June 16, 2020
Biosecurity of our national swine herd is a major focus of our response to African swine fever outbreaks that have swept through China and elsewhere. Another critical component is surveillance and rapid detection of infected pigs, should ASF break through our firewalls.
What would ASF testing look like through the lens of our veterinary diagnostic laboratories? Interestingly, we know exactly what it looks like because it's already being done at sites such as the Animal Disease Research and Diagnostic Laboratory at South Dakota State University.
Viral detection is particularly important with a disease like ASF, which has no pathognomonic signs. Put another way, the effects of ASF in pigs are similar to common diseases we face every day: porcine reproductive and respiratory syndrome or porcine circovirus, for example. The only way to definitively identify it is through directly detecting ASF virus in an animal.
With this in mind, the USDA has already implemented active surveillance for ASF (Classical swine fever surveillance occurs simultaneously with this process), and veterinary diagnostic laboratories are key players. Considering the most likely pig populations ASF would first pop up in, different diagnostic "streams" have been identified. Examples of these targeted populations include pigs condemned at slaughter, feral swine and pigs from garbage-feeding operations. Veterinary diagnostic laboratories are the source of another stream: sick pigs submitted from veterinarians for routine diagnostics.
At the ADRDL, submitted pigs or tissues are first evaluated to see whether they meet USDA's criteria to be included in the surveillance stream. The submission form must include a USDA premises identification number. The history provided by the submitting veterinarian is then scanned to determine whether any ASF attributes are present in the pigs or herd: acute septicemia, skin discoloration, swollen lymph nodes, enlarged spleen or abortions, for example. Pigs from a group experiencing increased mortality and one of the signs in the group also qualify. Again, these signs are also typical of a variety of regular domestic swine diseases.
Then the question becomes whether the desired samples are available. The USDA specifies tonsils, spleen (preferred by the ADRDL) or lymph nodes as acceptable for the surveillance program. This requirement is usually easily met when whole pigs are submitted to the ADRDL, but submissions consisting of tissues sent in from field necropsies don't always contain the specified tissue samples.
If the PIN, compatible signs and tissue types are all acceptable, then ASF testing can proceed. At the ADRDL, the tissues are taken from the necropsy floor to the molecular diagnostics lab section. Nucleic acid from the sample is obtained through an extraction process, then the purified nucleic acid is subjected to real-time polymerase chain reaction testing to determine whether the ASF-specific portion of the nucleic acid is present.
The ASF PCR protocol used by the ADRDL is the same as that used by the National Veterinary Services Laboratory and all labs belonging to the National Animal Health Laboratory Network and includes positive and negative controls. This test methodology uses strictly non-infectious materials: no live ASF virus is involved.
Should an ASF sample test PCR-positive at a cycle threshold of lower than 40, it will be considered presumptively positive. Results would be reported to USDA's Foreign Animal Disease Diagnostic Laboratory on Plum Island, as well as to NAHLN. The tissue would be packaged up and overnighted to FADDL, where the sample would be re-tested and the result confirmed.
A positive ASF confirmation at FADDL — needless to say — would set off a remarkable chain of events. Samples found ASF-negative at the VDLs are not subject to confirmation, and the results are reported directly to the submitting veterinarian. In most cases, ASF test results are available within a week of submission. There is no charge to the veterinarian or producer for ASF testing.
Veterinarians and swine producers, therefore, are assisting our country's ASF preparedness when they make regular submissions (provided they include tonsil, spleen or lymph nodes) to their VDLs. From a wider standpoint, if more conventional disease surveillance samples — such as oral fluids — could be approved as ASF-appropriate samples, ASF surveillance in the United States could be taken to a whole new level.
Every one of those submissions helps VDL personnel keep their skills honed — not only with PCR testing protocols, but with result messaging and communications as well. A robust approach to dealing with a foreign animal disease incursion means preparing our diagnosticians for the challenges an ASF incursion would present while we also work on biosecurity and other surveillance functions.
Sources: Russ Daly and Travis Clement, who are solely responsible for the information provided, and wholly own the information. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.
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