Emerging Porcine Sapelovirus
Sapelovirus (SPV) is an emerging virus in the family Picornaviridae, and is detected in several animal and bird species. These pathogens are highly stable in multiple environmental conditions and are spread easily to susceptible animals, mainly through the feco-oral route. Porcine sapeloviruses (PSVs), previously named porcine enterovirus 8, are the most pervasive in porcine populations throughout the world, and have been isolated from swine fecal samples in Brazil, Australia, Spain, Italy, China, the United Kingdom, and the United States. A new strain was recently isolated from a United States swine farm, and designated as PSV KS18-01.
Clinical signs
PSV infections are often subclinical, but viral symptomatic manifestations can occur, including neurological disorders, diarrhea, reproductive failure, and pneumonia. When causing gastrointestinal issues, PSV commonly has co-infections with other enteric pathogens, such as porcine kobuvirus, porcine enterovirus, and porcine teschovirus. Experimental studies showed that intravaginal inoculation at day 15 of gestation caused early embryonic death and fetal resorption, and inoculation at day 30 resulted in a significant increase in fetal death. In one study, PSV was isolated in the intestinal contents of stillbirth animals, indicating that the virus can also be transmitted transplacentally. The most common symptomatic manifestation is neurological disorders, including spinal cord damage, ataxia, mental dullness, paresis, paralysis, and decreased response to environmental stimuli. Gastroenteritis and respiratory distress may also be seen in pigs affected by neurological issues.
Polioencephalomyelitis strain in the United States
A novel PSV isolate from the United States in 2016 was found to be associated with typical polioencephalomyelitis. An acute outbreak of atypical neurologic disease in 11-week-old pigs in a finishing barn was investigated by veterinary diagnostic labs from Iowa State University, Kansas State University, and the University of Minnesota. The clinically affected pigs originated from a single nursery, and had been placed in two different finishing barns two weeks prior to manifestation of signs. Over three weeks, clinical signs included decreased water and food intake, ataxia, mental dullness, paresis, paralysis, and decreased response to environmental stimuli. Despite their mental dullness, the pigs maintained central awareness, and had no detected nystagmus or cranial nerve deficits. The herd veterinarian reported a 20 percent morbidity rate and a 30 percent case mortality rate. Samples that were collected and submitted revealed a genetically novel sapelovirus in the central nervous system tissues of affected pigs. The virus’s genetic sequence appeared closely related to a previously reported Korean sapelovirus. Over a six-month period, 65 cases were submitted for neurological disease investigation, and 30 had histologic neurological lesions attributed to a viral infection.
The emerging strain
The Swine Health Information Center (SHIC) recently funded a project for genetic characterization and diagnostic tool development for an emerging PSV strain. Isolated in a diagnostic specimen from a U.S. swine farm, the emerging virus was designated as PSV KS18-01. Researchers at Kansas State University and the University of Illinois collaborated to obtain a full-length genome sequence through next-generation sequencing. Phylogenetic evaluation showed the virus is closely related to two previously identified Japanese strains, and is distantly related to two unknown U.S. strains. This information suggests that PSV KS18-01 emerged as a novel strain in the United States. To perform these tests, PSV specific diagnostic tools were developed, including the monoclonal antibodies against viral proteins, VP1 and VP2, and a VP1-VP2 antigen-based indirect enzyme linked immunoassay (ELISA). The assay was used to test 604 serum samples from experimental animals, showing 503 positives and and 101 negatives. The animal’s infection status was verified by immunofluorescence assay (IFA).
Using this new assay, the PSV antibody’s dynamic response was investigated in post-weaned pigs naturally exposed to PSV. Ten post-weaned piglets were monitored from 3 to 9 weeks old. Blood samples were collected weekly, and results showed that antibody response was weak during the first week post-weaning. Antibody response increased in the ensuing weeks, peaking at 28 days post-weaning. These results correlated with the amount of viral RNA detected in fecal samples. The piglets’ clinical signs were also monitored daily. One piglet exhibited neurological signs, including circling and abnormal gait for two days during the six weeks post-weaning, while the remainder of the piglets exhibited no clinical signs. The relationship between the neurological signs and the PSV infection are unclear.
Further studies are required to determine how different PSV strains, especially newly emerging strains, will affect pigs. The new diagnostic reagents and assays generated during this study will be beneficial tools in future pathogenesis studies, as well as when vaccines and therapeutics are developed against PSV infection.
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