Host and Vector
Host and Vector
The terms host and vector refer to the route of transmission of some infectious diseases to humans and animals.
The host is the living being that the bacteria, virus, protozoan, or other disease-causing microorganism normally resides in. Some bird species, for example are normal hosts to arboviruses such as West Nile virus. Typically, the microorganism does little or no harm to the host, which is important if the disease-causing organism is to successfully persist in that host over time. Occasionally, the host population maintains the organism even though some members suffer from infection caused by it. Several species of birds in North America have experienced West Nile infection although they are considered the natural host.
A reservoir host, or simply a reservoir, refers to a living (human, animal, insect, or plant) or non-living (soil, water) entity where a disease-causing organism can normally live and multiply. A host in which a parasite resides to sexual maturity is called a primary host, and a host in which a parasite spends only part of its life cycle or does not reach sexual maturity is called an intermediate host. Certain species of snails, for example, are the intermediate host of the Schistosoma larvae that are responsible for causing the disease bilharzia in humans.
A vector is an organism that helps transmit infection from one host to another. For example, the mosquito serves as the vector to infect humans with the West Nile virus. The mosquito acquires the virus from birds when it takes a blood meal. If the same mosquito subsequently feeds on a human, the virus can be transferred, and the result can be West Nile disease in humans.
The host-vector route of transmission is responsible for a number of diseases including several types of encephalitis that sicken humans and horses (Western equine encephalitis, Eastern equine encephalitis, and St. Louis encephalitis). Malaria, which is caused by a number of protozoans of the genus Plasmodium (the most common and serious forms of malaria are caused by P. falciparum and P. vivax) is also a vector-borne disease. The vector for transmission of the malaria protozoa is also the mosquito. Typically, the host is another human whose blood harbors the protozoan. As with encephalitis, the mosquito acquires the microbe when it feeds on the infected host, and transfers the microbe to a susceptible human host when it seeks another blood meal.
Mosquitoes also function as the vectors in the transmission of arbovirus species that cause Yellow fever and Dengue fever in humans. Other examples of potential disease vectors include flies, mites, fleas, ticks, rats, skunks, and even dogs.
The host-vector route of disease transmission occurs globally. Some diseases are confined to certain regions of the world. One example is malaria, which is associated with equatorial regions. Malaria's influence is huge; the World Health Organization estimates that 350 to 500 million cases of malaria and up to three million deaths occur each year. Other vector-borne diseases can be present even in colder climates. For example, West Nile disease is increasing in Canada.
The best way to eliminate host-vector diseases is to break the vector-mediated chain of transmission between the infected host and the susceptible person or animal that will become a new host. In the case of malaria, for example, spraying areas that are breeding grounds for mosquitoes can help curb their population, and so reduce the likelihood of disease transmission. In some malaria-prone areas of Africa, the use of dichlorodiphenyltrichloroethane (DDT) is being advocated as a means of mosquito control. Despite the infamous history of DDT due to its overuse and resulting environmental harm, its controlled application may be a relatively safe means of host-vector control.
Another means of malaria host-vector control that is becoming more widely practiced is the use of mosquito netting to protect people while they sleep. This inexpensive and easy-to-use method prevents the mosquito from feeding on a sleeping person and interrupts the transmission path of the Plasmodium protozoan.
Similarly, protective clothing can minimize the chance that a vector will be able to get access to unexposed skin.
More exotic vector control approaches are being explored by scientists. An example is an ongoing program to breed and release male mosquitoes that cannot breed into malaria-prone regions. The intention is that, since malaria is transmitted only by female mosquitoes, the lack of availability of a male breeding partner will drive down the female population over time.
Changing the behavior of vectors influences the transmission of a disease. Knowledge of a vector's habitat, life cycle, behavior, and migratory patterns, for example, is vital to efforts to curb the spread of disease. Vectorborne diseases with simple transmission cycles can be difficult to treat and prevent. This is because the vectors are living things that are often capable of moving from one location to another, sometimes over thousands of miles.
Threats from vector-borne diseases with complicated transmission cycles that involve one or more intermediate hosts are sometimes easier to eliminate. This is because breaking only one link in the disease transmission chain will result in fewer infections. Guinea worm disease, for example, infected 3 to 5 million people in Asia and Africa about 20 years ago. Through an international effort, ponds in endemic areas were treated with a simple insecticide that eliminated the intermediate host, a copepod or “water flea”, but left the water potable (drinkable). By 2006, cases of Guinea worm infection numbered fewer than 12,000 in Africa, and the disease was eliminated from Asia.
A looming issue for host-vector diseases involves climate change. As vector-borne diseases such as malaria are associated with warmer climates, some researchers have warned that the increasing warming of the Earth's atmosphere could expand the habitat of mosquito species, and so increase the prevalence of mosquito-borne diseases such as malaria.
WORDS TO KNOW
INTERMEDIATE HOST: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.
PRIMARY HOST: The primary host is an organism that provides food and shelter for a parasite while allowing it to become sexually mature, while a secondary host is one occupied by a parasite during the larval or asexual stages of its life cycle.
RESERVOIR: The animal or organism in which the virus or parasite normally resides.
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.
IN CONTEXT: REDUCING COSTS AND RISKS OF VECTOR CONTROL
Integrated vector management (IVM) strategies are emerging as part of an effort to achieve effective disease-control at costs countries can afford and at the same time minimize potential negative impacts on biodiversity, ecosystems, and public health (e.g., reduce risks related to pesticides, bioaccumulation of toxic or potentially toxic chemicals).
The World Health Organization (WHO) Global Strategic Framework for Integrated Vector Management defines IVM as a strategy to “improve the efficacy, cost-effectiveness, ecological soundness and sustainability of disease vector control. IVM encourages a multi-disease control approach, integration with other disease control measures and the considered and systematic application of a range of interventions, often in combination and synergistically.”
The IVM approach is also designed to reduce the development of vector resistance to vector control measures (e.g., increasing resistance to pesticides).
Cost effectiveness is an important aspect of IVM strategy. For example, officials in Sri Lanka initially indicate that “costs of periodic river flushing to eliminate mosquito breeding habitats compared favourably with the use of insecticide-impregnated bednets as a mosquito-control measure.”
SOURCE: World Health Organization
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: Epidemics Deadly Diseases throughout History. New York: Rosen Publishing Group, 2003.
Centers for Disease Control and Prevention. “Division of Vector-Borne Diseases” <http://www.cdc.gov/ncidod/dvbid/> (accessed April 2, 2007).