Rapid and reliable detection of African swine fever virus infected pigs is critical for successful control. One desirable property of a diagnostic test is the capacity to detect viral infection, especially during the incubation time, even before the infected animals may be displaying clinical signs. In this SHIC study conducted in Vietnam using funds from a USDA-Foreign Ag Service grant, three pen-side tests for ASFV detection were evaluated - one PCR test for detection of viral genomic DNA and two lateral flow tests for detection of viral antigens. Results show the PCR pen-side test performed better than the other two options.
The first objective of the study, directed by Dr. Hiep Vu, University of Nebraska-Lincoln, was to determine the time from infection to the earliest detection. Ten pigs were experimentally infected with an ASFV strain circulating in Vietnam. Whole blood and oral swab samples were alternatively collected from five pigs every other day post-infection (dpi) and tested with the three pen-side assays. The pen-side PCR test detected infected pigs starting at 2 dpi when using whole blood and at 3 dpi when using oral swabs. It consistently continued to detect infection until the end of the study (10 dpi).
Using whole blood, the antigen test identified infected pigs starting from 3 dpi but no longer detected infection at 10 dpi . The antigen test did not work well when using oral swabs. Compared with the reference laboratory real-time PCR test, the pen-side PCR test exhibited 97.8% sensitivity and 100% specificity. The lateral flow antigen tests had 100% specificity but only 47.8% sensitivity, mainly because of failure to detect virus from samples collected early or late after infection.
The second study objective was to evaluate the diagnostic performance of the tests using samples collected from the field. Whole blood and oral swabs were collected from 205 pigs with 34 positive pigs and 171 negative pigs identified by the reference laboratory real-time PCR. All of the pen-side tests had 100% specificity, regardless of the sample types tested. The sensitivity of the pen-side PCR test was 88.2% and 70.4%, respectively when testing whole blood and oral swab samples. However the sensitivity of the antigen tests when using whole blood or oral swab samples was only 50% and 11.11%, respectively.
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