African swine fever is one of the deadliest viral diseases in pigs, capable of killing entire herds within days of the first signs of illness. Caused by a large, complex DNA virus that targets the immune system’s own defense cells, ASF carries fatality rates approaching 100 percent in domestic pigs exposed to virulent strains. The disease does not infect humans, but its economic and agricultural damage is enormous, having spread across Africa, Europe, Asia, and the Caribbean over the past century.1PubMed. One hundred years of African swine fever in Africa: Where have we been, where are we now, where are we going? With no widely available vaccine and no cure, prevention still depends almost entirely on biosecurity, early detection, and culling.
What the Virus Does Inside a Pig
ASFV is unlike most viruses that farmers and veterinarians deal with. It is a large double-stranded DNA virus with a complex multilayered structure, belonging to a family with no close relatives in other livestock diseases.2Cell Host & Microbe. Cryo-EM Structure of the African Swine Fever Virus Capsid Once it enters a pig’s body, it homes in on monocytes and macrophages, the very immune cells that are supposed to destroy invaders. By hijacking these cells, the virus triggers massive cell death, severe depletion of lymphoid tissue, and a runaway inflammatory response. The resulting cytokine storm damages blood vessel walls throughout the body, leading to hemorrhages in multiple organs.3PubMed Central. African swine fever virus: Virology, pathogenesis, clinical impact, and global control strategies
Research in piglets has shown that the virus also infects epithelial cells and liver cells, not just immune cells. Massive tissue necrosis and the formation of tiny blood clots (microthrombi) in multiple organ systems contribute to the rapid deterioration and high mortality seen in acute infections.4PubMed Central. From hemorrhage to apoptosis: understanding the devastating impact of ASFV on piglets The virus also actively sabotages the pig’s antiviral defenses. At least one viral protein, E120R, blocks the production of interferon-beta, a key signaling molecule that normally rallies the immune response against viral infections. It does this by interfering with the activation of a protein called IRF3, essentially cutting the alarm wire before the cell can call for help.5PubMed Central. African Swine Fever Virus E120R Protein Inhibits Interferon Beta Production by Interacting with IRF3 To Block Its Activation This kind of immune evasion is one reason the virus is so lethal and so difficult to vaccinate against.
Recognizing the Symptoms
The clinical picture depends heavily on the strain involved. Highly virulent strains cause acute disease: pigs develop high fevers (often above 40.5°C), stop eating, become lethargic, and huddle together. Within a few days, reddish-purple blotching appears on the skin, especially on the ears, tail, belly, and legs. Internal hemorrhaging is widespread, particularly in the spleen, kidneys, liver, and lymph nodes. Experimental infections with organ homogenate from infected pigs produced 100 percent mortality, with gross lesions in the spleen, kidneys, liver, gallbladder, and multiple lymph nodes visible at necropsy.6PubMed Central. Experimental comparison of African swine fever virus exposure through contaminated feed and infected pig organ homogenate Vomiting, bloody diarrhea, nasal discharge, and labored breathing may all appear in the final stages. Death usually follows within a week or two of the first fever.
Lower-virulence strains, which have become more common in some regions as the virus evolves, can cause a subacute or chronic form. Pigs may show intermittent fevers, joint swelling, skin ulcers, and slow wasting over weeks or months. These milder forms complicate detection because farmers may not immediately suspect ASF, and affected pigs can shed virus for extended periods. Because the symptoms of acute ASF overlap with several other pig diseases, especially classical swine fever, laboratory confirmation is always required.
How ASF Spreads
Understanding transmission routes is critical for prevention, and the virus has an uncomfortable number of ways to move between herds.
Direct Contact and Contaminated Feed
The most efficient route is direct contact between an infected pig and a healthy one, whether in the same pen, at a market, or during transport. Contact with wild boar is another major pathway. Evidence supports direct contact with infectious domestic pigs and wild boars, along with consumption of contaminated feed, as the primary routes by which the virus reaches new herds.7PubMed Central. Transmission routes of African swine fever virus to domestic pigs: current knowledge and future research directions
The feed question is more nuanced than it first appears. ASFV has long been associated with swill feeding, the practice of giving pigs kitchen scraps or waste products that may contain uncooked pork. Outbreaks on smallholder pig farms have been linked to swill feeding.8PubMed Central. Thermal Inactivation of African Swine Fever Virus in Swill However, a recent experimental study found that pigs given commercial dry or wet feed spiked with ASFV did not develop infection, while pigs fed infected organ homogenate all died within about eight days on average. Virus showed up in blood less than three days after exposure, followed quickly by shedding from the mouth, nose, and rectum.6PubMed Central. Experimental comparison of African swine fever virus exposure through contaminated feed and infected pig organ homogenate The practical takeaway is that the real danger lies in feeding raw or undercooked pig-derived products, not in standard commercial feed. Swill-feeding bans exist in many countries for exactly this reason.
Tick Vectors
Soft ticks of the genus Ornithodoros serve as both vectors and natural reservoirs for ASFV, capable of harboring the virus for years and passing it to pigs through their bite.9Peer Community Journal. Experimental design impacts the vector competence of Ornithodoros ticks for African swine fever virus: a meta-analysis of published evaluations In Africa, these ticks maintain the virus in a cycle involving warthogs, which tolerate infection without becoming seriously ill. In regions where domestic pigs are housed in rudimentary shelters with cracks and burrows where ticks can hide, tick-borne transmission is an ongoing concern. This pathway is also a worry for countries that have not yet seen outbreaks. In the United States, at least three species of Ornithodoros ticks have been shown to be competent ASFV vectors under laboratory conditions, and contact between these ticks and invasive wild pigs has been documented in the southeastern states.10PubMed Central. Contact between soft tick vectors of African swine fever virus and invasive wild pigs in the southeastern USA
Fomites and Environmental Persistence
ASFV is remarkably stable outside a host. It survives for weeks or months in blood, meat, and other organic material, and it can persist on contaminated equipment, clothing, boots, and vehicles. This environmental toughness means that a farmer walking between pens, a truck that carried infected pigs to market, or a pair of boots that stepped in contaminated material can all serve as carriers.11PubMed Central. African Swine Fever Virus – Persistence in Different Environmental Conditions and the Possibility of its Indirect Transmission The virus is also resistant to a wide range of pH levels and can survive freezing, which is why cured, smoked, or frozen pork products from affected regions remain a biosecurity risk. Human-driven factors along the pig value chain, including trade in infected products and poor disposal of carcasses, have been identified as the dominant obstacles to controlling and eradicating the disease.1PubMed. One hundred years of African swine fever in Africa: Where have we been, where are we now, where are we going?
Wild Boar and Warthogs as Reservoirs
Domestic pigs and European wild boar are both highly susceptible to ASFV and develop severe, often fatal disease when exposed to virulent strains. African wild suids like warthogs, on the other hand, typically tolerate the virus with few or no symptoms, acting as a silent reservoir that maintains the virus in the environment.12Veterinary World. African swine fever in the era of global expansion: Epidemiology, transmission dynamics, viral evolution, and One Health control strategies This difference in susceptibility is a defining feature of ASF epidemiology. In sub-Saharan Africa, the warthog-tick-pig cycle has been the baseline for decades. In Europe and Asia, where there are no warthogs, wild boar populations have taken on a parallel role: once infected, they spread the virus through direct contact, through carcasses left in forests, and through contaminated wallows and feeding sites.
Controlling ASF in wild boar is extraordinarily difficult. You cannot vaccinate or medicate free-ranging wildlife at scale, and traditional culling has often failed to reduce wild boar density enough to break transmission chains. Some European countries have invested in carcass search-and-removal programs combined with fencing to limit wild boar movement, with mixed results.
Diagnosing ASF
Because ASF mimics other hemorrhagic diseases in pigs, clinical suspicion alone is never enough. Laboratory confirmation relies on two broad categories of tests: those that detect the virus itself and those that detect antibodies the pig produces in response to infection.
For virus detection, PCR-based tests are the gold standard. During outbreaks in Eastern European Union countries, three different PCR methods showed very high agreement with each other, and a newer test called UPL-PCR proved particularly sensitive for catching the disease early and for identifying pigs that had survived infection and might still carry the virus.13PubMed Central. Assessment of African Swine Fever Diagnostic Techniques as a Response to the Epidemic Outbreaks in Eastern European Union Countries: How To Improve Surveillance and Control Programs For antibody detection, the immunoperoxidase test (IPT) outperformed several commercial ELISAs, picking up antibodies earlier in the course of infection. The same study found that IPT was useful for testing tissue fluid from dead wild boar, an important capability given that surveillance in wild boar populations often depends on sampling animals found dead in the field.
Newer diagnostic approaches are being developed, including rapid pen-side tests that could give results in the field without sending samples to a laboratory.14PubMed. Advances in the diagnostic techniques of African swine fever Speed matters enormously for a disease that can rip through a herd in days: every hour between suspicion and confirmed diagnosis is time the virus can spread to neighboring farms.
Prevention Through Biosecurity and Disinfection
Without a widely deployed vaccine, the entire weight of ASF prevention falls on biosecurity. The goal is to keep the virus from reaching pigs in the first place, which means controlling every possible entry point: people, vehicles, feed, water, equipment, and wildlife.
For commercial farms, this means perimeter fencing to exclude wild boar and stray animals, designated vehicle wash stations, boot and clothing changes at entry points, strict all-in/all-out herd management, quarantine periods for newly purchased pigs, and sourcing feed from certified suppliers. For smallholder farms in Africa and Asia, where resources are limited and pigs often roam freely, biosecurity advice is the same in principle but far harder to implement. Outbreaks continue to be reported in these settings, where farm practices and market behaviors contribute to the virus persisting in pig populations.15PubMed Central. The Context of Application of Biosecurity for Control of African Swine Fever in Smallholder Pig Systems: Current Gaps and Recommendations
Disinfection plays a supporting role, especially during and after an outbreak. A wide range of chemical classes can inactivate ASFV, including sodium hypochlorite (bleach), glutaraldehyde, caustic soda, and potassium peroxymonosulfate, all of which achieved reductions in virus levels of more than five orders of magnitude in laboratory testing.16PubMed Central. Effectiveness of Chemical Compounds Used against African Swine Fever Virus in Commercial Available Disinfectants Overall, effective disinfectants fall into eight broad categories: acids, alkalis, aldehydes, chlorine compounds, iodine compounds, oxidizing agents, phenol compounds, and quaternary ammonium compounds.17PubMed Central. Disinfectants against African Swine Fever: An Updated Review However, performance can drop dramatically in the presence of heavy organic matter such as blood or manure, and some compounds that work well in clean laboratory conditions fail in real-world settings. Classic work on this problem showed that in protein-rich material mimicking farm conditions, many commonly used disinfectants failed to inactivate ASFV within an hour at room temperature, and contact times of at least 60 minutes were needed even with effective products.18PubMed Central. Effects of some disinfectants on African swine fever virus Thorough physical cleaning to remove organic matter before applying disinfectant is essential.
Where Vaccines Stand
The absence of a safe, effective, mass-producible vaccine is the single biggest gap in ASF control. The virus’s complexity is a fundamental obstacle: ASFV has a massive genome encoding more than 150 proteins, many of which are devoted to evading the immune system. Researchers still do not fully understand which immune responses are actually protective, and there is substantial variation between ASFV strains circulating in different regions.19PubMed. Challenges for African swine fever vaccine development—”… perhaps the end of the beginning.” Killed-virus vaccines have consistently failed to protect pigs. Subunit vaccines, which use individual viral proteins to stimulate immunity, have also struggled to produce strong enough protection.
The most promising approach so far involves live attenuated vaccines, where specific genes are deleted from virulent ASFV strains to weaken them while preserving the ability to trigger immunity. Several candidates are moving through safety and efficacy testing. One candidate, ASFV-G-ΔI177L/ΔLVR, was tested in pregnant sows and caused no adverse effects, induced strong antibody production, and did not interfere with normal delivery. Piglets that consumed colostrum from vaccinated sows received maternal antibodies, though there was evidence of virus crossing the placenta, highlighting the need for continued vigilance around safety.20PubMed Central. Safety of Live Attenuated ASFV-G-ΔI177L/ΔLVR Vaccination in Sows With Advanced Pregnancies
A Chinese candidate called HLJ/18-7GD was assessed in a field trial involving 201 sows over more than 11 months across two parities. Vaccinated sows showed no disease, no changes in reproductive performance, and no adverse effects on piglet health or growth. The vaccine virus replicated at low levels and was cleared from sows within 84 days and from piglets by 56 days of age.21PubMed Central. Live-attenuated African swine fever vaccine HLJ/18-7GD is safe in pregnant sows with no impact on reproductive performance or offspring health at a field farm These are encouraging results, but field safety is only part of the equation. Another candidate, ASFV-G-ΔMGF, underwent a reversion-to-virulence study where the virus was forcibly passed through multiple animals. A variant emerged with increased replication and shedding and was associated with transient fever, though all animals recovered and full virulence did not return. Genomic changes clustered in the terminal regions of the genome rather than at the engineered deletion site.22PubMed Central. Assessment of African swine fever vaccine candidate ASFV-G-∆MGF in a reversion to virulence study The finding underscores that live attenuated vaccines always carry some risk of evolving back toward virulence, and each candidate needs long-term safety data before wide deployment. The biology of ASFV, its immune evasion toolkit, and the incomplete understanding of protective immunity all continue to slow progress.23PubMed Central. Advances in African swine fever vaccine development: challenges and prospects
Not a Risk to Humans, but a Massive Economic One
ASF is not zoonotic: it cannot infect people, and eating pork from an infected pig, while a food-safety concern for other animals and a biosecurity risk, does not cause human illness.24PubMed Central. African Swine Fever: A One Health Perspective and Global Challenges Public confusion on this point has occasionally depressed pork demand in affected countries beyond what the actual supply disruption would warrant. The real damage is economic and agricultural.
When ASF hit China in 2018, the consequences were staggering. The price of pork per kilogram jumped from an average of about $3.03 in 2019 to roughly $4.93 in 2020 as production gaps disrupted the supply-demand balance.25Exploration of Foods and Foodomics. Assessing the price effects of African swine fever in the China market China is the world’s largest pork producer and consumer, so these ripples were felt globally. Modeling of a hypothetical outbreak in the United States projects that a small outbreak would cause welfare losses of roughly $310 to $563 million, while a large outbreak could reduce U.S. hog production by about 7 to 9 percent, push hog producer prices up by more than 40 percent, and generate welfare losses of $10.9 to $11.4 billion. Global hog and pork prices would also rise, with trade shifting toward alternative exporters.26PubMed Central. Potential economy-wide impacts of an African swine fever outbreak in the United States
Why Eradication Has Proved So Elusive
Countries that have eliminated ASF in the past, including Spain and Portugal, which declared freedom from the disease in the 1990s after decades of effort, relied on aggressive culling, strict movement controls, and surveillance. But these successes happened before the disease reached the scale it occupies today. With infections now established in wild boar populations across much of Europe and Asia, the virus has a reservoir that no culling program can easily reach. In Africa, the warthog-tick sylvatic cycle has never been interrupted. And the global trade in pork products, legal and illegal, constantly threatens to introduce the virus to new regions. Its appearance in the Caribbean in 2021 was a reminder that no major pig-producing country is immune to the risk.1PubMed. One hundred years of African swine fever in Africa: Where have we been, where are we now, where are we going?
The combination of a resilient virus, multiple overlapping transmission pathways, wild reservoirs, tick vectors, and the absence of an approved vaccine creates a control challenge with few parallels in veterinary medicine. Progress on live attenuated vaccines is real but cautious, and even an effective vaccine would need years of production scale-up and distribution before it could change the trajectory of outbreaks already underway. For now, the boring fundamentals of biosecurity, surveillance, rapid diagnosis, and responsible management of the pig value chain remain the only tools that reliably work.