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African swine fever (ASF)

This diseases can affect pigs of any age. High mortality rates. The key clinical signs include blue-purple cyanosis of snout, ears, tail and lower legs; high fever; and heavy discharge from eyes and nose. This disease is notifiable – contact your vet and local authorities if you believe this disease is affecting your herd.

Background and history

African swine fever (ASF) resembles classical swine fever (CSF) (hog cholera) so closely that laboratory tests are required to differentiate them. The clinical signs and post-mortem lesions of the two diseases are almost indistinguishable. ASF is caused by a unique virus which is distinct from that of CSF and which infects only domestic and wild pigs and a variety of soft bodied ticks. The virus is endemic in Africa south of the equator, in warthogs and bush pigs, but the infection in them produces no clinical disease. It circulates between warthogs and the soft bodied ticks which inhabit their burrows. The ticks transmit it through all stages of their life cycle and perpetuate it. It is also endemic in the domestic pigs of some African countries.

The pig (and its close relatives, boars and hogs) is the only natural host of the double-stranded, Asfarviridae family of viruses, meaning the virus does not cause harm to humans or other animals. This does not mean that humans and other animals cannot spread the virus as carriers; African swine fever (ASF) is commonly carried by arthropods, such as the soft-bodied tick, through uptake of blood from infected pigs.

Contamination generally occurs via direct contact with tissue and bodily fluids from infected or carrier pigs, including discharges from the nose, mouth, urine and faeces or infected semen. It also spreads through transport and consumption of contaminated food products, and some cases have originated from failure to comply with biosecurity standards by feeding waste food to domestic pigs. It is believed that a highly pathogenic strain of ASF was introduced to domestic pigs and, subsequently, wild boar populations in the port of Poti, Georgia, in 2007 when waste food from a ship originating in South Africa was fed to local pigs.

Although the virus in wild boar and hogs does not manifest any signs of the disease, it remains highly contagious across all swine species and can survive in pigs for long periods of time post-slaughter – even in frozen carcasses. It is also important to note that curing and smoking pork products does not destroy the virus.

It is vital to immediately distinguish the disease that is infecting a herd; ASF and classical swine fever are caused by very similar viruses which are only distinguishable by laboratory testing. Notifying a vet as soon as any signs arise is the best way to ensure the correct quarantine and treatment procedures are followed – it could save the rest of your pigs.

Clinical signs

  • High fever 40-42°C.
  • Loss of appetite.
  • Depression.
  • Lethargic- sometimes refusal to stand or move.
  • Very unsteady when stood up.
  • Vomiting and/or diarrhoea with bloody discharge.
  • White skinned pigs: extremities (nose, ears, tail and lower legs) become cyanotic (blue-purple colour).
  • Discrete haemorrhages appear in the skin particularly on the ears and flanks.
  • Group will huddle together and are usually shivering.
  • Abnormal breathing.
  • Heavy discharge from eyes and/or nose.
  • Comatose state and death within a few days.
  • Some pigs can show conjunctivitis with reddening of the conjunctival mucosa and ocular discharges.

Pregnant sows commonly undergo miscarriage or deliver stillborn piglets that are malformed – piglets can be tested for the virus.

Mortality rate in infected groups of pigs is high and there is no vaccination proven to prevent or cure infection, therefore, it is crucial that control begins on-farm. European, South American and Caribbean countries which have been infected have adopted a slaughter policy to eradicate the virus within the herd. Mild strains of the virus also occur which cause a milder but equally serious disease in domestic pig herds – individuals from these herds must also be slaughtered to prevent pathogenesis.

Diagnosis

Pigs that die early in an outbreak may not have any noticeable lesions but as the disease progresses the lesions then are striking. Bright red haemorrhages in the lymph nodes, kidneys, heart and linings of the body cavities are common findings. There may also be excess haemorrhagic fluid in the body cavities and gelatinous fluid in the lungs. The spleen may be enlarged, darkened and crumble on slight pressure.

The veterinarian will have to send samples to a laboratory which specialises in CSF and ASF diagnosis. The best samples to send are blood, lymph nodes, spleen and, in chronic cases, serum for serology. In the case it is CSF and not ASF, the tonsils might also be sent. The veterinarian should consult the appropriate veterinary authorities on how best to send these.

The tonsils of the pig are very easy to find. Laying the dead pig on its back, cut away the skin and flesh under and between its lower jaw bone and tongue. The pair of tonsils are two large red patches each about the size of the end half of your thumb or perhaps slightly bigger. Their surfaces are covered with small pits or depressions.

In South Africa and countries outside Africa it is essential to isolate and identify the virus. Only about six laboratories in the world can do this. In African countries where the disease is endemic in the domestic pig population, the veterinarian may only send serum samples for antibody detection.

The virus may be isolated in primary cultures of pig bone marrow or peripheral blood leucocytes. Infected cells haemadsorb ie, pig red cells will adhere to them. Virus can also be detected in infected cells by fluorescent antibody tests. ELISA tests are also used to detect antibodies. In doubtful cases samples can be injected into experimental pigs.

Serum antibody titres may be tested in a number of ways. The indirect immunofluorescence (IIF) and the ELISA tests seem to be the most favoured.

Note that porcine dermatitis and nephropathy syndrome, which occurs from time to time in most pig rearing areas can resemble ASF and CSF clinically and at post-mortem examination. Laboratory examination may be necessary to eliminate them form the diagnosis.

Cause

African swine fever is caused by the Asfarviridae family of viruses which are distinct from the viruses associated with Classical swine fever. There are 22 known types of the ASF virus, allowing the epidemiological tracing of outbreaks to the source.

The infection can be introduced to uninfected herds in a number of ways:

  • the feeding of contaminated feed and contaminated food waste used to supplement feed;
  • through the bites of soft-bodied ticks, lice and flies;
  • through inoculation with contaminated syringes and use of contaminated surgical equipment; and
  • through the introduction of infected pigs to the herd.

Transmission of the virus within the herd is generally through direct contact with infected bodily discharges, faeces and vomit.

Prevention

There is no live or attenuated vaccine for the prevention of ASF therefore control of the virus is reliant on strict biosecurity.

  • Do not feed domestic pigs food waste; this is illegal in the UK, other EU regions and some states within the US
  • Where ‘permitted garbage feeding’ is legal in US states, pigs fed this way are prohibited from exportation.
  • Do not leave food waste exposed for wild swine species to access. Dispose of food waste properly.
  • Abide by strict biosecurity rules. Do not take pig meat onto farms, or restrict all food (and consumption of food) to a canteen. All staff on farm should be inducted onto a strict programme of hand and equipment sanitisation before and after contact with pigs.
  • Follow rules and regulations on disposal of food waste at ferry ports and airports.
  • Provide the means for staff and visitors to thoroughly sanitise their hands and equipment.
  • Ensure that wild boar, warthogs and wild pigs, and materials potentially contaminated by such wild species do not come into contact with domestic pigs.
  • Check infected regions before import of goods that could potentially be contaminated.
  • Advise and educate people on the risks of bringing back pork products from infected regions.

Treatment

There is no treatment.

All infected animals must be isolated and culled immediately upon confirmation of presence of the virus.