Dynamic Models of Infectious Diseases: Volume 1: Vector-Borne Diseases
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Figures and Tables. References Publications referenced by this paper. Cost-effectiveness projections of single and combination therapies for visceral leishmaniasis in Bihar, India. Visceral leishmaniasis: what are the needs for diagnosis, treatment and control? A review of the clinical and epidemiologic burdens of epidemic malaria. Anthony E. Kiszewski , Awash Teklehaimanot. Clinical and epidemiological aspects of Chagas disease. Predicting St. Factors such as social economics, health-seeking behaviour, geographical location, and population growth will determine the vulnerability of populations to climate change.
This may suggest a low resource allocation for health at the institutional and individual levels. Moreover, chloroquine, the mainstay of malaria treatment for many decades, has proved to be ineffective in many parts of the world, particularly against falciparum malaria. These phenomena can have serious consequences on malaria transmission in the African highlands. At equatorial latitudes, e. At lower latitudes in southern Africa, more intense transmission is likely to be felt above m.
Extreme weather events that cause flooding will intensify the transmission of desert malaria and Rift Valley fever Since , there have been numerous reports of malaria epidemics in east and southern Africa. For example, malaria epidemics have spread from 3 to 13 districts in western Kenya and, in some areas, outbreaks have become annual events Githeko, unpublished data, Further climate-linked malaria epidemics have been reported in Rwanda 28 and the United Republic of Tanzania Theoretically, further warming should affect areas above m in east Africa.
Current episodes of climate variability in Africa are likely to intensify the transmission of malaria in the eastern and southern highlands but its effects on transmission of other less climate-sensitive vector-borne diseases is not yet clear. While climate is an important co-factor in malaria epidemiology, drug resistance, reduced purchasing power and poor health infrastructures may be more important, as these are the tools and resources that reduce the impact of disease.
Moreover, while climate is likely to primarily affect the highlands, drug resistance affects the entire malaria landscape. Europe has warmed by a mean of 0. These changes have not been uniform, with the greatest warming has occurred in winter and in the north. If this trend continues, it is likely to reduce the high over-wintering mortality of vectors and new areas will become suitable for transmission.
Changes in rainfall patterns are less predictable, although it is likely to become wetter in winter and drier in summer. While it is likely to become wetter in the north, it will get drier in the south and east of the continent 2.
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The consequences of these changes are difficult to predict. For example, in areas where rainfall declines and wetlands dry out, there may be fewer potential mosquito-breeding sites. However, such a reduction in mosquito production may be partly offset if mosquitoes find alternative breeding sites, such as pools created in a drying stream-bed or in water butts used by gardeners to conserve rainwater.
The most important vector-borne diseases in Europe and some of the countries of the former Soviet Union are malaria and Lyme disease, which are transmitted by mosquitoes and ticks, respectively. The evidence that climate change has increased the risk of these diseases is weak, because of the relatively subtle changes in climate to date and the overriding impact of major environmental changes created by expanding populations, alterations in agricultural practice and changing socioeconomic conditions.
However, there is no room for complacency, as the capacity exists for an increase and expansion of many vector-borne diseases in many parts of the continent.
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Malaria was once common in many parts of Europe 30, 31 and occurred almost as far north as the Arctic circle 32 , although it was most common on the northern fringes of the Mediterranean shore and in eastern continental Europe. Recurrent outbreaks have occurred in eastern Europe, in Armenia, Azerbaijan, Tajikistan and Turkey However, none of these outbreaks was associated with climate change but rather with deteriorating socioeconomic conditions, hydro-agricultural development schemes, movement of infected cases and the cessation of malaria-control activities.
Local transmission of malaria in western Europe is possible, but is likely to be restricted to a few individuals and be sporadic in nature. In Italy, where malaria was eradicated 40 years ago, local transmission of vivax malaria has recently occurred 34, The climate in western Europe is more suitable for transmission of vivax malaria, a more benign parasite, than the frequently lethal, falciparum malaria, mainly because it can develop more quickly at lower temperatures The dynamics of transmission are further complicated because vectors may transmit only specific strains of a parasite.
Climate change may contribute to the expansion of the disease to northern latitudes However, of far greater importance in the newly independent states in eastern Europe is the increasing poverty, the mass movement of refugees and displaced people, and the impoverished health systems, which all contribute to an increase in malaria. Occasional outbreaks of malaria in Europe arise when infective mosquitoes are imported from the tropics by aircraft.
Since , there have been 60 such cases reported from a number of European countries A far greater problem is the growing number of patients who contract malaria overseas. In the United Kingdom there are around cases each year D. Warhurst, personal communication. This is particularly worrying because of the rapid spread of multidrug-resistant strains of the parasite; the occurrence of cases of untreatable malaria remains a distinct possibility. A major vector of dengue fever, Aedes albopictus , has spread to 22 northern provinces in Italy since being introduced eight years ago Arboviruses transmitted by mosquitoes can cause significant morbidity and mortality in Europe West Nile virus caused outbreaks in France in the s and in Romania in There have also been outbreaks of Sindbis virus disease in northern Europe over the past two decades, and numerous other viral infections have been reported.
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Predicting when and where such outbreaks will occur is extremely difficult, but it is possible to define suitable areas if the climate envelope in which the vector resides can be identified and mapped Tick distributions are also closely linked with climate, and there is growing concern that tick-borne diseases, such as Lyme disease and tick-borne encephalitis, may be increasing in northern Europe Although adult female ticks are most often infected, it is the more abundant nymphs that are the most important source of infection.
Tick larvae and nymphs feed on small vertebrates, such as mice and birds, while adults feed on larger hosts, such as deer and cattle While milder winters will reduce tick and host mortality and extend the period when ticks are active, drier summers will increase tick mortality. There is recent evidence that the northwards movement of the tick Ixodes ricinus in Sweden was related to the milder climate experienced in the s However, a note of caution is necessary since this change could also be related to the greater abundance of hosts, such as roe deer.
Leishmaniasis is endemic in many parts of southern Europe and is an important co-infection with human immunodeficiency virus HIV. Since , there have been co-infections reported largely from Spain, southern France and Italy As the climate becomes warmer, the sandfly vectors of leishmaniasis may become more abundant, spreading north. Long hot summers are also ideal for other flies and the possibility exists of increasing cases of diarrhoea transmitted by houseflies, Musca domestica , and other species of synanthropic flies.
Although sporadic incidents of malaria, Lyme disease and leishmaniasis have been observed in western Europe, good surveillance systems and health infrastructure will be able to contain large-scale outbreaks. However, this may not be the case for some countries in eastern Europe. Moreover, declining economies and civil unrest may precipitate conditions conducive to disease outbreaks.
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The most important climate-sensitive vector-borne-diseases in South America as far as the numbers of people affected are concerned are malaria, leishmaniasis, dengue fever, Chagas disease and schistosomiasis. The numbers of cases of these diseases reported to the Pan American Health Organization in are shown in Table 1 Other vector-borne diseases with relatively low number of cases occurring each year, and which may be sensitive to climate shifts, are yellow fever cases in , plague 55 cases in , Venezuelan equine encephalitis 25 cases in , and other arboviral infections Up to , in the Brazilian Amazonian region alone, different types of arbovirus were isolated and 34 are known to cause human disease, sometimes in explosive epidemics.
One of these, Oropouche fever virus, is known to occur in cycles associated with the beginning of the rainy season The most widespread and severe climate-sensitive vector-borne disease in South America is malaria. This has been documented in Colombia 54 , 55 and Venezuela Confalonieri, unpublished data, Furthermore, in Ecuador, indirect factors, such as population migration and the breakdown of the heath services, contributed to the epidemics Changes in the temperate ecosystem of southern South America brought about by climate change would allow Anopheles darlingi to extend its habitat southwards 61 , The impact of climate change on the annual run-off over South America has been studied using different general circulation models The climate change scenarios consistently projected increases in run-off over the north-west of South America where malaria is known to be endemic.
Although the importance of soil moisture for the breeding of Anopheles vectors has been demonstrated in Africa 65 , the association between the water cycle and malaria transmission has not been studied empirically in the Americas. In the north-east of South America, a region subject to periodic droughts, the resurgence of visceral leishmaniasis kala-azar has been observed, for example, in some urban areas of Brazil 66 , The most plausible explanation for these increases is drought-driven human migration.
In the case of the kala-azar outbreaks, people migrated from rural endemic areas to the cities in search for jobs or government aid, owing to ruined crops, while in the case of the increase in imported malaria cases, immigrants seem to have moved to the neighbouring Amazon, an endemic area, to find temporary work and then, with the end of the drought, took new cases of the disease to their homelands U.
Confalonieri, unpublished data, There is little information about the possible impacts of climate change on the tropical forest of the Amazon, a natural source of dozens of known sylvatic arboviral infections, restricted mostly to the rainforest, with many more probably remaining to be discovered. Temperature increases on a local level due to deforestation in the Amazon can be even higher than those predicted by global climate change models under a doubling of carbon dioxide emissions.
With continued deforestation, drier conditions are expected that will have an impact on the dynamics of infectious diseases, especially those associated with forest vectors and reservoirs, such as malaria, leishmaniasis and arboviral infections. Possible mechanisms for this interference are changes in the physical conditions for the survival of the vectors humidity, breeding sites , and influences upon insect predators and vertebrate reservoirs Climate oscillations can affect the dynamics of dengue fever 72 , which is transmitted by the predominantly urban mosquito Aedes aegypti.
The influence of rising temperatures on the intensity and distribution of dengue fever transmission in the different continents has been estimated New areas of transmission areas are also expected to develop in the southern part of the continent. Human migration resulting from drought, environmental degradation and economic reasons may spread disease in unexpected ways, and new breeding sites for vectors may arise due to increasing poverty in urban areas and deforestation and environmental degradation in rural areas.
Climate change will exacerbate these effects. Since , average daily temperatures in the contiguous USA have increased by approximately 0. Recent studies have shown that the hydrological cycle is changing, with increases in cloud cover and precipitation Extremes in precipitation have also changed with more frequent heavy precipitation events and fewer lighter precipitation events 74 , It is becoming increasingly apparent that measurable changes in climate trends are occurring The health risks arising from such climatic changes will differ between countries depending on health infrastructures.
In Canada and the USA, good surveillance and vector-control programmes limit endemic transmission of diseases such as malaria and dengue fever. The health infrastructures of Mexico and other less developed nations are not so effective. Even in developed countries, increasing international travel and documented underreporting demonstrate a continued risk and need for strong surveillance The recent importation of West Nile viral encephalitis into the New York area in marked the first time West Nile virus had been found in North America It is not yet known whether the extreme record-breaking summer drought along the east coast affected the Culex mosquito populations that can carry the virus.
Birds are the natural hosts for West Nile virus. The hard tick, Ixodes scapularis , transmits Borrelia burgdorferis , a spirochaete, and the causative agent for Lyme disease, the most common vector-borne disease in the USA, with 15 cases in Other tick-borne diseases are Rocky Mountain spotted fever and ehrlichiosis, the latter having first being recognized in the mids. The relationships between vector life stage parameters and climatic conditions have been verified experimentally in both field and laboratory studies According to one modelling study, Rocky Mountain spotted fever may decline in the southern USA owing to tick intolerance of high temperatures and diminished humidity A temperature relationship for sporadic local malaria transmission was observed in New York and New Jersey during the s; common to these outbreaks was exceptionally hot and humid weather that reduced the development time of malaria sporozoites sufficiently to render these northern anopheline mosquitoes infectious 83 , However, even when climate conditions have favoured local transmission, the size of outbreaks has so far remained small.
Dengue and dengue haemorrhagic fever are on the rise in the Americas 85 , Puerto Rico averages 10 dengue fever cases annually, and the condition now occurs in nearly all Caribbean countries and Mexico, and has been periodically endemic in Texas in the past two decades. As mentioned above, global modelling studies have addressed transmission potential under climate change scenarios 73 , However, dengue fever is highly dependent on local environmental factors.
Thus far, only the analysis for Brownsville is complete. The modelled transmission potential for dengue fever decreased for this site. This may not be the case, however, for the island situation of Puerto Rico. Of reported encephalitis cases in the USA, most are mosquito-borne. Saint Louis encephalitis is the most prevalent 91 ; La Crosse encephalitis, and western, eastern, and Venezuelan equine encephalomyelitis also occur. Computer analysis of monthly climate data has demonstrated that excessive rainfall in January and February, in combination with drought in July, most often precedes outbreaks Such a pattern of warm, wet winters followed by hot, dry summers resembles some of the general circulation model projections for climate change over much of the USA 97 , Eastern equine encephalitis has been associated with warm wet summers along the east coast of the USA The pulmonary hantavirus epidemic in the south-west of the USA was believed to be due to an upsurge in rodent populations related to climate and ecological conditions , ; six years of drought followed by extremely heavy spring rains in resulted in a fold increase in the population of deer mice, which can carry hantavirus.
Similarly, the incidence flea-borne plague has been positively associated with preceding periods of heavy precipitation in the region Leptospirosis, carried by rodents, has been associated with flooding in Central America. For instance, in Nicaragua a case-control study of the epidemic found a fold risk of the disease associated with walking through floodwaters Leptospirosis is rarely reported in the USA.
However, the disease is under-diagnosed Lyme disease and encephalitis will increasingly become public health threats in the USA, as suitable conditions for transmission increase. However, as understanding of the links between climate and these diseases increases, and climate predictions become better, methods of preventing outbreaks, e.
Although Lyme disease is treatable, it remains difficult to diagnose. Currently available standard laboratory tests are not fully satisfactory in that they lack sensitivity and specificity and are not well standardized. Under-diagnosis is a problem in parts of the USA where the disease is not endemic or is relatively uncommon. The number of mice and deer in a region influences the number of ticks found there.
The recent resurgence of the deer population in the north-east USA and the incursion of suburban developments into rural areas where deer ticks are commonly found have probably been major contributors to rising prevalence.
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