Recently, the number of calls and emails about growing pigs showing a wide range of neurologic signs of disease possibly related to three somewhat lesser-known viruses have increased. Veterinarians report growing pigs presenting symptoms that range from mild muscle tremors with mental alertness to lethargy and ataxia, with the most extreme cases progressing to paralysis and death. Reports of morbidity have been as low as 5 percent to as high as 20 percent. Case fatality rate has ranged from 30 percent to 100 percent. Among the possible infectious causes are porcine teschovirus (PTV), porcine sapelovirus (PSV) and atypical porcine pestivirus (APPV). Although these viruses are not new to the United States, historically confirmed cases have been reported infrequently. [Source: SHIC, June 29, 2016]
Porcine teschovirus (PTV)
Porcine teschovirus (PTV) is a non-enveloped, positive-sense single-stranded RNA virus in the genus Teschovirus of the family Picornaviridae. There are 13 known serotypes of PTV. Pigs can be co-infected with more than one serotype and PTV is commonly isolated in healthy swine. Highly virulent strains of PTV-1 can cause teschovirus encephalomyelitis. Less virulent strains of PTV-1, in addition to PTV-2, PTV-3, and PTV-5, are associated with Talfan disease (also known as benign enzootic paresis), a milder presentation of polioencephalomyelitis than teschovirus encephalomyelitis.
In teschovirus encephalomyelitis, fever, anorexia, listlessness and locomotor ataxia can be seen prior to paralysis/paresis. Caudal ataxia leading to paresis or paralysis can be seen as early as two to three days post infection. Commonly, death occurs three to four days after the onset of clinical signs,1 but recent suspected cases progressed quickly to death within 24 hours.
Abortion and SMEDI syndrome (stillbirth [S], mummified fetus [M], embryonic death [ED], infertility[I]) have been linked to the variety of reproductive disorders that can be caused by PTV serotypes. SMEDI syndrome is also seen with parvovirus infections, which more frequently cause reproductive disorders in conventional herds than PTV.
Porcine sapelovirus (PSV)
Porcine sapelovirus (PSV) is a non-enveloped, positive-sense single-stranded RNA virus belonging to the genus Sapelovirus in the family Picornaviridae. PSV is closely related to the genus Enterovirus and was previously classified as porcine enterovirus 8 (PEV-8). There are three species within the Sapelovirus genus: porcine, simian and avian. Pigs, monkeys and ducks are the only known hosts for each species.
Polioencephalomyelitis syndrome, characterized by ataxia and limb paralysis, with or without other clinical symptoms (diarrhea or pneumonia) is suggestive of PSV infection.2 A recently reported incident resulted in high case mortality within 24 to 48 hours. Like PTV, SMEDI syndrome has also been linked to the virus. Litters with few to several stillborn or mummified fetuses may be suggestive of PSV-induced reproductive disorder3 when no other more common cause is identified.
Atypical porcine pestivirus (APPV)
A study by Arruda et al., published in 2016, identified an atypical porcine pestivirus from piglets with congenital tremors.4 This virus was closely related to a novel pestivirus reported in serum samples from pigs involved in a PRRS metagenomics sequencing study. Phylogenetic analysis showed the greatest similarity to a newly described pestivirus in bats in China.
Samples from growing pigs submitted to the Iowa State University Veterinary Diagnostic Laboratory for routine testing were screened for APPV RNA via RT-PCR and 6 percent (22/362) of the pigs tested positive.4 Also in this study, pregnant sows were inoculated with APPV (intravenous, intranasal, and inoculation of fetal amniotic vesicles) in an attempt to cause disease. Inoculated sows farrowed pigs affected with congenital tremors while controls did not. APPV was also consistently detected in tissues from affected piglets via RT-PCR.4
Just recently, an APPV was isolated from a pig with uncontrollable shaking coming from a herd in which approximately 700 affected pigs in the herd had died with no other diagnosed cause. Notably, this outbreak occurred in pigs 5 to 14 weeks-of-age, which is significantly older than piglets in which congenital tremors occur.5
Help Bridge the Knowledge Gap
There are significant knowledge gaps about the pathogenicity and epidemiology of these three viruses. The data from the veterinary diagnostic labs are yet to be analyzed to determine if the recent communications are part of an increasing trend or part of the more historic, sporadic outbreaks. Although individual cases may seem to be isolated, sporadic incidents, they could be indicative of a more important emerging disease trend. This is why it’s important that everyone who experiences an outbreak communicates it so the information can be aggregated to create a comprehensive assessment of the disease in question.
If you encounter any cases with clinical signs similar to these, keep these three viruses in mind as potential differential diagnoses. And, should the veterinary diagnostic lab discover that any of these three are the confirmed etiology, please let the Swine Health Information Center know by sending an email to [email protected] or calling the Center at (855) 211-4333. Only aggregate information, not individual identifiers, would be communicated if a trend is identified. This will be a great help to the industry as a whole to understand if any of them are an emerging pathogen in the U.S.
Posted under the Emerging Diseases tab of the Swine Health Information Center’s website, https://www.swinehealth.org are fact sheets with more information about these viruses and information about SHIC financial support for additional diagnostic testing. The mission of the Swine Health Information Center is to protect and enhance the health of the United States swine herd through coordinated global disease monitoring, targeted research investments that minimize the impact of future disease threats, and analysis of swine health data. For more information, visit https://www.swinehealth.org or contact Dr. Sundberg at [email protected].