is contagious and appears to persist in the herd even after vaccination has been discontinued. “Antibodies are detectable two to three weeks after vaccina- tion. The 5–20% or so of pigs that survive a full-fledged out- break of ASF do generally have antibodies, and the ASF wild type virus can be detected weeks and months later in their blood and tissues.”
Detection of the vaccine virus Detecting the new vaccine virus by PCR can be rather diffi- cult, Dr Johnson says. “Oral swabs are not reliable for ASF and are even less sensitive for the vaccine virus. The dou- ble-gene-deleted vaccine virus also may not be present in the blood but it is often possible to find it in lymph nodes and other tissues when the blood is negative.” Three genes usually give away whether ASFv is present in a pig. Dr Johnson says, “ASFv is detected by a PCR-test for the P72 gene. If P72 is detected, a second test for either CD2v, MGF360, or both will be conducted if it is desired to know if it is the field (wild type) virus or the vaccine virus. “Wild type ASFv should be positive for all three genes (P72, MGF360 and CD2v). The double-deletion-vaccine virus does not possess MGF360 or CD2v but does have P72, so the P72 will be PCR positive and the MGF360/CD2v will be negative.”
Official Chinese research to a vaccine It is known that the Chinese authorities have been working on the creation of an ASF vaccine as well – but that develop- ment appears entirely unrelated to what is now happening
in the field with illegal vaccines. After all, scientists at Harbin Veterinary Research Institute have openly published about their progress during 2020, and Pig Progress published about the progress of this vaccine in March and July. International experts expressed moderate optimism about their vaccine. The Harbin live-attenuated virus has a deletion of seven genes. In doing so it builds on the approach taken by the Plum Island Animal Disease Center (PIADC), part of the US Department of Agriculture’s Agricultural Research Service (USDA ARS). PIADC researchers deleted six genes but thought deleting a seventh (CD4) did not improve the results. The Harbin vaccine is still being researched, notes Dr John- son, “As far as we know the Harbin vaccine with seven genes deleted has not been released into the market. It is missing the two genes cut from the double-deleted vaccine plus five
New Hope Liuhe also reports the variant
Dr Johnson is not the only one to have reported a mutation of ASFv. Pig integra- tor New Hope Liuhe, in the top five of swine producers in China, also reported this in an article by Reuters. The article speaks of two new strains of ASFv, which would have infected more than 1,000 sows on several farms and contract farms. New Hope found strains of the virus missing both the MGF360 genes and the CD2v genes.
▶ PIG PROGRESS | Volume 37, No. 1, 2021 7
Healthy finisher pigs on a farm in Jiangsu prov- ince, China. The farm is unrelat- ed to the recent health issues.
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