXTINCTION VS. SURVIVAL -
(page 4)
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Disease (continued):

    Pence & Ueckermann (2002) note:

    "Although short term mortality may appear devastating, in a self sustaining population, mortality is non compensatory and a mange epizootic generally does not affect long term population dynamics.  Alternatively, the net effect of mange epizootic can have serious consequences in remnant or fragmented populations of CITES listed, threatened, or endangered species where loss of even a few individuals can be critical to the survival or restoration of the species".

    The Mercury newspaper of the 12th April 1904 (p. 7) reported on the outbreak of sarcoptic mange brought to Tasmania with infected horses from New South Wales.

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    Could these infected horses have been the primary source of the epizootic disease that decimated thylacine numbers?

    Sleightholme & Campbell (2015) state that: "Endangered populations are unlikely to sustain an epizootic disease without the presence of a common host.  Since mange is transmitted by direct or indirect contact, it is density dependent.  As thylacine population densities in the wild

sarcoptic scabies in Hobart - 1904
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1904 Mercury newspaper clipping reporting the outbreak of sarcoptic mange in Hobart.
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were relatively low, one would assume that the impact of the disease would have been less severe than that experienced in captive stock.  With sarcoptic mange, the intensity of mite infestation correlates with the severity of clinical signs seen in infested animals".  Other diseases often cited as the cause of the epizootic are viral pneumonia and Toxoplasmosis.  Guiler (1998) notes that a type of pleura-pneumonia spread through the dasyure population in 1908-1909.  Sleightholme & Campbell (2015) state that: "It is entirely plausible that two pathogens were acting in concert, producing a combination of symptoms".  They conclude: "Further research is required on museum specimens collected during the early part of the 20th century to establish the underlying cause.  Until this is undertaken, attempts to identify the true nature of the disease are somewhat speculative" and that "Irrespective of the underlying cause, the disease is known to have had a high mortality rate in captive stock".  Paddle (2012) notes that the greatest loss to captive animals occurred at the Melbourne Zoo, where in the two-year period from 1901 to 1903, sixteen of the zoo's seventeen thylacines succumbed to the illness and died.  This loss equates to a mortality rate of 94%.

    Paddle (2012) states: "Captive thylacine records suggest that some thylacines exposed to the disease never picked it up, whilst others experienced its effects only mildly or were naturally immune".  Sleightholme & Campbell (2015) contend: "This observation is important in that it demonstrates that there were levels of immunity within the thylacine population to the disease".

    The thylacine shown in the photograph below (often erroneously depicted as being Benjamin in the literature) is now believed to have been a carrier of the epizootic disease.  It arrived at the Beaumaris Zoo (QD) on the 24th January 1928, and died the day after this photograph was taken.  It is highly probable that this animal was the source of infection responsible for the deaths of at least seven of the zoo's other thylacines that year.  Alison Reid, the daughter of Arthur Reid, the curator at Beaumaris (QD), was firmly of the opinion that the epizootic disease was responsible for their deaths.

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Beaumaris Zoo (QD) - 1928
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Emaciated thylacine at the Beaumaris Zoo (QD), 1928.  Photo: Ben Sheppard.

    Sleightholme & Campbell (2016) state: "The vulnerability of island species to disease, and the part that this can play as a lever in the extinction process, is adequately demonstrated by the collapse in thylacine numbers observed in the bounty records.  The extent to which the epizootic disease contributed to this collapse is not known, but it appears to have been a significant factor".

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References
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