|
| ||
| Home | Search | |
African swine fever (ASF) is one of the three most dreaded epidemic diseases of pigs. The other two are swine vesicular disease (SVD) and foot-and-mouth disease (FMD). ASF is dangerous because of its highly infectious nature, variety of means of spreading, high morbidity rate (= percentage of infected swine population becoming ill), high mortality rate and the lack of specific treatment or vaccine.
African Swine Fever (ASF)
African Swine Fever (ASF) - European Union to revise & harmonise tests:
European Commission news January 2002: Report (PDF format)
ASF outbreaks in Africa
In recent years African swine fever epidemics have devastated pig herds in the Côte d'Ivoire, Benin, Nigeria, Togo and, most recently, in Gambia, Ghana and Madagascar. The 1999 epidemic in Ghana was finally brought under control by a major surveillance operation, strict border controls, heightened public awareness, and a market value compensation scheme (with World Bank assistance) for slaughtered pigs.African swine fever is subclinically (without producing disease symptoms) endemic in warthogs in parts of southern and eastern Africa (not South Africa or Zimbabwe). Biting soft-bodied Ornithodoros ticks spread the virus from warthogs to domestic pigs. Other wild African suids (bush pigs and giant forest hogs) are also sublinically affected by ASF, but their role, if any, in the epidemiology of African swine fever is unclear.
Since 1996 the Food and Agriculture Organisation of the United Nations (FAO) has assisted a number of countries in Africa with Technical Cooperation Projects to control and eradicate ASF.
In the first six months of 2001, ASF outbreaks were reported in Benin, the Democratic Republic of the Congo, Mozambique, Senegal and Togo.
African swine fever in European Union in 2000: overview
ASF outbreaks in Italy
African Swine Fever is enzootic (endemic) in Sardinia (classical swine fever is also enzootic there). There were 28 outbreaks of African swine fever in Sardinia in 1998 and 22 outbreaks in 1999. Most outbreaks have been in the province of Nuoro, but Cagilari has also been affected. Antibodies to the virus have been found in the local wild boar population.
On 23 December 1999, Dr Romano Marabelli, Director General of Veterinary Services, Ministry of Public Health, Rome, reported that Italy (except for the Island of Sardinia) was now considered free from African swine fever and classical swine fever, with effect from 30 October 1999.
ASF in Portugal
ASF surveillance in Portugal 2000
A surveillance programme for African swine fever was approved by the Standing Veterinary Committee (SVC) of the European Commission (EU) on 24/11/1999. During the year 2000 several thousands of pigs were sampled and farms were inspected. No antibodies or virus were found. The Portugueses authorities continue to take ASF control surveillance measures in the Alentejo region, including:
- monitoring and testing of domestic pigs of the Alentejano breed;
- testing of feral pigs (wild boar);
- measures to ensure that old premises where ticks of the genus Ornithodorus may be present are not used to keep pigs
The Portuguese authorities will keep the European Commission and the SVC informed on the results of these measures on a regular basis."
ASF recurrence in Portugal (November 1999)
Date of last reported outbreak: August 1993
In November 1999, Dr Rui Marques Leitão, Director General of Veterinary Services, Ministry of Agriculture, Rural Development and Fisheries, Lisbon reported:
An outbreak of African swine fever was detected on 5 November 1999 (estimated date of first infection: 3 November) at Aldeia das Fernandes (in Alentejo region of south Portugal).
There were 44 pigs on the farm, seven became ill with ASF and six of these died. The remaining 38 pigs were destroyed.
Diagnosis was confirmed by laboratory isolation of African swine fever virus on 15 November 1999.
Control measures put into effect: slaughter and destruction in situ (by burying) of all pigs present on the farm.
Data source: Office International des Epizooties (OIE)
Clinical effects of ASF
Incubation period is 5-15 days, followed by one or (usually) more of these forms of disease:
- Peracute
Infection with highly virulent virus strains can result in some pigs being suddenly found dead, or close to death.
- Acute
There is fever (pyrexia), loss of appetite (anorexia) and inactivity. Areas of red or blue skin discoloration may appear on the ventral chest or abdomen, tips of ears or tail, or distal limbs. Diarrhoea, vomiting, coughing, breathing difficulty and abortion may also occur. Almost 100%. of pigs with these symptoms will die within 7 days. Pigs that recover can be lifelong carriers of virus.
- Subacute
Seen particularly with virus strains of moderate virulence. Affected pigs are only mildly ill, but sows may abort. There can be intermittent fever for up to one month, followed in most cases by recovery. Mortality ranges from 30-70%. Recovered pigs can still be excreting the virus up to six weeks after infection.
- Chronic
Although having occasional episodes of fever, these pigs show little illness apart from reduced growth, stunting or emaciation. There may be necrotic patches of skin or chronic skin ulcers. They are vulnerable to secondary infections, pneumonia and lameness (arthritis). Infection lasts for two to five months but mortality is less than 30%.
Diagnosis of African swine fever
Autopsy findings
Petechial (pinpoint) haemorrhages are very typical, especially in the cortex, medulla and pelvis of the kidneys, the mucous membranes of the larynx and bladder and on visceral surfaces of organs.
Lymph nodes are enlarged and haemorrhages may be seen in the gastrohepatic and renal lymph nodes.
haemorrhagic swollen lymph node - medial retropharyngeal
(courtesy of Dr. Stan Done, Veterinary Laboratories Agency UK)Blood splashes (ecchymoses) may be present in the skin of the legs or abdomen. The spleen is often enlarged and there can be excess fluid in the abdominal, pleural and pericardial cavities. Oedema of the gall bladder and mesentery of the colon is also quite typical of ASF. In chronic cases, lungs may show focal lesions of caseous necrosis and even mineralisation.
Differential diagnosis
African swine fever is very similar to classical swine fever (hog cholera) and porcine dermatitis and nephropathy syndrome (PDNS) in clinical signs and autopsy lesions. These diseases must be distinguished by means of laboratory tests, such as virus isolation, serology, or a direct fluorescent antibody test on diseased tissues.
Laboratory tests
Virus isolation from serum, spleen, kidney or lymph nodes.
Virus antigen demonstration by direct immuno-fluorescence on tissues.
PCR (polymerase chain reaction) is a sensitive test to detect viral genetic material.
Serological (antibody) tests on serum from recovering pigs, 8-21 days after infection: ELISA, indirect fluorescent antibody test, immunoblotting, immunoelectrophoresis.
Differential diagnosis
Other pig diseases that can show superficially similar signs to ASF are erysipelas, pasteurellosis, acute Salmonella infection and septicaemia. .
ASF virus
African swine fever virus (AFSV) is a DNA virus, formerly classified as an iridovirus (Iridoviridae ) it has recently been re-classified into a newly created family of viruses called Asfarviridae - a name derived from "African Swine Fever And Related Viruses" Within that family it has been allocated to the Genus Asfivirus of which it is the only member.
Different strains of AFSV vary in their ability to cause disease, but they there is one serotype of the virus detectable by blood antibody tests.
Epidemiology and Control of ASF virus
Spread is by pig to pig contact, mechanical carriage by humans or equipment, contaminated injection needles, ticks (and other biting insects and lice) or uncooked swill (garbage).
High concentrations of ASF virus are shed in secretions and excretions from acutely infected pigs. The virus survives well in the environment and in meat, so spread can occur via contaminated livestock pens or by feeding poorly cooked garbage (particularly waste food from the catering areas of international airports or seaports).
There is no effective treatment or vaccine for ASF, so during outbreaks it is vital to slaughter infected herds rapidly and dispose effectively of carcases and litter. Pens and equipment must be thoroughly cleaned and disinfected before re-stocking. It is usual to designate an infected zone around outbreaks in which pig movements are banned or controlled. A surveillance operation is mounted to identify any other infected herds in the area, combined with tracing and testing of all contacts with infected herds.
Border and importation controls need to take into account that live pigs, semen, embryos and products of pig origin all pose a risk of introducing ASF.
NEW books
on
Diseases of Pigs
detailed reviews
of CSF & ASF
