- Vector-borne diseases (who)
- Vector-borne diseases account for more than 17% of all infectious diseases, causing more than 1 million deaths annually.
- More than 2.5 billion people in over 100 countries are at risk of contracting dengue alone.
- Malaria causes more than 400 000 deaths every year globally, most of them children under 5 years of age.
- Other diseases such as Chagas disease, leishmaniasis and schistosomiasis affect hundreds of millions of people worldwide.
- Many of these diseases are preventable through informed protective measures.
Main vectors and diseases they transmit
Vectors are living organisms that can transmit infectious diseases between humans or from animals to humans. Many of these vectors are bloodsucking insects, which ingest disease-producing microorganisms during a blood meal from an infected host (human or animal) and later inject it into a new host during their subsequent blood meal.
Mosquitoes are the best known disease vector. Others include ticks, flies, sandflies, fleas, triatomine bugs and some freshwater aquatic snails.
- Dengue fever
- Rift Valley fever
- Yellow fever
- Japanese encephalitis
- Lymphatic filariasis
- West Nile fever
- Sandfly fever (phelebotomus fever)
- Crimean-Congo haemorrhagic fever
- Lyme disease
- Relapsing fever (borreliosis)
- Rickettsial diseases (spotted fever and Q fever)
- Tick-borne encephalitis
- Chagas disease (American trypanosomiasis)
- Sleeping sickness (African trypanosomiasis)
- Plague (transmitted by fleas from rats to humans)
- Onchocerciasis (river blindness)
- Schistosomiasis (bilharziasis)
Vector-borne diseases are illnesses caused by pathogens and parasites in human populations. Every year there are more than 1 billion cases and over 1 million deaths from vector-borne diseases such as malaria, dengue, schistosomiasis, human African trypanosomiasis, leishmaniasis, Chagas disease, yellow fever, Japanese encephalitis and onchocerciasis, globally.
Vector-borne diseases account for over 17% of all infectious diseases.
Distribution of these diseases is determined by a complex dynamic of environmental and social factors.
Globalization of travel and trade, unplanned urbanization and environmental challenges such as climate change are having a significant impact on disease transmission in recent years. Some diseases, such as dengue, chikungunya and West Nile virus, are emerging in countries where they were previously unknown.
Changes in agricultural practices due to variation in temperature and rainfall can affect the transmission of vector-borne diseases. Climate information can be used to monitor and predict distribution and longer-term trends in malaria and other climate-sensitive diseases.
WHO responds to vector-borne diseases by:
- providing the best evidence for controlling vectors and protecting people against infection;
- providing technical support and guidance to countries so that they can effectively manage cases and outbreaks;
- supporting countries to improve their reporting systems and capture the true burden of the disease;
- providing training on clinical management, diagnosis and vector control with some of its collaborating centres throughout the world; and
- developing new tools to combat the vectors and deal with the disease, for example insecticide products and spraying technologies.
A crucial element in vector-borne diseases is behavioural change. WHO works with partners to provide education and improve awareness so that people know how to protect themselves and their communities from mosquitoes, ticks, bugs, flies and other vectors.
For many diseases such as Chagas disease, malaria, schistosomiasis and leishmaniasis, WHO has initiated control programmes using donated or subsidized medicines.
Access to water and sanitation is a very important factor in disease control and elimination. WHO works together with many different government sectors to control these diseases.
Insect vectors are responsible for some of the most devastating diseases in developing countries.
Mosquitoes, blackflies, sandflies, ticks, and lice are effective vectors of disease, transmitting pathogens via their blood meals. These Neglected Tropical Diseases (NTDs), are mostly diseases of poverty, and responsible for major economic burdens through disability, death of principal earners and missed educational opportunities for children and young adults, helping to maintain the poverty trap. It is no coincidence that the countries most affected by these diseases are also amongst the poorest countries in the world.
Malaria is the most serious and costly of the insect-borne diseases with over 200 million cases of people being sick, and currently causing about 660,00 deaths per year, mostly children under the age of 5. The tragedy is, that all these deaths could be stopped with a determined and co-ordinated approach .
Vector- borne diseases, most of which are transmitted in and around the home, are best controlled by a combination of vector control (use of public health insecticides on bednets, or by spraying), medicines and vaccines.
Historically, successful vector-borne disease prevention resulted from management or elimination of vector populations. Malaria was driven out of the USA and most of Europe in this way.
Where vector control has been consistently applied in the past, the results have been dramatic, especially with early efforts to control malaria by spraying the inside surfaces of houses with insecticides. Indoor Residual Spraying (IRS) and long-lasting insecticide treated bednets have been very effective over the last 10 years and are widely regarded as one of the main contributors to the more than 1 million lives saved.
In contrast to expenditure and effort on medicine, diagnostic and vaccine development, relatively little attention was given to vector control in the past. The foresighted establishment of IVCC in 2005, with a grant from the Bill and Melinda Gates Foundation began the process of bringing Vector Control into the mainstream strategy for future eradication of malaria and other vector-borne diseases.
Although WHO emphasizes that new strategies for prevention and control of vector-borne diseases should be through “integrated vector management”, most technologies are at least 25 years old.
Vector-borne diseases are infections transmitted by the bite of infected arthropod species, such as mosquitoes, ticks, triatomine bugs, sandflies, and blackflies1. Arthropod vectors are cold-blooded (ectothermic) and thus especially sensitive to climatic factors. Weather influences survival and reproduction rates of vectors2, in turn influencing habitat suitability, distribution and abundance; intensity and temporal pattern of vector activity (particularly biting rates) throughout the year; and rates of development, survival and reproduction of pathogens within vectors. However, climate is only one of many factors influencing vector distribution, such as habitat destruction, land use, pesticide application, and host density. Vector-borne diseases are widespread in Europe and are the best studied diseases associated with climate change, which is reflected in this Review.
West Nile fever is caused by the West Nile virus, a virus of the family Flaviviridae which is part of the Japanese encephalitis antigenic group. West Nile fever mainly infects birds and infrequently human beings through the bite of an infected Culex mosquito.
In numerous European countries the virus has been isolated in mosquitoes, wild rodents, migrating birds, hard ticks, horses and human beings3. Since roughly 80% of cases are asymptomatic, the rate of West Nile virus infections in human beings remains largely unknown, and probably only some of the epidemics with tens or hundreds of West Nile fever cases have been documented4. Past entomologic data have been linked to meteorological data in order to model a West Nile fever outbreak in Southern France in 2000; the aggressiveness of the vector (Culex modestus) was positively correlated with temperature and humidity, and linked to rainfall and sunshine, which were particularly high during the epidemic period5.
An outbreak in 1996-97 in southeastern Romania6 resembled a subsequent outbreak in Israel in 2000, which was associated with a heat wave early in the summer with high minimum temperatures7. These observations are in agreement with a climatic model for West Nile virus with mild winters, dry spring and summers, heat waves early in the season and wet autumns8. Dry spells favour reproduction of city-dwelling mosquitoes (e.g. Culex pipiens) and concentrate vectors with their avian hosts around water sources, which leads to arbovirus multiplication9. Explanatory models have assisted public-health practitioners in making decision about the spraying of preventative of preventive larvicides10.
Dengue is the most important arboviral human disease, however, mainly due to nearly universal use of piped water the disease has disappeared from Europe11. Dengue is frequently introduced into Europe by travelers returning from dengue-endemic countries but no local transmission has been reported since it would also depend on the reintroduction of its principal vector, the mosquito Aede. aegypti (also the yellow fever mosquito) which is adapted to urban environments. However, over the last 15 years another competent vector Ades. albopictus (Asian tiger mosquito) has been introduced into Europe and expanded into several countries, raising the possibility of dengue transmission12.
Epidemiological studies have shown that temperature is a factor in dengue transmission in urban areas13. Climate change projections on the basis of humidity for 2085 suggests dengue transmission to shift the latitudinal and altitudinal range14. In temperate locations, climate change could further increase the length of the transmission season15. An increase in mean temperature could result in seasonal dengue transmission in southern Europe if A aegypti infected with the virus were to be established.
Chikungunya fever is caused by a virus of the genus Alphavirus, in the family Togaviridae, which is transmitted to human beings by the bite of infected mosquitoes such as A aegypti, and A albopictus.
A confirmed outbreak of chikungunya fever was reported in August 2007 in north-eastern Italy, the first chikungunya outbreak on the European continent16 17 18. Vector surveillance in the vicinity of the cases identified large numbers of A albopictus mosquitoes in traps, but no sandflies or other vectors. While introductions of A albopictus and chikungunya virus into Italy were accidental events, a climatic model with five scenarios has been developed for possible further establishment of A albopictus in Europe with main variables such as mild winters, mean annual rainfall exceeding 50 cm and mean summer temperatures exceeding 20°C19. Vector population density, an important determinant of the epidemic potential, is also linked to duration of the seasonal activity; therefore, the weeks between spring egg hatching and autumn egg diapause are also factored in. This model defines the potential for further transmission and dispersion of the vector under favourable climatic conditions in temperate countries and outlines the geographic areas potentially at risk of future outbreaks.
Malaria is caused by one of four species of the Plasmodium parasite transmitted by female Anopheles spp mosquitoes. Historically malaria was endemic in Europe, including Scandinavia, but it was eventually eliminated in 1975 through a number of factors related to socioeconomic development20 21. Any role that climate played in malaria reduction would have been small. Nevertheless, the potential for malaria transmission is intricately connected to meteorological conditions such as temperature and precipitation 22. For example, conditions for transmission in Europe have remained favorable as documented by sporadic autochthonous transmission of a tropical malaria strain by local vectors to a susceptible person23 24.
The potential for malaria and other “tropical” diseases to invade southern Europe is commonly cited as an example of the territorial expansion of risk due to climate change (socioeconomic, building codes, land use, treatment, capacity of health-care system, etc). Projections of malaria under future climate change scenarios are limited in Europe. An assessment in Portugal projected an increase in the number of days per year suitable for malaria transmission; however, transmission would depend on infected vectors to be present 25. For the UK, an increase in risk of local malaria transmission based on change in temperature projected to occur by 2050 was estimated to be 8 to 14%, but malaria re-establishment is highly unlikely26. Thus, while climatic factors may favor autochthonous transmission, increased vector density, and accelerated parasite development, other factors (socioeconomic, building codes, land use, treatment, etc) limit the likelihood of climate-related re-emergence of malaria in Europe27.
Leishmaniasis is a protozoan parasitic infection caused by Leishmania infantum that is transmitted to human beings through the bite of an infected female sandfly. Temperature influences the biting activity rates of the vector, diapause, and maturation of the protozoan parasite in the vector28 29. Sandfly distribution in Europe is south of latitude 45oN and less than 800 m above sea level, although it has recently expanded as high as 49oN30 31. Historically, sand-fly vectors from the Mediterranean have dispersed northwards in the postglacial period based on morphological samples from France and northeast Spain and sandflies have been reported today also from northern Germany32. The biting activity of European sandflies is strongly seasonal, and in most areas is restricted to summer months. Currently, sandfly vectors have a substantially wider range than that of L infantum, and imported cases of infected dogs are common in central and northern Europe. Once conditions make transmission suitable in northern latitudes, these imported cases could act as plentiful source of infections, permitting the development of new endemic foci. Conversely, if climatic conditions become too hot and dry for vector survival, the disease may disappear in southern latitudes. Thus, complex climatic and environmental changes (such as land use) will continue to shift the dispersal of leishmaniasis in Europe33 34.
Tick-borne encephalitis (TBE) is caused by an arbovirus of the family Flaviviridae and is transmitted by ticks (predominantly Ixodes ricinus) that act both as vectors and as reservoirs35. Similar to other vector-borne diseases, temperature accelerates the ticks’ developmental cycle, egg production, population density, and distribution. It is likely that climate change has already led to changes in the distribution of I ricinus populations in Europe36. I ricinus has expanded into higher altitudes in the Czech Republic over the last two decades, which has been related to increases in average temperatures37 38 39.
This vector expansion is accompanied by infections with TBE virus40 41. In Sweden, since the late 1950s all cases of encephalitis admitted in Stockholm County have been serologically tested for TBE45. An analysis of the period 1960–98 showed that the increase in TBE incidence since the mid-1980s is related to milder and shorter winters, resulting in longer tick-activity seasons. In Sweden, the distribution-limit shifted to higher latitude ; the distribution has also shifted in Norway and Germany43 44.
Climate models with warmer and drier summers project that TBE will be driven into higher altitude and latitude, although certain other parts of Europe will be cleared of TBE45. However, these climatic changes alone are unlikely to explain the surge in TBE incidence over the last three decades, and it is endemic in 27 European countries today54 . There is considerable spatial heterogeneity in the increased incidence of TBE in Europe, despite observed uniform patterns of climate change46. Potential causal pathways include changing land use patterns; increased density of large hosts for adult ticks (e.g. deer); habitat expansion of rodent hosts; alterations in recreational and occupational human activity (habitat encroachment); public awareness, vaccination coverage, and tourism47 48. These hypotheses can be tested epidemiologically and tackled through public-health action.
Lyme Borreliosis is caused by infection with the bacterial spirochete Borrelia burgdorferi which is transmitted to human beings during the blood feeding of hard ticks of the genus Ixodes. In Europe, the primary vector is I ricinus, also known as deer tick, as well as I persulcatus from Estonia to far eastern Russia. In Europe, Lyme borreliosis is the most common tick-borne disease with at least 85 000 cases yearly, and has an increasing incidence in several European countries such as Finland, Germany, Russia, Scotland, Slovenia and Sweden. Although detection bias could explain part of this trend, a prospective, population-based survey of cases in southern Sweden has serologically confirmed such an increase49 50.
A shift toward milder winter temperatures due to climate change may enable expansion of Lyme borreliosis into higher latitudes and altitudes, but only if all of the vertebrate host species required by tick vectors are equally able to shift their population distribution. In contrast, droughts and severe floods will negatively affect the distribution, at least temporarily. Northern Europe is predicted to experience higher temperature with increased precipitation while Southern Europe will become drier, which will impact tick distribution, alter their seasonal activity and, shift exposure patterns 51.
Crimean-Congo hemorrhagic fever (CCHF) is caused by an RNA virus of the Bunyaviridae family and transmitted by Hyalomma spp ticks from domestic and wild animals. The virus is the most widespread tick-borne arbovirus and is found in the Eastern Mediterranean where there have been a series of outbreaks in Bulgaria in 2002 and 2003, in Albania and in Kosovo in 200152 53 54. Milder weather conditions, favouring tick reproduction may influence CCHF distribution55. For example, an outbreak in Turkey was linked to a milder spring season (a substantial number of days in April with a mean temperature higher than 5°C) in the year before the outbreak. However, other factors such as land use and demographic changes have also been implicated. There have been new records of spotted fever group rickettsioses with new pathogens such as Rickettsia slovaca, R. Helvetica, Rickettsia aeschlimannii and flea-borne rickettsioses (Rickettsia typhi, Rickettsia felis)57 58. However, this emergence is most likely detection bias due to advancements in diagnostic techniques. Since ticks, flees, and lice serve as vectors as well as reservoirs they might contribute to disease amplification under favorable climate change conditions. There has been a geographic expansion of rickettsial diseases throughout Europe 59, and while underlying reasons for this expansion are still unclear, it is possible that wild bird migration could play a part 60.
Human Granulocytic Anaplasmosis is caused by Anaplasma phagocytophilum, a bacterium usually transmitted to humanbeings by I ricinus. In Europe, this disease was known to cause fever in goats, sheep, and cattle until it emerged as a disease in human beings in 1996 61. It has now shifted to new geographical habitats throughout Europe, and migrating birds have been implicated in its expansion 62. Spatial models have been developed to project the geographical distribution under climate change scenarios for North America but not for Europe 63 64.
Based on the vector-borne disease articles reviewed, here it is clear that climate is an important geographic determinant of vectors, but the data do not conclusively demonstrate that recent climatic changes have resulted in increased disease vector-borne disease incidence on a pan-European level. However, the reports indicate that under climate change scenarios of the last decades ticks have progressively spread into higher latitudes in Sweden and higher elevation in the Czech Republic; they have become more prevalent in many other places and intensified the transmission season. Conversely, the risk for Lyme borreliosis is projected to be reduced in drought and flood-ridden locations. The articles reviewed here do not support the notion that climate change has altered the distribution of sandflies and visceral leishmaniasis but since sandfly vectors expand further than L infantum this hypothesis cannot be discounted. The risk of reintroduction of malaria into certain European countries is very low and determined by other variables rather than climate change. Introduction of dengue, West Nile fever, and chikungunya into new regions in Europe is a more immediate consequence of virus importation into competent vector habitats; climate change is one of many factors that influence vector habitat.
The lack of published articles for other vector-borne diseases makes an assessment difficult; for example, tick-borne relapsing fever caused by spirochaetes of the genus Borrelia could spread from its current endemic area in Spain since its tick vector is sensitive to climatic changes but no climate models have been developed for this disease 65. In the case of yellow fever the existence of an effective vaccine makes the establishment in Europe very unlikely; conversely, an existing human vaccine for Rift Valley fever is not available (veterinary vaccines are used in Africa). These multifactorial events call for a case by case assessment and targeted interventions.
Source: Semenza JC, Menne B. Climate Change and Infectious Diseases in Europe. Lancet ID. 2009;9:365-75.
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– See more at: http://ecdc.europa.eu/en/healthtopics/climate_change/health_effects/Pages/vector_borne_diseases.aspx#sthash.Wch4Oln0.dpuf