Disease vector
In epidemiology, a disease vector is any living agent that carries and transmits an infectious pathogen such as a parasite or microbe, to another living organism. Many familiar vectors, such as mosquitos, ticks, and certain flies rely on blood-feeding and can acquire or pass on pathogens during that process. Disease vectors remain a major global health challenge. The World Health Organization reports that these illnesses make up over 17% of all infectious diseases worldwide, and are responsible for hundreds of thousands of deaths every year.
Agents regarded as vectors are mostly blood-sucking arthropods such as mosquitoes. The first major discovery of a disease vector came from Ronald Ross in 1897, who discovered the malaria pathogen when he dissected the stomach tissue of a mosquito.
The process of proving that a vector is responsible for transmitting pathogens is called vector incrimination. Transmission depends on interactions among a host, a pathogen, and a vector species that is capable of carrying the infection. Some pathogens may multiply or undergo part of their life cycle inside of the vector, while others are transferred from one surface to another without developing inside the carrier.
Shifts in climate, expanding cities, and land-use changes are reshaping where vectors can survive. Warmer temperatures and altered rainfall patterns can boost mosquito and tick populations, which can extend their breeding seasons and geographic reach. Urban green spaces and infrastructure can create new breeding grounds for vectors such as mosquitos, thus bringing them closer to dense human populations. Additionally, international trade and human movement can rapidly transport vectors and pathogens across continents, which may introduce them to naive populations. Naive populations refer to a set of participants who have not previously encountered or been exposed to a particular substance, treatment, or condition, and therefore have no built-up immunity or prior experience with it.
Due to these shifting conditions, public health agencies are encouraging broad and coordinated approaches to reducing vector-borne diseases. Strategies implemented include monitoring vector populations, improving environmental management, increasing community participation, and adopting newer tools and technologies where appropriate.
Arthropods
s, including mosquitoes, ticks, biting midges, blackflies, sandflies, tsetse flies, triatome bugs, lice, fleas, and thrips, form a major form a major group of pathogen vectors, transmitting a huge number of pathogens. Many such vectors are haematophagous, meaning they feed on blood at some or all stages of their lives. When the insects and ticks feed on blood, the pathogen enters the blood stream of the host. These pathogens replicate within the vector and the vector is often a carrier for the rest of its life. The pathogen is spread to new hosts from the vector during subsequent blood meals.When a mosquito takes a blood meal from a human or animal, a pathogen from that host can pass from the gut of the mosquito into the mosquito's body, if that pathogen is able to grow within a mosquito. There, the pathogen multiplies and eventually moves to the salivary glands of the mosquito. When the mosquito next takes a blood meal from a human or animal, the pathogen is transferred from the mosquito's salivary gland to the new host.
Different mosquito genera act as vectors for different diseases. The Anopheles mosquito transmits malaria, lymphatic filariasis, and O'nyong'nyong virus. Malaria is caused by Plasmodium parasites, and lymphatic filariasis is caused by the filarial nematodes Wuchereria bancrofti, Brugia malayi, and Brugia timori.
The Aedes mosquito transmits chikungunya, dengue, lymphatic filariasis, Rift Valley fever, yellow fever, and Zika. The chikungunya virus is related to the O'nyong'nyong virus that is carried by Anopheles mosquitoes, with both viruses being in the Alphavirus genus. Dengue, Rift Valley fever, yellow fever, and Zika are all caused by viruses.
Culex mosquitoes act as vectors for Japanese encephalitis, lymphatic filariasis, and West Nile fever. Japanese encephalitis and West Nile fever are both caused by viruses.
Ticks are known to carry over one hundred different pathogens, including viruses, bacteria, protozoans, and parasites. These pathogens are found in Europe, Asia, and North America. Ticks act as vectors for diseases such as Lyme disease, tick-borne encephalitis, Crimean-Congo hemorrhagic fever, relapsing fever, rickettsial diseases such as spotted fever, and tularemia. Lyme disease, relapsing fever, rickettsial diseases, and tularaemia are caused by bacteria. Lyme disease is caused by the bacteria Borrelia burgdorferi, while relapsing fever is caused by several different species of Borrelia bacteria. Rickettsial diseases come from bacteria within the order Rickettsiales and tularemia is caused by the bacteria Francisella tularensis. Two of the viruses carried by ticks are tick-borne encephalitis virus and Crimean-Congo hemorrhagic fever virus.
Although Aedes mosquitoes are able to carry the oropouche virus and play a role in the spread of the virus in wild animals such as three-toed sloths, primates, and birds, the disease is mainly spread between humans in urban environments by biting midges, specifically Culicoides paraensi. These biting midges are much smaller than mosquitoes, but their bites are often more painful. Culex quinquefasciatus may also play a role in spreading Oropouche virus between humans in urban settings, however, biting midges are the main vector.
Blackflies, also known as Simulium rasyani, are the vector for onchocerciasis, which is caused by the nematode Onchocerca volvulus. The blackfly carries O. volvulus when it takes a blood meal from an infected human and ingests microfilariae. These microfilariae move to the blackfly's midgut and then thoracic muscles, where they can develop into larvae and, later, infective larvae. These infective larvae then move to the proboscis of the blackfly. From there, the infective larvae are able to spread to a new host the next time the blackfly takes a blood meal.
Sandflies are vectors for leishmaniasis and sandfly fever. Leishmaniasis is caused by parasites of the genus Leishmania, while sandfly fever is caused by viruses in the genus Phlebovirus.
Both sleeping sickness and Chagas disease are trypanosomatid diseases, caused by the protozoan parasites Trypanosoma brucei and Trypanosoma cruzi, respectively. However, these two diseases are spread through different vectors. Tsetse flies act as the vector for sleeping sickness, while triatome bugs spread Chagas disease. In the case of Chagas disease, triatome bugs defecate during feeding and the excrement contains the parasites, which is accidentally smeared into the open wound, eyes, or mouth by the host.
The body louse Pediculus humanus acts as a vector for the bacteria Rickettsia prowazekii, which causes epidemic typhus, and Rickettsia typhi, which causes murine typhus. The same species of louse also spreads the bacteria Borrelia recurrentis, which is the causative agent of louse-borne relapsing fever.
Plague, caused by the bacteria Yersinia pestis, is spread between humans and small mammals by infected fleas.
There are several species of Thrips that act as vectors for over 20 viruses, especially Tospoviruses, and cause all sorts of plant diseases.
Mollusks
act as vectors for trematode worms of the genus Schistosoma, which cause schistosomiasis. These snails release the larval form of these worms into water, which are then able to penetrate the skin of humans that have contact with this water. These larvae develop into adult schistosomes in the human host and then release eggs, which can be released back into water through urine and feces, thus continuing the life cycle.Plants and fungi
Some plants and fungi act as vectors by transmitting pathogens between susceptible hosts. Fungal and plant vectors can influence disease cycles in many agricultural systems by carrying plant viruses that spread through soil, roots, or direct plant contact. These vectors can not be considered passive carriers, since many of them have life cycles that align closely with host plant growth. Their transmission patterns often follow root development, nutrient flow, and plant age, which strengthens their ability to maintain pathogens within a cropping system. For example, the big-vein disease of lettuce was long thought to be caused by a member of the fungal division Chytridiomycota, namely Olpidium brassicae. Eventually, however, the disease was shown to be viral. Later it transpired that the virus was transmitted by the zoospores of the fungus and also survived in the resting spores. Since then, many other fungi in Chytridiomycota have been shown to vector plant viruses.Several soil dwelling fungi transmit plant viruses through motile spores and long lived resting structures. Species of Olpidium are important fungal vectors, and Olpidium brassicae produces zoospores that attach to plant roots and release virus particles into the host during infection. These zoospores often locate host roots by following chemical gradients in the soil, which increases their efficiency as vectors. Other fungal vectors include Polymyxa species. Polymyxa graminis is an obligate parasite, and it can transmit viruses such as barley yellow mosaic virus and soil borne wheat mosaic virus. These fungi can survive for extended periods in soil, which supports ongoing transmission cycles in grain producing regions. Because Polymyxa depends completely on living host tissue, its presence in soils ensures that cereal viruses remain active year after year and can re-infect crops even after rotations.
Many plant pests that seriously damage important crops depend on other plants, often weeds, to harbour or vector them; the distinction is not always clear. In the case of Puccinia graminis for example, Berberis and related genera act as alternate hosts in a cycle of infection of grain. Sexual reproduction on Berberis shrubs generates new genetic combinations in the pathogen, and this process can create rust races capable of overcoming wheat resistance genes. This sexual stage was historically so important that large scale programs removed millions of Berberis shrubs in order to reduce stem rust epidemics in wheat growing regions. The ability of Puccinia graminis to recombine genetically on Berberis increases the diversity of spores released into surrounding environments and contributes to frequent shifts in virulence patterns. This relationship shows how non crop plants can play essential roles in disease epidemiology and pathogen evolution.
More directly, when they twine from one plant to another, parasitic plants such as Cuscuta and Cassytha have been shown to convey phytoplasmal and viral diseases between plants.
Some parasitic flowering plants also act as vectors for plant pathogens. Cuscuta species form haustoria that connect their vascular systems to those of multiple host plants, and these connections allow movement of viruses and phytoplasmas through shared tissues. Because a single dodder vine can attach to multiple hosts, it can contribute to rapid pathogen spread in natural and agricultural environments. The haustoria of Cuscuta forms by penetrating host cortex tissue and establishing direct phloem level continuity with the host vascular system. This structure allows not only viruses but also proteins, metabolites, and other macromolecules to move between connected plants. Dodder is frequently used in research as a tool for experimentally transmitting viruses between plant species because it bypasses natural host resistance barriers and enables direct phloem to phloem movement.