Improving our understanding of the risks of LSD incursions via non-regulated pathways

Background: What is lumpy skin disease and how is it spread?

Lumpy skin disease is not in Australia but is present in South-East Asia. As set out in Australia’s National LSD Action Plan, the growing threat of this disease to Australia’s cattle and buffalo herds is taken very seriously. As part of this Plan, we’re working to improve our understanding of the risks and uncertainties associated with non-regulated pathways. The emergence of LSD in South-East Asia in 2019, and its ongoing spread, means the threat and the significance of different risk pathways to Australia have been changing.

To rapidly characterise the threat Australia faces, high level structured expert judgement exercises were held in March 2021 and again in March 2022 facilitated by the Centre of Excellence for Biosecurity Risk Analysis. These exercises are discussion based, using an internationally recognised process and provide an estimate of the level of threat posed by LSD, including from its presence and spread in our region. The exercises estimated the probability of an LSD outbreak in Australia in the following 5 years. An actionable threat was indicated, as well as highlighting several important uncertainties and unknowns.

There are many global knowledge gaps surrounding LSD and its epidemiology. Its recent emergence in the South-East Asia region means there are some uncertainties about how it may behave and spread in this new context. Different LSD virus strains likewise may also have the potential to affect the virus’s transmission and impacts.

To better understand the risk of LSD to Australia, identify gaps in our evidence base and to help guide our preparedness efforts, we commissioned epidemiological modelling of the future rate of LSD incursions under a hypothetical scenario where LSD has spread and is endemic throughout South-East Asia, Timor-Leste and Papua New Guinea.

The pathways examined in the model were windborne entry of insects, and infectious insects hitch-hiking on either commercial vessels, returning live export vessels, or vessels moving under the Torres Strait Treaty. The pathway for insects arriving as hitchhikers on commercial airplanes was not considered in this assessment, given the existing control measures and lack of significant cattle populations around international airports. Insect incursions via Torres Strait Treaty movements were examined and the risk of LSD introduction via this route was estimated to be negligible.

While the probability of LSD entering Australia was estimated to be lower than previously thought – and it would depend on the number of insect vectors required to initiate an infection – the parameters used in the modelling are subject to considerable uncertainty and limitations. It is important to note that while this modelling is not a forecast, it does help us better understand critical points of potential entry pathways and the sensitivity of risk calculations for key parameters like infection rates.

We now know more as a result of the detailed modelling and this will help us to target research and mitigate the risk of LSD arriving and spreading in Australia.

As mentioned above, there are still many global knowledge gaps about LSD. This uncertainty does not diminish the value of the modelling work, rather it shows the need for considered judgement of the analysis, including next steps. Critically, it provides us with a better understanding of how some of these uncertainties could significantly influence the threat to Australia. This will help us target further work and strengthen Australia’s preparedness.

LSD remains a significant exotic animal disease which necessitates our continued vigilance, consistent with the recently adopted National LSD Action Plan.

Key findings from the modelling work

• It matters if it takes more than one insect to start an infection. There is strong scientific evidence that a single insect is unlikely to start an incursion of LSD. Instead, it is very likely that several insects must land on a single animal to begin an infection. This is hard to achieve when insects are blown across several hundred kilometres, so it becomes a sensitive parameter in the epidemiological assessment.
• It matters where the infections are in Indonesia. LSDV was restricted to Sumatra and Central Java province as of October 2022. Atmospheric modelling suggests windborne dispersal of insects to Australia from these regions is very unlikely. However, it was anticipated that LSDV will continue to move eastwards. Since the modelling was done, LSD has now been reported in East Java, which is closer to Australia, and windborne dispersal is possible from this region.  

Modelling helps us to better plan for outbreaks if they occur

• We know where and when to look. We know from the modelling that windborne entry of infectious insects from countries to our north presents the highest risk of LSDV incursions, compared to entry on ships. The Tiwi Islands and regions east of Darwin to the Cobourg Peninsula are most at risk of windborne incursion, particularly during the northern Australian monsoon season between December and March. This information can help to prioritise LSD surveillance activities to look for and quickly respond to outbreaks if they do occur.
• The modelling also tells us which group of insects to look at. Mosquitoes represent a higher risk based on current laboratory studies by leading researchers overseas. Midges are more likely to be blown across, but there is little scientific evidence yet to show that they spread LSDV.
• There are key knowledge gaps around LSD virus and how it spreads between regions.
  • For example, it is not known exactly how many infectious insects need to bite an animal to transmit disease. Current scientific evidence suggests that multiple insects are required and the modelling results presented here depend on that assumption.
  • Another key assumption is that LSDV does not replicate inside an insect. The virus simply survives on mouthparts of the insects and there is wide consensus in the scientific research on this issue.
  • The risk assessment model helps us to identify the most uncertain and highest risk steps in the hypothetical spread pathway and therefore priorities for further work.

Next steps

New information from this recent work we commissioned provides critical insights to further support our preparedness and target our efforts. Under the National LSD Action Plan and Northern Australian Biosecurity Strategy, work on key research gaps and assumptions has already commenced. The epidemiological modelling will be revisited to consider new information as it emerges, such as the type and number of infectious insects needed to transmit the disease and whether the virus can replicate in the insect vector. The modelling work will also be used to inform our surveillance strategies for LSD and support our continued efforts to assist Indonesia and neighbouring countries to manage its spread.