| 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. |
.
|
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 |
.
| 1904
Mercury newspaper clipping reporting the outbreak of sarcoptic mange in
Hobart. |
. |
| 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 1936, 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. |
| Click
the film icon above to view a short excerpt about disease in thylacines
at the Beaumaris Zoo from a January 1996 interview with Alison Reid. |
|
|
.
.
|
Emaciated
thylacine at the Beaumaris Zoo (QD), 1936. 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". |
