Unresolved Porcine Pasivirus Disease Investigations
Yield Valuable Insights
A recent Swine Health Information Center (SHIC)-assisted investigation into a case of increased death loss associated with acute onset of CNS signs, initially described as tremors, in a grow-finish herd illustrated how SHIC can offer assistance in the quest to gain insights about unresolved cases, regardless of outcome. While everyone always desires a conclusive etiology to any unknown case, that outcome is not guaranteed. Despite this reality, there is always value in the ongoing professional investigatory process itself and discovery of any new facts.
Tremors Case Offers Multiple Possibilities
In late spring 2015, a grow-finisher flow experienced increased death loss with acute onset of central nervous system (CNS) signs initially described as tremors. From one submission, a diagnosis of streptococcal septicemia exacerbated by metabolic bone disease was offered as a presumptive diagnosis. Another submission showed no lesions of significance in multiple tissues and ultimately, no etiologic diagnosis was identified that were suggestive of CNS infection.
Additional tissue and serum samples from groups of clinically affected pigs were submitted for next-generation sequencing (NGS). A novel pestivirus was identified from the brain samples, but no other viruses were detected. The pestivirus detected was similar to that reported in pigs born with congenital tremors caused by a novel pestivirus and similar to a pestivirus previously named “atypical porcine pestivirus” or APPV. Although APPV was found in samples from affected pigs in this case, causal proof of this clinical disease was absent.
SHIC Supports More Testing
Because symptoms and losses remained ongoing in this herd, SHIC provided diagnostic support. Three pigs with typical early signs of tremors and two normal cohort pen mates were euthanized and examined. Blood, oral fluids and other tissues were collected from these pigs. On examination, lesions that could be related to clinical presentation were not detected in any tissue of the five pigs. In addition, PCR testing was negative for PCV2, but positive in a pooled sample from unaffected cohorts. Meanwhile, NGS of tissues and serum could not find APPV but the sera samples showed fairly high levels of swine pasivirus (a member of the Picornaviridae family) as well as lesser amounts of parvovirus type 7.
Definitive Diagnosis Illusive
Although a definitive diagnosis of cause for this unusual and severe fatal tremor disease was not found by this investigation, the investigation did provide some key insights into this case. This included the detection of an abundance of pasivirus detected by NGS in serum samples. This isn’t a common finding and should be emphasized that many viruses are endemic in swine populations. Merely detecting a novel virus with molecular techniques in a sample is not definitive evidence for disease causation.
Unfortunately, proper investigation tools are not in place to uncover pathogenicity, virulence and prevalence for most of these newly discovered viruses, including pasivirus. Also, serology tests have not been developed nor animal studies performed. Additional research is needed to determine if pasivirus may have played a role in this case. SHIC will continue to monitor for diagnostic trends that might help to better determine if pasivirus is implicated as an emerging disease issue.