Stochastic epidemiological models to test the origin and demographic impact of smallpox on Aboriginal Australians in the 18th Century
Nitschke, MC, AN Williams, SD Ingrey, B Griffiths, N Pitt, L Russell, S Ulm, K Beller, JR Hore, MI Bird, SH Fatima, IJ McNiven, F Saltré, A Bashford, C Wilson, CJA Bradshaw. Stochastic models indicate rapid smallpox spread and mass mortality of Indigenous Australians after colonial exposure. In review
The impact of smallpox (variola) on Aboriginal peoples of Australia in 1789 was catastrophic and irreversible. Historically biased perspectives of the disease’s introduction and spread impede modern-day reconciliation efforts. Understanding whether the disease entered and spread from pre-colonial Makassan (Indonesian) trade along the north coast, or from the First Fleet’s arrival in south-eastern Australia is necessary to infer the likely rate of spread and magnitude of mortality. We developed stochastic, multi-patch epidemiological models, supported by historical observations of the disease, to test hypotheses regarding possible entry points, spread rate, and demographic impacts. Our models suggest that entry from south-eastern Australia was more likely, with a probability < 0.02 of reaching Sydney from a northern entry, and an unrealistic chronology. The spread of the disease was rapid, and suggests a single interaction could have resulted in the widespread impacts of the disease. Initial entry to extinction of the disease took between 1,200 and 1,400 days (< 4 years), and due to its high mortality rate, appears to have been limited to the eastern margins of Australia and along major intersecting river systems. There was a lower likelihood of transmission into semi-arid, arid, or tropical areas. We found no evidence that smallpox would have reached the central or western half of Australia. Assuming a 60% lethality based on global data, the loss of > 150,000 people would have occurred in these regions in < 4 years, more than previously proposed, but in alignment with recent projections of demographic change during that period.
- main script:
OneStage.m
Movement1Stage.m: simulates movement between neighbouring patchesSpread_beta.m: simulates disease spread through contact within each patch (susceptible to exposed S → E)RecoverDeath.m: simulates the other stages of the disease (exposed to infected E → I, and infected to recovered I → R)
- Populations.mat: projected population vector for each patch (1×592)
- Q.mat: conditional probability of movement to each neighbouring patch (∑ for each patch = 1).
- Tindale.mat: area for each patch based on Tindale (1974) map
We acknowledge the sovereign Traditional Owners and custodians (First Nations) of the unceded lands and seas where we live and work, including Kaurna in Tarndanya/Adelaide (M.C.N., C.J.A.B., F.S., S.H.F.), Bidiagal in Warrane/Sydney (A.N.W., N.P., J.R.H., A.B.), Dharawal in Kamay/Botany Bay (K.B., S.D.I.), Kulin Nation in Naarm/Melbourne (B.G., I.J.M., L.R.), Yirrganydji and Gimuy Walubara Yidinji in Gimuy/Cairns (S.U., M.I.B.), Dharawal in Woolungah/Wollongong (F.S.), Ngarrindjeri of Murrundi/lower Murray River, Kurangk/Coorong, and eastern Fleurieu Peninsula (C.W.), Palawa in Nipaluna/Hobart, lutruwitra/Tasmania (C.W.), and Peramangk in Bukatila/Mount Lofty Ranges (C.J.A.B.). We also recognise the deep historical and cultural harm our truth-telling exposes, and we commiserate with all First Nations peoples of Australia. We thank J. Llewelyn for initial discussions.
Funded jointly by the Australian Research Council Centre of Excellence for Indigenous and Environmental Histories and Futures (CE230100009), and the Australian Research Council Centre of Excellence for Australian Biodiversity and Heritage (CE17010001). L.R. supported by an Australian Research Council Laureate Fellowship (FL190100161). B.G. supported by an Australian Research Council Discovery Early Career Researcher Award (DE220100203). A.B. and N.P. supported by an Australian Research Council Laureate Fellowship (FL200100144).
