A120 - AFRICAN SWINE FEVER

IDevice Icon

A120 - AFRICAN SWINE FEVER

Nature of the disease
African swine fever (ASF) is a highly contagious virus disease of pigs, caused by a unclassified virus (sharing similarity with Iridovirus and Poxvirus). The disease is characterised by fever, hyperaemia of the skin, haemorrhages of the internal organs and, frequently, very high mortality.
Classification
OIE List A disease
Susceptible species
In this region, domestic (Sus scrofa domesticus) and wild pigs (Sus scrofa scrofa) are the only susceptible species.
Distribution
ASF occurs in most of Africa south of the Sahara. The disease has spread to Europe, Central and South America and the Caribbean countries.

In most cases the disease has been eradicated after extensive control programs. In it is no longer found in the New World and in Europe is only present in the island of Sardinia (Italy) where it is endemic.

ASF has never occurred in the Pacific region.

Clinical signs
Clinical signs are very similar to those caused by classical swine fever.

According to the virulence of the virus, the disease can vary from a chronic form to an acute form where there are high levels of morbidity and mortality (approaching 100%). In some cases all that may be seen are large numbers of dead pigs.

In the acute forms the following symptoms are observed:

  • High fever (up to 42°C),
  • Depression and loss of appetite,
  • Hyperaemia and cyanosis (seen as reddening) of the skin, particularly ears and snout
  • Coughing and increased respiratory rate
  • Vomiting and diarrhoea (sometimes bloody)
  • Abortions
  • Discharges from the eyes and nose
  • Mortality occurs after 1 to 2 weeks, can reach 100% and survivors are life carriers of the virus.

In the subacute forms the following symptoms are observed:

  • Same symptoms than in the acute form but usually less sever.
  • Abortion,
  • Mortality occurs between 2 and 7 weeks and ranges from 30 to 70%

In the chronic forms the following symptoms are observed:

  • Loss of weight,
  • Irregular fever,
  • Pneumonia-like symptoms
  • Pericarditis
  • Necrosis and ulcers of the skin
  • Low mortality and long disease (over 1 year).
Post-mortem findings 
Lesions cannot be differentiated from classical swine fever. The haemorrhages predominate:
  • Cyanosis of the skin,
  • Haemorrhages in the internal organs: liver, spleen, lymph nodes, kidneys, larynx, bladder,
  • Splenomegaly,
  • Oedema of the digestive tract, and effusion in natural cavities (ascites, pleuratitis, hydropericardium).
Differential diagnosis
  • Classical swine fever (cannot be differentiated from clinic or postmortem),
  • Erysipelas,
  • Salmonellosis,
  • Pasteurellosis
  • Thrombocytopaenia purpura,
  • Warfarin poisoning
Specimens required for diagnosis
Identification of the virus can be performed from whole blood in EDTA (0.5%) or Heparin (10 IU/mL) anticoagulant from live sick animals. From dead animals, tonsils, spleen, lymph nodes or kidney samples (2-5 g) can be submitted if possible kept at 4°C without freezing. Virus identification include techniques such as Haemadsorption test, Fluorescent Antibody test and PCR.

Sera from suspect cases (8 to 21 days after infection) can also be collected for testing for the presence of antibodies by ELISA (recommended by OIE), Indirect Fluorescent Antibody test, Immunoblotting test and Counter Immunoelectrophoresis test.

Transmission
Infected pigs shed large amounts of virus in secretions, specially nasopharyngial. In the acute and subacute forms all secretions from sick animals are considered as infective. Therefore the virus spreads very effectively between pig by contact.

The virus is shed in semen and could be transferred this way. Risk of transmission via embryos is considered negligible.

ASF virus also survives well under most environmental conditions (2-3 months in carcases and in sheds). It can survive for many months in meat (3 months in ham, 6 months in brown marrow of ham, 18 months in blood meal). It resists to freezing process but is inactivated by high temperature (30 min at 60 °C). Spread of the disease by ingestion of products from infected pigs is an important way that the virus spreads to new countries. The contamination by fomites such as premises, vehicles, implements or clothes.

The virus is also spread by soft tick from the genus Ornitodoros where it is vertically transmitted.

Risk of introduction
ASF could be introduced by:
  • Illegally imported pig meats and other pig products,
  • Garbage from international aircraft and ships (in 1957 sewage from an aircraft coming from Angola introduced the disease in Portugal from where it spread in Western Europe),
  • Biological products,
  • Infected husbandry materials,
  • Illegally imported pigs or semen,
  • Introduction of infected ticks.
Control / vaccines
There is no vaccine available for ASF.

To prevent introduction of the disease the sterilisation of garbage and the destruction of waste food coming from infected countries by ship or aircraft should complement the import policy.

The only method to eradicate the disease is by the slaughter and safe disposal of carcases, banning swill feeding, sterilisation of garbage, restricting movements of pigs, cleaning and disinfecting affected premises.

ASF has been successfully eradicated from island countries by total depopulation of the pigs and restocking after a rest period.

References
  • African Swine Fever, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 504-506
  • African Swine Fever, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 935-940
  • GANIERE JP (2001), La peste porcine africaine In Cours de Maladies Réputées Contagieuses des Suidés, Ecole Nationale Vétérinaire de Nantes, p.31-34
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, p. 37-45
  • Office International des Epizooties, 2002