Climate-change-tick-borne-encephalitis-and-

Ecological Modelling110(1998)55–63

Climate change,tick-borne encephalitis and vaccination needs in

Sweden—a prediction model

Elisabet Lindgren*

Natural Resources Management Institute,Department of Systems Ecology,Stockholm Uni6ersity,S-10691Stockholm,Sweden

Abstract

A future,global,climate change may indirectly lead to changes in the transmission and incidence of several vector-borne diseases.This paper presents an example of a modeling tool for projections of possible changes in the incidence of tick-borne encephalitis(TBE),and the subsequent changes in vaccination needs,during the next half-century in Sweden.The model is based on the Hadley Center’s regional temperature predictions for the year 2050,taking into account the IPCC IS92‘non-intervention scenario’.The model has been constructed into STELLA, a graphical dynamic-simulation,soft-ware program.

The model project an increase in TBE incidence in Stockholm County,a high-endemic region in Sweden,during the next50years.According to this simplified model,the annual vaccination rate need to increase by3–4-fold during the next half century in order to prevent the projected increases in TBE incidence in the region from a climatic change.©1998Elsevier Science B.V.All rights reserved.

Keywords:Climate;Diseases;Human;Encephalitis;Tick-borne;Sweden;Vaccination

1.Introduction

Since the beginning of industrialization human activities have led to an enhancement of the greenhouse gases in the atmosphere(Intergoven-mental Panel on Climate Change(IPCC I,1996). As a consequence the planet’s climatic system will be altered,although the magnitude and tim-ing will depend on the amount of future gas emissions.Based on the IPCC‘non-intervention emission scenario’(1592a)the global mean tem-perature is predicted to rise with an average of 0.3°C/decade within the next century,or0.2°C/ decade if the cooling effect of sulfate aerosols are taken into account(The Hadley Center,1995; IPCC I,1996).Regional predictions from a global climate change are difficult to make. However,the highest increases in surface air temperature,especially during the winter season, are expected to occur over the higher northern latitudes(Maskell et al.,1993;The Hadley Cen-ter,1995).

*Tel.:+468161290;fax:+468158417;e-mail:

elisa@system.ecology.su.se

0304-3800/98/$19.00©1998Elsevier Science B.V.All rights reserved. PII S0304-3800(98)00041-6

E.Lindgren/Ecological Modelling110(1998)55–63

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Fig.1.Diagram of the main relations between the influence of changes in climate on the ecology of ticks and on other factors of importance for the incidence of tick-borne diseases.

Disturbances in the climatic system would cause a whole range of consequences,including effects on human health(IPCC II,1996).The spread and geographical distribution of many in-fectious diseases that are transmitted to humans by insects or other arthropods may change.Sev-eral of these so called vector-borne diseases that are now predominant in the tropics may spread into countries and regions that at present have a climate that is not suitable for the survival and reproduction of the disease vector and the patho-gen,as illustrated in some climate-disease models (Martens et al.,1994;Matsuoka and Kai,1994; Martens,1995;Martens et al.,1995;Martin and Lefebvre,1995).

Sweden is located within both the temperate and subarctic zones,between55°N and69°N. Next to Lyme disease,tick-borne encephalitis, TBE,is the most important vector-borne disease in Sweden.Both diseases are transmitted by the same tick species(Ixodes spp.).The incidence and distribution of tick-borne diseases may change with a future climatic change.However,the weather-dependent ecological interactions of im-portance for the transmission of these diseases are complex(Fig.1).

The distribution of ticks,their development and survival,and the time between blood meals are highly temperature dependent(Dobson and Carper,1993).Tick survival and activity also depend on adequate soil moisture and relative air humidity(Lindsay et al.,1995).The population density of host and reservoir animals,such as roe deer and rodents,may in turn be affected by winter temperatures and snow conditions(Ceder-lund,1981).

The objective of this GCC-TBE-VACC model is to provide an example of a modeling tool for use in the evaluation of the relations between changes in climate,changes in TBE incidence,and subsequent changes in vaccination needs in an endemic region in Sweden.

2.Methods

The model was constructed in STELLA1,a graphical dynamic simulation software program. The structure of the model is shown in Fig.2. 1STELLA II.Copyright©1990–1994by High Performance Systems