What is Going on with African Swine Fever?

By: Dr. Jim Lowe, Production Animal Consultation, and Dr. Ben Blair, University of Illinois

African Swine Fever (ASF) is a viral disease of swine that is endemic to Africa and subsequently was introduced into Europe and Asia in the last 10 years. The current ASF epidemics in Europe and Asia appear to be a result of a single ASF introduction into Georgia in 2007. ASF has spread outside of wild boars and into farmed pig populations.

African Swine Fever is a highly contagious, rapidly spreading, fatal disease of pigs of all ages and breeds. It was first described in 1921 and contained in Africa until 1957 (Manso Ribeiro et al. 1958).

ASF is an untreatable disease and would have large economic im plications on the swine industry if the disease was to break in a country or region and, therefore, is of utmost importance.

There have been major outbreaks of ASF outside of African herds in Europe, South America, the Caribbean, and the Middle East, and starting in August 2018, there have been major outbreaks in China, Romania, Belgium, and other parts of eastern Europe. Current endemic regions include more than 20 countries in sub-Saharan Africa, including Madagascar, Sardinia (Italy), and the Caucasus region of Eurasia following an outbreak in Georgia in 2007 (Fernandez 2016).

Several principles about the African Swine Fever Virus (ASFV) make it different from Classical Swine Fever (CSF, Hog Cholera) and Foot and Mouth Disease (FMD). First, ASF is much less contagious than CSF or FMD. ASF will spread at a slower rate and produces fewer new infections from a single infected animal.

African Swine Fever Virus is transmitted directly by contact with secretions or excretions of infected pigs and indirectly through fomites, tick exposure, inhalation, environmental contamination, or consumption of infected pork (Spicket 2016). A large amount of virus is shed in secretions and excretions, including nasal discharge, conjunctiva discharge, saliva, urine, feces, and blood. ASFV is known to survive on fomites, such as vehicles, feed, and equipment. The tick species Ornithodoros (soft tick) is well documented for transmitting ASFV and being a main factor in transmission of feral warthogs (Plowright et al. 1969). Other blood sucking insects, mosquitoes or biting flies, can act as vectors, although this is somewhat debated. According to one source, Stomoxys calcitrans (stable fly) can carry high levels of virus for 2 days (Spicket 2016).

Incubation periods for ASFV depend on the isolate, virulence, and route of exposure but are typically between 4-19 days and as short as 3-4 days in the acute form (Mebus et al. 1983). The ability for ASFV to survive outside the host and in the environment is a major influence of its persistence in feral herds and the ease of its transmission. ASFV can survive in feces up to 11 days, urine up to 15 days (at 4°C), boned in meat for 150 days at 4°C, salted dried hams for up to 140 days, and frozen carcasses for several years. One study showed that ASFV can persist in feed ingredients, such as soybean meal, choline, cat food, dog food (both moist and dry), pork sausage casings, and complete feed, using a Trans-Atlantic transportation model (Dee et al. 2018).

The immune response of ASFV is poorly understood. ASFV specific antibodies have never been demonstrated to neutralize the virus in a classical sense of neutralization. Therefore, the main difficulties in protective immunity through natural infection or vaccine are the lack of neutralizing antibodies and the variability of ASFV isolates. Currently, there are no effective vaccines or means of providing a protective herd immunity against this disease due to the lack of proper immune response. Preventing the virus from introduction into a herd is the most crucial step to controlling spread of this disease.

Common disinfectants are ineffective against ASFV. Sodium hypochlorite, citric acid (1%), formalin, 3% orthophenylphenol, and select iodine and quaternary ammonium compounds destroy ASFV on non-porous surfaces. Two-percent citric acid has been documented to work well on wood surfaces. Unprocessed meats must be heated to 70°C for 30 minutes. Virus in serum or other body fluids can be inactivated after 30 minutes in 60°C. In serum-free medium, the virus will die in a pH below 3.9 or above 11.5 (Spicket 2016); however, serum increases resistance of the virus.

Spain was successful in eliminating ASF in 1995. The key actions to success in Spain (1985-1995) were summarized by Arias and Sanchez-Vizcano (2002b) and BechNeilsen et al. (1995). They included:

• Elimination of ASF outbreaks, ID, slaughter of ASFV-carrier animals and depopulation of infected herds

• Adequate compensation to pig producers

• Improvements in animal holding facilities to prevent spread of disease o Foot baths, sanitary enclosures, residue and slurry disposal o Biosafety and sanitary measures to avoid transmission between herds

• Network of mobile veterinary field teams in charge of sanitary control of holdings o Animal ID, epi surveys, sample collection for serological surveillance, serological control at slaughterhouses

• Strict control of animal movement and ID of animals moved for fattening or breeding

• Vehicular washes and disinfection

• Direct involvement and active participation of farmers

The OIE employs a standardized reporting system that treats the ASF outbreak in Europe as a single outbreak for a multitude of reasons. Upon deeper inspection, it is apparent that there are multiple layers to the current outbreak of ASF in Europe. The distinction of unique populations on the contact rate is important in the current ASF outbreak due to the presence of both wild and domesticated pigs. With this insight, it quickly becomes apparent that what we are observing in Europe is not a single outbreak but multiple contemporaneous outbreaks in different populations, each with their own contact rates and risk factors for transmission. This is a crucial distinction, as the primary spread of ASF is occurring in different populations within different geo-spatial clusters (distinct areas) in Europe, implying that effective control and prevention measures will be unique for each cluster.

Unlike North America, where nearly all pigs are reared in commercial production, both commercial (both intensive and extensive systems depending on region) and backyard pigs in Europe are significant parts of the industry. These two parts of the swine industry have very little direct pig contact but, depending on the region, have extensive indirect contact though people, supplies, and markets.

With this understanding, it is useful to consider that three distinct populations of pigs – wild, commercial, and village pigs – coexist in Europe depending on region.

The current outbreak of ASF in Europe reveals patterns about the last three years of behavior. These patterns of behavior have led to the development of the following three conclusions:

1. Outbreaks of ASF within predominantly wild boar populations are focal with limited radial expansion of the affected zone over time.

2. Outbreaks of ASF within predominantly domestic swine populations move rapidly and asymmetrically giving it the capability to spread over large distances.

3. Trends within the ASF epidemiological data support its repeated spread from domestic swine to wild boar populations.

Biosecurity is the key to stopping the ASF spread. A simple definition of biosecurity is a series of physical and procedural barriers implemented to enhance the economic output of an independent production system by reducing the probability of the introduction of a novel infectious disease. Biosecurity is about risk reduction, probabilities, and improving economic outcomes. The keys to a successful biosecurity system are the integration of both the physical barriers on the farm and procedural actions the farms take. Those two pieces must work together in concert to minimize the risk of disease introduction onto a farm. The practical challenges are the numerous routes for pathogen introduction onto a farm.

There is a direct correlation between the need for biosecurity practices and procedures on farms to the amount of viral challenge in that farm’s ecosystem. Unfortunately, the effectiveness of those practices and procedures is inversely proportional to the amount of viral challenge. A discordance between the need and value of a given practice and its relative effectiveness will be established.

Domestic pigs and their products appear to be the primary source of ASF for other domestic pig farms. This spread is both direct (pig to pig, meat to pig) and indirect with fomites. Within smallholding domestic pigs, all routes are likely to be important, and for commercial farms, meat and meat products and fomites are the likely routes of transmission in Europe. In China and greater Asia, domestic pigs are the sole source of ASF for other domestic pigs. While the data is incomplete for China, an understanding of cultural practices and industry structure would suggest that both direct and indirect transmission are highly likely.

Looking at the entire situation, the spread of ASF in Asia and Europe is likely to continue for an extended period due to the structure of the industry, the lack of an effective vaccine, and limited investments in biosecurity practices in many farms in these regions. ASF is here to stay; our challenge is to figure out how to protect the US industry from this ongoing threat.