The term "Small Round Structured Virus" (SRSV) is widely used for the viruses causing viral gastroenteritis and food intoxication. This term originates from the terminology advocated by Caul et al of England based on the morphological classification observed with the aid of the electron microscope in 1982. (Figure 1)
    In the 1980s, morphological observation by means of the electron microscope was the mainstream of the laboratory diagnosis of viral diarrhea. The term Small Rounded Structured Virus was used as the generic name for the group of viruses that are small and have a surface structure, which were detected from the stools of various patients afflicted with diarrhea.
About twenty years after the viral particles were detected with the aid of the electron microscope, Jiang et al .determined the complete base sequence of the Norwalk virus genome, the prototype virus of SRSV, which had not been extensively studied due to the difficulties of growing the virus in cell culture. This opened a new chapter in the history of research into viral gastroenteritis. Following these studies, various small round structured viruses were genetically analyzed in different countries. Now a classification combining the genetic classification with the morphological one has been conceived, as shown in Figure 1. Besides a new virus classification / designation method for the family Caliciviridae has been reported at the ICTV this year. (Please refer to the Eighth International Designation Committee Report; http://www.ictvdb.iacr.ac.uk/lctv/fs_calic.htm)
    Thus the investigation into the genetic structure has led to new findings. In the present situation where all SRSVs genes have not been deciphered, group specific RT-PCR primers, immunoassay techniques(ELISA, etc.) and kits allowing to detect all SRSVs have not yet been established. There still exist SRSVs, which cannot be detected by means of the current PCR primers but can be detected only by direct observation with the aid of conventional electron microscopy. (Figure 2, Table 1). The presence of SRSVs undetectable by either genetic or immunological diagnosis constitutes a major problem. Thus electron microscopy is one of the key screening techniques indispensable to the detection of SRSVs, whose culture is not possible, and its usefulness remains highly appreciated.
    Conventional electron microscopy requires a complex pretreatments such as concentration of the virus by means of ultra centrifugation and elimination of foreign matters in order to examine it directly under the microscope from the feces of patients. More over, the morphological identification of SRSVs under the field of observation requires a high level of technical expertise, and it cannot be considered as a general and extensively employed detection procedure. We have developed software for electron microscope manipulations utilizable even by beginners having no technical skill, and it has been put into practical use (Udagawa, 2002). We are continuing to work on improvements in order to upgrade its precision. It is expected that electron microscopes equipped with this new device and software will be highly useful for the identification of small round structured particles that cannot be detected by means of other screening techniques.
Figure 1 - Morphological and genetic classification
Morphology - Genetics - Detection technique
1) Eighth ICTV Report 2002
Family Caliciviridae
(Former Norwalk-like virus changed to Genus Norovirus)
(Former Sapporo-like virus changed to Genus Sapporo)
Table 1 - Number of detection cases of pathogenic virus for viral gastroenteritis during the 2000-2002 season (IASR)