Approximately 850 tick species have been described worldwide. The family Ixodidae is by far the largest and economically most important family with 13 genera and approximately 650 species.
The most important species of hard ticks in North America as parasites of companion animals are: Ixodes pacificus, Ixodes scapularis (dammini) (Black-legged deer tick), Dermacentor variabilis (American Dog tick), Rhipicephalus sanguineus (Brown Dog tick) and Amblyomma americanum (Lone Star tick).
Ixodes ricinus (Castor bean tick, sheep tick) is the most common tick in Northern Europe and is an important vector of both animal and human diseases. Further ticks of importance in Northern Europe are: Dermacentor reticulatus, D. marginatus, Haemaphysialis concinna, H. punctata, Ixodes canisuga and I. hexagonus.
For Southern Europe the following ticks should be mentioned: Boophilus annulatus, Dermacentor marginatus, Haemaphysialis leachi, H. parva, H.punctata, Hyalomma anatolicum anatolicum, Hy. marginatum marginatum, Ixodes canisuga, I. hexagonus, I. ricinus, Rhipicephalus bursa, R. sanguineus, R. turanicus.
A large number of hard tick species have been identified as carriers of Borrelia burgdorferi, the causative agent of Lyme disease. Among these four Ixodes species are recognised as important vectors of Lyme disease world-wide: I. persulcatus and I. ricinus in the Old World, and I. pacificus and I. scapularis (dammini) in the New World.
The risk for tick-borne diseases is measurable as a function of two epidemiologic parameters: the entomologic risk and human exposure.
Entomologic risk (here with the example of Lyme disease) is defined as the density per unit area of host-seeking nymphal ticks infected with the pathogen (here: Borrelia burgdorferi). Limited resources preclude the direct measurement of entomologic risk over large geographic areas; therefore, indirect measures are used to estimate risk in order to develop risk maps. In the case of Lyme disease data on vector distribution, abundance, B. burgdorferi infection prevalence, and human exposure were compiled. Then geographic information systems (GIS) technology was used to combine these data and categorise each of the 3,140 counties into four risk classes (here for Lyme disease), resulting in:
- High risk: where I. scapularis or I. pacificus populations are established and where prevalence of infection is predicted to be high, and which are in the top tenth percentile of counties reporting human cases.
- Moderate risk: where I. scapularis or I. pacificus populations are established and where the prevalence of infection is predicted to be high.
- Low risk: where I. scapularis populations are established, but infection prevalence is predicted to be low, or where I. scapularis populations are reported but not established, or where I. pacificus populations are either established or reported.
- Minimal or no risk: where neither I. scapularis nor I. pacificus are established or reported.