The African swine fever threat to Australia

Keywords: African swine fever virus, biosecurity, epidemiology, pig, risk assessment, suid, surveillance, transboundary spread.

African swine fever (ASF) was first documented as a disease of domestic pigs in Africa 100 years ago.¹ Despite some spread events outside of Africa – southern Europe in the 1950s and 1960s, and North and South America in the 1970s and 1980s – during the twentieth century ASF mostly was considered a disease affecting Africa, which involved a wild suid reservoir, tickborne spread, and high morbidity and mortality when the ASF virus (ASFV) spilled over to domestic pig populations. In most cases when ASFV spread outside of Africa, it was eradicated.

However, ASF has emerged and spread through much of the world during the first two decades of this century. A pivotal event was an ASFV incursion into the Republic of Georgia in 2007 and subsequent spread throughout parts of Eastern Europe and Russia, the Baltic and western Europe. ASFV also spread eastwards, entering China in 2018 and quickly spreading throughout east and south Asia. By early 2020 ASFV had reached Indonesia, Timor Leste and Papua New Guinea. Throughout Asia, ASFV appeared to spread in phases, as a propagating epidemic. Besides a clear hotspot developing in China, southeast Asia and in particular northern Vietnam, Laos and Cambodia have become hotspots of ASFV spread. Average mortality rates of about 25% were reported from this pandemic of ASF in Asia.² Despite the devastating effect of ASF on pig production globally that has resulted in a serious risk to food security in places, ASF has been described as ‘The Forgotten Pandemic’.³ To date, a vaccine has not been successfully used to control this pandemic. Control options are limited to biosecurity measures.

ASFV has not been detected in Australian pigs, although virus fragments have been detected in food products that individuals have attempted to illegally import into Australia, and even in pork product for retail in Melbourne.⁴ ASFV has reached our doorstep, but what does the future hold?

Although ASFV is highly resistant in the environment, the transboundary spread of this virus has been linked more often to the trade in animal products. For example, the incursion in Brazil in 1978 was likely via food waste from transcontinental flights or animal products imported by tourists.⁵ Similar routes have been suspected in the incursion of ASFV into China and in the recent incursion in the Dominican Republic; spread within infected countries mostly seems to be via the movement of live pigs and pig products.^6,7 The role that wild pigs play in ASFV spread is debatable. In Europe, wild boar (including hunting activities associated with this species) appear to play a role, albeit probably minor in most situations. In Asia the role of wild pigs at present is uncertain. Overall, the evidence that ASFV is transmitted between free-living and domestic pigs is limited.⁸ The evidence that is available is mostly relevant to the European situation.

To address this information gap, a review was recently conducted of the potential role that wild (including feral) pigs in Asia and the Pacific might play in ASFV spread.⁹ It involved a review of the current literature, analysis of the official reporting resources and a survey of the World Organisation of Animal Health Member delegates in Asia and the Pacific. Twelve species of endemic wild pig in Asia were documented – the abundant Sus Scrofa, seven other Sus spp. and three Babyrousa spp. (endemic to South-east Asia) and Porcula salvinia (endemic to North-East India). It was noted that these species make a substantial contribution to the diversity of Suidae species globally and are an important conservation resource. Except for Sus scrofa, all these endemic species appear to be declining in population distribution and abundance. Between July 2018 and June 2022 there were 2112 reports to the World Animal Health Information System (WAHIS) of ASF in wild pigs from four countries in the region: China (4), Laos (2), Malaysia (54) and the Republic of Korea (2052). This suggests that wild pigs are important in the spread and maintenance of ASFV in the Republic of Korea, whereas in other countries wild pigs are unlikely to play a role, or that infection within wild pig populations suffers from poor surveillance and is underreported. Feral pig populations might be an important risk factor – based on the Korean experience – but native species of wild pigs are not. Rather, ASFV might be a threat to these small populations of native pig species. In addition to WAHIS reporting, in this study member countries also reported ASF in wild pigs in Bhutan, India, Vietnam and The Philippines.

Risk assessments of ASFV incursions focus on pathways that include the movement of live pigs, pig products (mostly pork), movement of people, feed and wild pigs. In the case of the ASF threat to Australia, the introduction of this virus via live pigs – domestic or wild – appears unlikely because of Australia’s isolation and quarantine restrictions, in contrast to its spread throughout mainland Asia. And given the nature and distances involved in transportation, introduction via people as vectors and feed appears less likely. The greatest risk appears to be the illegal import of pork products. This is why a strong regulatory and surveillance system to control and monitor pig products imported into Australia is important. A program to test pork products seized at international airports and mail centres was initiated in 2018 by the Australian Government. During the period 2018–2019 there were a total of 853 samples tested, and 248 (29%) samples were found to contain ASFV fragments.¹⁰ Although this does not necessarily suggest that the material was contaminated with live virus, it does indicate that illegally imported food is a risk pathway. To put this in perspective, between 5 November 2018 and 31 October 2020 there was more than 43 tonnes of pork products carried by air travellers entering Australia that was intercepted.¹¹ In addition, the final step in such a disease introduction pathway – exposure of domestic and wild pigs to contaminated food – is critical to control. For example, swill feeding is banned. Also, there are ongoing efforts to improve surveillance and reporting,¹¹ as timely detection of an incursion is critical in eradicating the disease via a stamping out response. For example, an online training resource (Fig. 1) was recently released.¹²

Fig. 1.  An emergency animal disease surveillance online training module for African swine fever (https://eadonline.com.au/modules/pigs/sampling_for_disease_exclusion.html).

The wild pig problem remains. Although in most situations wild pigs do not appear to play an important role in ASFV maintenance, Australia has a very large feral pig population and how ASFV might behave within our unique environment is unknown. Disease spread modelling based on field data collection (Fig. 2) has been undertaken on both classical swine disease and foot-and-mouth disease in northern Australia.^13,14 Like ASF, both of these are serious transboundary animal diseases that currently do not exist in Australia. These studies suggest that feral pigs could be infected, which could create issues in disease eradication efforts and demonstration of freedom from disease. The control of feral pigs in Australia has not been highly successful. If feral pig populations become endemically infected with one or more of these transboundary diseases, then compartmentalisation (whereby domestic pig production can be considered separate from feral pig populations) would be highly challenging. The example of The Republic of Korea, where wild boar appear to be capable of maintaining the circulation of ASFV, is a situation well worth considering.

Fig. 2.  Field data collection to inform disease spread models of transboundary disease swine in Australian feral pig populations.

The Australian pig industry is worth about $5.3 billion annually, with 3400 production sites, and employs approximately 36 000 people.¹⁵ Considering how quickly ASFV has spread across Europe and Asia during the past two decades, its impact on pig production and its effect on trade, ASF is one of the most important disease threats currently facing Australia.

Data sharing is not applicable as no new data were generated or analysed during this study.

The authors declare no conflicts of interest.

This research did not receive any specific funding.