Recently, many small-scale pig farmers have reported being hit again byAfrican swine feverSome 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 prompted some farmers to bring pigs to market sooner, increasing the supply of live pigs. Pig prices remain sluggish, and many farms are operating at a loss. Clinical signs caused by currently circulating strains are becoming atypical, making early identification more difficult. To date, there are still no effective commercial treatments for African swine fever, and vaccine protection is limited. Understanding how the virus spreads and how the disease develops, and identifying it early, remain central to prevention and control. Typical ASF symptoms and basic preventive measures were reviewed previously. This article focuses on the transmission characteristics and disease patterns of African swine fever at this stage, for reference by pig farms.
Main transmission characteristics of African swine fever
African swine fever virusTransmission efficiency is highly correlated with ambient temperature and humidity, the virus’s survival time outside the host, stress levels in the pig herd, and the movement of personnel, supplies, and live pigs. The current outbreak is primarily spreading through indirect contact, with undetected carriers being the main cause of the outbreak at this farm.
1, the virus is highly resistant in the environment, and transmission risk varies with the season and environmental conditions
African swine fever virus tolerates low temperatures but not high temperatures. It can survive for long periods in low-temperature environments of 0–10℃, for more than 110 days in feces, and for several years in frozen pork products. In northern regions, temperatures are low in winter and spring, allowing the virus to survive for long periods in pig housing, transport vehicles, supplies, feces, and bedding, greatly increasing the probability of contact infection.
Note: African swine fever does not have an absolute seasonal pattern. In summer, high temperatures and humidity, heavy rain, and flooding can spread environmental contaminants through runoff. Combined with heat stress and reduced disinfectant effectiveness in humid conditions, outbreaks can also occur at multiple locations in summer, so vigilance must not be relaxed. In dry conditions above 30℃, the virus’s survival time outside the host decreases significantly.
2, Various types of stress reduce the herd’s resistance and increase the risk of infection
During the cold winter and spring, pig farms seal up housing to retain heat. Inadequate ventilation leads to ammonia buildup, causing significant respiratory stress in pigs and reduced immunity. In low temperatures, pigs expend more energy to maintain body temperature. If nutritional supply is insufficient, their disease resistance declines further. Once exposed to a virus, they become ill more quickly, and the disease spreads faster within the farm. Heat stress in summer, as well as stress from regrouping, breeding, farrowing, and high-density housing, also increases pigs’ susceptibility.
3, The movement of people, vehicles, and goods is the main route of transmission between farms, with the risk of undetected virus carriage difficult to detect
Inadequate vehicle cleaning and disinfection, and incomplete disinfection of personnel’s clothing and shoe soles when entering a farm, can easily lead to virus carriage and the spread of disease between farms. Viruses can attach to carriers such as tires, transport equipment, feed packaging, and incoming supplies, enabling indirect transmission between farms through the movement of people and vehicles. This type of undetected virus carriage is difficult to identify with the naked eye. Even when basic biosecurity measures are in place on a farm, viruses brought in by outside supplies, personnel, 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 should not be overlooked.
What are the current patterns of African swine fever outbreaks?
At this stage, highly virulent and moderately virulent strains are both circulating. Many cases no longer involve the rapid, large-scale, explosive deaths typical of the past. Subacute, sporadic, gradually developing cases have become more common, and clinical signs are atypical, making them easily confused with PRRS or mixed respiratory infections. A comprehensive assessment based on multiple types of information is needed. Confirmation must rely on laboratory PCR testing; a diagnosis cannot be made based on clinical signs alone.
1, pigs at different stages tend to become affected in a particular order
Generally, sows in poorer condition are the first to become affected. Postpartum and non-pregnant sows are at the highest risk, followed by pregnant sows; then come growing-finishing pigs and nursery pigs. Unweaned piglets are relatively less likely to become affected, with infections mostly appearing later. However, when moderately virulent strains are circulating, this order can be disrupted, so the order of cases alone cannot be used to make a determination.
2 Persistent high fever, with slow, sporadic spread within the facility; easily confused with mixed infections such as PRRS
When an outbreak occurs nearby, the condition of the herd needs to be monitored continuously. Sick pigs have a high fever that is difficult to bring down; fever-reducing medication lowers the temperature only briefly, after which it quickly rebounds. PRRS and respiratory mixed infections: These often begin with underlying symptoms such as prolonged coughing and labored breathing, then rapidly erupt across a large area when conditions are favorable. ASF (currently subacute cases): Cases often occur sporadically and spread gradually. Early on, the wider herd shows no obvious underlying signs such as coughing or reddening of the skin; deaths then begin to occur over time. Note: ASF and PRRS coinfections are increasing year by year and can add respiratory symptoms, making differential diagnosis significantly more difficult. ASF cannot be ruled out based solely on whether coughing is present.
3 Bleeding from injection sites that does not stop after a sick pig is injected is an important warning sign
Many farmers give pigs injections to reduce fever and inflammation after discovering that they have a fever. The ASF virus causes systemic septicemia, damages platelets, impairs blood clotting, and darkens the blood. Oozing or persistent bleeding from an injection site that cannot be stopped for a long time is a highly suspicious sign. Note: Other septicemic diseases, such as streptococcal infections, 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, sows may die suddenly and acutely without obvious warning signs; the risk is highest around farrowing
Sows undergo substantial physical exertion and a decline in immunity during farrowing and the postpartum period, making it easier for viruses to invade. If a sow is normally in good condition and shows no obvious signs of illness beforehand, but dies suddenly and acutely during or after farrowing or around weaning, collect samples immediately to investigate African swine fever.
5, medium- to large-sized finishing pigs are susceptible and may die suddenly without symptoms
In large-scale farming, finishing pigs consume high-energy feed, have low activity levels, and have poor stress tolerance. When infected with a moderately virulent strain, symptoms are mild in the early stages and difficult to detect; the disease then progresses rapidly in the middle to late stages, leading to unexplained sudden deaths. This is also a very common presentation currently seen on pig farms.
The above 5 items are reference points for on-site clinical screening and are intended only for risk alerts. If a suspected case occurs, isolate the sick pig immediately, collect samples from dead or dying pigs in accordance with standard procedures and submit them for testing, and rely on laboratory testing to confirm whether they are infected with African swine fever.
African swine fever poses a major threat to the pig farming industry,Biosecuritymust be the top priority in pig farm disease prevention and control. Implement controls for people, vehicles, and materials; disinfect the environment; and reduce various sources of stress. Improve the biosecurity system to help prevent African swine fever while also reducing the risk of other diseases.