Written by Dr Andrew McKay
African Swine Fever (ASF), is a highly contagious viral disease which affects domestic and wild pigs of all ages. It is caused by a double stranded DNA virus belonging to the Asfarviridae family that replicates in the cytoplasm of infected cells.
Forms of African Swine Fever
There are 5 clinical forms of the disease. Although ASF is highly virulent and most often lethal, each of the 5 clinical forms have differing degrees of lethality and virulence (refer to the below table). Wild pigs can be asymptomatic but are still able to transmit ASF to other wild and domestic pigs.
|PERACUTE||90 – 100%||HIGH|
Clinical Signs of African Swine Fever
ASF is often difficult to diagnose given the non-specific clinical signs displayed. Clinical signs are similar to classical swine fever, and the two need to be distinguished by laboratory diagnosis.
In a peracute infection, pigs may die even before clinical signs develop. They can often become depressed, recumbent & have difficulty breathing before dying.
During an acute infection, vomiting, diarrhoea and coughing may be seen while a blue/red discolouration of the skin often develops particularly around the ears, snout, abdomen and extremities. This is due to skin haemorrhaging. Abortions are also seen in the sows. Death usually occurs within 7 days of clinical signs developing. Mortality is almost 100% however some pigs may survive and can carry the disease for several months progressing to subacute or chronic stages.
Weight loss, irregular fever, respiratory impairment and joint swelling are among some of the variable signs of a chronic infection. Mortality rates in a chronic infection are still as high as 10%.
African Swine Fever is Difficult To Control
The incubation period for ASF is 3-19 days. Large amounts of virus are shed in all secretions and excretions (particularly those that contain blood) 24-48 hours before clinical signs develop. It is difficult to inactivate. 60 degree heat for 30 minutes, a pH of less than 4 or more than 12 and some disinfectants will inactivate the virus.
It is highly resistant to the environment. It can survive up to 11 days in faeces and more than a month in contaminated pens. It will survive in meat (muscle) from an infected animal for 150 days, in blood for 18 months and 3 years in frozen meat. These long incubation periods, its huge virus shedding capability and long survival times increase the risk of the spread of the disease enormously and makes the ability to control the disease a real challenge.
Transmission via contact
Virus is transmitted principally by direct contact with infected pigs via faeces and body fluids (including semen), while indirect transmission occurs via contact with fomites, such as equipment (vehicles etc) and personnel (boots, clothing etc), or following ingestion of infected pig meat or products.
Transmission via food
The feeding of food scraps or food waste that contains or has come into contact with meat or meat products (swill feeding) is illegal in Australia, to prevent a range of diseases including ASF. The carcasses of infected pigs have also been linked to the spread of the virus. This may be an important pathway for spread of ASF between feral pig populations. Infected biting ticks and flies may also have a part to play in the transmission of the disease.
Incidence/Prevalence of Afrcian Swine Fever
ASF was first recorded in Kenya in the early 1900’s. It was limited to sub-Saharan Africa until 1957 when the disease occurred in Portugal. From the 1960’s it was reported in Belgium and then in other parts of Europe up to the late 1900’s. Outbreaks have occurred since 2007 in a number of countries including Russia but it wasn’t until 2018 when it was detected in China that it spread very quickly and devastated the Chinese pig industry before spreading further throughout Asia.
It has now established itself in Mongolia, Vietnam, Laos, Cambodia, Myanmar, North Korea, South Korea, the Philippines, Timor Leste, Indonesia and Papua New Guinea.
African Swine Fever in Australia
ASF has never occurred in Australia. However, its changing distribution means it’s a significant biosecurity threat. The disease would be difficult and costly to eradicate so vigilance is required to prevent it getting into the country.
Preventing African Swine Fever
Like many emergency animal diseases, there is currently no cure for African Swine Fever and no vaccine to protect the animals from contracting the disease.
It is therefore critical to prevent ASF from coming into countries currently free of the disease. The most significant risk of entry of ASF virus into Australia is via illegally imported contaminated pork products that are swill fed to domestic pigs and/or accessed by wild pigs.
Therefore, the best defence against ASF is to maintain & continually improve Australia’s strong biosecurity standards to prevent people bringing pork into the country.
Strong biosecurity standards are everyone’s responsibility. The Australian Federal Government is responsible for border protection. The State and Territory Governments are responsible for biosecurity within their jurisdictions while producers are responsible for ensuring good biosecurity practices on farm.
A biosecurity program forms the basis of any disease control strategy and needs to be practical & cost effective. There are two components to on farm biosecurity.
- External biosecurity – reducing the risk of introducing disease on to the farm
- Internal biosecurity – reducing the spread of disease within the farm
External biosecurity measures include:
- Animal proof fence around a pig farm – strong enough to keep wild boar out.
- Restrict and enforce non-essential personnel’s access to the farm
- Strict entry system for all personnel
- wash hands/forearms
- Provide boots and clothes on farm
- boot dips
- Reduce staff contact with pigs outside the farm
- No entry for 2 weeks after overseas travel
- Vehicle/equipment cleaned and disinfected when entering and leaving the property
- Loading to be done from extended ramp outside the clean area
- Do not bring meat products into your piggery – do not swill feed.
- Ensure the feedmill follows good manufacturing and quality assurance protocols
Internal biosecurity measures include:
- No swill feeding
- Quarantine pigs entering farm (ideally 2 weeks)
- Keep age groups together
- Ensure farms have clear ‘clean’ and ‘dirty’ zones
- Record the movement of pigs and people – traceability
- Reduce other possible vectors – ticks, flies, rodents etc
- Trained and reliable employees
- Practise good on-farm hygiene – Good cleaning and disinfection products & protocols
African Swine Fever is a notifiable disease and the presumptive diagnosis needs to be reported immediately to the respective authorities (Chief Veterinary Officer) for further investigation and confirmation of diagnosis and action. A confirmed diagnosis will result in depopulation of the infected herd and appropriate sanitation of the farm.
Although the virus is not harmful to humans, an outbreak would be devastating for the pig industry. If the disease does reach our shores, a good cleaning and disinfection protocol will play a very important part in preventing Emergency Diseases from entering your farm and spreading within your farm. As mentioned earlier, only a few disinfectants have the ability to inactivate the ASF virus. Ensure your farm is using approved APVMA products that are easy to use and can inactive the virus at a low concentration.
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