African Swine Fever: Symptoms, Treatment, and the Search for a Vaccine (A Complete Guide)

Imagine a virus so ruthless that it can wipe out an entire pig herd in just days, leaving farmers and communities reeling in its wake. African Swine Fever (ASF) is precisely that threat a highly contagious disease that wreaks havoc on domestic pigs with alarming speed and severity.

In this complete guide, we’ll explore the warning signs, the challenges of treatment, and the ongoing global quest for a safe and effective vaccine, shedding light on a crisis that affects not only animal health but livelihoods and food security worldwide.

What Is African Swine Fever?

African Swine Fever (ASF) is a highly contagious and often fatal viral disease that affects domestic pigs and wild suids, including wild boars, warthogs, and bushpigs. The causative agent, African Swine Fever Virus (ASFV), is a large, double-stranded DNA virus, classified as the sole member of the genus Asfivirus within the family Asfarviridae. ASFV is notable for its resilience, being able to survive in a wide range of environments and in various pork products, such as ham, bacon, and sausages, for extended periods.

ASF is not a threat to human health, but its impact on pig populations can be devastating, with mortality rates ranging from 0% to 100% depending on the virulence of the strain. The disease can manifest in acute, subacute, chronic, or peracute forms, with clinical signs ranging from high fever, incoordination, and prostration to hemorrhages, cyanosis of ears and snout, and sudden death.

ASF was originally endemic to sub-Saharan Africa, maintained in a sylvatic cycle involving warthogs and soft ticks (Ornithodoros species). It was first detected outside Africa in Portugal in 1957 due to the feeding of waste from airline flights to pigs, and it subsequently spread to other European countries, including Spain, Italy, France, and Belgium, throughout the 20th century.

After a period of relative containment, ASF resurfaced in Georgia in 2007 and rapidly spread to central and eastern Europe, Russia, and subsequently to Asia. China, the world’s largest pork producer, experienced its first outbreak in 2018, followed by rapid transmission to neighboring countries such as Mongolia, Vietnam, Laos, Cambodia, the Philippines, North and South Korea, Myanmar, Timor-Leste, and Papua New Guinea. ASF has also reached parts of the Caribbean, including Haiti and the Dominican Republic. Today, the disease is considered endemic in many wild boar populations across Europe and Asia, posing a continuous threat to domestic pig herds worldwide.

Differences Between ASF and Other Swine Diseases

ASF is often confused with Classical Swine Fever (CSF, also known as hog cholera) because both diseases can present with high fever, hemorrhages, and high mortality. However, there are key differences:

• Causative Agent: ASF is caused by ASFV (a DNA virus in the Asfarviridae family), while CSF is caused by a pestivirus (an RNA virus in the Flaviviridae family).
• Host Range: ASF affects domestic pigs and wild suids but does not infect humans or other livestock. CSF similarly affects pigs but can have different transmission patterns.
• Environmental Resistance: ASFV is highly resilient in the environment and in pork products, while CSF virus is less resistant outside a host.
• Epidemiology: ASF has a sylvatic cycle in Africa involving soft ticks and wild pigs, which CSF lacks. ASF outbreaks often result from movement of contaminated meat or wild boars, while CSF spreads more readily via direct contact among domestic pigs.
• Vaccines: There is currently no widely available vaccine for ASF with proven safety and efficacy, although some modified live vaccines are under field evaluation. CSF has effective vaccines widely used in pig populations.

Understanding these differences is critical for accurate diagnosis, rapid response, and implementation of effective biosecurity measures to control ASF outbreaks.

Causes and Transmission

Causative Agent: African Swine Fever Virus (ASFV)

African swine fever (ASF) is caused by the African swine fever virus (ASFV), a large, double-stranded DNA virus belonging to the genus Asfivirus in the family Asfarviridae. ASFV is highly resilient and can persist in the environment for extended periods. It remains infectious in raw or undercooked pork products, contaminated equipment, clothing, and even in carcasses, which makes it particularly difficult to control. The virus affects domestic pigs, feral swine, and wild boars, as well as certain African wildlife such as warthogs and bushpigs. Importantly, ASFV does not infect humans or other non-suid species.

Common Routes of Transmission

ASFV spreads through several primary routes, reflecting its adaptability and high virulence:

1. Direct Contact:
   • Pigs can become infected through direct contact with blood, secretions, or excretions from infected pigs or wild boars.
   • Even minimal contact with contaminated bodily fluids can result in transmission.
2. Fomites (Contaminated Objects):
   • The virus can survive for months on clothes, boots, farm equipment, vehicles, and other materials.
   • Contaminated pork products, such as raw or undercooked meat, sausages, or ham, can serve as sources of infection if fed to pigs.
3. Tick Vectors:
   • Certain soft ticks of the genus Ornithodoros can act as biological vectors, maintaining ASFV in a sylvatic cycle with wild pigs in Africa.
   • Tick-borne transmission is particularly significant in regions where these vectors are endemic, allowing ASFV to persist even in the absence of domestic pigs.
4. Environmental Persistence:
   • ASFV is extremely resistant to heat, pH extremes, and desiccation. It can survive for months at room temperature or refrigerated conditions and is only reliably inactivated by cooking to at least 70°C for 30 minutes.
   • Contaminated environments, including pig pens or wild boar habitats, can act as reservoirs for the virus.

Risk Factors for Outbreaks

Several factors increase the likelihood of ASF outbreaks in domestic and wild pig populations:

• Movement of Infected Pigs or Pork Products: Transport of live pigs, carcasses, or contaminated meat across regions can introduce ASFV to previously uninfected areas.
• Poor Biosecurity Practices: Farms with limited control over access to animals, vehicles, and feed are at higher risk. Feeding uncooked food waste or swill containing infected pork significantly elevates the risk.
• Presence of Wild Suids: Wild boars and feral pigs can maintain ASFV in natural ecosystems, making eradication difficult once the virus spreads beyond domestic herds.
• Tick Infestation: In areas with Ornithodoros ticks, the virus can persist in wildlife reservoirs and re-emerge in domestic pigs despite strict farm biosecurity.
• Proximity to Endemic Regions: Countries neighboring ASF-affected regions face higher risks due to potential cross-border movement of animals, products, or contaminated materials.

Symptoms of African Swine Fever

Incubation Period and Disease Progression

After pigs are exposed to ASF virus (ASFV), the incubation period is typically 3–14 days, depending on the viral strain and dose. During this time, the virus replicates in the tonsils and lymph nodes before spreading throughout the body via the bloodstream. Highly virulent strains often result in rapid disease progression, with pigs dying within a few days of the onset of clinical signs. Less virulent strains may produce a slower disease course, sometimes extending over several weeks and causing chronic or subclinical infections.

Clinical Forms of ASF

ASF manifests in five main forms, each with distinct characteristics:

1. Peracute Form:
   • Most severe and rapid form.
   • Pigs may die suddenly without showing obvious clinical signs.
   • Mortality approaches 100%, often within 2–3 days after infection.
2. Acute Form:
   • High fever (up to 42°C), lethargy, loss of appetite, and weakness.
   • Red or bluish discoloration of ears, snout, and body.
   • Hemorrhages in the skin, lymph nodes, kidneys, and internal organs.
   • Vomiting, diarrhea, respiratory distress, and abortion in pregnant sows.
   • Mortality ranges from 70–100%.
3. Subacute Form:
   • Clinical signs are milder and less specific.
   • Fever, reduced appetite, and moderate lethargy.
   • Occasional hemorrhages, abortions, or respiratory symptoms.
   • Mortality rates are 30–70%, depending on the strain.
4. Chronic Form:
   • Results from infection with low-virulence strains or survivors of acute infection.
   • Non-specific symptoms such as emaciation, swollen joints, respiratory difficulties, or intermittent fevers.
   • Pigs may appear healthy while still shedding virus.
5. Asymptomatic or Subclinical Form:
   • Pigs may carry ASFV without obvious clinical signs.
   • Such animals can act as silent carriers, complicating disease control and surveillance.

Common Signs and Symptoms

Affected pigs may display a combination of the following:

• High fever, lethargy, and loss of appetite.
• Red or bluish skin discoloration (ears, snout, body).
• Hemorrhages in lymph nodes, kidneys, spleen, and internal organs.
• Sudden death, especially in highly virulent outbreaks.
• Vomiting, diarrhea, and respiratory distress.
• Abortions in pregnant sows.
• Weakness, incoordination, and prostration in surviving pigs.

Influence of Virulence and Host Factors

The severity of ASF symptoms is influenced by:

• Viral strain: Highly virulent isolates produce rapid onset, severe hemorrhagic disease, and near-total mortality. Low-virulence strains lead to milder symptoms, prolonged illness, or chronic infection.
• Host age and breed: Pigs of all ages are susceptible, but younger pigs may show more severe clinical signs. Certain wild or local breeds may show variable resistance.
• Environmental and management factors: Stress, co-infections, and poor biosecurity can exacerbate disease severity and spread.

Understanding the spectrum of ASF symptoms is critical for early detection and prompt intervention, as rapid recognition can help contain outbreaks and minimize economic losses.

Treatment and Management

African swine fever (ASF) is a viral disease for which no effective antiviral treatment currently exists. Because of its high contagion and rapid progression, management of ASF focuses primarily on prevention, containment, and supportive care, rather than curative therapies.

Absence of Effective Treatment

Currently, there is no approved antiviral drug or universally effective therapy to treat pigs infected with ASFV. Experimental vaccines and modified live virus vaccines have shown promise in some countries, such as Vietnam, but these remain limited in availability and require rigorous testing for safety and efficacy. Consequently, infected pigs cannot be cured, and survival depends largely on the virulence of the viral strain and host factors.

Supportive Care and Biosecurity Measures

While specific treatments are unavailable, supportive care can help pigs with milder infections survive longer. Supportive measures include:

• Providing clean water and nutrient-rich feed to reduce stress and maintain immune function.
• Minimizing handling and transportation to prevent further physiological stress.
• Ensuring strict biosecurity protocols, such as disinfecting equipment, clothing, and vehicles, and restricting movement of pigs, people, and materials between farms.

Biosecurity is critical in controlling ASF outbreaks, as the virus can survive for months in contaminated environments, pork products, and fomites. Measures include:

• Preventing contact between domestic pigs and wild boars.
• Avoiding the feeding of uncooked food scraps or waste containing pork products.
• Implementing double fencing and controlled access points in pig farms.

Culling and Sanitation as Core Management Strategies

Due to the lack of treatment, culling infected animals remains the primary method to control and eradicate ASF during outbreaks. This approach is combined with strict sanitation measures to reduce viral spread:

• Safe disposal of carcasses through incineration or deep burial.
• Thorough disinfection of contaminated housing, equipment, and transport vehicles.
• Quarantine of affected farms until the virus is eradicated.

Prompt culling, when combined with surveillance and movement restrictions, has historically been the most effective strategy for preventing ASF from spreading to other farms or regions.

Economic and Animal Welfare Considerations

Managing ASF outbreaks carries significant economic implications, including:

• Loss of pigs due to culling or natural mortality.
• Trade restrictions on pork products from affected regions.
• Costs associated with enhanced biosecurity, sanitation, and monitoring.

Animal welfare is also a major concern. Rapid detection and humane euthanasia of infected pigs are critical to prevent prolonged suffering from the hemorrhagic and systemic effects of the virus. Additionally, support for farmers, including compensation schemes, is essential to mitigate economic losses and ensure compliance with control measures.

Progress and Challenges in Vaccine Development

Recent Breakthroughs and Ongoing Research

Researchers have explored a variety of approaches to develop vaccines against ASF, including:

• Live attenuated vaccines (LAVs): These vaccines use weakened forms of ASFV to stimulate immunity without causing full-blown disease. Some LAVs have shown promising safety and efficacy profiles in controlled studies.
• Genetic and molecular approaches: Advances in gene editing and viral genomics have allowed scientists to selectively delete virulence genes from ASFV, producing candidate vaccines that are safer and more immunogenic.
• Recombinant subunit vaccines: Efforts are ongoing to identify key viral proteins that could elicit protective immunity, though these approaches have yet to achieve the same efficacy as LAVs.

Field evaluations of some of these vaccines have already begun in certain countries. For example, Vietnam has licensed two modified live virus vaccines for restricted field use, marking a significant step toward practical ASF vaccination in regions heavily affected by outbreaks.

Major Challenges

Several factors complicate the development and deployment of ASF vaccines:

1. Genetic diversity of ASFV: Over 24 distinct genotypes exist, and a vaccine effective against one genotype may not protect against others.
2. Safety concerns: Live vaccines must not revert to a virulent form or cause adverse effects in pigs. Ensuring safety in both domestic and wild populations is critical.
3. Effectiveness in the field: Vaccines must confer immunity under variable farm conditions, including exposure to different viral loads and environmental stressors.
4. Regional specificity: ASFV strains differ by region, necessitating vaccines tailored to the genotypes circulating locally.

These obstacles underscore why no single, universal ASF vaccine has yet been approved for global use.

Role of International Standards

To guide safe and effective vaccine development, the World Organisation for Animal Health (WOAH) adopted an official ASF vaccine standard in 2024. This standard establishes minimum requirements for safety, immunogenicity, and regional suitability, ensuring that vaccines used in the field:

• Reduce disease severity and transmission.
• Provide protective immunity without adverse effects.
• Correspond to the genotype circulating locally, minimizing the risk of viral recombination and emergence of new strains.

WOAH standards provide a framework for countries and manufacturers, fostering innovation while maintaining safety and efficacy in vaccination programs.

Field Trials and Commercial Vaccine Candidates

Several countries have begun field trials and limited vaccination programs to evaluate candidate vaccines in real-world conditions:

• Vietnam: Licensed two modified live virus vaccines for restricted field use, targeting highly affected provinces.
• Targeted regional vaccines: Research in Europe and Asia is exploring vaccines designed for the ASFV strains circulating in local wild boar and domestic pig populations.

Although these vaccines are not yet universal, these advances represent critical steps forward. When combined with biosecurity measures, import controls, and movement restrictions, vaccines once widely approved could play a central role in mitigating ASF outbreaks, protecting both pig health and the livelihoods of farmers.