Molecular epidemiology of norovirus in symptomatic and asymptomatic population in Burkina Faso

Background
Noroviruses (NoV), belonging to the family Caliciviridae, are now recognized as the leading cause of gastroenteritis outbreaks worldwide, and represent an important cause of sporadic gastroenteritis in both children and adults.
Many studies describe NoV epidemiology. However, few data are available about the NoV strains circulating in most of African countries, in particular in Burkina Faso. The population of Burkina Faso is characterized by the young age of its habitants, and most are living in rural environment.
Objectives
The purpose of this epidemiological study was to determine the prevalence of NoV in Bobo Dioulasso (Southern part of Burkina Faso) by molecular diagnosis methods in patients presenting or not gastroenteritis symptoms, to quantify the excreted viral load, and to genotype the circulating strains.
Methods
Patients with and without gastro-intestinal disorders were selected in several Health Care Centres of Bobo Dioulasso. Clinical and epidemiological data, as well as stool samples, were collected during 8 weeks through March to April 2011.
Viral genomic RNA was automatically extracted with a Maxwell® (Promega) instrument. Molecular detection of genogroups (G) I, II and IV NoV in stool samples was performed by a home-made real-time RT-PCR targeting the ORF1-ORF2 polymerase junction region. For each positive sample, viral load was estimated by using standard curves (successive dilutions of recombinant GI and GII plasmids). Molecular characterization was performed on the detected strains, using both polymerase and capsid regions.
Results
NoV were detected in 21.6% of the 453 collected stool samples, with a distribution of 21.0% and 23.1% in the samples from the 319 symptomatic (SP) and the 134 asymptomatic patients (AP) respectively.
Genogroup distribution was 7.2% for GI, 10.7% for GII and 3.1% for both GI and GII among SP’s samples, and was 11.2% for GI, 10.4% for GII and 1.5% for both GI and GII among AP’s samples.
Average viral load values were higher for GI NoV in SP than in AP (p=0.02), when they were higher for GII NoV in AP than in SP (p=0.04).
Phylogenic analysis showed a high degree of genotypical diversity in both groups of patients. One recombinant strain GII.7/GII.6 was also detected, to our knowledge, for the first time.
Conclusion
Even if a true pathogenic role of NoV could not be showed from the study design, it allowed to precise the molecular epidemiology of NoV strains prevalent in a representative country of the East African region. It also showed that asymptomatic patients could play an important role as a NoV “reservoir”. Despite the fact that GII strains, and more precisely those belonging to GII.4 genotype, are nowadays highly reported worldwide, the surprising proportion of NoV GI detected in this study suggests that GI and GII strains should be excreted in equal proportion in the environment. The origin of this epidemiologic difference, even if partially explained by the difference in immunity and genetic sensitivity of the population, is still to be solved.