Editorial: Biological Drivers of Vector–Pathogen Interactions

1 Biology Centre, Institute of Parasitology, Czech Academy of Sciences, České Budějovice, Czechia, 2 Faculty of Science, University of South Bohemia, České Budějovice, Czechia, 3 Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, TX, United States, 4 Department of Pediatrics, National School of Tropical Medicine, Baylor College of Medicine, Houston, TX, United States, 5 UMR BIPAR, INRAE, ANSES, Ecole Nationale Vétérinaire d’Alfort, Université Paris-Est, Maisons-Alfort, France


Editorial on the Research Topic Biological Drivers of Vector-Pathogen Interactions
Blood feeding arthropods are a highly diverse group of animals that use blood as the main nutrient source. During this process, they transmit various viral, bacterial, and protozoal pathogens that are responsible for some of the worlds' deadliest diseases leading to millions of human deaths as well as that of livestock every year. Understanding the relationships between vectors, pathogens, and the vector microbiota is an area of research, that used to be overlooked and now takes center stage when fighting vector borne diseases. The interactions include those seen at the time of acquisition, dissemination, and persistence of the pathogen within the vector and in the transmission to the vertebrate host. This basic research would help guide investigations into effective methods that would help block pathogen transmission (Shaw and Catteruccia, 2019). We believe the 'biological drivers' highlighted below are important in helping researchers develop tools that would alleviate the disease burden associated with vector-borne pathogens.

VECTOR MICROBIOMES
Vector microbiomes drive key factors of the invertebrate host physiology, development, nutrition, vector competence, and pathogenesis. Since most of the information known regarding pathogenmicrobiome interaction in mosquitoes has been conducted in laboratory reared insects, the work by Rodriguez-Ruano et al. compared mosquito sampling methods on microbiome profiles of wild caught mosquitoes. Furthermore, the research team compared the impact of insect preservation methods and pooled versus individual samples on the microbiome diversity. The work indicated that microbiome analysis of individual tissues produced little variation compared to whole specimens. Collectively, the study signified the importance of utilizing standardized methods for sample preservation, processing to minimize sampling bias.
Most microbiome studies in ticks are restricted to the taxonomic analysis of the tick microbiota with little insight about the functional roles of the non-pathogenic microbes associated with ticks. Obregoń et al. analyzed the gut microbiome metagenomes of three tick species within the family Ixodidae. The results suggest that the tick microbiome forms a complex metabolic network that may increase microbial community resilience and adaptability. For example, the genes for biosynthesis of vitamin B, essential for

VECTOR IMMUNITY
Argasid ticks are an understudied vector of emerging pathogens including African swine fever virus and relapsing fever spirochetes. Research on immunity in the New World argasid tick Ornithodoros turicata led to the identification of four defensin molecules which shared homology to defensins from the Old World tick Ornithodoros moubata (Armstrong et al.).
The findings indicated that subsets of defensin molecules are produced after blood feeding while others are up-regulated in flat molted ticks. This work sets the foundation to now determine the role of defensins after pathogen acquisition.  (Regmi et al.). The authors further showed that I. scapularis ticks decrease IsSMase expression both in vivo and in vitro, which in turn produced an accumulation of sphingomyelin that supported membrane-associated viral replication and exosome biogenesis in tick cells. Acinetobacter species have been identified in several insect species including human body lice. However, whether lice can acquire A. baumannii from the skin or blood of infected individuals and are able to transmit the bacteria to an uninfected host remain a topic of research. In their study, Ly et al. found a strong association between body lice infestation and the presence of A. baumannii DNA in the skin of homeless individuals from Marseille, France. The bacterium was not present in the blood of skin positive and/or lice infested individuals. The results of this study suggest that lice acquire A. baumannii while biting the skin of colonized individuals and likely transmit the bacteria in their feces.

VECTOR-PATHOGEN INTERPLAY
Mixed infections within a vector can lead to competitive interactions and change the dynamics of how the pathogenic strains are transmitted or abundant within an ecological area. Dolores et al. showed that when strains of Borrelia afzelii were coinfecting the hard tick I. ricinus, it led to the reduction in spirochete load for both strains and seasonal treatment of the ticks did not lead to any effect on the intensity of inter-strain competition.
In order to understand how various pathogens are able to establish infection within their mammalian or arthropod host, it is important to determine what factors help them avoid the immune systems. The review by Lin et al. condenses current data studying the roles of various Borrelia associated Complement Regulator Acquiring Surface Proteins (CRASPs) and the importance of these molecules in tick-borne transmission and dissemination of the spirochete in mammalian hosts.

COMPLEXITIES OF A VECTOR
An extremely well timed and extensive review on human head and body lice by Amanzougaghene et al. covers a host of sections connected with phylogenetics, epidemiology, disease-vector interactions as well as insecticide resistance. It certainly is a great resource collection of the current data that is available on one of the world's oldest human parasites. It certainly will help researchers push forward with research that is lacking when it comes to understanding the human louse and its vectorial nature.
Finally, we would like to show our appreciation to all authors who contributed to the Research Topic and have opened more doors in the study of vector-pathogen interactions by providing major insights into these complex relationships.

AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.