Critical epitopes in transmissible gastroenteritis virus neutralization.
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TGEV replicates in intestinal enterocytes and causes diarrhea in young pigs. PRCV, a spike (S) gene deletion mutant of TGEV with an altered respiratory tissue tropism, causes mild or subclinical respiratory infections. Comparisons of TGEV and PRCV strains suggest that tropism and pathogenicity are influenced by the S gene and ORF3, respectively. Recently, outbreaks of TGE of reduced virulence were reported in the field. We investigated a similar suspect TGEV outbreak of reduced virulence in nursery pigs from a swine herd in the Midwest. A TGEV strain (BW021898B) was isolated in swine testicular cells from gut contents of a diarrheic pig and three PRCV strains (BW126, BW154, BW155) were isolated from nasal swabs from normal TGEV-seronegative sentinel pigs in contact with the diarrheic pigs. Sequence analysis of the TGEV isolate in the partial S gene and ORF3/3a and ORF3-1/3b revealed high homology with enteropathogenic TGEV strains. Gnotobiotic pig inoculation and histopathological results revealed that this TGEV isolate retained virulence even though in the field outbreak the diarrheal disease was of reduced severity. Sequence analysis of the S gene deletion region of the three PRCV isolates revealed identical deletions between nt 105-752, which differ from deletions previously reported among PRCV strains. The three PRCV isolates had variable sequence changes in ORF 3/3a and ORF 3-1/3b, affecting the ORF size and amino acid sequence. Thus, sequence analysis and pathogenicity studies indicate that this TGEV isolate resembles other enteropathogenic TGEV strains. Therefore, the reduced severity of TGE observed in this herd may be due to the ongoing PRCV infections, which induce antibodies cross-reactive with TGEV and result in decreased disease severity. The results outlined in this study highlight the need to monitor the molecular epidemiology of TGEV/PRCV strains with sensitive differential diagnostic assays, followed by sequence analysis of the critical regions to identify changes and pathogenicity studies to confirm the disease potential of the TGEV isolates.
Piglets of eight sows vaccinated by different routes with the attenuated TGE mutant coronavirus, Nouzilly (N) strain, and piglets from two field seropositive sows were challenged with a virulent TGE strain. On the day of challenge and 10 days after challenge, milk and serum samples from sows were analysed for their level of neutralizing antibodies, total immunoglobulin classes and TGE antibody classes by an ELISA. No direct relationship was seen between the level of protection of the litters and the titres of the different antibody classes on the day of challenge. However, an inverse correlation was seen 10 days after challenge between protection and the level of TGE antibodies.
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Four virus-induced non-structural proteins with apparent molecular weights of 11-14 kilodaltons (kDa) were identified by two-dimensional electrophoresis in cells infected by TGEV. Differences in the number of non-structural proteins were observed among virulent and attenuated TGEV strains as well as with two antigenically related feline and canine coronaviruses.
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TGE seronegative pregnant gilts were vaccinated by intramammary inoculations with a chemically inactivated (betapropiolactone) cell culture virus vaccine and their nursing pigs were exposed to virus at three days of age. The pig morbidity and survival rates were compared with those of pigs nursing gilts exposed orally to virulent TGE virus during late gestation and also with pigs nursing non-vaccinated seronegative sows. The morbidity rates were 100%, 19% and 19%, while the survival rates were 33%, 100% and 17% respectively. The intramammary vaccine used to vaccinate pregnant gilts in this study did not stimulate sufficient antibody response to provide an acceptable level of protection to nursing pigs against experimental challenge with virulent TGE virus. The serum, colostrum and milk TGE neutralizing antibody responses are reported.
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