Meredith G. Morrow Environmental Public Health Topic Diseases transmitted by arthropod vectors are of major importance to the health of both humans and animals. Every year, more than 500 million people become severely ill with malaria (WHO, 2007). In addition, Lyme disease is currently the fastest growing infectious disease in the world (Vanderhoof-Forschner, 1997): between 1991 and 2000, the incidence of Lyme Disease in the United States nearly doubled and the 17,730 cases reported for the year 2000 was higher than any other previous year (Schwan, 2003). Disease patterns differ from one geographic zone to another, and may change over time due to a plethora of factors. The objective of this paper is to evaluate various environmental factors and how they may impact vector-borne disease outbreaks in the future. Issues and Facts Anthropogenic environmental change is acknowledged as a primary factor in the emergence of diseases (Derraick and Slaney, 2007). The ecology and epidemiology of vector-borne diseases are affected by the interrelations between three major factors comprising the environment, pathogen, and the host (human, animal or vector). For instance, the emergence of malaria in a population can be associated with an increased number of mosquito vectors due to stagnant water (the environment), the development of drug-resistant parasites (the pathogen), or human migration (the host).
Important factors that play a role in the emergence and spread of vector-borne diseases include habitat changes, alterations in water storage and irrigation habits, atmospheric and climate changes, immunosuppression by HIV, development of insecticide and drug resistance, globalization and the significant increase in international trade, tourism and travel (Harrus et al, 2005). War and governmental management failure are also major contributors to the spread of disease (Harrus et al, 2005). While the breadth of this issue obviously stretches outside the scope of this paper, I will focus on two anthropogenic environmental changes not discussed in class (anthropogenic
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eutrophication and urbanization) and how these have and will most likely continue to effect vector-borne disease infection rates.
Anthropogenic Eutrophication Anthropogenic eutrophication refers to increasing nutrient concentrations (primarily phosphorus, nitrogen and carbon) in receiving bodies of water and the consequent rise in organic productivity (Williams and Hecky, 2001; Paerl, 1997). Runoff from agricultural fields, urban lawns, golf courses, and untreated, or partially-treated, domestic sewage are all major sources of excess nutrients. Certain aquatic plants, (specifically water lettuce and water hyacinth) thrive in these nutrient-rich bodies of water and can double its biomass in a matter of days depending on temperature (Fonkou et al, 2002; Aoi and Hayaski, 1996). This uncontrolled growth produces extensive mats capable of blocking navigational channels, and impeding water flow in irrigation and flood canals (Stuckey and Les, 1984). This overgrowth is an indirect threat to the public health since certain species of mosquitoes are dependent upon these aquatic plants for their survival (Lounibos and Escher, 1993). Unlike other immature mosquitoes which breathe at the surface of the water and are susceptible to chemical controls, the larvae and pupae of Mansonia mosquitoes are unique because they remain attached to the roots of water lettuce in order to obtain oxygen (Lounibos and DeWald, 1989). Mansonia spp. are opportunistic feeders with a broad host range (Lounibos and Escher, 1985). Mansonia spp. will take a blood meal from a wide variety of hosts and therefore pose a more serious risk to humans and other animals. Since they readily feed on possible reservoirs of disease and humans (with no preference) they increase the chance of mosquito-borne disease transmission (Edman, 1971).
Urbanization Urbanization can also be implicated in the spread of the many diseases within human and animal populations. As human population continues to steadily increase, we are forced to expand into areas which were once untouched (Patz et al, 2004). We are living with, and around, various plants and animals that were once in a separate ecosystem. As a consequence, we are increasingly exposed to various reservoirs and vectors of disease, thereby increasing the likelihood of transmission. Human expansion
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has also resulted in a gradual reduction of the predators that normally keep disease reservoir populations in check. For instance, the main Lyme disease reservoirs are deer and the white-footed mouse. The white footed mouse also has a wide habitat tolerance; therefore, a species-poor community tends to have more mice, whereas a species-rich community has mice in addition to many other potential hosts (LoGiudice et al, 2003). This may serve to dilute the impact of Lyme disease on humans if there is an abundance of other hosts a tick is able to feed on.
This theory is called the Dilution Effect
hypothesis which states that infection prevalence of ticks is decreased with increasing host diversity (LoGiudice et al, 2003). In effect, if there are a plentiful number (plethora) of species to choose from within the community (rather than just an overabundance of the pathogen-carrying reservoir) ticks may never even become infected with Borrelia burgdorferi (the bacterium which causes Lyme disease).
Opinion and Conclusion I suspect that in the coming years, there will be an increase in vector-borne disease outbreaks, along with an increase in the number of humans affected by these diseases. While Public Health agencies in the developed countries tend to keep the number of mosquitoes controlled in areas where West Nile Virus (and other mosquitoborne diseases) have been known to occur (via aerial spraying, larvicidal applications, etc.), who is to say that this control structure will always be available? As was mentioned earlier in class, if the impending energy crisis does affect the United States as drastically as LaPuma predicts, one can assume that the number of vectors and the diseases they spread will not be as highly controlled in such a systematic way as they are today. A perfect example of this can be seen in the aftermath and the chaos of Hurricane Katrina; it took months before a mosquito control routine was formed and adequately implemented. As globalization continues to escalate, accidental importation of disease vectors and the introduction of new pathogens into areas where they were previously not found will become inevitable. All of these and the aforementioned factors will play a role in our future, and the future of vector-borne disease control.
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References Cited Aoi T, Hayashi T. 1996. Nutrient Removal by Pistia stratiotes (Pistia stratiotes). Wat Sci Tech. 34(7-8): 407-412. Derraick and Slaney. (2007) Anthropogenic Environmental Change, Mosquito-borne Diseases and Human Health in New Zealand. EcoHealth. 4(1): 72-81. Edman JD. 1971. Host-feeding patterns of Florida mosquitoes. I. Aedes, Anopheles, Coquillettidia, Mansonia and Psorophora. Journal of Medical Entomology.8:687-695. Fonkou et al. 2002. Potentials of Pistia stratiotes (Pistia stratiotes) in domestic sewage treatment with macrophytic lagoon systems in Cameroon. Proceedings of the Sixth International Symposium on Environmental Pollution and Contaminated Wastewater Management. 7(10): 709-714. Harrus et al. (2005) Drivers for the emergence and re-emergence of vector-borne protozoal and bacterial diseases. International Journal for Parasitology. 35(11-12): 1309-1318. LoGiudice et al. (2003) The ecology of infectious disease: effects of host diversity and community composition on Lyme disease risk. Proceedings of the National Academy of Sciences of the USA. 100(2): 567-571. Lounibos, L.P. and Escher, R.L. 1985. Mosquitoes Associated with Water Lettuce in Southeastern Florida. The Florida Entomologist. 68(1): 169-177. Lounibos, L.P. and DeWald, L.B. 1989. Oviposition site selection by Mansonia mosquitoes on water lettuce. Ecological Entomology. 14(4): 413-422. Lounibos, L.P. and Escher, R.L. 1993. Insect associates of Pistia stratiotes (Arales: Araceae) in southeastern Florida. Florida Entomologist. 76(3): 473-500. Paerl HW. 1997. Coastal Eutrophication and Harmful Algal Blooms: Importance of Atmospheric Deposition and Groundwater as "New" Nitrogen and Other Nutrient Sources. Limnology and Oceanography. 42(5): 1154-1165. Patz et al. (2004) Unhealthy landscapes: Policy recommendations on land use change and infectious disease emergence. Environmental Health Perspectives. 112(10):1092-1098. Schwan, T.G. (2003). Temporal regulation of outer surface proteins of the Lyme-disease spirochaete Borrelia burgdorferi. Biochemical Society Transactions. 31(1): 108-112. Stuckey RL, Les DH. 1984. Pistia stratiotes (Pistia stratiotes) recorded from Florida in Bartram’s travels, 1765-1774. Aquaphyte 4: 6. WHO. (2007) World Health Organization: Global Malaria Programme. Avail from: http://www.who.int/malaria. Accessed 08/06/07. Williams AE, Hecky RE. 2001. Invasive Aquatic Weeds and Eutrophication: The Case of Water Hyacinth in Lake Victoria. University of Waterloo: Ontario, Canada: 1-50. Vanderhoof-Forschner, K. (1997). Everything You Need To Know About Lyme Disease (and other tick-borne disorders). pp. 16, 145-147. John Wiley and Sons, New York.
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