Vector-Borne Diseases Understanding the Environmental, Human Health, and Ecological Connections: Workshop Summary (2008) / Chapter Skim
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Summary and Assessment
Summary and Assessment
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These diseases profoundly restrict socioeconomic status and development in countries with the highest rates of infection, many of which are located in the tropics and subtropics. From the perspective of infectious diseases, vectors are the transmitters of disease-causing organisms; that is, they carry pathogens from one host to another. By common usage, vectors are normally considered to be invertebrate animals, usually arthropods, but they may also include fomites, which are defined as "[a]
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For the purposes of this discussion, a disease that is transmitted to humans, plants, or animals by any agent, arthropod, or fomite is a vector-borne disease. Over the past 30 years -- following decades during which many mosquitoborne human illnesses were controlled in many areas through the use of habitat modification and pesticides -- malaria and dengue fever have reemerged in Asia and the Americas, West Nile virus (WNV)
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The adoption of vector control measures, including the
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The effective application of these environmental control measures greatly reduces the reliance on pesticides for vector control (Center for Science and Environment, 1999)
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FIGURE SA-1 Deaths from vector-borne diseases. SOURCE: Reprinted with permission from the World Health Organization (2004c)
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* Animals that live in close association with humans (Montana State University Entomology Group, 2007)
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. Yellow fever, which along with dengue was controlled in the Americas by a variety of mosquito abatement techniques through the mid-20th century, remains However, chikungunya, hantavirus pulmonary syndrome, and Lyme disease may have a higher percentage of new cases based on local populations that are immunologically naïve being exposed to and acquiring these newly emerging diseases (IOM, 2003; Chretien et al., 2007)
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Impact of Vector-Borne Animal and Plant Diseases The majority of emerging, reemerging, and novel human infectious diseases are zoonoses (diseases that can be transmitted from animal reservoirs to humans) , of which vector-borne diseases comprise a large percentage (IOM, 2003)
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For example, Sudden Oak Death (SOD) -- an emerging infectious disease that has been spread across wild lands by hikers, mountain bikers, and equestrians (i.e., human "vectors")
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Hallmarks of Vector-Borne Disease Vector-borne diseases are transmitted among their human, animal, or plant hosts by arthropods, usually insects. A broader definition of vector-borne disease recognizes that other animals can serve in the role of infectious disease vector by harboring pathogens that cause disease only in susceptible populations.
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These discussions focused on the vector's paramount importance to the ecology and epidemiology of vector-borne diseases, a role which complicates transmission patterns, but which also provides opportunities for disease control. Dynamics of Disease Transmission A standard graphic representation of the ecology of infectious disease features host, pathogen, and environment as circles intersecting in a common zone that defines permissive conditions for disease transmission (see Figure SA-3)
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His findings, which are discussed in detail in Chapter 2 (see Scott and Morrison) , as well as in a subsequent section of this summary, "Lessons Learned: Case Studies of Vector-Borne Diseases," support the notion that heterogeneity in exposure to infection can be exploited to optimize vector control.
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Based on these findings, they developed a cost-efficient spray plan that targeted those areas of the city that would have the largest impact; the timely implementation of this plan by local authorities appears to have averted an epidemic. This result, and those of similar studies, imply that careful modeling of transmission patterns and vector life cycles may suggest FIGURE SA-4 Factors affecting plant disease outbreaks.
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that have recently been launched for malaria and dengue and which could potentially be extrapolated to other vector-borne diseases, according to speaker Barry Beaty of Colorado State University. The development of both DSSs is supported by the Innovative Vector Control Consortium (IVCC)
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. In his contribution to Chapter 1, Gubler presents case studies of three reemergent arboviral diseases -- West Nile viral fever, dengue and DHF, and yellow fever -- that illustrate the epidemiological effects of pristine populations and environmental change, which include animal and wildlife hosts.
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SA-5 Trade and transportation have greatly contributed to the global spread of plant disease vectors, as described below, but as Almeida observes in his contribution to Chapter 1, many emerging infectious diseases of plants -- and indeed of humans and animals as well -- can also be viewed as a byproduct of agriculture. "The expansion of agricultural land and increased pesticide, irrigation, and fertilizer use have been the major controllable inputs to increase crop yield," he states, but these gains have come at a price.
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. 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 (Marin/ Sonoma Mosquito and Vector Control District, 2005)
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As long as the deer population of the eastern United States was limited by farming and hunting to a few small, isolated bands, Lyme disease -- though probably present -- was unrecognized. The decline of agriculture 13 Human-induced land use changes are the primary drivers of a range of infectious disease outbreaks and emergence events and also modifiers of the transmission of endemic infections (Patz et al., 2000)
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. 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 (CIESIN Thematic Guide, 2007; Epstein et al., 1998; Gubler, 1998)
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If immature mosquito habitats remain flooded for a month or more secondary Culex mosquito vector populations will surge and become infected after feeding on viremic domestic animals, and potentially cause an epizootic and/or epidemic. With this information, the researchers developed an operational model capable of predicting RVF outbreaks based on ocean temperatures, rainfall anomalies, and vegetation characteristics.
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These events illustrate 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 vectorborne diseases by tracking and integrating factors critical to disease transmission, Linthicum said.
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. The future effects of climate change and extreme weather events on disease emergence and resurgence, a subject of debate among researchers, was raised by workshop participants in a discussion that anticipated a detailed exploration of this topic at a December 2007 Forum public workshop entitled Global Climate Change and Extreme Weather Events: Understanding the Potential Contributions to the Emergence, Reemergence, and Spread of Infectious Disease.
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FIGURE SA-6 Map of the distribution of bluetongue throughout Europe as of November 28, 2007. SOURCE: Copyright European Communities (2007)
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Surveillance and Detection As is true for all infectious diseases, the early detection of vector-borne disease outbreaks is essential to their control. In Mexico, for example, the detection
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An important, and under-recognized, impact of the emergence of WNV in the United States is the threat it poses to the nation's blood supply. Within 3 years of its arrival, Petersen said, WNV had become the most common transfusion 17 For a discussion of the strengths and challenges of infectious disease detection and surveillance, see the summary report of the Forum's recent workshop, Global Infectious Disease Surveillance and Detection: Assessing the Challenges -- Finding Solutions (2007)
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. Speaker Charles Calisher of Colorado State University discussed findings that suggest ENSO influences outbreaks of HPS, a rodent-borne viral disease of which the first human epidemic was reported in the spring of 1993 (see Calisher in Chapter 2)
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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, IVCC supports the development of novel insecticides and deployment methods as one of its foremost goals (see Eisen and Beaty in Chapter 2)
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"Therefore an integrated program with vector reduction and immunization will more effectively prevent epidemic dengue and is more sustainable than either strategy alone." However, he noted, 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." Multidisciplinary Research and Management Research on infectious diseases must often be conducted in the midst of epidemics and in concert with management efforts. This challenging process was described by speaker David Rizzo of the University of California, Davis, who has worked to understand and mitigate the effects of SOD in California since shortly after its emergence there in the mid-1990s (see also Chapter 2 Overview)
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Needs and Opportunities Insofar as vector-borne diseases represent the more general class of emerging infectious diseases, they entail a host of needs and opportunities all too familiar to workshop participants, and well characterized in numerous reviews and reports (Karesh and Cook, 2005; NRC, 2005) , including the founding documents of the Forum on Microbial Threats, Emerging Infections: Microbial Threats to Health in the United States (1992)
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. Roger Nasci, chief of the Arboviral Diseases Branch at CDC's Division of Vector-Borne Infectious Diseases, described multidisciplinary teams as "essential" to addressing international health problems, but also noted the difficulties in coordinating such teams (e.g., the previously discussed response to RVF in Kenya)
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. The following information was deemed essential by workshop presenters: • 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 Field studies of vectors are crucial to answering many of these questions; however, as several participants who engage in such research attested, this work is not well funded.
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. Barriers to Implementation There is a general lack of infrastructure for implementing vector-borne disease interventions in most settings, Morens observed, whether they are cities in the United States -- as revealed by the introduction of Lyme disease -- or impoverished countries where vector-borne diseases cause major morbidity and
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WNV is certainly not the last mosquito-borne virus that will invade the United States, Fish predicted, but without sustained federal support for surveillance and control of such diseases, "we will again be vulnerable to threats, accidental or not, and incapable of prompt action that could curb or prevent epidemics." Reflecting on this situation, Forum member George Korch wondered aloud how governments and industry might be convinced to invest in addressing vectorborne diseases. "What is the product that medical entomology and infectious disease studies provide?
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2004. Emerging infectious diseases of plants: pathogen pollution, climate change and agrotechnology drivers.
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2007a. Questions and answers about Rift Valley fever, http://www.cdc.gov/ncidod/dvrd/spb/ mnpages/dispages/rvf/rvf_qa.htm (accessed October 19, 2007)
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2007b. Animal health disease cards: bluetongue, http://www.fao.org/ag/againfo/subjects/en/ health/diseases-cards/bluetongue.html (accessed October 19, 2007)
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Presentation at the Forum on Microbial Threats workshop entitled "Vector-borne diseases: understanding the environmental, human health, and ecological connections," June 19-20, 2007, Ft. Collins, CO.
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Emerging Infectious Diseases 4(3)
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Emerging Infectious Diseases 9(1)
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Emerging Infectious Diseases 12(10)
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