In the epidemiology investigation of this event, we observed more cases of upper respiratory tract infectious disease occurred from 1 January to 6 February in 2009 (389 cases) than in 2008 (261 cases), an increase of 48.8%, from the Xixiang Chinese Medicine Hospital and Hanzhong Central Hospital (unpublished data). We found climate factors changed such as drought showed little rain and lower temperature in the same season than previous years in this area. Other virus infections, such as human respiratory syncytial virus, human rhinovirus and seasonal influenza virus may contribute to the outbreak for the other forty-nine patients whose clinical specimens were unavailable for pathogen detection. In addition, nosocomial infection can not be precluded during the outbreak. This is because six of 21 patients visited the same hospital at almost the same time after the index case. As prolonged shedding of adenovirus and its hardy nature make it an ideal agent for nosocomial transmission, nucleotide sequence comparison strongly suggested that all six patients have the same strain of adenovirus in their pharynx swabs give strong information for the nosocomial transmission of infection. A surveillance network for adenovirus infection has not been established; and adenovirus vaccines are presently unavailable in China. Most of the adenovirus infections especially severe pneumonia in infants was diagnosed clinically without laboratory confirmation, especially in county hospitals. Additionally, no HAdV-7 strains have been isolated and no population immunity survey has been reported from the Hanzhong areas. In the outbreak, there was no close correlation among most of the patients where they presented a diffused distribution and with higher occurrence in Xixiang County of the Hanzhong area. The parents of the infants denied having contact histories with similar patients or any history of travel. Therefore, it is difficult to determine the adenovirus origin for the outbreak.
HAdV-7 has multiple genome types, in the early 1980s, a new genome type Ad7d became the prevalent dominant strain. Ad7d was isolated only in China from 1958-1984 and was dominant during 1980-1994. It was the representative genome type in Asian nations until 1998. In a long-term survey of adenoviral pneumonia in Beijing (1958-1990), HAdV-7 was associated with a higher fatality rate than HAdV-3. In Taiwan from 1980 to 2001, Ad7 and Ad4 were two emerging viruses, Ad7b was the predominant genotype of Ad7, while in some provinces of mainland China, such as Jiangsu, Hubei and Jiangxi, most of isolates from respiratory diseases outbreaks were Ad3[7, 8, 13]. Outbreaks of adenovirus serotype 7 infection have not been reported in China during the previous ten years; whereas a sporadic case of HAdV-7 infection has been reported in Beijing recently, and In 2002, Erdman et al. reported two emergent genome types of adenovirus type 7; both genome types were associated with epidemics, severe illness, and deaths outside of the United States. There was a wide outbreak of adenovirus infection with five dead in Japan in 1995. Then in 1998, the first report of an adenovirus 7d2 infection outbreak occurred in a pediatric chronic-care facility and tertiary-care hospital in Chicago with 67 infected and eight dead.
Although genome typing of the adenovirus serotype 7 isolates in this study has not been performed because reference strains were unavailable, a comparison with the available entire hexon gene sequences from the GenBank shows strain HAdV-7 0901 HZ isolated from the outbreak has the highest homology with HAdV7d2 from Israel, a 1993 isolate, HAdV-7d from Japan in 1998 and HAdV-7i from Korea in 1999 (GenBank accession number AF321311, AF053086 and AY769946, respectively) (Figure 2B). Comparison of the predicted amino acid sequences with other adenovirus 7 genotypes shows strain 0901 HZ lost glutamine at site 253 similar to the Korean strain; and at site 495, arginine took the place of serine. The role of these changes in the adenovirus antigenicity is not known and requires further study. An adenovirus infection surveillance programme is going conducted in five provinces of China, including Shaanxi province, which will be helpful for chasing the transmission origin and more molecular epidemiology baseline data establishment in China.