African Swine Fever (ASF) is an infectious disease caused by African swine fever virus (ASFV) in pigs of all ages. The disease was first described in 1921 in Kenya, Africa (Montgomery, 1921) with symptoms of high fever, hemorrhage of the skin and internal organs, and respiratory and digestive dysfunction. For more than three decades, the disease occurred only in African countries, until 195, when ASF spread beyond the borders of Africa, and from 200, the disease was detected in wild and domestic pigs in Asia, Europe, and Africa (Solenne et al, 2009).
From 2016 to 2020, according to WAHIS (World Animal Health Information System) reports, ASF outbreaks occurred in many countries in Africa, Europe, and Asia, in both domestic pigs and wild boars.
In Vietnam, ASF was confirmed in late February 2019 in Hung Yen and Thai Binh and became an outbreak nationwide just 3 months later. In 2019, the number of pigs that died and were culled due to ASF nationwide reached 6 million head, accounting for 23% of the total herd (Ministry of Agriculture and Rural Development, 2020). Currently, ASF in Vietnam still occurs sporadically in some localities across the country, causing losses to the pig farming industry.
ASF CHARACTERISTICS
PATHOGENESIS
The most common route of ASFV entry into the body is through the oral mucosa. Infection can also occur via the upper respiratory tract or damaged skin. The initial replication sites of the virus are monocytes and macrophages in the lymph nodes near the site of entry. Viral replication in macrophages leads to dysfunction of these cells (Arias et al., 2018).
After infection, macrophages are destroyed, but before they are destroyed, hemadsorption occurs, involving the attachment of red blood cells to their surface (Sierra et al.,1991). Based on this phenomenon, it can be concluded that after replication in macrophages, the virus is transported in the blood and spreads throughout the body. Viremia usually begins four to eight days after infection. Because pigs lack neutralizing antibodies, the virus can persist for a long time (Dimmoch, 1993)
The acute form has a high mortality rate, the subacute form has a moderate mortality rate and less typical lesions (Sánchez-Vizcaíno et al.,2012), and this form can lead to animals becoming virus carriers, contributing to the continuous presence of the virus in the environment and increasing the risk of outbreaks spreading to other areas that are not yet affected (Gallardo et al.,2015).
SYMPTOMS
The incubation period of ASF in nature is usually 4–19 days.
- High fever 40–410C
- Hemorrhage in the skin and internal organs
- Bleeding from natural openings (nose, anus)
SOME SIGNS WHEN INFECTED
- Loss of appetite, weight loss, lethargy
- Sudden fever. High fever 40–41 oC
- The ear margins, tail, lower legs, and skin on the lower chest and abdomen show spots of congestion and hemorrhage
GROSS LESIONS
- The spleen is dark, enlarged, and congested.
- Lymph nodes are hemorrhagic and edematous.
- Kidney petechial hemorrhages in the renal cortex.
- Hemorrhagic gallbladder
- Petechial hemorrhages on the mucosa of the bladder, lungs, and pericardium.
BIOSECURE PIG FARMING
Currently, ASF still occurs in many places; therefore, applying biosecurity measures, including full vaccination against infectious diseases according to the procedures of each farm, is necessary to enhance the animals’ resistance. In particular, preventing factors that spread the disease is essential, especially disinfecting barns and preventing disease vectors.
BIOSECURITY MEASURES FOR ASF PREVENTION
To prevent ASF and reduce losses when outbreaks occur, pig farms should implement the following procedure:
Pig barns must be built separately, at a minimum distance from residential areas and neighboring farms. They should be away from main roads, markets, and centralized livestock slaughterhouses. | Barns must have fences, perimeter walls, and barrier nets to control and prevent intermediate carriers of pathogens from entering the farm. Livestock areas, offices, and feed receiving areas should be arranged into separate zones. | Disinfection footbaths should be placed at the farm entrance and before livestock areas. Disinfectant solution should be changed daily or when it becomes dirty and loses the product’s indicator color. Carry out disinfection and sterilization. |
Limit the introduction of breeding stock. If breeding stock is introduced, only healthy pigs that test negative for ASFV should be brought in. Quarantine for 14–28 days before mixing with the herd. | During rearing, veterinary hygiene, biosecurity, and environmental protection must be carried out in accordance with regulations. Vaccinate periodically against mandatory diseases (swine fever, PRRS, FMD). Conduct periodic serological testing of the breeding herd (ASF) to cull carrier pigs. | Supplement biological products including interferon, β-glucan, and probiotics during rearing to enhance resistance and reduce the risk of ASF. |
It is recommended to follow the “all-in, all-out” principle for movement in and out. | Feed transport vehicles and vehicles used for pig movement in and out must be disinfected before entering the farm and must park in the designated area. | |
Workers and technical staff must disinfect and change into clean protective clothing before entering the livestock area. | Limit visitors to the farm as much as possible. If visits are necessary, visitors must have had at least 72 hours without visiting other livestock facilities or contacting disease-spreading risks. Follow the disinfection procedure and wear protective clothing, clean shoes, and boots before entering the livestock area. | |
REPOPULATING PIG FARMS
To reduce the risk of ASF outbreaks, when raising replacement pigs, it is necessary to:
- Renovate, clean, and disinfect barns and livestock equipment according to regulations. It is best to disinfect 2–3 times and let them dry for at least 7 days before introducing pigs.
- The repopulation time must be at least 30 days from the date the last sick pig was culled and all disease control measures have been implemented as required.
- Initially, only 10% of the total number of pigs that the facility can raise should be stocked. After 30 days of repopulation, collect blood samples for testing; if all test samples are negative for ASFV, then increase the herd according to the farm’s capacity.
- Replacement pigs must be purchased from disease-free facilities with negative ASFV test results. Quarantine for at least 14–28 days to monitor disease status before mixing with the herd.
- Raise pigs according to the “Biosecurity” approach
DISINFECTION AND SANITIZATION OF LIVESTOCK BARNs
Disinfection of barns must be carried out properly to ensure effectiveness
1) Principles of cleaning and disinfection:
Dry cleaning: Sweep daily, collect trash and waste (manure, garbage, etc.) and place it in the designated area for proper technical treatment.
Wet cleaning: Scrub equipment and barns clean with water and soap or detergent. Do this after dry cleaning and follow the principle from top to bottom, from inside to outside.
Only use disinfectants after the surface has been cleaned and must be completely dry; do not leave standing water on the disinfected surface.
2) Steps for disinfection and sterilization:
Step 1: Mechanical cleaning
- Remove organic matter before washing:
- Wash thoroughly with water.
- Clean with soap or bleach.
Step 2: Disinfection
- Spray disinfectant after cleaning thoroughly.
- Use the correct concentration and dosage, ensuring a contact time of at least 10 minutes on clean surfaces.
- Only use recommended disinfectants; prepare the solution at the correct concentration (according to the manufacturer’s instructions).
- Spray downwind. Spray from top to bottom, from inside to outside.
- Spray in a Z pattern, overlapping each pass slightly so the disinfectant penetrates evenly over the entire surface to be disinfected.
* For barns with animals: Spray as an aerosol over the entire ceiling, walls, air, and housing area.
* For empty barns, the ground around the livestock area, and transport vehicles: Spray disinfectant over the entire surface of the floor, walls, feeders, drinkers, ceiling, and roof of the barn. Wet all surfaces to be disinfected.
Procedure for cleaning and disinfection steps (FAO, 2023)
SOME DISINFECTANT ACTIVE INGREDIENTS IN LIVESTOCK FARMING
Preventing intermediate disease vectors
- Build fences, perimeter walls, and barrier nets to control and prevent intermediate carriers of pathogens from entering the farm
- Periodically spray insecticides, especially against flies and mosquitoes
- Control free-ranging animals, mainly poultry, birds, and rodents, which may come into contact with food materials contaminated with ASF virus or carry ASF virus on their feathers, claws, footpads, etc., and transmit the disease to pigs.
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