Vector-borne diseases

Presence of Aedes albopictus (the tiger mosquito) in Europe in January 2008

Note: Developed by Francis Schaffner (BioSys Consultancy, Zurich), in partnership with Guy Hendrickx/Ernst-Jan Scholte (AviaGIS, Zoersel, Belgium) and Jolyon M Medlock (Health Protection Agency, UK) for the ECDC TigerMaps project

Data source:

Schaffner, F.; Hendrickx, G.; Scholte, E.J.; Medlock, J.; Angelini, P.; Ducheyne, E., 2008. Development of Aedes albopictus risk maps. TigerMaps project report. Stockholm: European Centre for Disease Prevention and Control. http://ecdc.europa.eu/.

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Areas of possible establishment of Aedes albopictus (the tiger mosquito) in Europe for 2010 and 2030

Note: Developed by Francis Schaffner (BioSys Consultancy, Zurich), in partnership with Guy Hendrickx/Ernst-Jan Scholte (AviaGIS, Zoersel, Belgium) and Jolyon M Medlock (Health Protection Agency, United Kingdom) for the ECDC TigerMaps project

Data source:

Schaffner, F.; Hendrickx, G.; Scholte, E.J.; Medlock, J.; Angelini, P.; Ducheyne, E., 2008. Development of Aedes albopictus risk maps. TigerMaps project report. Stockholm: European Centre for Disease Prevention and Control. http://ecdc.europa.eu/.

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Past trends

Mosquito-borne
Higher temperatures can contribute to higher virus replication rates in mosquitoes, increased mosquito populations, expansion of the mosquito distribution, and easier establishment and replication of vectors.
Chikungunya: Aedes albopictus (the tiger mosquito) has extended its range in Europe substantially over the past 15 years (Scholte and Schaeffner, 2007) and is now present in 12 European countries (see Figure 1).
This mosquito can transmit a variety of diseases. The risk of local transmission of mosquito-borne viruses is the result of the simultaneous presence of the virus, competent mosquitoes, susceptible human hosts, and contacts between these three entities. In 2007, a cluster of cases of Chikungunya (a virus that is highly infective and disabling but not transmissible between people) was observed in the Emilia-Romagna Region of Italy. This is the first example in continental Europe of an imported human disease case being followed by sustained local mosquito transmission (ECDC, WHO, 2007; Menne et al., 2008).
Dengue: Aedes aegypti, one of the many vectors that transmit dengue, closely follows the 10 ^oC winter isotherm and is extending its range. Currently, Ae. aegypti is absent in Europe, but was well-established until after World War II. Dengue is only one of a variety of diseases transmitted by Ae. aegypti. Today, dengue is frequently introduced into Europe by travelers returning from Dengue-endemic countries. No locally-transmitted dengue cases have been reported; one can thus assume that the risk of locally-transmitted dengue is currently low, and any increase would depend on the re-introduction of Ae. aegypti into Europe. In addition, local transmission could occur if the dengue virus were introduced into the Ae. Albopictus population (Semenza and Menne, 2008).
Malaria: Anopheles mosquitoes, the malaria vectors, are and have long been present in all European countries. In recent decades, conditions for the transmission of malaria in Europe have remained favorable, as documented by repeated rare autochthonous transmission of a tropical malaria strain by local vectors to a susceptible person. Currently, autochthonous malaria continues to pose a challenge in Turkey. However, the risk of local transmission depends on the simultaneous presence in a given area of anthropophilic, high-longevity and genetically-competent vectors, and human parasite carriers (Menne et al., 2008; Ebi and Menne, 2006).
West Nile Virus (WNV): is primarily transmitted through bird-feeding mosquitoes (particularly Culex spp.). Climate change has been implicated in changes in the migratory and reproductive phenology (advances in breeding and migration dates) of several bird species, their abundance and population dynamics, as well as a northward expansion of their geographical range in Europe. There are two potential consequences: a) shifts in the geographic distribution of the vectors and pathogens due to altered distributions or changed migratory patterns of bird populations; b) changes in the life cycles of bird-associated pathogens due to a mistiming between bird breeding and the breeding of vectors, such as mosquitoes. Higher transmissions of WNV have been observed along major bird flyways. However human cases of WNV are rare in Europe and occur mainly in wetland and urban areas (Hubalek et al., 2006).

Tick-borne
Climate change can increase tick survival and thus tick density, prolong the season of tick activity, prolong host activities, and shift ticks toward higher altitudes and northern latitude. Under climate change, a shift towards milder winter temperatures may enable expansion of the range of Lyme disease and tick-borne encephalitis into higher latitudes and altitudes. In contrast, droughts and severe floods will negatively affect the distribution, at least temporarily. There is some observational evidence of northern or altitudinal shifts in tick distribution from Sweden and the Czech Republic. However, climate change alone is unlikely to explain recent increases in the incidence of tick-borne diseases in Europe, as there is considerable spatial heterogeneity in the degree of increase of tick-borne encephalitis (Daniel et al., 2006).

Sandfly-borne
While there is no current compelling evidence that sandfly and visceral leishmaniasis distributions in Europe have altered in response to recent climate change, cCASHh analysis points to a considerable potential for climate-driven changes in leishmaniasis distribution. Sandfly vectors already have a wider range than the pathogen (L. infantum), and imported dogs infected with it are common in central and northern Europe. Once conditions make transmission possible in northern latitudes, the imported dog cases could act as a source of new endemic foci. Climate-induced changes in sandfly abundance may thus increase the risk of emergence of new diseases in the region (Lindgren and Naucke, 2006).

Projections

Projections of climate-change-related vector-borne diseases use different approaches to classify the risk of climate-sensitive health determinants and outcomes. For malaria and dengue, results from projections are commonly presented as maps of potential shifts in distribution (see Figure 2). Health-impact models are based typically on climatic constraints on the development of the vector and/or parasite, and include limited population projections and non-climate assumptions. Models with incomplete parameterisation of biological relationships between temperature, vector and parasite often over-emphasise relative changes in risk, even when the absolute risk is small. Several modelling studies used the IPCC SRES climate scenarios, a few applied population scenarios, and none incorporated economic scenarios. Few studies incorporate adequate assumptions about adaptive capacity. The main approaches used are inclusion of current 'control capacity' in the observed climate-health function and categorization of the model output by adaptive capacity, thereby separating the effects of climate change from those of improvements in public health (Confalonieri et al., 2007).
The range of Aedes albopictus is projected to be further extended. Schaffner et al., 2008 estimated areas of further A. albopictus extension for 2010 and 2030 (see Figure 2). However, whether or not there will be outbreaks of Chikungunya in the next years will depend very much on the global circulation of the virus and global travel.
An empirical model estimated that, in the 2080s, 5-6 billion people would be at risk of dengue as a result of climate change and population increase, compared with 3.5 billion people if the climate remains unchanged (Hales, 2002). This projection includes an extension of the risk of dengue for Mediterranean countries.
Several climate-change-related models project an increase in malaria risk: For example, in the United Kingdom it was estimated that, with temperature increases, the risk of local malaria transmission could increase by 8-15 % by 2050. In Portugal, the number of days suitable for survival of malaria vectors is projected to increase. Nevertheless, there is agreement that the risk of transmission of malaria related to localised climate change is very small. Risks are greater in countries where importation of malaria coincides with socioeconomic degradation, disintegration of health and social services, uncontrolled cross-border migration and lack of environmental management for mosquito control.