Abstract
A selective review of recent concepts, events and major recent research methodologies, and educational approaches in the field of vector-borne disease are drawn together in this article. Since vector borne disease is a major contributor to world disease burdens, and also comprises list of neglected diseases, recent research in the field elucidates the uncertain and far-reaching consequences of these diseases to human health and well-being. Some of the specific findings included in this review are the following: Chickungunya virus disease range is changing as a result of global climate change; Tick-borne disease vaccinations are being pursued with the help of PCR techniques; the wide availability of remote sensing and ecology are providing habitat surveillance tools to improve predictability of risk areas; environmental health education approaches are incorporating community and cultural aspects to improve success and reduce risk.
Developing countries in the world today contend with environmentally related diseases and conditions that are considered by the developed world to be in categories such as “reemerging diseases”, “neglected diseases” and “emerging diseases”. In the World Health Organization's 2006 publication “Preventing Disease through Healthy Environments: Towards an estimate of the environmental burden of disease”, Pruss-Ostun and Corvalan present information on global disease risk and burden. They estimate that 24% of the global disease burden and 23% of deaths can be attributed to environmental factors. Often this is the result of unregulated industrialization by-products such as hazardous waste, hospital waste, air pollutants, and waste water. The list of diseases with the largest absolute environmental source includes: diarrhea, lower respiratory infections, injuries, and malaria. The environmental causes of these diseases places one quarter of the world's population at high risk. Additionally, in many places, even where industrialization has not developed, infectious disease, especially vector borne disease, is on the top of disease burden lists because of mosquito borne malaria.
In a recent article, authors Beyrer, et al 1 discussed a list of emerging and reemerging diseases, calling them “neglected” diseases, and emphasized their close association with civil conflicts. A concept both sets of authors introduced was the “right to health”.
In the vision statement of the Bill and Melinda Gates foundation “all people deserve a chance to live a healthy and productive life”. Environmental Health professionals, especially those in developed countries, have important responsibilities to apply existing experience, knowledge and research to old and new environmental health problems, with a global goal of bringing health education to our world neighbors. To address these and other problems in our present rapidly changing civilization, we need to first ask “What are the diseases, concepts, and methods in environmental health that are receiving research attention?” A second question would be: “Is this research addressing the crucial environmental factors contributing to disease?” And finally, a question that cannot be fully answered in a short review: “Are environmental health and health education professionals using that research to addressing the needs of people in the greatest need?
The remainder of this article is a review of some concepts, events and major recent research methodologies, and educational approaches in the field of vector-borne disease related these questions. Vector borne disease is a major contributor to world disease burdens, and also comprises the list of neglected diseases in Beyrer et al.
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The political and economic perturbations of humans to the environment and society often result in uncertain and far-reaching consequences to human health and well-being, many times resulting in increased disease vectors. The specific topics included in this review are:
the link of vector borne disease burdens with global climate change, a result of industrialization in part, the disease vectors mosquitoes, ticks and triatome bugs, the research methodology of genetic mapping and molecular analysis, remote sensing and ecology, Successful environmental health education approaches.
An important new global event affecting environmental health research is the nearly unassailable legitimacy of global climate change, which has an important effect on vector borne disease. In Harrus and Baneth's 2 review of the drivers for emergence and reemergence of vector borne diseases, atmospheric and climate changes are primary. In addition, Harrus and Baneth include habitat changes, alterations in water storage and irrigation habits, immunosuppression by HIV, pollution, insecticide and drug resistance, war and civil unrest, increase in international trade, tourism and travel, and government management failure. This review recommends that physicians, veterinarians, and biosecurity officers in addition to health care workers play an important part in prevention of vector borne diseases. Topics for research at the Southeastern Center for Emerging Biologic Threats Conference in the US 3 included vector borne disease and epidemiological surveillance techniques, including modeling and mapping.
One item in this list of global disease dispersion drivers that may be changing is global traffic. Tatem, Hay and Rogers 4 as well as Harrus and Baneth 2 saw an increased potential for vector borne disease dispersal in travel, tourism and trade. An example used was the range change in the mosquito Aedes albopictus from Asia to the US through ship and air traffic. Comments by scientists on the program “60 Minutes” in 2009, 5 displayed a convincing argument that Ae. Albopictus could have arrived by aircraft as well as in truck tires. Slower distribution increases in other vectors, such as Anopheles gambiae from Africa were explained partly by the low volume of sea and air traffic from endemic areas to Europe and the US.
Since the aftermath of the NY twin towers bombing in 2004, when air travel and tourism dropped dramatically, political and economic events in the world have had continuing effects on many parts of our global life. It is possible that there may be less travel and tourism since the 2008 oil price climate change, and the 2009 recession than these authors imagined and thus less influence on the global dispersal of disease and vectors as well.
Concepts, Events and Current Methodologies
Global Climate Change
The effects of global climate change by itself continue to provide evidence of change in disease dispersal. Chretien, Anyamba and Bedno
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provide evidence of the spread of Chickungunya virus, a mosquito borne disease, from Kenya to the Indian Ocean Islands and India in 2005–2006 (Fig. 1). Drought conditions preceded the outbreaks as recorded by satellite data. Along with drought conditions were the infrequent replenishment of water stores (through pumping or transport of water) by the populations affected. As populations soared and water supplies shrank, unexpected consequences arose, including the spread of this reemerging disease, since the vector responded positively to the fluctuating water conditions. Knowledge and surveillance by environmental health professionals of the vector would have been useful in preventing a mosquito vector build up and in tracking the spread of the disease.
Chickungunya and Dengue in the Indian Ocean Islands and India.
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A method that is available worldwide to increase such knowledge is satellite-generated data. Anyamba, Chretien and Small, 7 used satellite data to describe climate anomalies such as the “unscheduled” El Nino advisory in 2006. El Nino/Southern Oscillation (ENSO) was then related to changes in infectious diseases such as those found in Indonesia, Malaysia and the Philippines from drought conditions. In other parts of the world, above normal rainfall was expected to occur. The authors list the following parts of the world at risk for disease outbreaks: Indonesia, Malaysia, Thailand, and most of the southeast Asia Islands for increased dengue fever transmission; Coastal Peru, Ecuador, Venezuela, and Columbia for increased risk of malaria; Bangladesh and coastal India for elevated risk of cholera; East Africa for increased risk of Rift Valley fever and malaria, southwest USA for increased risk for hantavirus and plague; southern California for increased West Nile virus transmission, and northeast Brazil for increased dengue fever and respiratory illness.
Other authors, e.g. Hunter 8 include waterborne diseases with vector borne diseases in predictions of increased incidence following climate change, though the “actual impacts of climate change on public health are still far from clear”. Vector borne and water borne disease outbreaks often lag behind direct climatic or geologic events, causing more widespread harm than the direct climatic event itself. Experts and insurance agencies are not yet sure how to relate distal causes and proximate effects of climate change and natural disasters that may follow.
Ecological shifts, particularly those towards decreased species diversity may have an increased effect on vector borne disease. Ostfeld and Keesing
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proposed a hypothesis of “dilution effect” due to diversity in intermediate hosts as a mediating factor in the transmission of Lyme disease in the Eastern US. The vector borne disease model typically includes RESERVOIR-VECTOR-HOST (Fig. 2). These relationships can be dampened or improved depending on the specialization of the vector and the transmission competence of the hosts. Because lowered diversity often follows disastrous climatic or political events, these protective ecological relationships may also be disrupted and open yet another pathway for increased vector borne disease transmission. In 2006, Keesing, Holt and Ostfeld
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listed a set of research priorities in the ecological area: 1) describing patterns of change in disease risk with changing diversity 2) identifying the mechanisms responsible for observed changes in risk; 3) clarifying additional mechanisms in a wider range of epidemiological models; and 4) experimentally manipulating disease systems to assess the impact of proposed mechanisms.
The classic relationship of the vector-borne disease triangle where a break in any of the lines can interrupt transmission.
Mosquito Borne Diseases
In the area of mosquito borne diseases (other than malaria, which continues to defy researchers and foundations, government organizations, and private corporations), West Nile virus, especially in the US, Dengue Fever in the rest of the Americas, and Chickungunya, an emerging or re-emerging disease are receiving research attention. Chickungunya is an important example of an emerging disease because it is in the beginning stages of emergence. The word “chickungunya” which is used for both the virus and the disease, means “to walk bent over” in some African languages. This meaning refers to the symptom of severe joint pain which characterizes this dengue-like infection. Aedes albopictus is the presumed vector in a recent outbreak of this disease in Reunion Pialoux, Gauzere, Jaurebuiberry, and Strobel. 11 Chickungunya has reemerged on the Indian subcontinent and in the Indian Ocean region, for unclear reasons. Possible explanations are similar to those for West Nile Virus (WNV): increased tourism, virus introduction into a naive population, and viral mutation.
Further investigation by Charrel, Lamballerie, and Raoult 12 found that the outbreak in the Indian Ocean region and in the Indian Subcontinent, which resulted in 2.3 million cases at the time of the article, was caused by a new variant of the virus genome. This change may also be responsible for the virus adapting to a new mosquito vector. Prior to its adaptation to Aedes albopictus, the vector had been Aedes aegypti. Both of these mosquitoes are anthropophilic, and extremely widespread. Aedes albopictus is the famed “Asian tiger mosquito” which was transported in tires to Texas in the 1990's. Since both mosquitoes are found in the US, and since Ae. Albopictus is the most common urban mosquito in many areas, the threat of imported Chickungunya disease through human travelers is an extremely important area of needed surveillance and research.
Research Methodologies: Molecular and Genetic Research
Another extremely important area of research is the methodology of molecular identification, and genetic mapping. In order to find new vaccines, repellents, and pesticides, PCR/Serology methodology is used. Nuttall, Trimnel, Kazimirova and Labuda 13 discuss the need for anti-tick vaccines. Their research in prevention of tick borne disease found a group of antigens that combines the properties of exposed and concealed antigens which offers the prospect of broad spectrum vaccines.
A tick responsible for the transmission of Lyme disease, an emerged disease in the US Northeast, Ixodes scapularis is the subject of a genome project.14,15 These authors affirm that genomics approaches are being applied increasingly to blood-feeding arthropod vectors. The National Institute of Allergy and Infectious Diseases (NIAID) has also made a firm commitment to genomic sequencing of bioterrorism agents, emerging and reemerging pathogens, and to arthropod vectors of human disease, giving this methodology an important place in the future of vector borne disease research. Another genomic sequencing project reported by Pittendrigh, Clark and Johnston 16 has a new target genome, Pediculus humanus humanus (Phthiraptera: Pediculidae), the body louse. This pest transmits the bacterial agents of louse-borne relapsing fever, trench fever, and epidemic typhus. In times of civil disturbance or war, these diseases take a toll on migrant populations and troops. Hill, Kafatos, Stansfield and Collins 17 state that “radical changes in vector-biology research are required if scientists are to exploit genomic data and implement changes in public health”. Surveillance patterns using morphologically-based taxonomic data are often riddled with misidentification, and molecularly-based taxonomic data impossibly slow and difficult to acquire. Scientists working together on these two approaches to biological diversity and distribution would improve predictive accuracy.
Tick and louse vector genomics is aimed at tracing patterns of evolution and distribution of vectors, but researchers are also using genetics in other ways. An example is to trace the evolution of a virus, St. Louis encephalitis, a mosquito borne disease. The genome of St. Louis encephalitis has just recently been completed at the Sackler Institute for Comparative Genomics at the American Museum of Natural History (AMNH). 18 Molecular methodology can advance genetic manipulation of vectors and diseases for control. For example, the vector of yellow fever, Aedes aegypti, was reported to be transformed with the Hermes element from the housefly in an article by Jasinskiene, Coates and Benedict, et al. 19 This line of investigation shows promise in reducing the capacity of Ae. aegypti to carry and transmit the yellow fever virus. Perhaps the virus itself could be altered. Another report from Aultman, Beaty and Walker 20 is an overview of genetically manipulated vectors of human disease. Because of the current lack of new pesticide development, the increasing resistance of vectors to currently used pesticides, and because of government regulatory processes, these authors recommend molecular and genomic research as vitally needed alternatives. Genomics can provide information and maps to assist in favorable genetic manipulation of vectors and of the diseases they carry.
Research Methodologies: Satellite Imagery and Modeling
A major area of recent research in vector borne disease is the use of satellite imagery data to study the distribution and ecology of disease vectors and their habitat. In 2005, 61 countries agreed to implement the global Earth Observation System of Systems (GEOSS). The purpose was to provide geo-referenced data and analysis software that could be used worldwide. The US contribution was the Integrated Earth Observation System (IEOS). Seventeen federal agencies comprised the working group to develop the plan for IEOS. Within this plan is an Integrated Ocean/Coastal Observing System (IOOS) to collect measurements of the air, water and land from the ground, air or from space. Of the societal goals listed, climate change monitoring and reduction of public health risks are high on the list. Other components are safety and homeland security.
Making use of this abundant world data, Kalluri, Gilruth, Rogers, and Szczur 21 reviewed the use of remote sensing techniques in the surveillance of arthropod vector borne infectious disease. The review reports a fertile environment for research found in the use of simple image classification techniques associating land characteristics with vector habitats and in the more complex image processing algorithms for analyzing multi-temporal meteorological observations with vector biology and ecology. Examples of vectors used in these studies were mosquitoes, ticks, black flies, tsetse flies and sand flies, which the authors claim as responsible for the majority of vector-borne diseases in the world. In one of the recent articles using remote sensing 22 researchers have applied remotely sensed vegetation indices (ASTER) to an analysis of West Nile vectors in the northeastern US. Multivariate analysis of the environmental variables vs. the mosquito distribution data explained 86% of the variance in mosquito/environmental data. The use of these complex analytical algorithms with remotely sensed data can help in pinpointing landscape features for maximum control effectiveness. These techniques involve considerable expense and expertise not available to many worldwide researchers, but show promise because of the worldwide availability of remotely sensed data.
Jouda, Perret and Gern 23 used a regional scale analysis of Lyme borreliosis (Lyme disease) on the Swiss Plateau and in an Alpine valley. They state that Lyme borreliosis is the most important vector-borne disease in the Northern hemisphere. Direct fluorescent antibody assay was used in this study to determine infection in collected ticks, and comparisons made to determine correlations between nymphal infection, location and density, and adult infection, location and density. Infection rates were higher for adults at high densities. All locations but one produced infected ticks. No satellite imagery was used in this case, but could have been added for a richer description of environmental variables.
Mathematical analysis of remotely sensed data also takes the form of modeling as Eisen and Eisen 24 illustrate in their article “Spatial modeling of human risk of exposure to vector-borne pathogens based on epidemiological versus arthropod vector data”. Their recommendations for risk modeling include the use of both epidemiological and vector data on a sub county level, using simulation or analytical models. The aim of the model is to predict critical vector abundance thresholds needed to maintain enzootic pathogen maintenance. This is a persistent question vector control managers are asked, but a difficult one to find for a particular site. The recommendation specifies sub county levels of input data, implying that models are not widely geographically applicable. Cost is again a factor for worldwide use of this technique, but it also shows promise.
Education and Community Involvement
In order to provide immediate improvement to high disease risk burdens, education and community involvement in vector and disease management techniques are needed. Chaves, Cohen, Pascual, and Wilson 25 investigated the changes in American cutaneous leishmaniasis concurrent with changes in forest cover. Their conclusion was that “social factors …. play a critical role that may ultimately determine disease risk.”
In a further development of community involvement, Van den Berg, von Hildebrand, Ragunathan, and Das 26 investigated the technique of empowering farmers in integrated vector management. The approach was to integrate education on improving rice yields with managing vector borne diseases. This was an important step in total integration using the “farmer field school” method to provide farmers with an ecologically realistic demonstration of all the aspects of their interaction with the environment.
Providing farmers first-hand experience with vector control actions also improved general sanitation and personal protection against pesticides. In a study done in Trinidad and Tobago, authors Rawlins, Chen, Rawlins, Chadee and Legall
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found that knowledge of prevention and protection techniques against Dengue Fever risk did not coincide with practice, and found a need for the demonstration of its efficacy, and a need for community involvement in the educational process. This confirmed the efficacy of “active learning” and community involvement. A more detailed model of the vector borne disease triangle includes these educational and community involvement details (Fig. 3).
Vector triangle including environmental health aspects.
Conclusions
The diseases, concepts, and methods in current environmental health vector borne disease research that are receiving research attention can be summarized by the examples of continued surveillance projects aided by GIS mapping and software modeling analysis, and by a movement to collaborate in these projects with molecular genomic analysis. Research is addressing the crucial environmental factors contributing to disease through efforts to understand effects of global warming on disease and vector distribution. And finally, a question that cannot be fully answered in a short review: “Are environmental health and health education professionals using that research to addressing the needs of people in the greatest need” is beginning to be answered with health educational translational projects such as the Van den Berg et al 26 research.
An extremely hopeful sign for global disease management is the convergence of methods regardless of the vector borne disease, environmental conditions, or geographic location that has emerged in recent research. Vector borne disease research consortia and global organizations have evolved to coordinate and disseminate important geographic and environmental information to researchers. Geneticists, molecular biologists, and mathematical modelers have combined methodology with epidemiologists, medical therapists and educators. A real effort has been made to involve communities in the management of their own food production and disease control. Vector borne disease in environmental health has thus provided a broad focus for collaboration on human public health problems that have direct research threads through all of the methodologies and diseases illustrated in this article. Convergent collaborative research has begun to emerge, where complex methodologies can provide answers to health education researchers who can empower communities with active learning techniques to manage their own local vector borne disease risk. Problems of application to local situations of great need are still very common, however, has Translational research now a priority in many US funding agencies (NSF for example) and in private foundations to aid in the transmission of research information.
Footnotes
The author reports no conflicts of interest.