In this report, we describe the identification of a torovirus of swine and initial morphological and genetic data. The virus was detected in the feces of piglets by immunoelectron microscopy and RT PCR. In fresh fecal samples, PoTV particles appeared elongated, measuring 120 nm in length and 55 nm in width. Two types of surface projections were observed, the longer of which was petal shaped and 18 to 20 nm in length and most likely represented oligomers of the S protein (12
). The shorter spikes were 6 nm long and presumably represented HE. Surface projections of this size are also seen in BoTV; they are absent in ETV (5
), where the HE gene is truncated at its 5′ end (27
). Preliminary observations from RT PCR amplification and sequence analysis indicate that, like BoTV, PoTV contains an intact HE gene (17
Comparative sequence analysis of the torovirus N-protein genes showed that BoTV and ETV were closely related, with 81% sequence identity in this region. In contrast, PoTV showed only 68% sequence identity with the other two viruses for this region. The NTRs of PoTV, BoTV, and ETV were highly conserved, with sequence identities of about 88%. We conclude that PoTV is antigenically and genetically related to but clearly distinct from the bovine and equine representatives of the torovirus genus and should therefore be considered a new member.
In a heterotypic in vitro neutralization assay, >80% of the adult sows in The Netherlands were positive for torovirus antibody; similar observations had been reported for Switzerland (32
). Torovirus infections are obviously as common and widespread in pigs as in cattle and horses (16
). Piglets are infected shortly after weaning, when protection by maternal antibodies and/or lactogenic immunity has waned. In this study, virus shedding in the feces, as monitored by RT PCR, lasted between 1 and 9 days, suggesting that PoTV predominantly causes acute enteric infections.
The high percentage of seropositive animals and the early occurrence of infection (shortly after weaning, when immunological protection has declined) are indicative of PoTV endemicity. The virus would persist in a herd because of the continuous presence of susceptible piglets and reinfection of partly immune animals. Also, chronically infected carriers may exist, as has been demonstrated for other members of the Nidovirales
, such as coronaviruses (6
) and arteriviruses (for a review, see reference 21
). A more sensitive nested RT PCR targeted to the conserved NTR may allow the identification of long-term shedders among the adult pig population. The physical stability of toroviruses in the environment is as yet unknown; ETV is surprisingly stable at a low pH but is less thermostable than TGEV (18
Piglets passed normal stools during PoTV shedding and did not show any sign of disease. As our longitudinal study included only a few animals and monitored only one herd, we cannot exclude the possibility that the virus causes disease at a low incidence, in other circumstances, or in combination with other agents. Future research will focus on the epidemiology and pathogenic potential of PoTV.