Symbiotic bacteria of helminths: what role may they play in ecosystems under anthropogenic stress?

Impact of anthropogenous environmental factors on the marine ecosystem of trophically transmitted helminths and hosting seabirds: Focus on North Atlantic, North Sea, Baltic and the Arctic seas

Alongside natural factors, human activities have a major impact on the marine environment and thus influence processes in vulnerable ecosystems. The major purpose of this review is to summarise the current understanding as to how manmade factors influence the marine biocenosis of helminths, their intermediate hosts as well as seabirds as their final hosts. Moreover, it highlights current knowledge gaps regarding this ecosystem, which should be closed in order to gain a more complete understanding of these interactions. This work is primarily focused on helminths parasitizing seabirds of the North Atlantic and the Arctic Ocean. The complex life cycles of seabird helminths may be impacted by fishing and aquaculture, as they interfere with the abundance of fish and seabird species, while the latter also affects the geographical distribution of intermediate hosts (marine bivalve and fish species), and may therefore alter the intertwined marine ecosystem. Increasing temperatures and seawater acidification as well as environmental pollutants may have negative or positive effects on different parts of this interactive ecosystem and may entail shifts in the abundance or regional distribution of parasites and/or intermediate and final hosts. Organic pollutants and trace elements may weaken the immune system of the hosting seabirds and hence affect the final host's ability to control the endoparasites. On the other hand, in some cases helminths seem to function as a sink for trace elements resulting in decreased concentrations of heavy metals in birds' tissues. Furthermore, this article also describes the role of helminths in mass mortality events amongst seabird populations, which beside natural causes (weather, viral and bacterial infections) have anthropogenous origin as well (e.g. oil spills, climate change, overfishing and environmental pollution).

Helminth-host-environment interactions: Looking down from the tip of the iceberg

In 1978, the theory behind helminth parasites having the potential to regulate the abundance of their host populations was formalized based on the understanding that those helminth macroparasites that reduce survival or fecundity of the infected host population would be among the forces limiting unregulated host population growth. Now, 45 years later, a phenomenal breadth of factors that directly or indirectly affect the host-helminth interaction has emerged. Based largely on publications from the past 5 years, this review explores the host-helminth interaction from three lenses: the perspective of the helminth, the host, and the environment. What biotic and abiotic as well as social and intrinsic host factors affect helminths? What are the negative, and positive, implications for host populations and communities? What are the larger-scale implications of the host-helminth dynamic on the environment, and what evidence do we have that human-induced environmental change will modify this dynamic? The overwhelming message is that context is everything. Our understanding of second-, third-, and fourth-level interactions is extremely limited, and we are far from drawing generalizations about the myriad of microbe-helminth-host interactions.Yet the intricate, co-evolved balance and complexity of these interactions may provide a level of resilience in the face of global environmental change. Hopefully, this albeit limited compilation of recent research will spark new interdisciplinary studies, and application of the One Health approach to all helminth systems will generate new and testable conceptual frameworks that encompass our understanding of the host-helminth-environment triad.

Eco-evolutionary implications of helminth microbiomes

The evolution of helminth parasites has long been seen as an interplay between host resistance to infection and the parasite's capacity to bypass such resistance. However, there has recently been an increasing appreciation of the role of symbiotic microbes in the interaction of helminth parasites and their hosts. It is now clear that helminths have a different microbiome from the organisms they parasitize, and sometimes amid large variability, components of the microbiome are shared among different life stages or among populations of the parasite. Helminths have been shown to acquire microbes from their parent generations (vertical transmission) and from their surroundings (horizontal transmission). In this latter case, natural selection has been strongly linked to the fact that helminth-associated microbiota is not simply a random assemblage of the pool of microbes available from their organismal hosts or environments. Indeed, some helminth parasites and specific microbial taxa have evolved complex ecological relationships, ranging from obligate mutualism to reproductive manipulation of the helminth by associated microbes. However, our understanding is still very elementary regarding the net effect of all microbiome components in the eco-evolution of helminths and their interaction with hosts. In this non-exhaustible review, we focus on the bacterial microbiome associated with helminths (as opposed to the microbiome of their hosts) and highlight relevant concepts and key findings in bacterial transmission, ecological associations, and taxonomic and functional diversity of the bacteriome. We integrate the microbiome dimension in a discussion of the evolution of helminth parasites and identify fundamental knowledge gaps, finally suggesting research avenues for understanding the eco-evolutionary impacts of the microbiome in host-parasite interactions in light of new technological developments.

Vertebrates as uninfected disseminators of helminth eggs and larvae

The passive dispersal of non-mobile organisms by vertebrates (zoochory) is a common mechanism used to explain their often widespread distribution. Transport occurs either internally via the vertebrate digestive tract (endozoochory), or externally be adhering to skin, feathers or fur (ectozoochory), and its success is due to both physiological and ecological factors associated with the disseminating 'hosting' animal. Helminth eggs and larvae are generally non-mobile stages that are largely dependent on the movement of another animal, typically a host, for geographical dissemination. Studies on the zoochory of helminths by vertebrates are extensive and particularly long-standing, stretching back to the 19th century, although this literature is often overlooked when considering the biogeography of parasites. This review assesses the potential of helminths to be dispersed passively by zoochory examining evidence from both laboratory and field studies. The physiological dynamics of the vertebrate intestines and skin surface as hostile environments, as well as the characteristics of eggs and larvae which may facilitate successful transport are evaluated. The various mechanisms of helminth endo- and ectozoochory are presented and the likelihood of long-distance dispersal determined. It is concluded that zoochory is a potentially important means of disseminating parasites.

Parasite-bacteria interrelationship

Parasites and bacteria have co-evolved with humankind, and they interact all the time in a myriad of ways. For example, some bacterial infections result from parasite-dwelling bacteria as in the case of Salmonella infection during schistosomiasis. Other bacteria synergize with parasites in the evolution of human disease as in the case of the interplay between Wolbachia endosymbiont bacteria and filarial nematodes as well as the interaction between Gram-negative bacteria and Schistosoma haematobium in the pathogenesis of urinary bladder cancer. Moreover, secondary bacterial infections may complicate several parasitic diseases such as visceral leishmaniasis and malaria, due to immunosuppression of the host during parasitic infections. Also, bacteria may colonize the parasitic lesions; for example, hydatid cysts and skin lesions of ectoparasites. Remarkably, some parasitic helminths and arthropods exhibit antibacterial activity usually by the release of specific antimicrobial products. Lastly, some parasite-bacteria interactions are induced as when using probiotic bacteria to modulate the outcome of a variety of parasitic infections. In sum, parasite-bacteria interactions involve intricate processes that never cease to intrigue the researchers. However, understanding and exploiting these interactions could have prophylactic and curative potential for infections by both types of pathogens.