What is the difference between African swine fever field virus and vaccine virus? Why is the success rate of depopulation and removal lower with vaccine virus?

The outbreak situation in southern China has been relatively serious recently. Some pig farms have successfully depopulated and removed affected pigs, while others have not. Pig farmers know that,African swine feverprevention and control, “depopulation and removal” is a key measure for limiting losses. However, many frontline farms report that when they encounter a classic field virus, timely detection may still make it possible to remove all infected pigs. Once vaccine virus (a gene-deleted attenuated strain) is involved, positive results often recur and the virus cannot be completely cleared. The virus remains in the farm for a long time, and the success rate of depopulation and removal drops significantly.

Field virus is a highly virulent strain with straightforward characteristics: rapid onset and a short incubation period. After infection, pigs quickly develop a high fever, stop eating, and show skin hemorrhages; they may even die rapidly. The symptoms are obvious, so caretakers can easily notice them right away. Once a typical case is seen, samples can be collected and the pig isolated and culled immediately. Contamination may still be confined to a small unit, and the environmental viral load is not especially high. As long asbiosecurityKeep up, and there is a chance to contain the outbreak. Put simply, wild-type virus makes a big show of itself—it is like fighting in the open.

Vaccine virus, by contrast, is a genetically modified strain with gene deletions and weakened virulence. Its most defining feature is that it is highly difficult to detect. Infected pigs rarely show typical ASF symptoms. They may show only sporadic reductions in feed intake, slight fluctuations in body temperature, or occasional deaths among weaker pigs. This can easily be mistaken for PRRS, stress, or a common bacterial disease, causing the optimal window for response to be missed. By the time repeated tests detect positive results, the virus may have been spreading quietly through the herd for a long time. The pig house, passageways, supplies, and staff clothing may all have been contaminated, and the extent of infection may be far greater than during an outbreak of wild-type virus.

The shedding characteristics of vaccine virus make the situation even more difficult. Unlike wild-type virus, which is shed in large amounts over a concentrated period, vaccine virus is shed intermittently and at low levels. A virus-carrying pig may test negative in one sample and positive again a few days later. Routine fixed-point sampling can easily miss it, leading to the mistaken conclusion that the herd is clear. If controls are relaxed, the outbreak may recur. Some pigs may also become persistent virus carriers—“reservoir hosts.” They may appear to eat and drink normally, but carry the virus long term and continually shed it into the environment, becoming an ongoing source of infection within the pig farm.

Differential diagnosis is also a major challenge. Standard PCR can only detectAfrican swine fever virus, it is impossible to directly distinguish wild-type virus from vaccine-derived virus. Genotyping requires gene sequencing or a specific differential PCR test. Many pig farms lack the means to test quickly, making it impossible to identify the strain type immediately, and the “tooth extraction” strategy can easily go off course. If diseased pigs are simply culled using the same approach as for wild-type virus, while asymptomatic carrier pigs are missed, outbreaks will inevitably recur.

There is another hidden risk: as vaccine-derived virus is continuously passed from pig to pig, there is a potential risk of reversion to virulence. If a mutation occurs and virulence increases, an outbreak that was originally mild may suddenly worsen, causing greater losses.

Many pig farms run into trouble with vaccine-derived virus. The root cause is not inadequate disinfection, but late identification and the difficulty of detecting carrier pigs. For this type of strain, the traditional “tooth extraction” approach used for wild-type virus cannot be applied. Sampling frequency should be increased, sampling coverage expanded, and saliva and environmental sample monitoring given due attention. Monitoring must be ongoing; a single negative result is not enough to determine that the farm is safe.

There is no room for luck in African swine fever prevention and control. Wild-type virus is fierce, but visible; vaccine-derived virus seems mild, yet is a hidden time bomb within the herd. Recognizing the differences between the two and adjusting monitoring and response plans is essential to protecting the pig farm.