At least two biotypes were observed at the 2nd passage stage after isolation of the foot-and-mouth disease virus (FMDV) O/JPN/2000 strain. These viruses differed in plaque phenotype and were distinguishable by using monoclonal antibody (MAb) 64G8, which was made for the FMDV O/JPN/2000 strain. One of these two biotypes formed small plaque (SP) and with immunostaining showed a positive reaction to MAb 64G8, while the other formed clear large plaque (LP) and did not react with MAb 64G8. The amino acid sequences of the capsid coding region (VP1-VP4) of the SP-forming virus (SPV) and the LP-forming virus (LPV) revealed two substitutions on the 133rd amino acid in VP2 and on the 56th amino acid in VP3. These amino acid changes of SPV and LPV were Asn to Asp, and Arg to His, respectively. The Arg of the 56th amino acid in VP3 is in a critical position of the cell culture-adapted virus. Only LPV showed high pathogenicity in suckling mice, and its LD50 was calculated to be about 102 TCID50/0.1 ml. These results showed that the SPV at the 2nd passage stage from isolation was a low-virulence virus, which may explain why cattle infected with O/JPN/2000 did not show clear symptoms.
(Research Team for Exotic Diseases, TEL +81-29-838-7708)
Reference:
Morioka, K., et al. (2008) J. Vet. Med. Sci. 70: 653-658.
