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% to as high as 20%. Case fatality rate has ranged from 30% to 100%.
Among the possible infectious causes are porcine teschovirus, porcine sapelovirus and atypical porcine pestivirus. Although these viruses are not new to the United States, historically confirmed cases have been reported infrequently.
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.
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
A study by Arruda et al., published in 2016, identified an APPV 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% (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