Vector-borne disease
Vector-borne diseases are illnesses caused by pathogens such as bacteria, viruses, or protozoa that are transmitted from one host to another through a third organism known as a vector. Common vectors include mosquitoes and ticks, which facilitate the spread of diseases like malaria, dengue fever, Lyme disease, and yellow fever. For instance, malaria is transmitted from infected humans to other humans via Anopheles mosquitoes, while Lyme disease is spread from animals like deer to humans through tick bites. These diseases are not limited to tropical regions; they can occur in temperate climates as well, as seen with West Nile virus infections in Canada.
The impact of vector-borne diseases is significant, with millions of cases and hundreds of thousands of deaths reported annually. Prevention strategies focus on interrupting the transmission cycle, including the use of insecticides, mosquito netting, and other protective measures. Integrated vector management approaches aim to optimize environmental conditions to reduce vector habitats. However, the adaptability of vectors and the effects of global climate change pose ongoing challenges for controlling these diseases, highlighting the need for continued research and innovative solutions.
Vector-borne disease
Definition
Vector-borne disease is a 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).
Types and Examples
With vector-borne disease, the host and recipient can be of the same species; a well-known example is malaria—an individual infected with the parasitic protozoan that causes malaria is bitten by a mosquito. The mosquito transfers the protozoan to the next person it bites. Sometimes, the original host and the subsequent recipient may be of different species, such as with western equine encephalitis, which can transfer from an infected bird to a human or a horse through a mosquito vector.
Another example of a vector-borne disease is dengue fever, in which a flavivirus is transmitted from the host to a human by the mosquito species Aedes aegypti. The bacterium Borrelia burgdorferi, causes a vector-borne disease called Lyme disease, which is relatively common in eastern North America. It is commonly transmitted by several tick species that bite deer infected with the bacteria. When these ticks fall off of deer and then bite a human, the disease is transferred. 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, which is harbored by rodents. The disease, which is caused by the bacterium Yersinia pestis, is described in passages from the Christian Bible's Old Testament. While rats can be a vector for the plague, transmitting the disease from rodents to humans, fleas that have bitten infected rats are the most common vector. If untreated, some types of plague, such as pneumonic plague which infects the lungs, are fatal.
Also caused by a flavivirus, yellow fever 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, 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 spread to Canada, where it could be transmitted by mosquitoes during warmer months and even during the cooler days of spring by mosquitoes that survive 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 Control
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. Malaria treatment and prevention strategies aim to eliminate mosquito vectors through several methods, such as by spraying breeding grounds with insecticide. Although dichloro-diphenyl-trichloroethane (DDT) and other persistent organic pollutants (POPs) have been widely banned, their use was still allowed into the mid-2020s as an insecticide for controlling malaria-bearing mosquitos. However, Hindustan Insecticide in India, the only DDT production site worldwide, began decreasing DDT production in 2024.
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.
One type of prevention program called the Sterile Insect Technique involves the release of laboratory-bred infertile male mosquitoes. Because malaria transmission requires female mosquitoes, the reduced reproductive success resulting from a greater population of infertile males decreases the number 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.
The World Health Organization’s Integrated Vector Management program seeks to prevent transmission of diseases like malaria, dengue, Zika, chikungunya, Japanese encephalitis, leishmaniasis, schistosomiasis, and Chagas disease by creating sustainable, cost-effective, and efficient initiatives. These include public education, monitoring and surveillance, and the reduction of vector breeding grounds. For example, reducing stagnant water (a breeding ground for mosquitoes), minimizing deforestation, and optimizing water quality protection. Implemented solutions are relevant to a particular region.
Impact
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, there were 263 million cases of malaria infections in 2023, with approximately 597,000 deaths. Infamously, plague led to millions of deaths worldwide in the fourteenth century. Yellow fever infected hundreds of thousands of people in developing tropical countries each year until a vaccine capable of long-term protection was able to be widely distributed in the 2010s.
Vector-borne diseases are difficult to treat. The vector is mobile and capable of moving over considerable distances. Additionally, as the use of insecticides to kill mosquitoes in malaria prevention programs has shown, vectors can become resistant to these compounds. Insects such as mosquitoes have been around for millennia, primarily because of their ready adaptation to change. Scientists continue to study the habitats, lifecycles, and migration patterns of vector-borne diseases to predict and limit adverse health outcomes. However, as climate change alters the natural habitats of vectors like insects, preventing and predicting infections of blood-borne diseases will become more complex.
Bibliography
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