In Vietnam, the African swine fever virus (ASFV) was initially detected in 2019 and has since become endemic. Although two live-attenuated ASFV vaccines have been recently approved in the country, their usage remains tightly regulated. Consequently, ASF control relies heavily on stringent biosecurity measures, including movement restriction, quarantine, and compulsive depopulation of affected herds, said Hiep Vu, University of Nebraska-Lincoln, during the 2023 Leman Swine Conference.

Rapid and reliable detection of ASFV-infected pigs is critical for the control of the disease. Additionally, efforts are being made to develop a safe and effective vaccine against this virus. Two projects were conducted in Vietnam, one focusing on assessing the performance of two pen-side diagnostic tests and the other exploring the mechanisms of vaccine-induced protection, Vu explained.

Project 1 evaluated the performance of a pen-side real-time PCR test for detecting viral genomic DNA, as well as evaluating a lateral flow test for detecting viral antigens, he said.

Ten ASFV-seronegative pigs were inoculated with a virulent ASFV strain currently circulating in Vietnam. Blood samples were collected from each pig every other day until they reached humane endpoints (within 10 days). These samples were tested using the two pen-side diagnostic tests immediately after collection. Additionally, the samples were tested using a reference real-time PCR test, Vu noted.

The pen-side PCR test detected infected pigs from 2 days post-infection (dpi) and consistently detected infection until the end of the study (10 dpi). In contrast, the antigen test began detecting infection at 3 dpi and no longer detected infection at 10 dpi, he said.

Vu concluded that in the Project 1 study, the pen-side PCR test exhibited greater sensitivity and detected infected pigs earlier and for a longer duration after infection than the antigen test.

In Project 2, to elucidate mechanisms of vaccine-induced protection, 30 ASFV-seronegative pigs were divided into three groups, Vu said. One group received an experimental live-attenuated vaccine (LAV), another group received two doses of an experimental killed virus vaccine (KV), and a third group was kept as non-vaccinated control. At 42 days post-vaccination, all pigs were challenged with a highly virulent ASFV strain and monitored for 21 days. Serum samples collected at 41 days post-vaccination were analyzed for reactivity against a panel of 29 viral structural proteins, he added.

The LAV vaccine conferred 100% protection against a lethal challenge with the virulent ASFV strain, while the KV vaccine did not confer the same protection. Pigs vaccinated with the LAV vaccine developed a broader antibody response against a diverse range of viral proteins than those receiving the KV vaccine. Notably, within the LAV vaccinated group, a negative correlation was observed between the intensity of antibody reactivity against specific ASFV antigens and the levels of viral DNA in the blood following a virulent ASFV challenge, he said.

Distinct antibody profiles were observed between pigs vaccinated with the LAV and those vaccinated with the KV vaccine. The viral proteins exclusively recognized by sera from the LAV group hold promise as potential markers of vaccine-induced protection, Vu and his Vietnamese research partners concluded.