Selection of Human Viruses by Oyster: When Specific Ligands Come Into Play
Pollution of coastal waters by human sewage can result in the contamination of shellfish with a variety of microbial pathogens, including human enteric viruses. Shellfish have been known as vectors for human pathogens for a long time and despite regulation based on enteric bacteria, they are still implicated in viral outbreaks. Among shellfish, oysters are the most common vector of contamination worldwide and the pathogens most frequently involved in these outbreaks are noroviruses which are responsible for acute gastroenteritis in humans. For a long time, oysters were believed to act as filters or ionic traps, passively concentrating particles. However long-term persistence of some viral strains and failure of depuration now suggest that viruses do not behave like bacteria. Through in vitro, in vivo experiments and the study of naturally contaminated samples in the environment, we have demonstrated that oysters as Crassostrea gigas are able to select some noroviruses based on a carbohydrate ligand that is shared with humans. This observation has also been confirmed in other oyster species such as C. virginica or sikamea, but not in Ostrea edulis which suggests that oysters not only act as a vector of norovirus transmissions, but can also serve as a reservoir of human norovirus. Further biochemical characterization of these ligands and more precise quantification of their impact on the preservation of viral infectivity are now under study.
Keywords: oysters, human viruses, sewage, norovirus, glycan, outbreaks.
Figure: Influence of oyster in the selection of norovirus transmission. 1) Shedding in the environment of large amounts of GII norovirus (blue) and much lower amounts of GI strains (red) due to the overwhelming predominance of NoV GII in human disease. Shedding of NoV GIII (green) in cattle is also shown ; 2) Viruses present in seawater are ingested by oysters; 3) NoV GI are more efficiently accumulated and persist longer than NoV GII or GIII via specific ligand; 4) Upon consumption of a NoV-contaminated oyster, infection caused by GI and GII strains occur with similar frequency because of the selective accumulation and retention of GI viral particles. GIII NoV transmission is unlikely to happen as few particles persist in oysters.