1 March 1997

Molecular Identification of Porcine Enteroviruses

- Are the Teschen-like Viruses Really Enteroviruses?

N.J. Knowles

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Porcine enteroviruses (PEV) have been classified into 13 serologically distinct types (Dunne et al., 1971; Knowles et al., 1979; Auerbach et al., 1994). Recently Honda et al. (1990) described four additional distinct types. All these serotypes have also been divided into three groups based on physico-chemical properties and type of cytopathic effect (CPE) produced in pig kidney cells (Zoletto, 1965; Rasmussen, 1969; Kadoi et al., 1970; Zoletto et al., 1974; Knowles et al., 1979; Honda et al., 1990). The three groups have different cell culture host ranges (Knowles et al., 1979; Honda et al., 1990). All attempts to demonstrate haemagglutination by PEV have been unsuccessful.
Recently we have sequenced the entire genome of PEV-9 (group III), 274 and 275 bases in the 5' NCRs of PEV-10 (group III) and PEV-8 (group II), respectively, and c.1400 bases at the 3' end of PEV-8 (J.H. Peng, J.W. McCauley, R.P. Kitching and N.J. Knowles, manuscript in preparation; J.H. Peng, R.P. Kitching and N.J. Knowles, manuscript in preparation; F. Lin, D.K.J. Mackay and N.J. Knowles, unpublished data). These three PEV serotypes are related to other human and animal enteroviruses but the members of the two PEV groups (II and III) are quite distinct. The remaining ten serotypes (1-7 and 11-13) belong to group I and do not appear to be typical enteroviruses for the following reasons:

  1. Heat stability: group I viruses are not inactivated by heating at 50�C for 1h as is the case with true enteroviruses.

  2. Effect of ions: normally enteroviruses are protected from inactivation at 50�C by the presence of 1M MgCl2. Group I viruses when heated at 50�C in the presence of halide ions are inactivated. In this they resemble encephalomyocarditis virus (EMCV) which is heat-inactivated when in the presence of halide ions but between certain pHs.

  3. Formaldehyde sensitivity: Treatment with 0.1% formaldehyde for 5-7 days at 37�C has been used to inactivate bovine enteroviruses prior to raising type specific reference antisera (Knowles and Barnett, 1985). This same treatment affects the capsid structure of group I PEVs (but not group II or group III viruses); the virions have an lowered Sverberg coefficient. A similar effect has been shown to occur with poliovirus but with formaldehyde concentrations about 100-fold higher (Wouters et al., 1973). The poliovirus particles appeared swollen when observed by EM.

  4. Growth properties/CPE: The time taken for the appearance of CPE following inoculation of 100 TCD50 is 4-5 days. This is generally slower than most enteroviruses. The PEV group I CPE is not like that of PEV group II or PEV group III or other enteroviruses.

  5. RT-PCR: Using an enterovirus group-specific RT-PCR (in the 5' NCR) we were able to amplify and sequence a genome fragment from a number of animal enteroviruses, including human, bovine and ovine enteroviruses and PEV-8, PEV-9 and PEV-10; however, the remaining PEVs (group I) were negative (F. Lin, D.K.J. Mackay and N.J. Knowles, unpublished data).

Conclusions

I believe that for the reasons presented above that the PEVs belonging to CPE group I, that is, types 1-7 and 11-13, are unlikely to belong to the enterovirus genus. However, only sequence analysis of significant areas of the genomes of these viruses will finally answer this question.

References

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