Discussion on a Selection of Key Scientific Articles

By John Atkinson,
Associate Director, Intergovernmental Veterinary Health
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African swine fever marches across much of Asia.
Normile, D. Science. 2019;364(6441):617-618. DOI: 10.1126/science.364.6441.617.

Key Thoughts:

It is likely to be some time before the real impact of African swine fever (ASF) is understood in Asia, a region with many pigs and high pork consumption. The scale of the problem facing those working to control the disease may be huge, but many approaches are being taken and it is clear that communication and collaboration will play a critical role in trying to change practices of pig keeping which have been utilized for generations.

Article Summary:

In less than a year since its entry into China in August 2017, ASF has moved across several countries in Asia, including Vietnam, Cambodia, Mongolia, and Hong Kong. It is expected that the disease will continue to spread to new countries because it is highly contagious, and many countries have limited veterinary infrastructure and surveillance systems.

The disease is often fatal, and many more pigs are being culled to try and control the disease. This means that food security is being threatened in addition to the significant economic impact of ASF. 

ASF spreads rapidly by various routes including: direct contact between pigs, direct contact with wild boar, contaminated feed and water, and through people movements (eg, via shoes, clothing, vehicles). A further threat is posed by pork products contaminated with the ASF virus, with such items having been confiscated at various airports across the region.

Without effective vaccines, control focusses on reducing the transmission of the virus. The banning of swill feeding and the initiation of culling, inspection/disinfection stations, and the closure of live pig markets have all been implemented in China. Such control measures can be difficult in some countries because small holders in Asia commonly feed kitchen and table waste to their pigs, which poses a high risk for ASF transmission.

As one of the most serious animal health diseases in the world in an area with limited human, financial, and material resources, these ASF outbreaks may take years to control and require a restructuring of the pig industry.

Wolves contribute to disease control in a multi-host system.
Tanner, E., White, A., Acevedo, P., Balseiro, A., Marcos, J., Gortázar, C. Scientific Reports. 2019;9:art. no. 7940.

Key Thoughts:

There are often many factors involved in the transmission of a disease, including multiple host species and wildlife reservoirs. This may mean that various approaches can be taken to control a disease according to the situation at hand. Viewing predators as an aid to reducing diseases in livestock certainly adds a different perspective to conservation conflicts and warrants further discussion.  

Article Summary:

Wildlife often play a key role as reservoirs of infection for pathogens that can be transmitted between different species. One such disease is tuberculosis (TB) in animals, which is important because it affects many species and causes severe economic losses to the livestock industry. Culling of wildlife has been undertaken as a control measure for TB in livestock; however, the impact of the ecosystem on disease transmission has not been fully explored.

This paper explores the reduction of TB in the presence of wolves in the region of Asturias in north-western Spain utilizing field observations and mathematical modelling. Data from the regional government were used regarding wolf and wild boar population sizes. Animal sampling took place post-mortem using animals shot by hunters, with serum antibodies against Mycobacterium tuberculosis complex (MTC) used to indicate TB prevalence in wild boar. Individual and herd-level data on cattle TB were used from the annual skin testing of all cattle herds in the region.

It was estimated that the wolf population grew from 196 in 2000 to 392 in 2014, and that the reports of wolf predation on livestock were unrelated to livestock numbers. It was assumed that the wild boar density was 50% higher in areas without wolves than those with wolves. In terms of TB prevalence in wild boar, the seroprevalence significantly reduced in areas with wolves, whilst the seroprevalence was initially lower and did not significantly change in areas without wolves. In cattle herds, the TB prevalence in areas with wolves remained stable whilst the TB prevalence increased by 56% in areas without wolves.

Modelling showed that as wolf density increases, the TB prevalence decreases, and wild boar density increases. Therefore, predation may be a key factor in reducing TB in wild boar and decreasing mortality in wild boar due to TB. Importantly for livestock living near wild boar, the level of the pathogen in the environment was reduced by more than 50% over the 14-year period. The model also highlighted how predators may have a much bigger impact on the prevalence of infection in prey species than on the density of those prey.

The authors call for increased consideration of the benefits of predators when managing conflicts associated with predator re-establishment. 



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