Question: I’ve recently read about an African swine fever outbreak in Zambia. Please tell me more about the disease.
African swine fever (ASF) is a highly contagious haemorrhagic disease that infects pigs of all ages, warthogs and European and American boars, as well as giant forest hogs. “Haemorrhagic” means that it is a disease that causes bleeding. Humans cannot be infected with the disease.
CLINICAL SYMPTOMS OF ASF
In its most pathological (disease-causing) form AFS causes the following symptoms:
- High fever.
- Loss of appetite.
- Redness of the skin on ears, stomach and legs.
- Difficulty to breathe.
- Bleeding from the nose and rectum, and sometimes diarrhoea.
- Bleeding in the skin and internal organs.
- Death occurs after between 2 and 10 days of infection.
- Mortality can be as high as 100%.
- Abortions may be the first sign of an outbreak.
In its less virulent forms ASF shows less serious symptoms, but can still result in mortality of between 30% and 70%.
Chronic ASF causes the following symptoms:
- Weight loss.
- Uninterrupted fever.
- Respiratory signs.
- Chronic skin lesions.
- Inflammation of the joints.
ASF is caused by a DNA virus from the Asfarviridae family. The disease is listed by the World Organisation for Animal Health (OIE) and must be reported to the OIE Terrestrial Animal Health Code.
Visit the OIE’s Information Database (WAHIS) Interface to see where the disease has spread.
TRANSMISSION AND DISTRIBUTION
- Warthogs (Phacochoerus africanus), bushpigs (Potamochoerus porcus) and giant forest hogs (Hylochoerus meinertzhageni) can act as natural reservoirs for ASF without displaying any symptoms.
- Hosts that show symptoms are the domesticated pig (Sus domesticus) and European and American wild boars.
- Ticks from the genus Ornithodoros (soft ticks, Ornithodoros moubata) are seen as the natural arthropod
- Host and there is speculation that ASF may really be an arthropod disease and that mammals are possibly only “casual hosts”.
SOURCES OF THE VIRUS
- Blood, tissue, secretions and excretions from sick or dead animals.
- Animals that recover from acute or chronic infections can become carriers of the virus, especially in areas where the disease occurs.
TRANSMISSION OF THE VIRUS
The virus can be transmitted directly from sick to healthy animals, or indirectly in the following ways:
- Feeding pigs with garbage that contains contaminated meat. ASF can remain infectious for 3 to 6 months in uncooked pork products.
- Biological carriers – soft ticks from the genus Ornithodoros.
- Contaminated premises, vehicles, implements and clothes.
- Transmission in ticks – transovarial, trans-stadial and through sexual (venereal) transmission.
- Biting flies.
DIAGNOSING THE DISEASE
ASF can be suspected on the basis of clinical signs and is confirmed through laboratory tests. It is especially important to distinguish between ASF and Classical swine fever.
PREVENTION AND CONTROL
There is no vaccination for ASF at present. Since the disease is caused by a virus, there is also no treatment for sick pigs.
Countries where the disease does not occur follow strict import policies to prevent infected live pigs and pork products from entering their borders. This includes food scraps from aeroplanes, vehicles and ships originating from contaminated countries.
It is difficult to control the virus in reservoir animals like warthogs in areas where the disease is endemic (regularly found). It is important to ensure that susceptible pigs are not fed the meat of warthogs or other infected animals.
All successful eradication programmes depend on rapid diagnosis, the culling of all pigs on the contaminated premises, thorough cleaning and disinfection, as well as movement control of pigs.
- Compiled by Dr. Faffa Malan, veterinary consultant, for Landbou.com on 29 June 2016. (firstname.lastname@example.org)