African swine fever was officially announced in Vietnam for the first time on February 19, 2019. Strict epidemiological control measures have been implemented; however, the disease remains complicated and continues to spread to new areas of pig production. The economic losses caused by African swine fever are extremely severe because there is still no vaccine for prevention, nor can it be treated with antibiotics. Effective control of the spread of African swine fever must begin at pig barns and farms. Under current regulations, pigs in households and farms that contract African swine fever will all be culled. However, if infection can be controlled and limited right from the barns and farms, transmission between farms and regions can be reduced and losses minimized. This article shares a few opinions to help pig farmers reduce the losses caused by African swine fever.
1. Causative agent
African swine fever (ASF) is caused by a DNA virus belonging to the genus Asfivirus, family Asfarviridae, and is enveloped. Because it is an enveloped virus, it is very easily inactivated and destroyed by many types of disinfectants, including detergents (laundry powder, soap, etc.).
2. Characteristics of ASF
ASF has an incubation period of about one week, so when bringing pigs into households or farms, great care must be taken to isolate them to prevent ASF virus from infecting the herd. The disease progresses very quickly; pigs develop a very high fever, stop eating, and may die within 3 days while still in good body condition. Sick pigs may show reddening of the skin, cyanosis, and hemorrhage in areas with thin skin (ears, rump, groin, etc.). However, in many cases in Vietnam, sick pigs do not show signs of cyanosis or hemorrhage. In Vietnam, during the early stage of the ASF outbreak, over 80% of ASF virus-positive cases showed high fever and loss of appetite, while only about under 20% of ASF virus-positive cases showed reddening of the skin and cyanosis.
Therefore, the first suspicious sign in pigs with ASF during the current severe outbreak is a very high fever, loss of appetite, and rapid death within 3 days, even after antibiotic treatment. This is considered an indicator for isolating the pigs, taking samples for testing, and culling early to avoid infection within the herd.
Although ASF causes rapid death, it spreads slowly. If suspected ASF-infected pigs are detected and isolated early, and ASF-sick pigs are culled promptly, ASF virus transmission within the farm can be limited, reducing the risk of ASF virus spreading outside.
3. Shedding and transmission of ASF virus
The virus is shed and transmitted mainly through blood, feces, and oral cavity secretions. Therefore, within the farm, ASF virus mainly spreads through three routes: bloodborne (injections), barn hygiene (boots, clothing, etc. contaminated with feces), and direct pig-to-pig contact. ASF virus spreads very slowly; if these three transmission routes are well controlled, the spread of ASF virus within the herd and the farm can be effectively limited. Necropsy for diagnosis is necessary; however, in the case of ASF, it is a highly risky factor for spreading and transmitting the virus. The sample for ASF virus testing in sick pigs is only a normal clotted blood sample. Therefore, it is recommended not to necropsy pigs suspected of ASF to observe lesions or collect samples unless the necropsy is performed by specialized authorities and under strict biosecurity conditions.
4. Intervention measures to limit ASF virus transmission within the herd and farm
4.1 Bloodborne route: ASF virus is present in large amounts in the blood of sick pigs and can survive and remain infectious for many weeks in containers contaminated with the blood of sick pigs. This is the main source of virus spread and transmission inside and outside the farm. By habit, when pigs are sick, farm staff often inject medication for treatment and usually use only one syringe, or even one needle, for all pigs in the pen or herd. Thus, with just one ASF-sick pig, the entire group or herd can become infected with ASF virus in a very short time. The outbreak will then be impossible to control. Therefore, the recommended solution is as follows:
- Isolate as early as possible: farm staff must closely monitor the condition of pigs in the farm. As soon as a pig with high fever and loss of appetite is detected, immediately isolate the sick pig in a separate pen with a solid partition.
- Take samples for testing: Do not necropsy. Collect blood samples and send them for ASF virus testing.
- Treatment: Since this is a viral disease, there is currently no specific treatment. The veterinary authority should be notified early; while waiting for test results, if permitted by veterinarians, the measure of using one syringe per pig may be applied. Inject broad-spectrum antibiotics (Tulathromycin (Tulavitryl), enrofloxacin (Enroxic LA), ceftiofur (Ceptifi), cefquinome (Cequin 250), etc.), antipyretics for pigs, and monitor the disease progression for 1–2 days. If there is no sign of improvement, culling is recommended.
- Cull as early as possible and do not necropsy: Immediately cull all pigs in the same isolation area. If possible, combine culling with burning and burial. Note strict biosecurity measures during culling for the pigs being destroyed (do not move culled pigs into areas with healthy pigs, etc.), and for the people and vehicles involved in the culling. The number of people participating in culling should be limited to reduce the risk of infection.
4.2 Barn and farm sanitation
Pig feces are the second major source of ASF virus transmission after blood. ASF virus can survive and remain infectious for one week in the feces of sick pigs. Items and equipment contaminated with sick pig feces will carry ASF virus and be transmitted by caretakers moving from one pen to another, or by company staff and transport vehicles carrying the virus from one farm to another, one place to another, etc. To minimize the risk of ASF virus transmission through feces and waste from sick pigs, farms must require staff to properly implement minimum biosecurity measures in barn sanitation and pig care. If possible, farms should increase the number of disinfectant trays at each pen, disinfect boots at each pen, or provide boots for each pen. Staff must disinfect or change boots every time they enter a new pen; medical gloves may be worn when caring for pigs and changed each time a new pen is entered. Otherwise, lime powder can be spread along all walkways to limit the survival of ASF virus when it comes into contact with lime.
Remove feces and wash the barn thoroughly, and spray disinfectant on the floor and barn surface at least once every 2 days. Staff caring for sick pigs must bathe thoroughly before returning to care for pigs again.
4.3 Direct pig-to-pig contact
To reduce ASF virus transmission from pigs in one pen to those in the next pen through direct contact, farms with open-partition pens should, if possible, convert them to solid partitions and reduce stocking density.
5. Intervention measures to limit ASF virus transmission in smallholder households
For smallholder households, biosecurity management conditions are not the same as on farms. A few basic measures, if applied, can also help limit ASF virus transmission within the herd as well as to other livestock households (Figure 3). ASF virus is easily inactivated and destroyed by disinfectants as well as detergents (laundry powder, soap, etc.). Therefore, if disinfectants are not available at the household, in suspected ASF cases, laundry powder or soap can be used as disinfectants.
Feed made from pig by-products or kitchen waste must be thoroughly cooked, at least at 70OC for 30 minutes, before it can be used. A common mistake among farmers using pig by-products or kitchen waste is that they focus only on eliminating ASF virus in the by-products while forgetting that the people and equipment used to transport the feed are also sources of ASF virus transmission and must also be cleaned and disinfected to eliminate ASF virus.
Pig caretakers; people, tools, equipment, etc. transporting kitchen waste feed for pigs, or those who have gone to other pig pens, ASF-affected areas, or before moving from their own pig pens to other areas, must be washed (tools, equipment) and bathed thoroughly (people) with soap before entering the barn to care for and feed the pigs.
Author & copyright belong to: Assoc. Prof. Dr. Nguyễn Ngọc Hải, Faculty of Animal Science and Veterinary Medicine, Nong Lam University, Ho Chi Minh City
