Genetic characterization and molecular clock analyses of the Crimean-Congo hemorrhagic fever virus from human and ticks in India, 2010–2011

A nosocomial outbreak of Crimean Congo hemorrhagic fever (CCHF) was reported among humans in Ahmadabad district, Gujarat, India during January, 2011. In the present study we provide the complete genomic sequences of four CCHFV isolates derived from two human patients and two pools of Hyalomma anatolicum ticks during the period of this outbreak and the complete S segment sequence of two retrospective human serum samples, positive for CCHFV in 2010. Sequence-based molecular characterization of the Indian CCHFV showed that they possessed the functional motifs known to occur in the S, M and L gene segment products as in other CCHF viruses. The S segment of the six Indian CCHF viruses showed 99.8% nucleotide identity. Notably both tick isolates shared 100% nucleotide identity with one of the Indian human isolates of 2011. Phylogenetic analysis based on the S segment demonstrated that the Indian CCHFV isolates formed a distinct cluster in the Asian-Middle East group IV of CCHF viruses. The S segment was closest to a Tajikistan strain TADJ/HU8966 of 1990 (98.5% nucleotide identity) and was of South-Asia 2 type while the M segment was of type M2. Both M and L segments were closest to an Afghanistan strain Afg09-2990 of 2009 (93% and 98% nucleotide identity, respectively). The Indian isolates were thus identified as a South-Asia 2/M2 far-east virus combination and the differing parental origin in the S and L/M segments is suggestive that it may be an intra-genotypic reassortant. Molecular clock studies further revealed that the ancestry of the viruses was not very recent and dated back to about 33 years on the basis of the S segment while it was about 15 years based on the M segment. Thus though the 2011 outbreak may not have resulted from a very recent introduction, considering that so far there is no evidence of multiple circulating strains in the country, the possibility of a recent re-introduction of the virus from any of the neighboring countries cannot be ruled out. The study thus warrants the need for continued surveillance and increased sampling of CCHFV in different parts of the country.