Anti-tick vaccines for the control of ticks affecting livestock

Abstract

Ticks are obligate hematophagous arthropod parasites affecting most terrestrial vertebrate species. Their importance as disease vectors is due to the abundance and diversity of organisms they transmit to their vertebrate host. In addition, secondary infections of the attachment site, direct toxicosis, and paralysis can occur due to tick feeding. The impact of tickborne diseases is most heavily felt in the livestock sector, where production is limited in many areas due to high tick infestations and tick-borne disease prevalence. Currently, large-scale tick control is achieved through acaricide application, but continued use has resulted in resistance to several active ingredients. As an alternative, the immunological control of ticks through vaccination has been proposed. Proof of concept has been shown culminating in commercial vaccines for the control of the cattle tick Rhipicephalus microplus. Despite this initial success, the development of anti-tick vaccines faces a number of unique obstacles due to complex interactions between tick and vertebrate hosts. This complexity does however allow novel areas of vaccine development to be followed that are unavailable to other vector-borne diseases. Tick antigens localized in the tick gut are concealed from the host immune response during normal tick feeding. With the uptake of the blood meal from a vaccinated host, damage to the gut wall occurs which may result in death or decreased reproductive capacity. The use of “concealed antigen” candidates has been successful in effectively reducing tick populations with successive generations. Furthermore, a number of “exposed antigen” candidates, present in the saliva or cement cone, have also been investigated for their ability to interfere with tick feeding and block parasite transmission. With the completion of tick genome sequencing projects the number of candidate antigens for evaluation as vaccine candidates increasing. In this chapter, the history and potential future of anti-tick vaccine development are outlined with particular reference to control of ticks and tick-borne diseases affecting livestock. Anti-tick vaccine advances for the major tick species affecting livestock, Rhipicephalus, Amblyomma, and Hyalomma, will be discussed.