Selective Culling as a Containment Strategy for African Swine Fever
Background
In Vietnam, following the widespread transmission of ASF, regulations have been adapted to permit partial and selective culling strategies. This adaptation allows for the retention of pigs that test negative for ASF, helping to conserve resources, mitigate the environmental consequences of mass carcass disposal, and preserve the economic stability of farmers’ operations. Such measures reflect a shift towards more sustainable and ethically considerate management practices in the face of continuing ASF challenges. Partial culling emerges as a viable strategy when ASF infection is detected early and confined to a few epidemiological units, such as individual stables on a farm. Partial culling can on average save over 50% of total stock with an 8-day prolongation of the implementation of control measures. Selective culling can then become an ASF control alternative if the disease has already spread too far on the farm and partial culling is no longer an option. This approach is particularly advantageous in sow farms where animals are housed individually, enhancing its effectiveness compared to fattening farms where animals are grouped together. The rationale behind selective culling is to minimize the number of animals removed, aiming to preserve valuable breeding sows, which represent significant investments for farmers.
Selective culling strategy
Selective culling, also known as the “tooth extraction approach”, is a strategy for controlling ASF by removing only sick and suspect animals instead of the entire herd, practiced in Vietnam. This method prioritizes preserving healthy animals, particularly valuable breeding pigs. Farms typically adapt this strategy based on their understanding of disease control, which can vary significantly. In response to an outbreak of ASF, culling strategies can be categorized into three primary methods:
(i) total culling, also known as “stamping out”, where all pigs within an infected holding are culled;
(ii) partial culling, targeting only pigs within a specifically infected epidemiological unit;
(iii) selective culling, where only those pigs exhibiting signs or suspected of infection are culled
Early diagnosis of ASF and systematic removal of infected pigs are critical components for the effective implementation of selective culling strategies. Additionally, a high level of fragmentation plays a key role in minimizing contact between animals. The optimal approach is to test conspicuous animals and separate them. Under no circumstances should suspect animals be left in the herd for several days before they become severely ill and succumb to the disease.
As an example, a swine farm housing 600 sows confirmed an ASF outbreak in March 2024. This farm successfully contained and eliminated the disease through selective culling over a three-week period. The accompanying video illustrates the eradication process.
References
Implications of partial culling on African swine fever control effectiveness in Vietnam
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