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Rotaviruses are non-enveloped viruses of the family Reoviridae and a major cause of severe dehydrating diarrhea both in developed and developing countries. The infection caused by these agents is common in both tropical and temperate climatic areas and shows distinct seasonality.
Rotaviruses are thought to cause more than 800 thousand deaths annually in children aged less than 5 years in developing countries, and are responsible for over 500 thousand visits to a medical practitioner annually in the United States alone. Therefore a major emphasis has been put on the development of a safe and effective vaccine for use in early infancy.
One of the major challenges in epidemiological studies is the rapid evolution of rotaviruses via different mechanisms. Interspecies transmission plays a crucial role in the diversity of rotaviruses, and the ability of rotaviruses to exchange genetic material between human and animal viruses during co-infections results in the generation of novel viruses.
By using various surveillance and diagnostic methods and techniques, the estimations state that rotavirus infections cause around half of all gastroenteritides in children less than five years of age. Furthermore, although there has been a downward trend in the number of bacterial and parasitic gastroenteritides, viral ones (particularly those caused by rotavirus) have remained stable.
Outbreaks of rotavirus group A diarrhea are widespread among hospitalized infants, young children in day-care centers and elderly people in nursing homes. A large outbreak occurred in Brazil in 1977, and in 1981 an outbreak caused by contaminated municipal water was noted in Colorado. During 2005, the largest recorded epidemic occurred in Nicaragua as a result of mutations in rotavirus A genome.
Rotavirus B is notable for causing major epidemics of severe diarrhea affecting thousands of people in China, primarily as a result of sewage contamination of drinking water. Infections due to endemic CAL-1 strain of rotavirus B occurred in 1998 in Kolkata, the capital of India's West Bengal state.
Group C rotaviruses were first recognized as a causative agent of gastroenteritis in animals. The outbreaks have occurred in Japan and England, but since then they have been occasionally detected in sporadic cases of diarrhea in the United States, South Africa and other countries, confirming its role as an emerging pathogen.
The range of rotavirus strains co-circulating in the world is diverse, with emerging and predominant strains varying between regions and from year to year. Over the last decade, rotavirus G1 has represented the most common genotype worldwide. Until recently, G serotypes 1-4 associated with P[8] and P[4] genotypes were the major circulating rotavirus strains globally.
In Europe, there are at least 15 G types and 28 P types circulating, with five predominating combinations: G1P[8], G2P[4], G3P[8], G4P[8] and G9P[8]. G1P[8] is responsible for 70% of infections in Europe. A high degree of diversity of co-circulating viral serotypes exists in different locations of Europe, and over different seasons.
In Kenya, the genotype G1 was mainly observed up to the year 2002, when G9 emerged as the most predominant genotype, followed by a less frequent genotype G8. In India, G9 strains were detected and usually found in combination with the P[11] or P[6] genotypes at a detection rate of about 20%.
In Thailand, G1 represents the most prevalent genotype as well, followed by G2, G4 and G3, respectively. At least three G genotypes (mostly G1, G2 and G4) are seen to coexist in Thailand each epidemic year and in some studies all four G-serotypes were reported in the same epidemics. Similar distribution is observed in children from Saudi Arabia.
All of the aforementioned genotypes can cause disease in humans, with no clear evidence of any consistent strain-specific differences in the severity of disease or the age of person affected, albeit several reports indicate that the duration of symptoms and severity of associated gastroenteritis is greater than for those not caused by rotaviruses in age-matched subjects.
The rapid evolution of rotavirus strains and the emergence of new ones, most likely through the transmission of viruses across species by reassortment between animal and human rotaviruses, makes it necessary to include intensive strain surveillance of as an important component of any vaccine implementation program.