Submitted to: Journal of Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2007
Publication Date: 10/1/2007
Citation: Carrillo, C., Lu, Z., Borca, M.V., Vagnozzi, A., Kutish, G.F., Rock, D.L. 2007. Genetic and Phenotypic Variation of FMDV During Serial Passages in a Natural Host. Journal of Virology. 81(20):11341-11351. DOI: 10-1128/jvi.00930-07.
Interpretive Summary: Our results show for the first time, the arising of new FMDV genetic variants with altered pathogenicity in pigs and the rapid replacement of the original consensus sequence by new variant genotypes with acquired mutations, mostly outside of the capsid coding region P1 were observed. The data indicate rapid accumulation of nucleotide substitutions, and fitness loss, suggesting bottleneck transmission effects, never before described in natural FMDV transmission. This observation, results of paramount significance for the epidemiology of FMDV and should seriously impact in the interpretation of the topotypes and molecular epidemiology of FMDV. We describe by the first time the establishment of an asymptomatic subclinical infection in pigs. Fixation of amino acid changes in nonstructural proteins likely resulted in deleterious effects for virus biology, leading to the establishment of a sub-clinical infection that resembles the carrier state described in cattle. Virus was isolated from tonsils and nasal swabs of the asymptomatic T15 pigs at 26 days post-contact, resembling a natural establishment of the carrier state previously described only for ruminants. Furthermore, we found significant differences in evolution parameters between in vivo and in vitro passaged virus, reflecting differences in selective pressures operating on virus populations, expressed as differences in synonymous vs. non-synonymous substitutions, frequency of transitions vs. transversions and a difference in tolerance for changes in specific viral proteins. Surprisingly, the region encoding for the capsid protein VP1 (1D) did not show amino acid changes during in vivo passages. These results are the first reported evidence of the possibility of using specific quantitative parameters to measure virus-host adaptation as epidemiological/forensic tools.
Technical Abstract: Foot-and-Mouth Disease Virus (FMDV) exhibits high mutation rates during replication that have been suggested to be of adaptive value. However, even though genetic variation in RNA viruses has been extensively examined during virus replication in a wide variety of in vitro cell cultures, very little is known regarding the generation and effects of genetic variability of virus replication in the natural host under experimental conditions and no genetic data is available regarding the effects of serial passage in natural hosts. Here we present the results of 20 serial contact transmissions of the highly pathogenic, pig-adapted, O Taiwan 97 (O Tw97) isolate of FMDV in swine. We examined the virus genomic consensus sequence for a total of 37 full-length viral genomes recovered from 20 in vivo passages. Unexpectedly, a significant reduction of virulence upon pig passages was observed and, finally, interruption of viral transmission chain occurred after the 14th pig passage (T14). Virus was however isolated from tonsils and nasal swabs of the asymptomatic T15 pigs at 26 days post-contact, resembling a natural establishment of the carrier state previously described only for ruminants. Surprisingly, the region encoding for the capsid protein VP1 (1D) did not show amino acid changes during in vivo passages. These data demonstrate that contact transmission of FMDV O Tw97 in pigs resembles the bottleneck effect and the fitness loss previously observed by others during plaque-to-plaque FMDV passage in vitro, suggesting that unknown mechanisms of virulence recovery might be necessary during the evolution and perpetuation of FMDV in nature.