What is vector-borne disease?
Vector-borne disease is disease caused by microorganisms, such as bacteria, viruses, or protozoa, that are transferred from one living thing (a host) to another living thing (a recipient) through a third living thing (a vector).
With vector-borne disease, the host and recipient can be of the same species; a well-known example is malaria, in which the parasitic protozoan is acquired from an infected person by a mosquito during a blood meal and transferred to another human that the mosquito subsequently feeds on. The host and recipient also can be of different species. An example is western equine encephalitis, in which the host is a bird that is infected by the disease-causing species of arbovirus and the recipient is a horse or a human; as with malaria, the vector is a mosquito.
Another example of a vector-borne disease is dengue fever, in which a flavivirus is transmitted from the host to the susceptible person by a species of mosquito called Aedes aegypti. Another vector-borne disease, and one that is spreading in eastern North America, is Lyme disease. Caused by the bacterium Borrelia burgdorferi, Lyme disease is transmitted from contaminated animals, such as deer, to humans by the bite of several species of tick. Although Lyme disease is easily treated early in the infection (when its hallmark is a bulls-eye pattern at the point of the tick bite), the disease becomes difficult to treat and is debilitating if not treated promptly. Symptoms include severe fatigue, joint pain, and heart trouble that can persist for years, even if diagnosed and treated.
An ancient vector-borne disease is plague. The disease, which is caused by the bacterium Yersinia pestis, is described in passages of the Old Testament. Rodents harbor the bacterium. The vector that transmits the bacterium from rodents to humans is another rat or, more commonly, a flea. Both can feed on an infected rat and subsequently spread the infection to a human through a bite. Several types of plague exist, depending on the site of the infection. Infection of the lungs (pneumonic plague) is almost always fatal within one week if not treated.
A final example of a vector-borne disease is yellow fever. Also caused by a flavivirus, the disease is transferred from the host (a species of monkey) to humans through a mosquito. Yellow fever has caused huge outbreaks in tropical regions; one notable outbreak occurred during the original construction of the Panama Canal. Each year, yellow fever sickens several hundred thousand people and kills an estimated thirty thousand people.
Vector-borne diseases occur worldwide. While some diseases, such as malaria and yellow fever, are concentrated in tropical equatorial regions of the globe, the transmission of other diseases can occur in more temperate climates. An example is mosquito-borne West Nile virus disease. The West Nile virus that causes the disease also has spread to Canada, where it can be transmitted by mosquitoes during warmer months and even during the cooler days of spring by mosquitoes that have survived the cold Canadian winter.
Global warming has led to an increase in territory that is habitable for vectors such as the mosquito. The expanding geographic distribution of malaria has been documented. As global warming continues, vector-borne diseases are expected to continue to expand geographically.
Vector-borne diseases can be treated and even prevented by interrupting the vector-mediated transmission between the infected host and the susceptible person or animal. Treatment and prevention strategies for malaria focus on the mosquito vector. For example, spraying mosquito breeding grounds with insecticide can be an effective control. Indeed, mosquito control now involves the carefully controlled use of dichloro-diphenyl-trichloroethane (DDT).
Another efficient and environmentally friendly means of controlling the mosquito-borne spread of malarial protozoa is the use of mosquito netting (sleeping nets) to protect people at night. Organizations such as World Vision have patron-sponsored campaigns to supply villages in Africa with mosquito netting. Similarly, protective clothing with overlapping upper and lower layers minimizes exposed skin, which is susceptible to a bite from a vector.
A trial prevention program involved the release of laboratory-bred infertile male mosquitoes. Because malaria transmission requires female mosquitoes, it is hoped that the reduced reproductive success resulting from a greater population of infertile males will decrease the numbers of females.
Other treatment and prevention strategies include vaccine development and the use of genetic material (known as morpholino antisense oligonucleotides) to compete with viral genetic material for control of binding sites to host sites, which are critical to the formation of new virus particles.
Organizations including the World Health Organization are promoting a vector-control program known as integrated vector management, which seeks to prevent disease transmission and to optimize the environment. An example of this approach is the rational design of drinking-water delivery systems to reduce the harmful presence of stagnant water (which is a breeding ground for mosquitoes), to minimize deforestation, and to optimize the protection of water quality. Implemented solutions are relevant to a particular region.
That some infectious agents can be moved from one organism to another by means of another organism (a vector) is critical to disease transmission. Classic examples of this form of transmission are malaria, plague, and yellow fever, which have exacted a huge toll on human life. For example, the number of malaria infections each year is about 500 million, leading to approximately 3 million deaths. Infamously, plague led to millions of deaths worldwide in the fourteenth century. Yellow fever continues to infect hundreds of thousands of people in developing tropical countries each year, despite the existence of a vaccine capable of long-term protection.
Vector-borne diseases are difficult to treat. The vector is mobile and capable of movement over considerable distances. Additionally, as has been clear by the use of insecticides to kill mosquitoes in malaria prevention programs, one can see vector resistance to these compounds. Insects such as mosquitoes have been around for millennia, primarily because of their ready adaptation to change.
To lessen the effects of vector-borne disease, science needs to understand vector habitats, life cycles, and migratory patterns. Global climate change is another major concern in the study of vector-borne disease.
Brower, Vicki. “Vector-Borne Diseases and Global Warming: Are Both on an Upward Swing?” EMBO Reports 2 (2001): 755-757. Examines the medical and political controversy about whether or not warmer global temperatures are increasing the incidence of vector-borne diseases.
Gratz, Norman. Vector- and Rodent-Borne Diseases in Europe and North America: Distribution, Public Health Burden, and Control. New York: Cambridge University Press, 2006. Details diseases that are spread by vectors. Covers the costs to public health and the control and distribution of vector-borne diseases.
Marquardt, William C., ed. Biology of Disease Vectors. 2d ed. New York: Academic Press/Elsevier, 2005. An excellent reference for understanding the role of vectors in the transmission of infectious diseases. Discusses prevention and control strategies and future implications.
Tolle, Michael A. “Mosquito-Borne Diseases.” Current Problems in Pediatric and Adolescent Health Care 39 (2009): 97-140. A thorough review of the life cycles of insects as disease agents. Includes discussion of the diagnoses, treatments, and vaccines for several mosquito-borne diseases. Special focus on the impact of the diseases on pregnant women and on children.