Effects of the Environment on Developmental Plasticity and Infection Success of Schistosoma Parasites - An Epigenetic Perspective

Abstract

Evidence of how environmental cues affect the phenotypes of, and compatibility between Schistosoma mansoni and their hosts come from studies in environmental parasitology and research on host diet and chemotherapeutic treatment. Schistosomes deal with a multitude of signals from the water environment as well as cues that come from their hosts, particularly in response to molecules that serve to recognize and destroy them, i.e., those molecules that arise from their hosts' immune systems. These interactions shape, not only the parasite's morphology, metabolism and behavior in the short-term, but also their infection success and development into different stage-specific phenotypes later in their life cycle, through the modification of the parasite's inheritance system. Developmental phenotypic plasticity of S. mansoni is based on epigenetic mechanisms which are also sensitive to environmental cues, but are poorly understood. Here, we argue that specific cues from the environment could lead to changes in parasite development and infectivity, and consequently, environmental signals that come from environmental control measures could be used to influence S. mansoni dynamics and transmission. This approach poses a challenge since epigenetic modification can lead to unexpected and undesired outcomes. However, we suggest that a better understanding of how environmental cues are interpreted by epigenome during schistosome development and host interactions could potentially be applied to control parasite's virulence. We review evidence about the role of environmental cues on the phenotype of S. mansoni and the compatibility between this parasite and its intermediate and definitive hosts.

Keywords: environmental cues; host-para site interactions; imune response; inheritance systems; schistosomiasis.

FIGURE 1

Life cycle of the human parasite Schistosoma mansoni. The life cycle starts when eggs (in green) are released by mammalian’s host and are affected by osmotic pressure in contact with freshwater (in blue) and deliver a free-swimming larva, the miracidium, that seek out an intermediate host, a freshwater snail of the Biomphalaria genus. Here, as a free-swimming larva, is the first time that the parasite is susceptible to an abiotic environment (in blue). After this first, miracidia have to penetrate the tegument of the snail host (in green) and transform into primary sporocysts while dealing with a sophisticated immune system with barrier functions in the epithelium, a cellular immune response and a humoral defense response. Cercariae (in green) are realized and it is the second time that schistosomes face water quality (in blue). Cercaria larva actively seek a definitive mammalian host (rodent, primate or human). After infection, schistosomules migrates through the venous environment to develop into adult parasites and to reproduce sexually while dealing with humoral and cellular immune responses. In blue – eggs and free-swimming stages under cues of the water environment. In green – parasitic stages under cues from the internal environment of the hosts.