African swine fever has been present in China for several years. The industry has gradually moved from “treating ASF as terrifying” toward routine prevention and control under “coexistence with the virus.” However, as attenuated strains have begun to appear in some areas, many pig farm managers have found that past disinfection procedures, testing plans, and response plans no longer seem as reliable as they were against virulent strains. An attenuated strain is not a different disease, but the same virus with a more elusive face. It has not changed the fatal logic of African swine fever, but it has changed how infections are detected. This is precisely where current prevention and control systems most need recalibration.
I. Attenuated strains and virulent strains: the difference is not just virulence
A so-called low-virulence strain refers to an African swine fever virus strain whose genome has mutated and whose virulence has decreased significantly. Compared with a highly virulent strain, the most obvious difference is the fatality rate: a highly virulent strain can cause a mortality rate close to one hundred percent in a susceptible pig population, whereas pigs infected with a low-virulence strain may show atypical signs such as no symptoms, mild fever, or reduced feed intake; some pigs may even survive while carrying the virus.
But “weak” absolutely does not mean “harmless.” The greatest danger of a low-virulence strain is not direct mortality, but the prolonged window it gives the virus to remain latent within the herd. Infected pigs show no typical symptoms, yet the virus continues replicating and shedding inside their bodies, making them mobile sources of infection. A fattening pig carrying the virus may continuously contaminate pens, personnel, and transport vehicles for months, while the entire farm remains unaware.
II. The “threefold difficulty” caused by low-virulence strains
First difficulty: detection. After infection with a highly virulent strain, pigs develop high fevers, hemorrhage, and die; the symptoms are immediately obvious, and farms can often identify the risk area within a few days. Attenuated strains are different: fluctuating body temperatures, feed intake dropping by a few percentage points, and individual pigs showing skin redness are signals that are too easily attributed to weather changes or ordinary stress. By the time testing confirms the infection, the virus has often been spreading on the farm for several weeks.
The second challenge: detection. Current mainstream nucleic acid testing uses tonsils, blood, and environmental swabs as samples. In the early stages of infection with a low-virulence strain, the viral load is low, and routine sampling often “misses” infections. Blood samples also have limited sensitivity for latent infections; once the sampling window is missed, a negative result can create a false sense of security.
The third challenge: response. When dealing with highly virulent strains, the industry-standard approach is rapid depopulation, thorough disinfection, and restocking after an empty period. But in farms testing positive for low-virulence strains, infected pigs appear normal and mortality is not high. Whether to depopulate, how large an area to depopulate, and how long to leave the facilities empty—each decision involves enormous costs, leaving managers caught between “gambling” and “enduring.”
Three. Is the existing prevention and control system still effective? A point-by-point analysis
First, the conclusion: the basic framework of the existing prevention and control system remains effective, but every part needs an “enhanced” upgrade tailored to weak strains.
At the biosecurity level, disinfection procedures for people, vehicles, and supplies remain the first line of defense. Attenuated strains can also spread through contact and contamination, so the underlying principles of physical isolation have not changed. What has changed is the implementation standard: whereas “disinfection upon entry” used to be enough, the requirement is now “isolation before entry, zoning within the site, and one-way movement,” treating every contact involving people, vehicles, or supplies as a potential route for virus introduction.
At the testing level, “targeted sampling + high-frequency monitoring” has replaced “sampling after disease onset” as the new principle. Regularly collecting nasal and oral swabs and environmental samples from high-risk pig populations for nucleic acid testing is far more effective than waiting until clinical symptoms appear before testing. Meanwhile, antibody testing can serve as a supplementary method, helping identify pig populations that have previously been exposed to the virus but have since tested negative by nucleic acid testing, and providing a second perspective for risk assessment.
At the response level, there is no longer a one-size-fits-all approach to farms that test positive for attenuated strains. Three strategies coexist: zonal depopulation, targeted culling, and continued rearing of infected pigs. The choice depends on the virus’s distribution within the farm, the value of the herd, and the restocking plan.
Four. The Hidden Financial Erosion of Attenuated Strains on Pig Farms
Losses caused by low-virulence strains are often not a one-time blow, but a sustained drain. Growth performance declines in subclinically infected pigs, the feed-to-meat ratio rises, time to market lengthens, and the cost of producing meat per pig quietly increases. Some estimates show that production performance at farms with stable low-virulence strain positivity may be ten to twenty percent lower than at negative farms. If these losses are not specifically calculated, they can easily be attributed to market conditions or feed.
More hidden are losses at the breeding stock stage. Once replacement pigs and boars carry the virus, their value for use drops significantly, and the cost of introducing them is sunk. If infected breeding pigs enter the mating herd, they will continue to contaminate offspring through vertical and horizontal transmission. For farrow-to-finish farms, a low-virulence strain is like a long-dormant landmine buried in the core breeding stock.
Five. Practical checklist for pig farms responding to low-virulence strains
First, upgrade the monitoring system. Shift testing from a “reactive” approach to a “routine” one. Set fixed sampling frequencies for the farrowing, nursery, and finishing stages; collect environmental and pig swabs in parallel; record test results in a log to create trend curves; and escalate the response immediately if any abnormal fluctuations occur.
Second, establish effective zoning and one-way flows. Divide the premises into clean and dirty zones according to risk levels. The movement of people, tools, and pigs must be strictly one-way, and crossing between zones must involve a disinfection buffer. The core of controlling low-virulence strains is not to eliminate the virus, but to confine it to the smallest possible area.
Third, make good use of sentinel pigs. For vacant areas or assessments before restocking, introduce a small number of sentinel pigs and observe them for one full cycle. Resume production only after testing confirms the environmental risk. Sentinel pigs do not cost much, yet they are the most direct “environmental safety indicator.”
Fourth, prepare contingency plans. Write in advance the response plan for farms testing positive for low-virulence strains, clearly specifying the circumstances under which sectional depopulation or whole-farm depopulation will be carried out, and which testing criteria must be met before restocking, to avoid rushed decisions and repeated disruption after an outbreak is confirmed.
VI. Industry and Policy Response Directions
At the industry level, the emergence of less virulent strains is driving testing technologies to become more sensitive, faster, and more portable. Automation in processes such as nucleic acid extraction and fluorescence quantification enables small and medium-sized pig farms to establish routine monitoring capabilities rather than relying on “occasional checkups” at external laboratories. Vaccine development is a long-term direction, but until then, the “combined approach” of monitoring, zoning, and response remains the only reliable line of defense.
At the policy level, more detailed guidance is needed on how to handle farms testing positive for low-virulence strains, to avoid the two extremes of “culling across the board” and “doing nothing.” At the same time, regional joint prevention and control mechanisms should cover cross-farm activities such as breeding pig transport and the harmless disposal of dead pigs, to prevent low-virulence strains from spreading more widely through the supply chain.
Seven. Conclusion: The defenses have not failed; they simply need to be upgraded
Answer the question raised in the title: the existing prevention and control system has not failed, but it is no longer facing “the same opponent.” In the highly virulent strain era, prevention and control depended on speed of response; in the low-virulence strain era, they depend on monitoring density and decision-making precision. Biosecurity, routine testing, zoning management, and tiered response are all still in place, but the implementation standards for every link need to be recalibrated to the characteristics of low-virulence strains.
For pig farms, the greatest danger has never been the virus itself, but using an old map designed for dealing with highly virulent strains to find new paths taken by low-virulence strains. Make monitoring more intensive, make zoning more effective, and make contingency plans more detailed: in this long-term contest with low-virulence strains, the winners will be pig farms willing to upgrade their understanding and implementation standards.