Evidence of opportunistic blood feeding in the parasitic nematode Heligmosomoides bakeri

Haem, which binds iron and oxygen, is essential for parasitic nematode growth. Nematodes lack endogenous haem synthesis pathways and acquire haem from their environment or intracellular symbionts. Genes involved in haem degradation and detoxification have been identified in parasitic nematodes who are either specialised blood feeders (haematophages) or reside in the blood stream. Targeting these genes, so limiting parasite growth and promoting parasite death, provides a new therapeutic avenue against blood feeding parasitic nematodes. Heligmosomoides bakeri, a model intestinal parasitic nematode, has not been considered a blood feeder. Adults live in the intestinal lumen and graze on host tissue. However, the earlier larval stages enter the intestinal tissue, where they grow and moult multiple times, an oxygen demanding process. We have shown that, in vivo, likely through nematode-induced damage, many tissue-dwelling H. bakeri are in close vicinity of red blood cells. During this time, infection induces host anemia. Further, tissue-dwelling H. bakeri that have fed on blood have an increased expression of collagen genes compared to those parasites that had not fed on blood. In vitro, this translates to a growth advantage. Our findings suggest blood feeding is more widespread among nematodes than currently described. We argue that biochemical adaptations previously considered to be limited to blood-feeders are found in other nematodes. Moreover, H. bakeri can serve as a model for studying blood-feeding mechanisms and developing anthelminthic strategies to them.