Stay up to date with African swine fever prevention - Veterinary Practice
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Stay up to date with African swine fever prevention

Promoting effective biosecurity practices is key to halting the spread of the devastating disease in Europe

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.

Clinical signs

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)
  • Inappetence
  • Depression
  • 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.

FIGURE (1) Pigs huddling together six days after infection with virulent African swine fever virus. Pigs have very high temperatures (over 41C), are lethargic and have lost their appetite, either not eating or picking at food. One pig on the left shows reddening of the pinnae of the ears
FIGURE (2) A pig seven days after infection with virulent ASFV with a very high temperature. This pig is very reluctant to get up, is depressed and has lost its appetite. Blotchy reddening of the pinnae is visible
FIGURE (3) In pigs infected with virulent ASFV, bloody diarrhoea (dysentery) is occasionally observed
FIGURE (4) Pigs infected with virulent ASFV can display skin redness and cyanotic areas (tip of ears, distal limbs, tail, perianal area, chest and abdomen). This pig has reddening of the skin of the ventral body and scrotum. It also had a high temperature

Post-mortem lesions

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

Treatment

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.

Preventing spread

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
FIGURE (5) An enlarged and partially haemorrhagic gastrohepatic lymph node in a pig affected by ASF. At later stages of disease the haemorrhagic area increases and the lymph node becomes almost black. Tracheo-bronchial lymph nodes are often nearly black
FIGURE (6) Pigs infected with acute ASFV can develop haemorrhagic gastritis as shown in this image
FIGURE (7) A typical finding in acute ASF is a very enlarged and haemorrhagic spleen
FIGURE (8) Excess yellow peritoneal and other body cavity fluids are common in pigs affected with acute ASF. The very dark and enlarged spleen can also be seen

Official guidance

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.

Details are published on gov.uk on what to do if you suspect ASF in a pig herd; this is a useful document to provide to the farmer.

Michaela Giles

Michaela Giles, PhD, has worked in the livestock health industry as a research scientist since 1985. She is a senior post-doc research scientist at the APHA, a director of the British Pig Association and is an active member of the British government’s Pig Expert Group


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