Median infectious dose of human norovirus GII.4 in gnotobiotic pigs is decreased by simvastatin treatment and increased by age

Human noroviruses (NoVs), a major cause of viral gastroenteritis, are difficult to study due to the lack of a cell-culture and a small-animal model. Pigs share with humans the types A and H histo-blood group antigens on the intestinal epithelium and have been suggested as a potential model for studies of NoV pathogenesis, immunity and vaccines. In this study, the effects of age and a cholesterol-lowering drug, simvastatin, on the susceptibility of pigs to NoV infection were evaluated. The median infectious dose (ID50) of a genogroup II, genotype 4 (GII.4) 2006b variant was determined. The ID50 in neonatal (4-5 days of age) pigs was ≤2.74×10(3) viral RNA copies. In older pigs (33-34 days of age), the ID50 was 6.43×10(4) but decreased to <2.74×10(3) in simvastatin-fed older pigs. Evidence of NoV infection was obtained by increased virus load in the intestinal contents, cytopathological changes in the small intestine, including irregular microvilli, necrosis and apoptosis, and detection of viral antigen in the tip of villi in duodenum. This GII.4 variant was isolated in 2008 from a patient from whom a large volume of stool was collected. GII.4 NoVs are continuously subjected to selective pressure by human immunity, and antigenically different GII.4 NoV variants emerge every 1-2 years. The determination of the ID50 of this challenge virus is valuable for evaluation of protection against different GII.4 variants conferred by NoV vaccines in concurrence with other GII.4 variants in the gnotobiotic pig model.