African swine fever is caused by the African swine fever virus (ASFV), a large, complex DNA virus that infects domestic pigs, wild boar, and wild African pigs. It is the only member of the Asfarviridae family, and no other virus like it exists in nature. The disease spreads through tick bites, direct contact between infected and healthy pigs, and contaminated meat products or equipment. Depending on the viral strain, mortality ranges from near zero to almost 100%.
The Virus Behind the Disease
ASFV is a double-stranded DNA virus with a genome that encodes more than 170 proteins. That makes it unusually large and self-sufficient compared to most animal viruses. It carries its own machinery for copying its DNA, which means it is less dependent on the host cell’s internal tools to reproduce. This complexity also makes it extremely difficult to develop vaccines against, since the virus has many ways to evade the immune system.
Once inside a pig, the virus targets macrophages, a type of white blood cell that normally acts as a first line of immune defense. By hijacking these cells, the virus essentially turns the pig’s own immune system into a virus factory. The destruction of macrophages triggers severe internal bleeding and organ failure in acute infections, which is why the disease is so lethal in domestic pigs.
How It Spreads Between Pigs
The most common way domestic pigs catch ASFV is through direct contact with an infected pig. In experimental settings, healthy pigs housed with infected animals became sick within one to nine days. Even when pigs were separated by solid partitions with no physical contact, the virus still spread through airborne droplets over short distances, though transmission took longer (six to 15 days).
Infected pigs shed enormous amounts of virus. Blood can contain up to a billion infectious doses per milliliter, and saliva, urine, and feces can carry up to 100,000 infectious doses per milliliter. The virus also persists for weeks in blood, feces, and urine left in the environment, meaning a pig pen can remain dangerous long after a sick animal has been removed.
The Role of Ticks
In Africa, the virus circulates in a natural cycle between warthogs and soft ticks of the genus Ornithodoros. These ticks live in warthog burrows and feed on young warthogs, passing the virus back and forth. Eight species of Ornithodoros ticks have been confirmed as biological vectors, meaning the virus actually replicates inside the tick rather than just hitching a ride.
What makes these ticks so effective at maintaining the virus is their longevity and ability to pass the infection to the next generation. Ticks in the O. moubata complex can stay infected for years. One species, O. erraticus, has been documented transmitting the virus after going five years without a blood meal. Some tick species also pass the virus to their offspring through their eggs or through mating, creating a self-sustaining reservoir that is nearly impossible to eliminate from an area.
The virus concentrates heavily in tick salivary glands, so transmission happens efficiently during feeding. In Europe, O. erraticus played a key role in sustaining outbreaks in Spain and Portugal during the 20th century. Outside of areas with these specific tick species, however, the virus spreads primarily through pig-to-pig contact and contaminated materials.
Wild Pigs as Carriers
Not all pigs respond to the virus the same way. African warthogs and bushpigs are the original hosts and carry the virus without getting sick. Warthogs develop low levels of virus in their blood and harbor it in their lymph nodes for life. Most adult warthogs in affected regions test positive for antibodies, confirming widespread, silent infection.
European wild boar and feral pigs are a different story. They are just as susceptible as domestic pigs, developing the same severe hemorrhagic disease with very high fatality. Wild boar populations cannot sustain the virus on their own for long. Outbreaks in wild boar tend to fade out unless there is ongoing contact with infected domestic pigs or other sources of fresh virus. This distinction matters because wild boar in Europe and Asia have become a major factor in spreading the disease to farms through environmental contamination.
Contaminated Meat and Feed
One of the most important ways ASFV crosses borders is through contaminated pork products. The virus is remarkably stable outside a living host. In chilled meat stored at 4 to 8°C, it remains infectious for 84 to 155 days. Frozen meat can harbor viable virus for over 100 days at standard freezer temperatures, and at deep-freeze temperatures (-70°C), the virus survives for years with almost no loss of potency.
Cured and processed meats are not safe either. The virus survived in cured ham (similar to Parma or Serrano ham) for over a year. Salami and pepperoni can remain infectious for up to 30 days. Even commercially cured pork belly and loin still contained live virus at 60 and 83 days, which falls within their normal shelf life. This is why discarded food scraps from international travel, particularly sandwiches and sausages, are considered a serious introduction risk. Swill feeding, the practice of giving food waste to pigs, has been linked to multiple outbreaks worldwide.
Infected spleen tissue stored in a refrigerator remained infectious for 204 days. Buried in soil at a shallow depth, it lasted 280 days. Skin and fat retained the virus for 300 days. These numbers explain why thorough cleaning and disinfection of farms, vehicles, and equipment are central to outbreak response.
Disease Severity Varies by Strain
ASFV exists in multiple genotypes, and the clinical outcome depends heavily on which strain a pig encounters. The disease occurs in four forms: peracute, acute, subacute, and chronic. The acute and peracute forms cause severe internal hemorrhaging and carry a fatality rate approaching 100%. Pigs with acute disease typically develop high fever, loss of appetite, skin reddening, and bloody diarrhea before dying within a week or two.
Less virulent strains cause a subacute form in which 30 to 50% of pigs survive. Chronic infections, associated with the least virulent strains, produce intermittent fever, joint swelling, and skin lesions over weeks or months. The strains circulating in most current outbreaks across Asia and Europe are highly virulent, which is why the disease has been so devastating to pig farming in affected countries.
Why There Is No Vaccine Yet
Despite decades of research, no fully approved commercial vaccine is widely available. The virus’s large genome and its ability to manipulate the pig’s immune response have made vaccine development extremely challenging. A promising live-attenuated vaccine candidate developed by USDA researchers passed key safety tests in 2022, showing it did not revert to a dangerous form after injection. That candidate was selected for commercial development in Vietnam, but regulatory approval and widespread availability remain ongoing processes. Each country must independently approve any vaccine before it can be used.
Effective Disinfection
Because no vaccine is broadly available, prevention relies on strict biosecurity. The good news is that ASFV, despite its environmental toughness, is vulnerable to several common disinfectants when applied correctly. Citric acid at 1 to 2% concentration eliminates the virus from hard surfaces like steel and plastic within 10 minutes. On porous surfaces like wood, a 2% solution needs 30 minutes. Bleach (sodium hypochlorite) at 500 parts per million works on hard surfaces in 10 minutes, while 2,000 ppm is needed for wood. Oxidizing agents like potassium peroxymonosulfate-based products are effective on steel, plastic, and concrete in 10 minutes. Iodine-based disinfectants can destroy the virus in as little as 5 minutes when sprayed directly.
These contact times matter. Simply splashing disinfectant and wiping it off is not enough. Surfaces must stay wet with the solution for the full duration. Thorough cleaning to remove organic material like blood, feces, and soil before disinfecting is equally important, since organic matter shields the virus from chemical contact.