Proposal for Sequencing the Genome of the Tick, Ixodes scapularis

Catherine A. Hill, Vishvanath M. Nene, Stephen K. Wikel

This proposal represents the cooperative efforts of the international tick research community to develop the first large scale genomic analysis of a medically significant tick, namely Ixodes scapularis. Ticks transmit the greatest variety of human and animal pathogens of any arthropod vector and are second only to mosquitoes as vectors of human disease (Fivaz et al., 1992; Sonenshine and Mather, 1994). Diseases transmitted by blood feeding ixodid ticks (subphylum Chelicerata; class Arachnida; subclass Acari; family Ixodidae) are global medical and veterinary health problems (Sonenshine, 1993) and include a wide variety of bacterial, rickettsial, viral and protozoan
diseases. Other forms of pathogenesis attributed to ticks include anemia, dermatosis, toxemia and paralysis (Gothe, 1999; Roberts and Janovy, 1996; Sonenshine, 1991; Sonenshine, 1993). Important tick-borne diseases include Lyme disease (LD), tick-borne relapsing fever, babesiosis, anaplasmosis, Rocky Mountain spotted fever, Boutonneuse fever, Queensland tick typhus, Q fever, and numerous
arboviruses (Sonenshine, 1993). The resurgence of LD and the emergence of other tick-borne diseases such as human granulocytic anaplasmosis (HGA) (Childs and Paddock, 2003; Gratz, 1999; Paddock and Childs, 2003) pose increasing public health concerns. Since the discovery of the causative agent of LD, Borrelia burgdorferi, fifteen previously unrecognized tick-borne bacterial pathogens have been described (Parola and Raoult, 2001). Furthermore, due to their efficiency as vectors of a wide variety of pathogens, broad vertebrate host range and worldwide distribution (Sonenshine, 1991), ixodid ticks are recognized as potential vectors of a number of pathogens considered to be possible bioterrorism agents for use against humans and livestock including Crimean-Congo hemorrhagic fever virus, Rickettsia rickettsii (Rocky Mountain Spotted Fever), the tick-borne encephalitis complex of flaviviruses (Central European tick-borne encephalitis, Far Eastern tick-borne encephalitis, Siberian tick-borne encephalitis, Kyasanur forest disease and Omsk hemorrhagic fever), Coxiella burnetii (Q Fever) and Francisella tularensis (tularemia) (Centers for Disease Control and Prevention Select Biological Agents and Toxins, 2004;

In the United States, I. scapularis is the most important tick species from a human health perspective. Ixodes scapularis transmits LD in the northeastern and north-central US, HGA and babesiosis and possibly the flaviviral agent of Powassan encephalitis (POW) which is related to West Nile virus. Recent studies by Anderson et al., (2003) also suggest that West Nile virus can be transmitted trans-stadially by I. scapularis although vector competence has yet to be established. LD is arguably one of the most important vector borne diseases in the US, Europe and Asia. Over 17, 000 positive LD cases were reported to the US in 2000 (Centers for Disease Control and Prevention, 2002).
LD and other tick-borne diseases have important long term health consequences. Of further concern is the fact that the incidence and geographic spread of LD and other tick-borne disease are increasing and many cases are suspected to be vastly under-reported or misdiagnosed (Walker, 1988). Ixodes scapularis is a member of the Prostriata, an evolutionarily primitive phyletic line of the Acari that includes a number of medically significant tick species. Ixodes scapularis genome data will be widely applicable to studies of other prostriates including I. pacificus, the vector of LD on the US Pacific Coast, I. ricinus and I. persulcatus, the Eurasian Ixodes spp. vectors of LD and tick borne encephalitis and I. holocyclus, an Australian ixodid responsible for transmission of Rickettsia and Borrelia and human cases of tick paralysis. These factors, particularly the wide range of human diseases that it transmits, make I. scapularis the best overall candidate for a genome project that seeks to have an ultimate impact on human welfare through development of novel vector suppression measures, therapeutics and vaccines.