African swine fever (ASF) is a non-zoonotic, highly contagious viral haemorrhagic disease of domestic and wild pigs. It is caused by a member of the Asfarviridae family of viruses, and has 22 known genotypes of varying pathogenicity, leading to acute, sub-acute and chronic forms of the disease. The acute form is the most prevalent, invariably causing death.
Studies within ASF-infected areas have shown that ASF is largely spread by the movements of infected wild boar and small-scale keepers feeding their pigs ASF-contaminated meat. Epidemiological tracing has demonstrated large geographical leaps, to previously uninfected areas, by the movement of people either via mechanical transfer or by allowing pigs or wild boar access to contaminated meat products. This method of spread is thought to be the highest risk to the UK herd and significant preventative measures have been put in place by Defra and industry, placing risk of entry into the UK as “medium” (“high” risk would require ASF to be confirmed in the UK).
As of April 2019, in Europe, ASF has been confirmed in Belgium, 1km from the French border (11 cases in wild boar); Romania (8 farms in domestic pigs and 36 cases in wild boar); Hungary (128 cases in wild boar); Ukraine (4 farms in domestic pigs and 1 case in wild boar); Bulgaria (1 wild boar); Latvia (9 wild boar); Lithuania (24 wild boar); and Poland (229 wild boar). Within Asia, China, Vietnam and Cambodia have reported new outbreaks of ASF in domestic pigs. ASF is currently a notifiable exotic disease in the UK.
The clinical signs and post-mortem lesions that occur with ASF are almost indistinguishable from classical swine fever (CSF), requiring laboratory tests to differentiate them. All ages of pigs are susceptible. The clinical signs include:
- High fever (>41°C; Figures 1 and 2)
- Lethargy – sometimes refusal to stand or move, potentially ataxia
- Vomiting and/or bloody dysentery (Figure 3)
- Extremities and ventral body may become cyanotic, noticeable on white-skinned pigs (Figure 4)
- Discrete skin haemorrhages appear, particularly on the ears and flanks
- Groups will huddle together and are typically shivering
- Dyspnoea and possibly coughing
- Heavy oculonasal discharge with sporadic epistaxis
- Occasional conjunctivitis with reddening of the conjunctival mucosa and ocular discharges
- Pregnant sows commonly undergo miscarriage or deliver stillborn piglets that are malformed – the piglets can be tested for the virus
- Comatose state and death within a few days
Pigs infected with less pathogenic strains may not show typical clinical signs; however, severe strains of the disease are generally fatal.
Lesions are variable and not all are present in each pig or in all groups of pigs. They include:
- Haemorrhages – skin, lymph nodes (Figure 5), kidney, bladder, larynx, heart, lung, liver and serosal and mucosal surfaces (Figure 6)
- Enlarged, reddened, almost black, lymph nodes
- Spleen enlarged, dark and more friable (Figures 7 and 8)
- Blood-stained effusions in pericardium, abdomen and thorax
Porcine dermatitis and nephropathy syndrome, which occurs occasionally, can resemble ASF and CSF clinically and at post-mortem examination. Laboratory examination may be necessary to eliminate from the diagnosis.
Transmission and spread
Transmission can occur through three main routes:
- Direct contact with infected pigs, or their faeces or body fluids. Spread from pig to pig is often slow, taking up to two weeks before mortality increases as the virus is not shed before clinical signs appear
- Indirect contact via fomites or people who work with pigs moving between pig farms with ineffective biosecurity, or people travelling from an endemic area bringing the virus back with them on their equipment or in pork products
- Pigs eating infectious meat or meat products. ASF can survive over two and a half years in frozen meat and there are very high amounts of virus in blood and tissues
Virus survival times
The ASF virus can survive long periods of time outside of the living pig:
- 1 day in faeces held at room temperature
- 70 days in blood on wooden boards
- 15 weeks in putrefied blood
- 150 days in boned meat held at 4oC
- 140 days in salted dried hams
- 18 months in pig blood held at 4oC
- 1,000 days in frozen meat
There is no cure or vaccine for ASF-infected pigs, although vaccines are being trialled. Control is through isolation, movement restriction, slaughter and incineration. As a notifiable disease, all pigs in the herd containing a confirmed case will be compulsorily slaughtered.
Recommended biosecurity measures include:
- Wearing protective clothing and boots and providing these for anyone coming onto the pig holding
- Cleaning and disinfecting vehicles and equipment that have been used in areas where pigs or other farmers are, such as agricultural stores
- Disposing of waste food in secure bins that pigs or wildlife cannot gain access to
The disease control strategy for African swine fever in Great Britain sets out the measures we would consider if it was suspected or confirmed in pigs in Great Britain.