Recently, some small-scale pig farmers have reported being hit by African swine fever again. Some pig farms have had to depopulate their herds to limit losses, while large-scale pig companies are also facing pressure from the outbreak. Disruptions caused by the outbreak have led some regions’ farmers to accelerate hog shipments, increasing pork supply and keeping hog prices persistently low. Many farms have fallen into operating losses. Clinical symptoms caused by currently circulating strains are becoming atypical, making early identification more difficult. There is still no effective commercial treatment for African swine fever, and vaccine protection is limited. Understanding the virus’s transmission characteristics and disease patterns, and ensuring early identification, remain central to prevention and control. Typical African swine fever symptoms and basic prevention measures have been reviewed previously. This article focuses on the transmission characteristics and disease patterns of African swine fever at the present stage, for farms’ reference.
Main transmission characteristics of African swine fever
The transmission efficiency of African swine fever virus is closely related to ambient temperature and humidity, how long the virus survives outside the host, stress levels in the herd, and the movement of people, supplies, and live pigs. The current outbreak is primarily spreading through indirect contact, and hidden virus carriage is the main trigger for outbreaks on farms.
1, the virus is highly resistant in the environment, and the risk of transmission varies with the season and environmental conditions
African swine fever virus is resistant to low temperatures but not high temperatures. It can survive for a long time at low temperatures of 0–10℃, for more than 110 days in feces, and for several years in frozen pork products. In northern areas, temperatures are relatively low in winter and spring, allowing the virus to survive for a long time in pig housing, transport vehicles, supplies, feces, and bedding, greatly increasing the probability of contact infection.
Please note: African swine fever is not strictly seasonal. In summer, high temperatures and humidity, along with heavy rain and flooding, can cause rainwater to spread environmental contaminants. In addition, heat stress and the reduced effectiveness of disinfectants in humid conditions mean that scattered outbreaks can also occur in multiple locations in summer, so vigilance must not be relaxed. In dry environments above 30℃, the virus's survival time outside the host is significantly shortened.
2, various forms of stress can lower pigs' resistance and increase the risk of infection
During the low temperatures of winter and spring, pig farms seal their housing to retain heat. Inadequate ventilation leads to ammonia buildup, causing noticeable respiratory stress in pigs and a decline in immunity. In low-temperature environments, pigs expend more energy to maintain themselves. When nutritional intake is insufficient, their disease resistance declines further. Once exposed to a virus, they develop illness more quickly, and it spreads faster throughout the farm. Stressors such as summer heat stress, regrouping, breeding, farrowing, and high-density housing also increase pigs’ susceptibility.
3, people, vehicles, and logistics are the main routes of cross-farm transmission, and the risk of undetected virus carriage is difficult to detect
Inadequate vehicle cleaning and disinfection, and incomplete disinfection of the clothing and shoe soles of people entering the farm, can easily result in virus carriage and cross-farm disease spread. Viruses can attach to carriers such as tires, transport equipment, feed packaging, and incoming supplies, enabling indirect transmission between farms as people and vehicles move. This kind of undetected virus carriage is difficult to identify with the naked eye. Even when basic biosecurity measures are in place on the farm, viruses brought in by external supplies, people, or vehicles may cause an outbreak across the entire farm. In addition, wild boar activity, contaminated pork products, and feeding pigs swill are also sources of infection that must not be overlooked.
What are the current patterns of African swine fever outbreaks?
At present, prevalent strains include both highly virulent and moderately virulent strains. Many cases no longer involve the rapid, large-scale, explosive deaths typical of the past; subacute cases and sporadic, gradual onset are becoming more common. Clinical signs are atypical and can easily be confused with mixed infections involving PRRS or respiratory pathogens. A comprehensive assessment based on multiple sources of information is needed. Confirmation requires laboratory PCR testing; a diagnosis cannot be made based on clinical signs alone.
1, the order in which pigs in different stages become ill tends to follow a pattern
Generally, sows in weaker physical condition are the first to become ill. Postpartum sows and non-pregnant sows are at the highest risk, followed by pregnant sows; next are grower-finisher pigs and nursery pigs. Unweaned piglets are relatively less likely to become ill and tend to become infected only later. However, when moderately virulent strains are circulating, this pattern of illness can be disrupted, so the order in which animals become ill cannot be used as the sole basis for diagnosis.
2, persistent high fever, slow and sporadic spread within the farm, easily confused with mixed infections such as PRRS
When an outbreak occurs nearby, the condition of the pig herd should be monitored continuously. Sick pigs have high fevers that are difficult to bring down; fever-reducing medication lowers their temperature only briefly, after which it quickly rebounds. PRRS and respiratory co-infection: Long-term coughing, wheezing, and other underlying disease symptoms often appear first, followed by a rapid, large-scale outbreak when conditions are favorable. African swine fever (current subacute cases): Cases often occur sporadically and spread gradually. In the early stages, the herd as a whole shows no obvious underlying disease symptoms such as coughing or skin redness; deaths then occur one after another. However, note that cases of African swine fever and PRRS co-infection are increasing year by year. Respiratory symptoms may occur alongside them, making diagnosis significantly more difficult. African swine fever cannot be ruled out solely on the basis of whether coughing is present.
3, persistent bleeding from an injection site in a sick pig is an important warning sign
Many pig farmers give pigs injections to reduce fever and inflammation when they discover that a pig has a fever. The systemic septicemia caused by African swine fever virus damages platelets and impairs blood clotting, causing the blood to become dark in color. Oozing or persistent bleeding from the injection site that cannot be stopped for a long time is a highly suspicious sign. Note: Other septicemic diseases, such as streptococcal disease, can also cause bleeding. This should be treated only as an on-site warning sign and cannot be used alone as a basis for diagnosis.
4, sudden acute death in sows without obvious warning signs; risk is highest around farrowing
During farrowing and the postpartum period, sows undergo substantial physical strain and reduced immunity, making them vulnerable to viral infection. If a sow is normally in good condition with no obvious early symptoms but suddenly dies acutely during farrowing, postpartum, or around weaning, collect samples immediately to investigate African swine fever.
5 and medium-to-large finishing pigs are susceptible and may die suddenly without symptoms
In large-scale farming, medium-to-large finishing pigs consume high-energy feed, have low activity levels, and tolerate stress poorly. When infected with a moderately virulent strain, early symptoms are mild and difficult to detect; the condition can deteriorate rapidly in the middle to late stages, leading to unexplained acute deaths. This is also a very common disease presentation on pig farms today.
The above 5 items are on-site clinical screening reference points for risk alerts only. If a suspected case occurs, isolate the sick pig immediately, collect samples from dead or dying pigs in accordance with proper procedures, and submit them for testing. Laboratory testing is needed to confirm whether the pigs are infected with African swine fever.
African swine fever poses a major threat to the pig farming industry. Biosecurity must be the top priority in pig farm disease prevention and control. Manage the movement of people, vehicles, and materials; disinfect the environment; and reduce all kinds of stress. A well-established biosecurity system can help prevent African swine fever while also reducing the risk of other diseases.