Chapter 4 - Blood feeding as a life choice and the multiple functions of tick saliva

Abstract

As obligate hematophagous arthropods, acquiring a blood meal is fundamental to transition from one life stage to the next. However, unlike many other blood-feeding organisms, ticks have evolved a pool-feeding strategy where the tick embeds itself into the skin of the host and consumes a volume of digested blood and tissue. Each immature life stage of the tick must find and feed from a vertebrate host before progressing to the next stage. Adult females require a blood meal to provide nutrition for egg development, and in some species, males will also take a blood meal. For hard ticks, this requires attachment to a host for long periods, often for days. Conversely, soft ticks have developed a little-and-often strategy, taking a brief blood meal, multiple times during each life stage. To achieve this, the tick must locate a host, select a site in which to pierce its dermal layers, and remain attached. It then alternates injection of saliva into the wound and ingestion of blood and digested tissue. Tick saliva plays a key role in this process and contains a vast array of molecules that enable attachment to the host, digestion of the blood meal, and suppression of many host responses that might be detrimental to the tick. Understanding the contents of tick saliva has received considerable attention due to its role in pathogen transmission. The potential to utilize salivary products as a component of a vaccine that could suppress tick feeding and prevent pathogen transmission has been a goal of many investigators. The range of functions that tick saliva deliver is remarkable and reflects the long and successful evolution of ticks. However, studying this complex cocktail of proteins, peptides, and lipids has proven challenging, and despite over 60 years of scientific endeavor, many aspects of both the contents and function of tick saliva remain unresolved. In addition, not all ticks are the same and there are clear differences between the two main tick families based on their different feeding strategies. There are also a number of anomalies represented by those ticks that have moved away from a strict hematophagous diet. This chapter provides a brief introduction to the subject and highlights what is known about tick feeding, and critically what remains to be discovered about this multifunctional secretion, tick saliva.