Vector-borne disease refers to the transmission of a disease that is caused by a microorganism from one organism (the host) to another organism via a third organism (the vector). Put another way, a vector is the means by which microbes can get from their normal place of residence, where they typically cause no harm, to a susceptible organism, in which an infection results.
There are numerous examples of vector-borne diseases that involve a variety of pathogens and vectors, including such well-known maladies as malaria, yellow fever, Lyme disease, plague, and West Nile disease.
Vector-borne diseases are characterized by the vectormediated movement of a microorganism (such as a bacterium, virus, or protozoa) from the host to a recipient. The host and recipient can belong to the same species. A well-known example of this is malaria, in which the protozoan that causes the infection is acquired from an infected person by a mosquito during a blood meal and transferred to another human that the mosquito subsequently feeds on. Alternatively, the host and recipient can belong to 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.
Dengue hemorrhagic fever is another example of a vector-borne disease. Dengue is caused by a virus in the Flavivirus genus. The virus is transmitted from the host to the susceptible person by a species of mosquito called Aedes aegypti.
Lyme disease is also a vector-borne disease. The disease, which is transmitted from contaminated animals such as deer to humans by the bite of several species of tick, is the most prevalent tick-borne ailment in North America. Lyme disease is caused by a bacterium called Borrelia burgdorferi. It is very debilitating if not treated promptly, and can cause severe fatigue, joint pain, and heart trouble that can persist for years even when the disease is diagnosed and treated.
Still another bacterial vector-borne disease is plague. The disease, which is caused by Yersinia pestis, is ancient. Passages found in the Old Testament of the Bible describe the ravages of epidemics of plague. 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 that they bite. There are several types of plague, depending on the site of the infection. Infection of the lungs (pneumonic plague) is almost always fatal within a week if not treated.
A final example of a vector-borne disease is yellow fever. Another viral disease caused by a member of the Flavivirus genus, the disease is transferred from the host (a type of monkey) to humans via a mosquito. In tropical regions, devastating outbreaks of yellow fever have occurred over the past several hundred years. A noteworthy outbreak occurred during the construction of the Panama Canal. Even today, several hundred thousand people are infected each year and about 30,000 die, according to the World Health Organization.
Vector-borne diseases occur worldwide. While some diseases, such as malaria, are concentrated in tropical equatorial regions of the globe, other diseases can occur in more temperate climates. One example of a vector-borne disease found in more temperate regions is the mosquito-borne disease called West Nile disease. The West Nile virus that causes this disease has spread as far north as Canada, where it can be transmitted by mosquitoes during the warmer months of the year and even during the cooler days of spring by mosquitoes that have survived the winter.
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, the carefully controlled application of dichloro-diphenyl-trichloroethane (DDT), a powerful insecticide used in the 1960s that effectively reduced malaria vectors, but also resulted in significant loss of bird populations, is again beginning to be used as a means of mosquito control.
Another efficient and environmentally friendly way of controlling the mosquito-borne spread of malarial protozoa is draping beds with insecticide-treated mosquito netting to protect people during sleep. Organizations such as World Vision have campaigns to supply villages in Africa with bed netting. Similarly, protective clothing with overlapping upper and lower layers minimize the amount of skin that is exposed to a bite from the vector.
WORDS TO KNOW
HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.
VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.
Another trial program aimed at preventing malaria involves releasing laboratory-bred infertile male mosquitoes. The program is based on the hypothesis that a greater population of infertile males will decrease the numbers of female mosquitoes due to reduced reproductive success. Since malaria is transmitted by female mosquitoes, fewer female mosquitoes should result in a reduction in the number of cases of malaria. There have been several successful small scale trials of this program, but the large number of sterile male mosquitoes needed is likely to make this approach impractical for larger scale implementation.
Other treatment and prevention strategies include vaccine development and the use of genetic material (known as morpholino antisense oligonucleotides) that can out-compete viral genetic material for binding onto cells of the host, blocking a crucial step in the formation of new virus particles.
Vector-borne diseases exact a large toll worldwide. For example, more than 500 million cases of malaria occur each year, with approximately three million deaths attributed to the disease. One million of these deaths are children. The disease is particularly prevalent in Africa. The World Health Organization (WHO) estimates that more than 2.5 billion people are at risk for malaria. Yellow fever continues to infect hundreds of thousands of people in less developed tropical countries annually, despite the fact that a vaccine exists that can provide long-term protection. In Asia, a form of encephalitis puts about three billion people at risk each year.
The burden of these and other vector-borne diseases have a substantial economic and social impact on areas of the world that are already destitute. In malariaprone areas, school attendance can be poor, as school-children are either sick, tending for other sick family members, or have been pressed into work as parents and older siblings can no longer work due to illness. With a lack of education, hope for a more promising future can diminish, and the economy of a nation can be undermined by the illness of a sizable portion of the work force.
IN CONTEXT: CHILDREN LESS THAN FIVE-YEARS-OLD SLEEPING UNDER INSECTICIDE-TREATED NETS
One means of preventing vector-borne disease is sleeping under treated nets. According to the U.S. Centers for Disease Control and Prevention (CDC), and more than one million people die from malaria (just one of many vector-transmitted diseases) per year. Young children are particularly at risk for many vector-borne disease. In some areas, bed nets are considered one of the most sustainable effective means to fight vector-borne disease.
Although disease risks vary, the list below reflects selected data from the World Health Organization that demonstrates the widest spectrum in results reported by WHO as of February 2007. Data was not available for all countries, including a lack of data for: Bangladesh, Brazil, China, Cuba, Egypt, Haiti, India, Mexico, Philippines, and not reported from some nations such as the United Kingdom and United States of America).
Lowest reported percentage of children under age 5 sleeping under insecticide-treated nets (with year data collected or reported):
- Indonesia 0.1% (2000)
- Swaziland 0.1% (2000)
- Madagascar 0.2% (2000)
- Uganda 0.2% (2000–01)
- Somalia 0.3% (1999)
- Sudan 0.4% (2000)
- Chad 0.6% (2000)
- Democratic Republic of the Congo 0.7% (2001)
- Equatorial Guinea 0.7% (2000)
- Cameroon 0.9% (2004)
- Niger 1% (2000)
Mid-range reported percentage of children under age 5 sleeping under insecticide-treated nets:
- Kenya 4.6% (2003)
- Rwanda 5% (2000)
- Zambia 6.5% (2001-02)
Highest percentage of children under age 5 sleeping under insecticide-treated nets:
- Viet Nam 15.8% (2000)
- Sao Tome and Principe 22.8% (2000)
- Malawi 35.5% (2004)
SOURCE: World malaria report 2005. Geneva, World Health Organization and United Nations Children's Fund, 2005
Through its Healthy Environments for Children Alliance, the WHO seeks to reduce environmental risks posed to children in under-developed countries; a major part of this program is directed at vector-borne diseases. Areas of concern include control of the spread of the vectors from their breeding grounds, the effect of increasing urbanization on the proximity of people to vector breeding grounds, and the poorer nutrition of under-developed regions (which can affect the efficiency of the immune system).
Vector-borne diseases are often difficult to treat. The vector is mobile and may be capable of movement over considerable distances. In addition, vectors can develop resistance to insecticides, as has occurred in some malaria prevention programs. Many vector insects, such as mosquitoes, have been around for millennia, and one reason for their persistence is their ability to adapt to changing environmental circumstances. Knowledge of a vector's habitat, life cycle, and migratory patterns is crucial in any effort to reduce the vector-borne spread of disease.
Global climate change is another factor in vectorborne disease. Some vectors, such as mosquitoes, thrive in warmer climates. The recent warming of the Earth's atmosphere could allow mosquitoes to inhabit more of the globe, which would undoubtedly increase the incidence of malaria and other diseases spread by these vectors.
The global nature of modern travel is an additional contributing factor to vector-borne disease transmission. Products and foods that harbor an insect vector can move virtually anywhere within hours. This increases the need for scrutiny of imported items at border crossings. In areas of the world where certain vector-borne diseases are endemic, aircraft are now routinely sprayed with insecticide after the hatches are closed and before takeoff to prevent any potential arthropod disease vectors aboard from reaching a new destination.
Honigsbaum, Mark. The Fever Trail: In Search of the Cure for Malaria. New York: Picador, 2003.
Marquardt, William H. Biology of Disease Vectors. 2nd ed. New York: Academic Press, 2004.
Marqulies, Phillip. West Nile Virus. New York: Rosen Publishing Group, 2003.
Centers for Disease Control and Prevention. “Malaria: Vector Control.” <http://www.cdc.gov/malaria/control_prevention/vector_control.htm> (accessed February 14, 2007).