Vector-borne diseases

European distribution of Borrelia burgdorferi in questing I. ricinus ticks

Note: The risks described in this figure are relative to each other according to a standard distribution scale. Risk is defined as the probability of finding nymphal ticks positive for Borrelia burgdorferi. For each prevalence quartile, associated climate traits were used to produce a qualitative evaluation of risk according to Office International des Epizooties (OIE) standards at five levels (high, moderate, low, negligible, and null), which directly correlate with the probability of finding nymphal ticks with prevalence in the four quartiles.

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Change in the distribution of Aedes albopictus in Europe

Note: Areas marked as ‘2011’ indicate that the tiger mosquito was detected in 2011 for the first time. They include areas of known geographical expansion of A. albopictus in France, northern Italy and Spain where vector surveillance has been in place since 2008 but also areas in Albania, Greece, and central and southern Italy, where the first detection of the vector in 2011 could be the result of increased vector surveillance rather than actual geographical expansion. ‘2008–2010’ refers to all areas where the vector has been present before 2011. Indoor presence corresponds to the presence recorded in greenhouses.

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Climatic suitability for the mosquitos Aedes aegypti and Aedes albopictus in Europe

Note: This figure shows the climatic suitability for the mosquitos Aedes aegypti (left) and Aedes albopictus (right) in Europe. Darker to lighter green indicates conditions not suitable for the vector whereas yellow to red colours indicate conditions that are increasingly suitable for the vector. Grey indicates that no prediction is possible.

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• The climatic suitability for dengue transmission in continental Europe provided by European Centre for Disease Prevention and Control (ECDC)

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Past trends

Vector-borne diseases are an emerging public health issue in Europe. Lyme borreliosis is the most common vector-borne disease in the EU, with a reported incidence of approximately 85 000 cases per year. The mean number of reported cases of tick-borne encephalitis (TBE) in Europe has been almost 2 900 per year during the period 2000–2010[i]. However, these numbers need to be considered with care due to difficulties in diagnosis and case definition. Thus, the overall burden of these tick-borne diseases in Europe remains unclear.

Mosquito-borne diseases have not been a substantial concern within Europe until recently. However, locally transmitted outbreaks of Chikungunya, Dengue and even malaria have occurred in recent years. Periodical outbreaks have been reported in Greece and possibly neighbouring countries for leishmaniasis, a disease transmitted by sandflies which naturally occur in southern Europe.

Tick-borne diseases

TBE and Lyme borreliosis are the two most important tick-borne diseases in Europe, transmitted primarily by I. ricinus. A key determinant is the abundance of ticks, which is sensitive to climatic variables, notably temperature. Climate change may shift the distribution range of I. ricinustowards higher latitudes and altitudes, as milder winter temperatures, longer vegetation seasons and earlier onsets of summer appear and warmer temperatures occur[ii]. There have already been reports on the northerly migration of the tick species in Sweden[iii], and to higher altitudes in the Czech Republic[iv]. Range shifts have also been observed in Germany and Norway[v]. 

Figure 1 shows the risk of the Lyme disease pathogen (Borrelia burgdorferi) in Europe. High riskis associated with mild winters, high summer temperatures, low seasonal amplitude of temperatures and high scores on vegetation indices [vi].

There are considerable differences between the distribution of ticks and the observed incidence of TBE[vii]. There has been a marked upsurge of TBE in recent years but it is not currently possible to assess the relative importance of climatic changes and of other factors influencing disease incidence, including vaccination coverage, tourism patterns, public awareness, distribution of rodent host populations and socio-economic conditions[viii]. There is limited evidence that two other tick-borne diseases may be sensitive to climate change. Some models have suggested that the Mediterranean basin has become suitable for an expansion of Crimean-Congo haemorrhagic fever[ix], but demographic factors and land-use change may be more important drivers. Rickettsia has also expanded in recent years, but the reasons for this are not yet well understood[x].

Mosquito-borne diseases

Mosquito habitats are influenced by temperature, humidity and precipitation levels. The Asian tiger mosquito (Aedes albopictus) is an important vector in Europe transmitting viral diseases, including Chikungunya and Dengue. Since its establishment in Italy in 1990,A. albopictus has substantially extended its range, aided by trade and travel; it is present in several EU countries and in some countries neighbouring the EU (Figure 2). Even larger parts of Europe are climatically suitable for A. albopictus (Figure 3, right).

Several disease outbreaks transmitted by the mosquito A. albopictus were recently reported in Europe: Chikungunya in Italy[xi] and in France[xii], as well as local transmissions of Dengue in France[xiii] and Croatia in 2010[xiv]. In all cases the virus has been imported to Europe by travellers. Some parts of Europe are currently climatically suitable to A. aegypti, a primary vector for Dengue (Figure 3, left).

Malaria was largely eradicated in Europe in the second half of the 20th century[xv]. However, the malaria vectors (Anopheles mosquitos) are still present in much of Europe, and a few cases of local transmission occur each year[xvi]. The risk of malaria re-establishment in a particular region depends on its receptivity, which refers to climatic and ecological factors favouring malaria transmission and to vector abundance, and on vulnerability to infection, which refers to either proximity to malarial areas or influx of infected people and/or infective mosquitoes[xvii].

Human cases of West Nile Virus (WNV) are relatively rare in Europe, and roughly 80 % of the cases are asymptomatic. The virus primarily infects birds and is transmitted to humans through mosquitoes (Culex sp.). WNV outbreaks in Europe have been associated with high temperature, rainfall and humidity[xviii]. Other factors influencing the WNV risk include the populations of migrating birds and reservoir hosts, and early detection and diagnosis.

Sandfly-borne diseases

Leishmaniasis is the most common disease transmitted by sandflies in Europe. Transmission of the two parasites responsible for this disease that are endemic in the EU (Leishmania infantum and L. tropica) is heavily influenced by temperature. L. tropica occurs sporadically in Greece and neighbouring countries, while L. infantumis endemic in the Mediterranean region of the EU. Sandfly vectors currently have wider distribution ranges than the leishmaniasis pathogens. The evidence of an impact of climate change on the distribution of sandfly in Europe is scarce[xix]. Climate change was suggested as one possible reason for the observed northward expansion of sandfly vectors in Italy[xx].

Projections

Tick-borne diseases

An expansion of the distribution range of ticks to higher altitudes and latitudes is projected under future warming[xxi] under the condition that their natural hosts (deer) would also shift their distribution. TBE is projected to shift up to higher altitudes and latitudes, potentially increasing the risk in some parts of northern and central Europe, unless targeted vaccination programmes and TBE surveillance are introduced. TBE risk is generally projected to decrease in southern Europe. Warmer winters may facilitate the expansion of Lyme borreliosis to higher latitudes and altitudes, particularly in northern Europe, but it would decrease in parts of Europe projected to experience increased droughts[xxii].

Mosquito- borne diseases

Various studies have found that warm seasonal and annual temperature and sufficient rainfall provide favourable climatic conditions for A. albopictus in Europe[xxiii]. The climatic suitability for A. albopictus is projected to increase in central and western Europe and to decrease in southern Europe[xxiv]. The risk of Chikungunya may also increase, particularly in those regions in Europe where the seasonal activity of A. albopictus matches the seasonality of endemic Chikungunya infections abroad[xxv], thereby potentially increasing the importation risk.

Climate-related increase in the A. albopictus density or active season could lead to a small increase in risk of Dengue in Europe. The risk could also increase should temperature increase facilitate the re-establishment of A. aegypti, the primary Dengue vector. Further modelling studies are required to assess whether climate change would increase or decrease the climatic suitability for A. aegypti in continental Europe. 

Some climate-related change in malariareceptivityin Europe is suggested, but probably not enough to re-establish malaria. The largest threat in Europe relates to populationvulnerability, which is influenced by sporadic introductions of the parasite through global travel and trade.

Climate change is not generally expected to strongly impact on WNV in Europe[xxvi]. However, it could influence the virus transmission through affecting the geographic distribution of vectors and pathogens, and changed migratory patterns of bird populations, as well as through changes in the life-cycle of bird-associated pathogens.

Sandfly diseases

Future climate change could impact on the distribution of leishmaniasis by affecting the abundance of vector species and parasite development. Recent modelling indicates that the central European climate will become increasingly suitable for Phelobotomus spp.sandflies, thereby increasing the risk of leishmaniasis, but such an expansion would be somewhat constrained by the limited migration ability of sandflies[xxvii]. The risk of disease transmission may decrease in some areas in southern Europe where climate conditions become too hot and dry for vector survival.

[i] ECDC,Annual epidemiological report 2011 - Reporting on 2009 surveillance data and 2010 epidemic intelligence data Surveillance report (Stockholm: European Centre for Disease Prevention and Control, 2011), http://www.ecdc.europa.eu/en/publications/Publications/Forms/ECDC_DispForm.aspx?ID=767.

[ii] Thomas G.T. Jaenson and E. Lindgren, „The range ofIxodes ricinus and the risk of contracting Lyme borreliosis will increase northwards when the vegetation period becomes longer“,Ticks and Tick-borne Diseases 2, Nr. 1 (März 2011): 44–49, doi:10.1016/j.ttbdis.2010.10.006.

[iii] E. Lindgren, L. Talleklint, and T. Polfeldt, „Impact of climatic change on the Northern latitude limit and population density of the disease-transmitting European tickIxodes ricinus“,Environmental Health Perspectives 108, Nr. 2 (2000): 119–123.

[iv] M. Daniel et al., „Shift of the tickIxodes ricinus and tick-borne encephalitis to higher altitudes in central Europe“,European Journal of Clinical Microbiology and Infectious Diseases 22, Nr. 5 (2003): 327–328, doi:10.1007/s10096-003-0918-2.

[v] J. C Semenza and B. Menne, „Climate change and infectious diseases in Europe“,The Lancet Infectious Diseases 9, Nr. 6 (Juni 2009): 365–375, doi:10.1016/S1473-3099(09)70104-5.

[vi] A. Estrada-Pena et al., „Correlation ofBorrelia burgdorferi sensu lato prevalence in questingIxodes ricinus ticks with specific abiotic traits in the Western Palearctic“,Applied and Environmental Microbiology 77, Nr. 11 (April 15, 2011): 3838–3845, doi:10.1128/AEM.00067-11.

[vii] Jochen Süss et al., „TBE incidence versus virus prevalence and increased prevalence of the TBE virus inIxodes ricinus removed from humans“,International Journal of Medical Microbiology 296 (Mai 2006): 63–68, doi:10.1016/j.ijmm.2005.12.005.

[viii] Sarah E. Randolph, „Tick-borne encephalitis incidence in Central and Eastern Europe: consequences of political transition“,Microbes and Infection 10, Nr. 3 (März 2008): 209–216, doi:10.1016/j.micinf.2007.12.005.

[ix] Helena C. Maltezou and Anna Papa, „Crimean–Congo hemorrhagic fever: Risk for emergence of new endemic foci in Europe?“,Travel Medicine and Infectious Disease 8, Nr. 3 (Mai 2010): 139–143, doi:10.1016/j.tmaid.2010.04.008.

[x] Frédérique Gouriet, Jean-Marc Rolain, and Didier Raoult, „Rickettsia slovaca Infection, France“,Emerging Infectious Diseases 12, Nr. 3 (März 2006): 521–523, doi:10.3201/eid1203.050911.

[xi] G Rezza et al., „Infection with chikungunya virus in Italy: An outbreak in a temperate region“,The Lancet 370, Nr. 9602 (Dezember 2007): 1840–1846, doi:10.1016/S0140-6736(07)61779-6.

[xii] Marc Grandadam, „Chikungunya virus, southeastern France“,Emerging Infectious Diseases (Mai 2011), doi:10.3201/eid1705.101873.

[xiii] G. La Ruche et al., „First two autochthonous dengue virus infections in metropolitan France, September 2010“,Eurosurveillance 15, Nr. 39 (September 30, 2010): pii=19676.

[xiv] I. Gjenero-Margan et al., „Autochthonous dengue fever in Croatia, August–September 2010“,Eurosurveillance 16, Nr. 9 (März 3, 2011): pii=19805.

[xv] Semenza and Menne, „Climate change and infectious diseases in Europe“.

[xvi] S. A. Florescu et al., „Plasmodium vivax malaria in a Romanian traveller returning from Greece, August 2011“,Eurosurveillance 16, Nr. 35 (September 1, 2011): pii=19954.

[xvii] WHO,Guidelines on prevention of the reintroduction of malaria (Cairo, Egypt: World Health Organization, Regional Office for the Eastern Mediterranean, 2007), http://applications.emro.who.int/dsaf/dsa743.pdf.

[xviii] Shlomit Paz and Iris Albersheim, „Influence of Warming Tendency onCulex pipiens Population Abundance and on the Probability of West Nile Fever Outbreaks (Israeli Case Study: 2001–2005)“,EcoHealth 5, Nr. 1 (Februar 12, 2008): 40–48, doi:10.1007/s10393-007-0150-0; Semenza and Menne, „Climate change and infectious diseases in Europe“; Shlomit Paz, „West Nile Virus Eruptions in Summer 2010 – What Is the Possible Linkage with Climate Change?“, inNational Security and Human Health Implications of Climate Change, ed. H. J. S. Fernando, Z. Klaić, and J.L. McCulley (Dordrecht: Springer Netherlands, 2012), 253–260, http://www.springerlink.com/index/10.1007/978-94-007-2430-3_21.

[xix] Paul D. Ready, „Leishmaniasis emergence in Europe“,Eurosurveillance 15, Nr. 10 (März 11, 2010): pii=19505.

[xx] Michele Maroli et al., „The northward spread of leishmaniasis in Italy: evidence from retrospective and ongoing studies on the canine reservoir and phlebotomine vectors“,Tropical Medicine & International Health 13, Nr. 2 (Februar 26, 2008): 256–264, doi:10.1111/j.1365-3156.2007.01998.x.

[xxi] Jaenson and Lindgren, „The range ofIxodes ricinus and the risk of contracting Lyme borreliosis will increase northwards when the vegetation period becomes longer“.

[xxii] Semenza and Menne, „Climate change and infectious diseases in Europe“.

[xxiii] Jolyon M Medlock et al., „Analysis of the potential for survival and seasonal activity ofAedes albopictus (Diptera: Culicidae) in the United Kingdom“,Journal of Vector Ecology 31, Nr. 2 (Dezember 2006): 292–304; David Roiz et al., „Climatic factors driving invasion of the tiger mosquito (Aedes albopictus) into new areas of Trentino, northern Italy“,Public Library of Science ONE 6, Nr. 4 (April 15, 2011): e14800, doi:10.1371/journal.pone.0014800.

[xxiv] Dominik Fischer et al., „Projection of climatic suitability forAedes albopictus Skuse (Culicidae) in Europe under climate change conditions“,Global and Planetary Change 78, Nr. 1–2 (Juli 2011): 54–64, doi:10.1016/j.gloplacha.2011.05.008.

[xxv] Rémi N Charrel, Xavier de Lamballerie, and Didier Raoult, „Seasonality of mosquitoes and chikungunya in Italy“,The Lancet Infectious Diseases 8, Nr. 1 (Januar 2008): 5–6, doi:10.1016/S1473-3099(07)70296-7.

[xxvi] P. Gale et al., „Assessing the impact of climate change on vector-borne viruses in the EU through the elicitation of expert opinion“,Epidemiology and Infection 138, Nr. 02 (Juli 7, 2009): 214, doi:10.1017/S0950268809990367; E.A. Gould and S. Higgs, „Impact of climate change and other factors on emerging arbovirus diseases“,Transactions of the Royal Society of Tropical Medicine and Hygiene 103, Nr. 2 (Februar 2009): 109–121, doi:10.1016/j.trstmh.2008.07.025.

[xxvii] Dominik Fischer et al., „Combining climatic projections and dispersal ability: A method for estimating the responses of sandfly vector species to climate change“,Public Library of Science, Neglected Tropical Diseases 5, Nr. 11 (November 29, 2011): e1407, doi:10.1371/journal.pntd.0001407.