African swine fever's global spread
African swine fever: the virus, how genotype II spread from Georgia to China and India, why vaccines are hard, and how biosecurity and zoning work.
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In August 2018, China, home to about half the world's pigs, reported its first case of African swine fever (ASF). Within a year the disease had spread to every province. Tens of millions of pigs died or were culled, and estimates suggest China's pig herd fell by around 40% at its lowest point. Pork prices doubled, global meat trade shifted, and the outbreak even pushed up world inflation figures.
ASF has since spread across Asia, including India from 2020, and returned to the Americas. It is now one of the most serious transboundary animal diseases in the world. It does not infect people, but it threatens the livelihoods of millions of smallholder pig keepers, many of them in India's north-eastern states, where pork is central to diets and culture.
The disease
ASF is caused by the African swine fever virus (ASFV). It is a large, complex DNA virus, the only member of the family Asfarviridae, and it has more than 150 genes. It infects only pigs: domestic pigs, wild boar, and African wild pigs such as warthogs and bushpigs.
- In domestic pigs and wild boar, virulent strains cause acute haemorrhagic fever: high fever, loss of appetite, reddening or bluish discolouration of the skin (especially ears and belly), vomiting, diarrhoea, abortion, and death within about 4–10 days.
- Mortality can approach 100% with highly virulent strains. Moderately virulent strains cause subacute or chronic forms, and some pigs survive as carriers.
- African wild pigs are infected without showing disease, and the virus circulates between them and soft ticks (Ornithodoros) in a sylvatic cycle.
ASF is a WOAH-listed disease that must be reported. Its signs resemble classical swine fever and other diseases, so laboratory confirmation by PCR is essential.
How it spreads
- Direct contact between infected and healthy pigs.
- Pork and pork products: the virus survives for weeks to months in chilled meat, frozen meat, and cured products such as ham. Feeding swill (kitchen or food waste) containing infected pork is a classic long-distance route.
- Fomites: contaminated vehicles, boots, clothes, equipment and feed.
- Wild boar populations in Europe and Asia.
- Soft ticks in parts of Africa and southern Europe.
Long-distance jumps, such as to Georgia in 2007 and probably to China in 2018, are usually traced to infected pork products, often in ship or airline waste or in travellers' luggage.
Why there is no simple vaccine
ASFV is large and complex, and it evades the immune system. Pigs that recover often do not have strong neutralising antibodies, and inactivated and subunit vaccines have failed to protect. Live attenuated vaccines, made by deleting virulence genes, have shown protection in trials. Vietnam licensed such vaccines in 2022 and has used them in the field. Concerns remain about safety: the vaccine virus could revert to virulence or recombine with field strains. In China, illegal or poorly made vaccines are suspected of producing new, milder but persistent variants. Most countries still rely entirely on biosecurity and control.
Control measures
- Early detection and reporting: farmers and veterinarians report sudden pig deaths, and samples go to laboratories. In India, these include ICAR–NIHSAD in Bhopal and regional laboratories.
- Culling and safe disposal of infected and in-contact pigs, with compensation for farmers, which is essential for encouraging reporting.
- Zoning: India's National Action Plan for ASF defines an infected zone of about 1 km around an outbreak, where pigs are culled, and a surveillance zone of about 10 km, where movement is restricted and pigs are monitored.
- Movement controls on pigs, pork and vehicles, with disinfection.
- Ban on swill feeding, or thorough cooking of food waste before feeding.
- Wild boar management: in Europe, carcass searches, fencing, such as along the German–Polish border, and hunting measures.
Setting up control zones after an outbreak
A village pig farm in a north-eastern district reports sudden deaths, and PCR confirms ASF. Authorities draw an infected zone with a radius of 1 km, about π × 12 ≈ 3.1 km2, and cull all pigs in it with compensation. The surveillance zone of 10 km radius covers about π × 102 ≈ 314 km2, and may include dozens of villages. There, pig movement and pork sales are restricted, and pigs are checked for signs. If no new cases appear for the period set in the action plan, after cleaning and disinfection, restrictions are lifted and restocking can begin.
Farm biosecurity
Biosecurity is the most effective defence, and it applies to small farms as well as large ones:
- Fence pig housing and keep pigs away from wild boar and free-roaming pigs.
- Do not feed kitchen waste containing meat unless it has been thoroughly cooked.
- Control entry: few visitors, dedicated boots and clothes, foot dips, and disinfection of vehicles.
- Quarantine new pigs for at least two to three weeks, and buy only from known healthy sources.
- Dispose of dead pigs safely, never in rivers or forests.
- Report sick or dead pigs immediately.
ASF in India
ASF was first confirmed in India in 2020, in Assam and Arunachal Pradesh. It then spread to Mizoram, Meghalaya, Nagaland, Manipur, Tripura and Sikkim, and later to Bihar, Kerala, Punjab and other states. Tens of thousands of pigs died or were culled in the north-east, where pig keeping is common among tribal households. India has a National Action Plan for Control, Containment and Eradication of ASF (2020), supported by the Department of Animal Husbandry and Dairying's livestock health programmes.
Key points
- ASF is a viral haemorrhagic disease of pigs caused by a large DNA virus (Asfarviridae). It does not infect people.
- Virulent strains cause up to 100% mortality. African wild pigs and soft ticks maintain the virus in a sylvatic cycle.
- It spreads through direct contact, pork and swill, fomites, wild boar and ticks. Genotype II spread from Georgia (2007) to Europe, China (2018), Asia and India (2020).
- No widely approved vaccine exists, though Vietnam uses live attenuated vaccines. Control relies on early detection, culling with compensation, zoning (1 km and 10 km), movement control and swill bans.
- Farm biosecurity, with fencing, no raw swill, controlled entry, quarantine and safe disposal, is the main defence.
What the exams ask
- Describe the cause, signs and transmission of ASF.
- Distinguish ASF from classical swine fever.
- Explain why ASF vaccines are difficult to develop.
- Describe control measures and India's action plan zones.
- List on-farm biosecurity measures for pigs.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can people catch African swine fever from pork?
No. ASF does not infect humans, and pork from infected pigs is not a human health risk. It is, however, a major risk to other pigs: pork products carry the virus long distances, so they must never be fed to pigs.
How is ASF different from classical swine fever?
Both cause similar haemorrhagic disease, but they are caused by unrelated viruses. Classical swine fever is caused by a pestivirus, an RNA virus, and good vaccines exist for it. ASF is caused by a large DNA virus for which no widely approved vaccine is available. Only a laboratory test can tell them apart.
Why is compensation so important?
If farmers lose their pigs without payment, they may hide sick animals, or sell them quickly, often at low prices, spreading the virus further. Fair, prompt compensation encourages early reporting, which is the basis of control.
Sources
- WOAH. "African swine fever." Terrestrial Manual, Chapter 3.9.1, and WAHIS situation reports. World Organisation for Animal Health, Paris, 2023–2025.
- Dixon, L. K., Stahl, K., Jori, F., Vial, L. and Pfeiffer, D. U. "African swine fever epidemiology and control." Annual Review of Animal Biosciences 8, 221–246 (2020).
- Penrith, M.-L. and Vosloo, W. "Review of African swine fever: transmission, spread and control." Journal of the South African Veterinary Association 80, 58–62 (2009).
- Department of Animal Husbandry and Dairying. National Action Plan for Control, Containment and Eradication of African Swine Fever. Government of India, 2020.
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