Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 4/1/2005
Publication Date: 5/23/2005
Citation: Carrillo, C., Lu, Z., Tulman, E., Borca, M.V., Vagnozzi, A., Kutish, G.F., Rock, D.L. 2005. Genetic and Phenotypic Variation of FMDV During Serial Passages in Pigs. European Study Group on the Molecular Biology of Picornaviruses. P. E06. Interpretive Summary:
Technical Abstract: RNA viruses evolve as complex distributions of genetically different but closely related variants termed viral "quasispecies". Due to the absence of low efficiency of proofreading-repair or post-replicative repair activities associated with replicating RNA, mutation rates of RNA viruses are in the range negative 3 to negative 5 Log 10 miss incorporations per nucleotide site and round of copying, in "in vitro" quantification experiments. Even though the high potential for variation of RNA viruses and, particularly, FMDV has been abundantly reviewed, very little has veen done regarding "in vivo" experimentally controlled serial passages of the virus and analyzing the genome sequences of the resultant progenies. Quantitative experimental data for the "in vivo" FMDV mutation rate are not available, and the interpretation of data from samples obtained in the field is difficult because it is the result of partial sequences from the capsid protein VP1 and lack the required background information. Here we present a novel experiment of 20 serial by-contact transmissions of FMDV in experimental groups of non immune pigs, housed in isolation units, using a natural, hyper virulent and highly porcine adapted FMDV O PanAsia isolate (O Tw97), that were never cloned or tissue culture adapted. A significant reduction in sensitivity of the pigs to the infection, characterized by lower fever and milder lesions were observed along the serial passages. Interruption of the by-contact infectious chain occurred after passages 15, 17 and 19. However, infectiousness was transitorily recovered when the vesicular fluid of the last infected pig was intra dermally injected into naive pigs. Full-length sequence of the serial viral progeny revealed definition of discrete genomic areas of rapid and permanent fixation of mutations as well as of transitory mutations, which are different for "in vitro" than "in vivo" replication of the O Tw97 isolate. Results indicated that non-structural proteins are the most informative regions for closely related isolates in non-immunized animals. We also detect significant differences in the syn/nonsyn rate ratio for "in vivo" and "in vitro" serial passages. The distribution of mutations along the genome was unusual with complete absence of replacements in the coding region for VP1 capsid protein. This work revealed that natural by-contact transmission of FMDV O Tw97 in pigs, resemble the effects of bottleneck passages of plaque to plaque "in vitro" transmission, and fitness loss described as the Muller's ratchet genetic effect, suggesting that other mechanisms like recombination and phenotypic masking might be more important than previously thought in the perpetuation of the disease in nature.