Overview

North America:
Ixodes pacificus (Champerlin, 1937), Ixodes scapularis (dammini) (Neumann, 1911), Ixodes ricinus, I. angustus (Gresson, 1956), I. cookei (Sonenshine et al., 1965), I. kingi (Chamberlin, 1937) I. sculptus (Chamberlin, 1937), Dermacentor variabilis, D. andersoni (Arthur, 1960), D. occidentalis (Arthur, 1960), Rhipicephalus sanguineus, Amblyomma americanum, A. maculatum ( Sonenshine et al., 1965 both), Haemaphysialis leporispalustris (Joyce u. Eddy, 1943), Boophilus annulatus, Boophilus microplus, Otobius megnini (Cooley u. Kohls, 1944).

Northern Europe (Skandinavia, Great Britain, Benelux, Switzerland, Austria, Northern France):
Dermacentor reticulates, D. marginatus (Pomerantzev, 1950), Haemoaphysialis concinna, H. punctata (Babos,1964), Ixodes canisuga, I. hexagonus, I. ricinus (Arthur, 1963)

Southern Europe (Spain, Portugal, Italy, Greece, Southern France, Turkey):
Boophilus annulatus (Starkhoff, 1958), Dermacentor marginatus (Starkhoff, 1958), Haemoaphysialis leachi (Babos, 1964), H. parva, H.punctata (Starkhoff, 1958), Hyalomma a. anatolicum (Pomerantzev,1950), H. marginatum marginatum (Pandazis, 1947), Ixodes canisuga (Starkhoff, 1958), I. hexagonus (Starkhoff), I. ricinus (Starkhoff), Rhipicephalus bursa (Pavlov, 1947), R. sanguineus (Starkhoff), R. turanicus (Papadopoulos, 1990)

Africa:
Amblyomma sparsum ( = hebraeum) (Santos Dias,1952), A. tholloni (Hoogstraal, 1956), A. variegatum (Hoogstraal, 1956), B. decoloratus (Hoogstraal, 1956), Haemaphysialis leachi (Hoogstraal, 1956), H. muhsamae (Hoogstraal, 1956), H. parmata, Hyalomma albiparmatum , H. impeltatum , H. marginatus rufipes , H. truncatum (Hoogstraal, 1956), I.lewisi (Walker, 1966), I. nairobiense (Hoogstraal, 1956), I. pilosus (Santos Dias, 1951), Ornithodoros savignyi (Pomerantev, 1950), Rhipicephalus appendiculatus (Hoogstraal, 1956), R. capensis (Hoogstraal, 1956), R. carnivoralis (Walker, 1966), R. compositus (Yeoman u. Walker, 1967), R. e. evertsi (Mettam, 1932), R. guilhoni (Morel u. Vassiliades, 1963), R. longus (Hoogstraal, 1956), R. lunulatus (Gamble, 1914), R. mühlensi (Santos...), R. muhsamae (Hoogstraal, 1956), R. planus (Hoogstraal, 1956), R.pulchellus (Neumann, 1911), R. sanguineus (Hoogstraal, 1956), R.simus (Hoogstraal, 1956), R. sulcatus (Hoogstraal, 1956)

Australia and New Zealand:Amblyomma t. triguttatum (Fielding, 1926), Aponemma auruginans (Robert, 1964), Boophilus microplus (Fielding, 1926), Haemaphysalis bispinosa (Roberts, 1963), H. papuana (Fielding, 1926), I. australiensis (Fielding, 1926), I. holocyclus (Neumann, 1901), I. robertsi (Roberts,1960), I. tasmani (Roberts, 1960)

Asia:
Alectorobius thollozani (Sen, 1938), Amblyomma testudinarium (Maa u. Noda, 1959), Boophilus microplus (Itagati u. Noda, 1959), Haemaphysialis bispiosa (Itagati u. Noda, 1959), H. concinna (Neumann, 1911), H. cornigera (Sen, 1938), H. flava (Sen, 1938), H. formosensis (Maa u. Kuo, 1966), H. hystricis (Pomerantzev, 1950), H. indica (Sen, 1938), H. leachi (Neumann, 1911), H. papuana (Fielding, 1926), H. verticalis (Itagati u. Noda, 1959), H. wellingtoni (Ressang et al., 1959), Hyalomma brevipunctata (Hoogstraal, 1956), H. detritum detritum (Hoogstraal, 1956), I.acutitarsus (Maa u. Kuo, 1966), I. ovatus (Neumann, 1911), I. persulcatus (Pomerantzev, 1950), I. ricinus (Pomerantzev, 1950), I. nosomma monstrosum (Sen, 1938), Ornithodoros savignyi (Hoogstraal, 1956), Rhipicephalus h. haemaphysaloides (Itagati u. Noda, 1959), R. h. pilans (Ressang et al., 1959), R. sanguineus (Pomerantzev, 1950)

The family Ixodidae (hard ticks) 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 are:Ixodes pacificus, Ixodes scapularis (dammini), Ixodes ricinus, Dermacentor variabilis, Rhipicephalus sanguineus, Amblyomma americanum, Haemaphysialis leporispalustris, Boophilus annulatus and Boophilus microplus.

A large number of hard tick species have been identified as carriers of Borrelia burgdorferi, the causative agent of Lyme disease.

Four Ixodes species are recognised as important vectors of Lyme disease world-wide: these are Ixodes persulcatus and I. ricinus in the Old World, and I. pacificus and I. scapularis (dammini) in the New World.

I. ricinus (Castor bean tick, sheep tick) a prostriate tick, is the commonest tick in Northern Europe and is an important vector of both livestock and human diseases. It acquired a new significance when in 1983 Burgdorfer et al identified I. ricinus as a vector of the newly described Lyme disease.

Geographical appearance

The risk for tick-borne diseases is measurable as a function of two epidemiologic parameters: the entomologic risk and human exposure.

Entomologic risk

Entomologic risk for Lyme disease i. e. is defined as the density per unit area of host-seeking nymphal ticks infected with Borrelia burgdorferi. Field studies needed for determination of entomologic risk require trained entomologists. Such studies are limited to a narrow seasonal window within the life-cycle of vector ticks. Limited resources preclude the direct measurement of entomologic risk over large geographic areas; therefore, indirect measures were used to estimate risk to develop national Lyme disease risk maps. First, data on vector distribution, abundance, B. burgdorferi infection prevalence, and human exposure were compiled on a county-unit scale for the United States. Then geographic information systems (GIS) technology was used to combine these data and categorize each of the 3,140 counties into four risk classes.

Vector Distribution

Vector data were obtained from a national distribution map of Ixodes scapularis and I. pacificus, which was previously published. These data delineate three classes of tick distribution based on all published and unpublished county collection records available to CDC (Centers for Disease Control). The three classes are as follows:

• established populations (less than or equal to 6 ticks reported or more than one life stage);

• reported occurrence (<6 ticks reported and only one life stage); and

• absence of ticks or missing data.

Infection Prevalence in Vectors

The prevalence of infection with B. burgdorferi is low throughout the distribution of I. pacificus with the exception of one California county. Within the entire southern distribution of I. scapularis, prevalence of infection with B. burgdorferi is low compared with the Northeast and upper Midwest.

One possible reason for these differences is the geographic variations in abundance of hosts that are competent reservoirs of infection for immature ticks. The white-footed mouse (Peromyscus leucopus) is the principal host for ticks in the Northeast and upper Midwest and is a competent reservoir for the spirochete. But in the Southeast and West Coast regions, reptiles appear to serve as major hosts for immature ticks. But reptiles are either inefficient or incompetent reservoir hosts for spirochetes. This pattern of tick-host association might result from the greater population density of lizards relative to rodents, resulting in reduced transmission rates in regions where lizards dominate.

Human Exposure to Risk

CDC case reports were used as a measure of human exposure to entomologic risk. 137 counties reported a minimum total of 23 cases. Expert decision rule was applied to construct the risk classification as follows:

Risk Classes

• High Risk: Counties 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: Counties where I. scapularis or I. pacificus populations are established and where the prevalence of infection is predicted to be high.

• Low Risk: Counties 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: Counties where neither I. scapularis nor I. pacificus are established or reported.

Summary:
The national map illustrates a clear focal pattern of Lyme disease risk with the greatest risk occurring in the Northeast and upper Midwest regions. Overall, 115 (4%) counties were classified as high risk, followed by 146 (5%) moderate risk, 1,143 (36%) low risk, and 1,736 (55%) as minimal or no-risk counties.