| ENVIRONMENTAL FACTORS IN THE VIEW OF VECTORS & VECTOR BORNE DISEASES |
|
|
Vector-borne diseases are among those infectious diseases causing highest disease burden today, and may be expected to represent the highest proportionate disease burden in the future. The distribution of the incidence of vector-borne diseases is grossly disproportionate, with the overwhelming impact in developing countries located in tropical and subtropical areas. These infections are major killers, particularly of children in developing countries. Over the past decade, more comprehensive and transparent methods of measuring health have improved understanding of the importance of these diseases. |
In recent years, vector-borne diseases (VBD) have emerged as a serious public health problem in countries of the South-East Asia Region, including India. Many of these, particularly Dengue fever, Japanese encephalitis (JE) and Malaria now occur in epidemic form almost on an annual basis. Filariasis is endemic in tropical regions of Asia,Africa, Central and South America whereas Yellow fever causes 200,000 illnesses and 30,000 deaths every year in unvaccinated populations. Despite technological advances and increasing affluence in many regions, vector-borne infectious diseases remain amongst the most important causes of global ill health.
Firm response measures to control infectious disease will be required for the foreseeable future and these measures will inevitably affect or interact with other ecosystem services. In addition, growing human populations and increased demand for ecosystem services/natural resources means that there will be continuing, and possibly increasing, human interactions with the natural processes that influence infectious disease transmission. Many diseases are influenced to some degree by the environment. However, vector-borne infectious diseases are especially ecologically sensitive, since environmental conditions, such as temperature, affect both the infecting pathogens that, depending on the interaction with human hosts, have the potential to cause clinical disease and the insects and other intermediate hosts that transmit them. |
|
| |
|
|
Climate change could significantly affect vector borne disease in humans. It also can affect the development of pathogens in vectors, as well as the population dynamics and ranges of the non-human vertebrate reservoirs of many vector borne diseases. Whether climate changes increase or decrease the incidence of vector borne diseases in humans will depend not only on the actual climatic conditions but also on local nonclimatic epidemiologic and ecologic factors. |
Many natural systems are being affected by regional climate changes, particularly temperature increases. The rate of warming of the earth’s surface over the past 50 years is nearly twice that over the past 100 years, and global average temperatures are projected to increase between 1.4 and 5.5°C by the end of this century. Temperature increases are in turn associated with rising sea levels and increased extremes of the hydrologic cycle (e.g., floods and droughts). |
Predicting the relative impact of sustained climate change on vector borne diseases is difficult and will require long-term studies that look not only at the effects of climate change but also at the contributions of other agents of global change such as changes in land use, water availability, and other issues. Adapting to the effects of climate change will require the development of adequate response plans, enhancement of surveillance systems, and development of effective and locally appropriate strategies to control and prevent vector borne diseases. |
Because transmission patterns of arthropod-borne diseases are strongly influenced by changes in ambient temperature, some researchers predict that certain vector-borne diseases, including malaria, yellow fever, and dengue, will expand their range to higher elevations and latitudes in response to global warming. It is also possible for hot weather to have a detrimental effect on vector populations and pathogen survival, which could result in a reduction of certain vector-borne diseases in some regions. Biological systems can amplify the effects of small changes in temperature to dramatic effect, a relationship that has inspired the creation of climate-based models to predict the disease range. But these models are severely limited by the fact that climate is not always the most important factor in defining the range of a vector-borne disease. In many cases, anthropogenic impacts on local ecology, such as deforestation and water use and storage, represent far more significant influences on the prevalence and range of vector-borne diseases; in addition, human behavior can significantly limit disease prevalence. Thus developing models that incorporate climate, geography, land use, and socioeconomic factors to predict malaria risk could advocate the careful examination of the complex ecological relationships involved in vector-borne disease transmission dynamics.
Impact of climate on epidemiology of vector borne disease
|
As with the climate-related health impacts, assessments of the impact of climate on vectorborne diseases are uncertain. Some vectors have very stringent requirements for their ecosystems. Some can't survive if the temperature gets too high or too low or if the rainfall is too high or low. 3°C warming in the United States could increase the range of Aedes aegypti, a mosquito species that can carry the viruses dengue and yellow fever. Freezing temperatures kills aedes aegypti and this mosquito is present only during the summer. A 3°C warming could increase the areas in which Aedes aegypti would be present during more of the year. |
Dengue virus requires a warm temperature in order to multiply within the mosquito. Up to a certain point, higher temperatures will shorten the period in which a mosquito, after consuming an infected blood meal, can transmit the virus. Warming might increase mosquito populations and improve conditions for disease transmission. |
The incidence and location of malaria outbreaks is also expected to be affected by climate change, according to the WHO task group malaria is the most prevalent, producing an estimated 270 million cases a year worldwide, primarily in tropical and subtropical areas. |
ENSO and disease forecast methods
|
ENSO is the most well-known phenomenon influencing global climate variability. Important aspects of interannual variability in global weather patterns are linked to ENSO. El Niño refers to a large-scale ocean-atmosphere climate phenomenon that is linked to periodic warming of SSTs across the central equatorial Pacific. Because of the large size of the Pacific Ocean, changes in SST patterns and gradients across the basin influence global atmospheric circulation. There is building evidence suggesting links between ENSO-driven climate anomalies and infectious disease, particularly those transmitted by arthropods, such as Murray Valley encephalitis, bluetongue, RVF, dengue, malaria and chikungunya. |
The El Nino weather cycle provides further evidence of the importance of climate on vector-borne disease transmission. While differences exist in the changes effected by anthropogenic climate change and El Nino and the time scale over which they occur, El Nino is a helpful model to investigate the impact of climate on infectious diseases given the short period over which it exerts its effects. El Nino has driven heat waves and drought in parts of Africa and Asia as well as heavy rains and floods in South America. In the Asia-Pacific region, El Nino has impacted dengue fever outbreaks. In South America, Asia, and Africa, malaria transmission is sensitive to the climate variability from El Nino. Additionally, the frequency and severity of El Nino events is expected to increase with anthropogenic climate change.
|
|
The increasing temperature has caused great concerns for the potential public health threatened from these vector-borne diseases. Global average temperatures have increased by over 0.5°C over the past half-century, but warming is not evenly spread. Further increases in temperature of 1-6°C (144) during the next century, will probably lead to changed geographical ranges, seasonal patterns, and intensity of transmission of vector-borne diseases. Trend is to continue, with altered vector breeding and disease transmission rates as vector and pathogen development rates increase with temperature though vector longevity decreases. |
Vectorial capacity is dependent upon the extrinsic incubation period of the virus in the mosquito vector, the time from virus ingestion to the time that virus infects the salivary glands of the mosquito. Global climate is significantly affected by the variability of sea surface temperatures (SSTs). Due to its location, the climate in Southeast Asia is influenced by the variability in both the Pacific and Indian Ocean temperatures. The impacts of the interannual variability in SSTs in these oceans are revealed in atmospheric circulation through outgoing long wave radiation (OLR) measurements. These satellite-derived measurements are a proxy indicator of cloudiness and hence rainfall. When expressed as anomalies with respect to reference long-term means, negative OLR anomalies in the tropics represent regions of precipitating clouds, whereas positive OLR anomalies are associated with dry conditions. Through such measurements the impacts of such phenomena as ENSO on global cloudiness and rainfall patterns can be observed. Various climate indicators, such as SST and OLR, can be measured with instruments on Earth-orbiting satellites. Large-scale variability in the climate regime producing either floods or droughts has the effect of enhancing the emergence and propagation of various disease vectors.
Temperature thresholds of pathogens and vectors:
- Tmin is minimum temperature required for disease transmission.
- Tmax for the pathogen is upper threshold beyond which temperatures are lethal.
- Temperatures are in degree Celsius
Temperatures assume optimum humidity; vector survival decreases rapidly as dryness increases. There is considerable variation in these thresholds within and between species (Purnell, 1966; Pfluger, 1980; Curto de Casas and Carcavallo, 1984; Molineaux, 1988; Rueda et al., 1990).
Disease |
Pathogen |
Tmin |
Tmax |
Vector |
Tmin for Vector |
Malaria |
Plasmodium falciparum |
16-19 |
33-39 |
Anopheles |
8-10 (biological activity) |
Malaria |
Plasmodium vivax |
14.5-15 |
33-39 |
Anopheles |
8-10 (biological activity) |
Dengue |
Dengue virus |
11.9 |
Not known |
Aedes |
6-10 |
Rainfalll
The following are the range of possible mechanisms whereby rainfall can impact on the risk of transmission of vector-borne disease:
• Increased surface water can provide breeding sites for vectors
• Low rainfall can also increase breeding sites by slowing river flow
• Increased rain can increase vegetation and allow expansion in population of vertebrate host
• Flooding may force vertebrate hosts into closer contact with humans
Decreased rainfall has been shown to be associated with epidemics of encephalitis when the vector, by supporting breeding in urban drainage systems.
Increased precipitation associated with global warming may benefit mosquito populations by increasing the number of breeding sites. Although in the past the lower temperatures at high altitudes have limited the expansion of malaria vectors, there are recent reports of malaria epidemics occurring in these previously unaffected areas, perhaps in part due to higher temperatures and increased precipitation.
|
|
A range of aquatic ecosystems supports the breeding of a great number of vectors that play a role in the transmission of diseases. Any vector-borne disease problems in irrigated areas can be traced to absent or inadequate drainage.
|
The various forms of surface irrigation all impose increased vector-borne disease hazards, while overhead irrigation and drip irrigation are virtually free of such hazards. Irrigated agriculture often requires additional chemical inputs for crop protection, and the application of pesticides can disrupt the ecosystem balance favouring certain disease vectors; it can also contribute to an accelerated development of resistance to insecticide in disease vector species. Water management practices that reduce the environment’s receptivity to the propagation of disease vectors and intermediate hosts can be the main contributor to reducing transmission risks of such vector borne diseases.
|
|
The presence of vegetation and floating plants are important for optimal breeding conditions. First, the plants are larval food and, more importantly, they provide shelter from predators and protection against wave movement. Therefore, mosquito larvae are not found on the open surfaces of large water bodies. The abundance of a number of species is linked to the presence of specific plants. |
Altitude and precipitation |
Altitude and proximity to the forest were independently associated with increased malaria risk in all years, including epidemic and non-epidemic years. Vector densities and transmission intensity in the highlands are generally much lower than in the adjacent lowland areas. In some highland area, areas of high malaria risk are consistent in epidemic and non-epidemic years and are associated with specific ecological risk factors. The perennial malaria transmission in the lowlands has been attributed to high vector densities throughout the year. The proportion of asymptomatic individuals is usually lower in highlands than in high-transmission areas where there is small among-season variation in mosquito prevalence and parasite densities. Thus, a small increase in the abundance of vectors may lead to a significant malaria outbreak in the highlands. |
|
Mosquitoes are highly sensitive to humidity and therefore the variations in their habitats (either wetland or drought regions) resulting from changes in relative humidity will have an impact on the breeding and transmission characteristics of vectors thereby paving way for altered entomological infection rates. |
All the variables discussed above are significant in their own regard, based on the period of the disease outbreak or region, latitude etc.. Analyzing in a better way the appropriate variables before, after or at the time of epidemic will yield a better knowledge on the depth of roles played by these environmental factors. |
|
That droughts cause famines is well recognized. Malnutrition remains one of the largest health crises worldwide, with approximately 800 million people, close to half residing in Africa currently undernourished. Droughts and other climate extremes not only have direct impacts on food crops but also can also indirectly influence food supply by altering the ecology of plant pathogens. While projections of the effect of climate change on global food-crop production appear to be broadly neutral, such change will probably exacerbate regional inequalities in the food supply. As there is a breakdown in sanitation as water resources become depleted, droughts can also increase the incidence of water and vector borne diseases. |
|
When ecosystems are altered, disease problems arise. The usual vertebrate hosts for most vector-borne pathogens that infect humans are wild or domestic animals; people may also become infected when they intrude on habitats where pathogens exist. It is therefore not surprising that the initial human occupation of remote ecosystems has resulted in the emergence of vector-borne diseases, given the potential for such circumstances to introduce vector-borne pathogens to immunologically naïve hosts and vectors. |
Deforestation and Reforestation
|
Land use changes such as deforestation, road construction, and dam building can trigger a cascade of secondary factors known to exacerbate infectious disease emergence, such as forest fragmentation, pathogen introduction, pollution, and human migration. The change in land use will alter mosquito biodiversity, in turn the numbers of the malaria vectors and thereby signaling the risk of infection. The change of agricultural practices and subsequent reforestations proved to be an ideal cause for increasing numbers of ticks in many areas. |
|
Warmer oceans also cause sea levels to increase, primarily as the result of the thermal expansion of salt water. Even if the mid-range predictions of climate change are correct and sea levels in the 2080s are, on average, ‘only’ 40 cm higher than the current values, the coastal regions at risk of storm surges will become much greater and the population at risk will increase from the current 75 million to 200 million. Rising sea levels will result in the salination of coastal freshwater aquifers and the disruption of storm water drainage and sewage disposal. |
Weather, Climate, and Prediction
|
Weather refers to short-term fluctuations in the atmosphere, whereas climate describes average weather over long periods of time. Climate tends to affect the geographic distribution of vector-borne diseases, while variations in weather such as temperature, rainfall, and humidity influence disease transmission dynamics, and thereby the timing and intensity of outbreaks. The usefulness of modeling the potential effects of climate change on disease transmission and spatial distribution is therefore appreciated in social and scientific regards. |
Ocean Temperatures and Outbreaks
|
Other than the seasons, the El Nino/Southern Oscillation (ENSO) is the primary source of global variation in temperature and rainfall. Decades of observation indicate that ENSO associated weather anomalies influence outbreaks of a variety of vector-borne. Upon recognitions from various case studies, that RVF outbreaks were associated with periods of heavy rainfall, the researchers started involved in developing operational models capable of predicting RVF outbreaks based on ocean temperatures, rainfall anomalies, and vegetation characteristics. |
Many other similar events elaborated the potential uses of vector-borne disease forecasting in reducing the impact and limiting the spread of disease. Environmental models may one day be used to identify imminent outbreaks of specific vector borne diseases by tracking and integrating factors critical to disease transmission. |
|
The control of disease vector populations by habitat modification, a mainstay of early 20th century public health, was replaced by chemical methods when they became relatively inexpensive and widely available. Pesticides remain the primary means to prevent or mitigate most vector-borne diseases, but resistance has increasingly limited the effectiveness of this strategy. Because insecticide resistance poses an especially significant barrier to controlling malaria and dengue, and because vector control measures could potentially reduce the incidence of additional vector-borne diseases, development of novel insecticides and deployment methods are supported by various research organizations. |
Emphasis on controlling mosquitoes indoors was one of several targeted vector control strategies. Many simulation models suggest that combining immunization with vector control would be advantageous. Vector control, in essence, slows the force of infection and makes the delivery of vaccine more effective. “Therefore an integrated program with vector reduction and immunization will more effectively prevent epidemic dengue and is more sustainable than either strategy alone. Implementing such programs will take “a change in mind set to start to get people who are working on vaccines to think about working together with people who are working on vector control.” |
|
The need to understand better the ecology of vector-borne diseases was identified as critical to a host of purposes:
· Targeting surveillance and control efforts
· Minimizing surveillance costs over large areas
· Forecasting risk and anticipating expansion of disease range (including globalization)
· Designing containment or exclusion strategies |
The following information was deemed essential to fill the knowledge gaps:
· Quantitative descriptions of endemic and epidemic disease cycles in all hosts
· Measurements of disease transmission potential by known and potential vectors
· Timing, distribution, and abundance of disease-competent vectors
· Mechanisms of host infection
· Mechanisms of pathogenesis
· Mechanisms of transovarial transmission
· Spatial and temporal distributions of vectors and environmental conditions in settings at risk for disease emergence |
Barriers to Implementation
|
There is a general lack of infrastructure for implementing vector-borne disease interventions in most countries. While acknowledging the global burden of vector-borne diseases, the daunting obstacles to much-needed research on their control, prevention, and treatment must also be expressed. Moreover, legal and bureaucratic barriers increasingly impede international research on and response to vector-borne disease. A major common goal for countries, or cultures, is to have a productive and healthy workforce that translates into the well being of the entire community, he suggested. Thus, in order to convince the pharmaceutical industry, as well as governments, that vector-borne diseases are worth solving, researchers will need to provide evidence of economic benefit and opportunities for strategic investment. |
|
| http://www.ncbi.nlm.nih.gov/pubmed/9701925 |
| http://www.sove.org/Journal%20PDF/December%202008/1-Moore%2008-56.pdf |
| http://nature.berkeley.edu/~rodrigo/Lab%20page/papers/Almeida08.pdf?41-126=&11950=. |
| http://www.parasitesandvectors.com/content/1/1/30 |
| http://www.who.int/heli/risks/vectors/vector/en/index.html |
| http://www.grida.no/publications/other/ipcc_sr/?src=/Climate/ipcc/regional/233.htm |
|
|
|
