Tick-parasite interactions at the host interface

P. A. Nuttall

CEH Institute of Virology and Environmental Microbiology, Oxford OX1 3SR, United Kingdom

Ticks were once described as “..disgusting parasitic animals..” (Aristotle) but now they are considered “supreme pharmacologists.” This change of attitude has arisen from a greater understanding of how ticks feed on the blood of their hosts, and how the pharmacological properties of tick saliva promote the transmission of tick-borne pathogens. Ixodid ticks in particular face one of the greatest challenges of all parasites. To survive, they must attach and remain feeding on a host for several days or weeks. The physical and chemical processes of feeding, and long duration of attachment, provoke host haemostatic, inflammatory and immune responses. Ticks counterattack with anti-haemostatic, anti-inflammatory and immunomodulatory substances secreted in their saliva. Included in this armoury are proteins that bind immunoglobulins, histamine, and serotonin, and various cytokine regulators that affect the production or activity of several cytokines, including interferons. As a result of the pharmacological activities of this rich cocktail of bioactive saliva molecules, the feeding pool within the skin becomes an immunologically privileged site. Any pathogens (viruses, bacteria, protozoa) transmitted by infected ticks into the feeding pool, or that are acquired by feeding ticks from infected hosts, will benefit from a supportive ecological niche created by the tick. Such a phenomenon, often referred to as ‘saliva-activated transmission,’ has been reported for several tick-borne pathogens. The challenge now is to determine whether this immunologically privileged feeding site can be destroyed, and pathogen transmission prevented.

Index terms: ticks, blood-feeding, immunomodulation, vector-borne pathogen transmission.

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