November 15, 2021
The Swine Health Information Center (SHIC) has released additional research results from a multi-phase African swine fever (ASF) field project in Vietnam. The project is being funded by a 2019 grant from USDA’s Foreign Agricultural Service division that was awarded to SHIC. National Pork Producers Council provided additional assistance.
One completed study evaluated the performance of ASF serum and/or oral fluid ELISAs for use in the surveillance and monitoring of ASF outbreaks in commercial farms in Vietnam and in preparation for the virus becoming endemic in the U.S. This study shows there is no single best diagnostic approach for ASF surveillance and demonstrated that the combined use of the Tetracore qPCR and indirect ELISA tests and serum/oral fluid sampling increase efficiency of ASF disease surveillance.
Another completed study modeled the risk of introducing ASF to a sow farm as a result of semen movement from apparently healthy boar studs located in an ASF disease control area. Results indicated the risk is negligible to low given study parameters, however, several factors with the potential to impact these results were acknowledged.
The study, “Evaluation of the diagnostic performance of an ASF serum/oral fluid antibody ELISAs under field conditions in Vietnam,” evaluated performance of these tests for surveillance and monitoring of outbreaks on commercial farms. A collaborative project between Innoceleris LLC and Tetracore Inc., the work addresses the complicated interpretation of ASF diagnostic results. A field team from Hanoi University collected 398 paired serum/oral fluid samples from individual animals, including 100 samples from 34 ASF-acute farms, 98 samples from 47 ASF-chronic farms, and 200 samples from 20 ASF-negative farms.
The samples were tested by Tetracore ASF iELISA and real-time PCR (qPCR). As expected, the detection rate by qPCR (74% serum; 69% oral fluid) was higher than by ELISA (16% serum; 11% oral fluid) in acute farms since most of the animals did not yet seroconvert. In contrast, in chronically affected farms, the detection rate of the ELISA was higher (72% serum; 57% oral fluid) than the qPCR (56% serum; 34% oral fluid). However, when researchers combined both qPCR and ELISA, the detection rate of ASF positive animal increased in acute (75% serum; 74% oral fluid) and particularly in chronic farms (85% serum; 74% oral fluid). All serum samples from negative farms were negative by both ELISA and qPCR (100% diagnostic specificity) while, for oral fluids, researchers obtained 100% and 99% diagnostic specificity for qPCR and ELISA, respectively. The high diagnostic specificity of the tests is particularly important for ASF surveillance. Absence of false positives avoids false alarms and disruption in production, and lack of confidence in the tests/surveillance system.
The study, “Determining the pathways for ASF introduction into boar studs and risk of ASF transmission via semen movements during an ASF outbreak,” included a proactive risk assessment (RA) that looked at the potential risk of semen movements during an outbreak. Researchers, led by staff at the University of Minnesota, established the ASF Boar Semen RA workgroup (WG). Together with researchers at the University of Hanoi, experts determined 10 potential entry pathways for ASF into boar studs as people, feed, water, geographic and/or aerosol transmission, fomites (such as tools, equipment, vehicles), mortality management, domestic animals (such as dogs, cats, replacement boars), biological materials (such as medicines and vaccines), insects/ticks, and wildlife. They further evaluated these pathways on this scale: Extremely High, High, Moderate, Low, Negligible.
Data from a boar stud in Vietnam were also obtained by working with a Vietnamese collaborator who was able to visit the site and perform an epidemiological investigation that included premises description, farm biosecurity, farm help/workers, farm equipment, manure handling, dead pig disposal, farm visitors, presence of wild animals, mortality data, description of the recent biosecurity practices, and a farm diagram. Of most value were data regarding clinical signs and diagnostic tests.
As a result of the pathway analyses, the proposed estimated likelihood of ASF infection of a boar stud operation in a Control Area due to water was negligible, as long as no surface water is being utilized in the boar stud operation. The likelihood of ASF introduction was negligible to low for feed, insects/arthropods, and wildlife (including infected feral pigs), as long as boar studs continue their standard biosecurity practices such as tandem feed bins, insect control, indoor housing, and double fencing.
The likelihood of ASF introduction was low for people, fomites, domestic animals (including replacement boars), and biological materials, as long as boar studs continue requirements and procedures including but not limited to shower-in/shower-out people entry with downtime from other pigs, decontamination and disinfection for materials entering the stud, and housing of replacement boars in isolation barns away from the boar stud and lab.
It is very important to note for seven potential entry pathways of ASF infection (people, feed, fomites, animals, insects/arthropods, wildlife, and domestic animals), there are suggested Enhanced Biosecurity Recommendations (EBRs) in the Secure Pork Supply (SPS) plan that, if followed and done correctly, are critical to lowering the risk of ASF infection. Therefore, following all EBRs was assumed to occur when these ratings were made, and examples of these biosecurity practices have been given above (for example, shower-in/shower-out).
On top of the EBRs in the SPS plan, the WG proposed putting into place targeted EBRs to further reduce the risk of ASF infection. When the WG decided these targeted EBRs were feasible by the vast majority of boar studs in the US swine industry, these protective actions were included in the estimates of the likelihood ratings.
This proactive risk assessment will be reviewed and open for comments. The comments will be considered and use to update the risk assessment as necessary before and during an ASF outbreak in order to incorporate the latest scientific information and preventive measures. If the Incident Command System (ICS) is activated in response to an ASF outbreak, Incident Command staff will review the risk assessment to assess industry requests for movement of liquid, cooled boar semen from a boar stud in a control area.
In total, the ASF-related research taking place in Vietnam is designed to help Vietnamese response and recovery from the ASF epidemic and U.S. pork producers learn lessons about ASF epidemiology and management, results continue to provide valuable insight.
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