The effect of age, weaning and postweaning diet on small intestinal growth and morphology were investigated in young swine. Small intestine weight and villus height, measured at the midpoint of the small intestine (i.e., jejunum), were determined in suckling and weaned pigs. Scanning electron microscopy was performed on jejunal specimens from suckling pigs killed at 2, 10, 21, 28 and 35 d of age and in 21-d and 35-d weaned pigs at various ages postweaning. A 2 X 2 factorial arrangement of postweaning diets also was used to investigate jejunal morphological measurements in a 21-d-old weanling pig group. These dietary treatments evaluated the effects of 0 or 25% added dried whey and 0 or 6% added corn oil. The morphology results demonstrated that jejunal villus height declined during the suckling period, with a marked reduction at 3 and 7 d postweaning for both 21-d-old and 35-d-old weaned pigs. Transmission electron microscopy also demonstrated long, uniform microvilli on the jejunal villi in suckling pigs at 2 and 21 d, with markedly reduced lengths upon weaning. Jejunal villi were shorter in weaned compared with suckling pigs at the same approximate chronological age. Scanning electron microscopy in suckling pigs at 2 and 10 d of age demonstrated long, thin, fingerlike villi with subsequently reduced heights and larger diameters by 35 d of age. At weaning, villi were in close apposition, resulting in an overall smoother villus luminal surface. Villus height subsequently increased by 14 d postweaning, coinciding with the appearance of morphologically tongue-shaped villi. Starter diet composition initially did not influence the villus height reduction response postweaning. Dietary corn oil addition was subsequently associated with shortened villus length (P less than .05) during the starter phase.

Virus particles morphologically similar to caliciviruses and rotaviruses were detected by electron microscopy (EM) in the intestinal contents of a 27-day-old diarrheic nursing pig. A third small spherical 23-nm virus-like particle was also observed. Calicivirus-like particles averaged 33 nm in diameter. Similar to rotaviruses, rotavirus-like particles were present as single-capsid 55-nm forms or double-capsid 70-nm particles. Most gnotobiotic pigs orally exposed to samples containing these three viruses developed diarrhea and villous atrophy of the small intestine, and all shed the three viruses in their intestinal contents. Attempts to propagate these viruses in cell culture were unsuccessful. The antigenic relationship of the rotavirus-like particles to known rotaviruses was explored by immune EM and immunofluorescent staining. By these techniques, the rotavirus-like particles did not cross-react with antisera to porcine, bovine, or human rotaviruses or to reovirus type 3. Antisera from gnotobiotic pigs exposed to all three viruses had enzyme-linked immunosorbent assay and virus neutralization titers of <4 against porcine rotavirus. Previous infection of gnotobiotic pigs with the mixture containing rotavirus-like particles failed to protect them against a subsequent challenge with porcine rotavirus. The antigenic relationship of the calicivirus-like particles to known caliciviruses was investigated by immune EM and virus neutralization. By these tests, the calicivirus-like particles did not react with antisera against feline calicivirus strain 255 or M-8. In a study conducted at Plum Island Animal Disease Center, antiserum against the three combined agents did not specifically neutralize any serotype of swine vesicular exanthema virus.

Some characteristics of a newly recognized porcine enteric virus are described. Tentatively, the virus was referred to as porcine pararotavirus (PaRV) because it resembled rotaviruses in respect to size, morphology, and tropism for villous enterocytes of the small intestine. However, it was antigenically distinct from porcine, human, and bovine rotaviruses and reoviruses 1, 2, and 3, and the electrophoretic migration pattern of PaRV double-stranded RNA was distinct from the electrophoretic migration patterns of the rotaviral and reoviral genomes. By passage in gnotobiotic pigs, PaRV was isolated from two suckling diarrheic pigs originating from two herds. After oral exposure of gnotobiotic pigs, villous enterocytes of the small intestines became infected as judged by immunofluorescence, resulting in villous atrophy and diarrhea. Mortality was high when gnotobiotic pigs less than 5 days old were infected. The C strain of this virus was serially passed 10 times in gnotobiotic pigs, and electron microscopy, immunofluorescence, and serological tests indicated no extraneous agents. The virus was serially passed five times in cell cultures which contained pancreatin in the medium, but replication was negligible or absent, as the number of immunofluorescent cells decreased with each passage. Since rotaviral infections are frequently diagnosed by direct electron microscopy of fecal specimens, the presence of other morphologically similar viruses, such as PaRV, should be considered. The use of immune electron microscopy is suggested as a means of helping recognize this situation.

The Cowden strain of porcine group C rotavirus (pararotavirus) was adapted to serial passage in a continuous monkey kidney cell line (MA104). Key factors in its successful adaptation included use of virus passaged in primary porcine kidney celis as the initial inoculum, use of roller tubes, and addition of pancreatin to the maintenance medium. A cell culture immunofluorescence test was used to quantitate the virus at each passage level, since a possible cytopathic effect was obscured by the effects of pancreatin. The virus titers dropped after initial passage into MA104 cells but increased thereafter, with peak titers evident after 16 passages (107 immunofluorescence U/ml). Immune electron microscopy and genome electropherotyping were used to identify group C rotàvirus particles and confirm group C rotavirus double-stranded RNA gel migration patterns, respectively, from infected cell culture supernatants. The electropherotype of the cell culturepropagated group C rotavirus was identical to that of the gut virulent virus from which it was derived. The cell culture-passaged group C rotavirus also retained its infectivity-for gnotobiotic pigs. No group A rotavirus was detected in the intestinal contents of the pigs or in cell culture fluids from group C rotavirus-inoculated monolayers with the two former techniques or the cell culture immunofluorescence test. This is the first verified report of serial propagation of a non-group A rotavirus in a continuous cell line.

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