Advances in vaccine development through the controlled human infection models for hookworm and schistosomiasis

Controlled human infection models can play an important role in vaccine development, particularly for neglected tropical diseases such as helminth infections. Currently, controlled infection models have been established for schistosomiasis and hookworm. This review highlights the developments in the controlled human schistosomiasis infection model (CHI-S) and the controlled human hookworm infection model (CHHI) and their contributions to vaccine development. In general, both models are considered safe and well-tolerated. Measures to decrease risk of potential adverse events were taken when developing the models. For both models, production of challenge agents follows the principles of Good Manufacturing Practice. Both models have proven to reliably detect infection and can be used to assess efficacy of immunization strategies. While hookworms and schistosomes are both helminths, the controlled human infection (CHI)-studies have also highlighted differences between these pathogens. Notably, schistosomiasis seems to induce more, dose-dependent, systemic symptoms, whereas in hookworm models skin symptoms are much more prominent. Infection levels for schistosomiasis are therefore limited and lower than those usually seen in endemic populations, whereas for hookworm it is possible to reach levels comparable to mild-moderate intensity infection in the field. Host responses to short-term infection were also different: short-term schistosome infection induced immune-tolerance, whereas short-term infection with hookworm larvae seems to induce a more pro-inflammatory response compared to that seen in the adult worms. Most studies have been performed in naïve non-endemic populations, however, currently the models are being expanded to endemic areas. This has raised new questions around the impact of non-native strains of parasites or vectors to the endemic parasite strains and the environment. Studies in endemic areas, however, will significantly contribute towards understanding the immunology of these helminth infections in pre-exposed individuals. In general, the success of these established models is encouraging to the further development of controlled human helminth infection models.