Tickborne diseases

Ticks transmit several bacterial, rickettsial and viral pathogens to humans (Fig. 10). Ticks are ectoparasites, and their geographical distribution depends on the availability of suitable habitat vegetation and host species, usually rodents and large mammals such as deer. The distribution and population density of ticks is also limited

Fig. 10. Interactions between tick-borne disease and the environment

Human society

Climate

Human society

Climate

V ■

4

Human behaviour

Ticks

& settlements

Host and reservoir animals

•r

,-i-' '__-

Pathogen transmission

Tick-borne disease incidence

Tick-borne disease incidence

Source: Lindgren (136).

by climatic factors. Tick vectors are long-lived and are active in the spring, summer and early autumn. The temperature must be sufficiently high to complete the tick's life cycle during the warmer part of the year and high enough in winter to maintain the life cycle. The humidity must be sufficient to prevent both eggs and ticks from drying out. Higher temperatures enhance proliferation of the infectious agent within the ticks, although temperatures above the optimum range reduce the survival rate of both ticks and parasites.

The northern limit of the distribution of ticks in Sweden changed between 1980 and 1994 (137). In regions where ticks were prevalent in the 1980s, population density increased between the early 1980s and mid-1990s. Changes in distribution and density over time are correlated with changes in seasonal daily minimum temperatures (138).

Ixodid ticks such as Ixodes ricinus and I. persulcatus, which are widely distributed in temperate regions, transmit tick-borne diseases in Europe. Most at risk of infection are those who spend time in the countryside or come into contact with the ticks in vegetation in periurban areas. People have also been infected in city parks. Tick populations are difficult to control directly using pesticides. Controlling the host animal populations is also difficult because many species can provide ticks with a blood meal. Tick populations may be controlled indirectly by modifying the type of local vegetation, but this can only be done on a small scale. Currently, the most effective public health measure is to raise public awareness about tickborne diseases and how to avoid infection.

Lyme borreliosis or Lyme disease is prevalent over much of Europe. The disease agent was described in 1975 after an outbreak in the United States, and the disease is therefore considered as an emerging infection. It is now the most prevalent arthropod-borne disease in temperate zones. The disease incidence has increased in several European countries, such as Finland, Germany, the Russian Federation, Scotland, Slovenia and Sweden. This may partly be due to increased reporting as well as a real trend. For example, an increase in Lyme disease during the last decade has been serologically confirmed in Sweden (139).

The risk of contracting the pathogen, Borrelia burgdorferi, from a single tick bite is 1 in 100-150 in endemic regions (140). Lyme borreliosis is a complex multisystem disorder and includes cardiac and nervous system disorders and arthritis. Most infections are asymptomatic and self-limiting, but the disease can be fatal if left untreated. Transmission occurs during the spring, summer and early autumn when the ticks are active. Climate change is likely to lengthen this transmission period. A vaccine has recently been developed in the United States, but this would not be applicable to Europe because the pathogen structures are more heterogeneous.

Tick-borne encephalitis is present in southern Scandinavia and central and eastern Europe. Tick-borne encephalitis is caused by a flavivirus with at least two subtypes: the central European type -prevalent in Europe - and the Russian spring-summer encephalitis subtype. The latter comprises other subtypes that cause diseases worldwide: louping-ill in Ireland, Norway and Scotland, Omsk haemorrhagic fever in Siberia, Kyasanur Forest disease in India and Powassan encephalitis in North America. The risk of contracting the disease from a single tick bite is 1 in 600 in endemic regions (140). The mortality rate for tick-borne encephalitis is 1%, and 10% of cases lead to permanent paralysis. Mortality rates are higher for the Russian spring-summer encephalitis subtype. A vaccine for tick-borne encephalitis is available, and persons at high risk of infection (such as those who live or work in endemic areas) are vaccinated in Sweden and other countries.

Tick-borne encephalitis virus is transferred mainly from small rodents to humans by ticks (Fig. 11). The virus has also been shown to infect humans via unpasteurized goat's milk, leading to some rare localized outbreaks in the eastern part of the European Region. A study lasting nearly four decades in a highly endemic region in Sweden found that the incidence of tick-borne encephalitis increased after

Fig. 11. Schematic overview of a 2-year tick life cycle in relation to the transmission of tick-borne encephalitis

Fig. 11. Schematic overview of a 2-year tick life cycle in relation to the transmission of tick-borne encephalitis

Source: Lindgren (136).
Table 4. Numbers of tick-borne encephalitis cases diagnosed annually from 1991 to 1997 in selected European countries

Year

Austria

Czech Republic

Estonia GermanyHungary Latvia

Lithuania

Poland

Slovenia

1991

128

356

68

44

288

227

14

4

245

1992

84

337

163

142

206

287

17

8

210

1993

102

621

166

118

329

791

198

249

194

1994

178

619

177

306

258

1 366

284

181

492

1995

109

744

175

226

234

1 341

426

270

260

1996

128

558

177

114

224

716

309

259

406

1997

99

416

404

160

99

874

645

200

274

Source: Baxter Immuno, personal communication, 1998.

Source: Baxter Immuno, personal communication, 1998.

milder winters (fewer days with temperatures below -7 °C) combined with extended spring and autumn seasons for two successive years (136). The range of tick-borne encephalitis in Europe may significantly contract, however, as well as shift to higher latitudes and altitudes (141), as its transmission depends on a particular pattern of tick seasonal dynamics, which may be disrupted by climate change.

Table 4 shows the numbers of tick-borne encephalitis cases diagnosed annually from 1991 to 1997 in selected European countries. The increase in the number of cases observed in many countries is largely likely to result from an increase in serological diagnostic capacity as well as an increase in awareness of the disease.

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