Disease vector

In epidemiology, a disease vector is any living^[1] agent that carries and transmits an infectious pathogen such as a parasite or microbe, to another living organism.^[2][3] Agents regarded as vectors are mostly blood-sucking (hematophagous) 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.^[4][5] The process of proving that a vector is responsible for transmitting pathogens is called vector incrimination.

A mosquito shortly after obtaining blood from a human (note the droplet of blood plasma being expelled as the mosquito squeezes out excess water). Mosquitos are a vector for several diseases, including viral malaria.

Arthropods

The deer tick, a vector for Lyme disease pathogens

Arthropods form a major group of pathogen vectors with mosquitoes, flies, sand flies, lice, fleas, ticks, and mites transmitting a huge number of pathogens. Many such vectors are haematophagous, which 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.^[6][7] 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.^[1]

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 the body of 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.^[8]

Different mosquito genera act as vectors for different diseases. The Anopheles mosquito transmits malaria, lymphatic filariasis, and O'nyong'nyong virus.^[1] Malaria is caused by Plasmodium parasites, and lymphatic filariasis is caused by the filarial nematodes Wuchereria bancrofti, Brugia malayi, and Brugia timori.^[9][10]

The Aedes mosquito transmits chikungunya, dengue, lymphatic filariasis, Rift Valley fever, yellow fever, and Zika.^[1] The chikungunya virus is related to the O'nyong'nyong virus that is carried by Anopheles mosquitoes, with them both being in the Alphavirus genus.^[11] Dengue, Rift Valley fever, yellow fever, and Zika are all caused by viruses.^[1]

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.^[1]

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.^[12] Ticks act as vectors for diseases such as lyme disease, tick-borne encephalitis, Crimean-Congo hemorrhagic fever, relapsing fever (also called borreliosis), rickettsial diseases such as spotted fever, and tularemia.^[1] 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.^[12][13] Rickettsial diseases come from bacteria within the order Rickettsiales and tularemia is caused by the bacteria Francisella tularensis.^[14][15] Two of the viruses carried by ticks are  tick-borne encephalitis virus and Crimean-Congo hemorrhagic fever virus.^[12][16]

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.^[17] 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.^[17]

Blackflies, also known as Simulium rasyani, are the vector for onchocerciasis (also called river blindness), 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.^[18]

Sandflies are vectors for leishmaniasis and sandfly fever (also called phlebotomus fever).^[1] Leishmaniasis is caused by parasites of the genus Leishmania, while sandfly fever is caused by viruses in the genus Phlebovirus.^[10][19]

Both sleeping sickness (also called African trypanosomiasis) and Chagas disease (also called American trypanosomiasis) are trypanosomatid diseases, caused by the protozoan parasites Trypanosoma brucei and Trypanosoma cruzi, respectively.^[20][21] However, these two diseases are spread through different vectors. Tsetse flies act as the vector for sleeping sickness, while triatome bugs spread Chagas disease.^[1] 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.^[22]

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.^[23] The same species of louse also spreads the bacteria Borrelia recurrentis, which is the causative agent of louse-borne relapsing fever.^[24]

Plague, caused by the bacteria Yersinia pestis, is spread between humans and small mammals by infected fleas.^[25]

There are several species of Thrips that act as vectors for over 20 viruses, especially Tospoviruses, and cause all sorts of plant diseases.^[26][27]

Mollusks

Freshwater snails 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.^[28]

Plants and fungi

Some plants and fungi act as vectors for various pathogens. 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.^[29]

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.^[30]

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.^{[31] [29]}

Mammals

Rabies is transmitted through exposure to the saliva or brain tissue of an infected animal. Any warm-blooded animal can carry rabies, but the most common vectors are dogs, skunks, raccoons, and bats.^[32]

Vector-borne zoonotic disease and human activity

This figure shows how the Flavivirus is carried by mosquitos in the West Nile virus and Dengue fever. The mosquito would be considered a disease vector.

Several articles, recent to early 2014, warn that human activities are spreading vector-borne zoonotic diseases.^[a] Several articles published in the medical journal The Lancet, discussed how rapid changes in land use, trade globalization, climate change and "social upheaval" are causing a resurgence in zoonotic disease across the world.^[33] Displacement due to conflicts, migration, or population movements can create situations where people are more exposed to disease vectors. Additionally, human activities such as deforestation, agricultural expansion, urbanization, and increased trade and travel, are creating environments where vectors can thrive and spread diseases to humans more easily.^[34] Rising temperatures due to climate change create more favorable conditions for mosquitoes to expand their ranges and increase their populations. This can lead to higher rates of disease transmission in areas where these diseases were previously uncommon or nonexistent and the emergence of new diseases.^[35]

Examples of vector-borne zoonotic diseases include:^[36]

• Lyme disease: Caused by the bacterium Borrelia burgdorferi, it is transmitted to humans by infected black-legged ticks, often found in wooded or grassy areas.
• Plague: Caused by the bacterium Yersinia pestis, it is primarily transmitted by fleas that infest rodents. The disease has had significant historical impacts, including the Black Death.
• West Nile virus: Transmitted by mosquitoes, it causes symptoms ranging from mild flu-like illness to severe neurological diseases, including encephalitis.

Several factors influence the incidence of vector-borne diseases, including environmental conditions, animal hosts, and the movement of people.^[36] The expansion of human settlements into previously undisturbed areas creates new habitats for vectors and animals that are potential hosts. Vector-borne zoonotic diseases are transmitted by a variety of vectors, including arthropods (mosquitoes, ticks, fleas) and rodents, with humans often acting as incidental hosts.

Humans can act as mechanical vectors for some diseases, such as Tobacco mosaic virus. TMV is a single-stranded RNA virus spread spread through physical contact. Humans physically transmit the virus with their hands or tools from plant to plant.^[37] The concept of humans acting as a vector for TMV requires understanding the transmission dynamics and how human activity can play a role in spreading the virus among plants. Humans do not usually act as primary vectors for zoonotic diseases; however, they contribute to indirect transmission via human travel or trade aiding the spread of vector-borne diseases.

Control and prevention

Public health agencies educate people about many different disease vectors. This artwork, at the London School of Hygiene and Tropical Medicine, shows 10 different animal vectors.

The World Health Organization (WHO) calls for the use of integrated vector management to improve the efficiency and sustainability of controlling vector-borne diseases. The goal of integrated vector management is to target vectors and intermediate disease hosts using methods that are as sustainable, efficient, and cost effective as possible. These methods involve using both chemical and non-chemical vector control methods; collaborating with public health sectors as well as other sectors to distribute resources, plan, and make decisions; advocating for public health and for communities; and building career structures and trainings at both local and national levels to manage integrated vector management programs.^[38]

Both insecticide-treated nets (ITNs) and indoor residual spraying (IRS) are common methods of controlling mosquito vectors.^[39] ITNs are used over beds and have the dual purpose of preventing mosquitoes from biting people and of reducing the number of mosquitoes. IRS involves regularly applying insecticides to the walls of a home, which then kills mosquitoes that land on those walls.^[40]

Between 2005 and 2017, ITNs treated with pyrethroids were distributed across the globe with the goal of preventing malaria. In 2017, the WHO updated their recommendation to combine these pyrethroids with piperonyl-butoxide (PBO) to make the nets more effective against mosquitoes that were gaining resistance to pyrethroids.^[41] In 2023, the WHO added recommendations for two new kinds of ITNs: pyrethroid-chlorfenapyr nets and pyrethroid-pyriproxyfen nets. Chlorfenapyr is an insecticide that works with pyrethroids to make the nets more deadly to mosquitoes. Pyriproxyfen is an insect growth regulator that disturbs the growth and reproduction of the mosquitoes.^[41]

There is research into infecting mosquito populations with the bacteria Wolbachia pipientis to reduce the number of mosquitoes. This bacteria infects a number of invertebrate species and consistently attacks the reproductive system of infected invertebrates. The progeny of an infected male and an uninfected female mosquito are often infertile, so this bacteria could be used for long-term management.^[42] There are two ways that this method could be used: in one, both male and female Wolbachia mosquito carriers would be released into the wild and would eventually replace the wild mosquito population. In the other method, a large number of male Wolbachia carriers would be released, thus creating infertile mosquitoes. This latter method would require consistent release of male Wolbachia carriers. The use of this bacteria to control the mosquito population is more complicated than other forms of vector control and costs more money, however, this method is considered more environmentally friendly and can still be effective in the medium or long term.^[42]

Other methods of vector control include veterinary health measures. Dogs can be vaccinated to prevent the spread of rabies. Livestock can be kept away from water sources that act as transmission sites for schistosomiasis. Animals such as cows and pigs can be treated for human African trypanosomiasis and insecticides can be used.^[43]

In addition, access to clean water and adequate sanitation is important in limiting the spread of certain diseases, such as schistosomiasis, since contaminated water is how worm eggs are transmitted. In addition, Culex mosquitoes breed well in poorly built latrines, thus contributing to disease spread. As such, ensuring access to safe water and sanitation is an important strategy against a myriad of diseases.^[44]

In 2014, the theme for the WHO's World Health Day was "small bite, big threat," urging for action against vector-borne diseases. This theme acted as a reminder of the scale of the vector-borne disease issue, given that, at the time of 2014's World Health Day, vector-borne diseases were responsible for one in six illnesses and disabilities worldwide and that over half the world's population was at risk of vector-borne diseases. In addition, they emphasized that the spread of these diseases is due to social, environmental, and economic factors.^[45]

See also

• Airborne disease
• Asymptomatic carrier
• Fomite
• Globalization and disease
• Insect vectors of human pathogens
• Insect vectors of plant pathogens
• VectorBase: genomic database of invertebrate vectors of human pathogens
• List of diseases caused by insects
• Natural reservoir
• Waterborne disease
• 2007 Yap Islands Zika virus outbreak

Notes

1. "Vector-borne zoonotic diseases are those that naturally infect wildlife and are then transmitted to humans through carriers, or vectors, such as mosquitoes or ticks."^[33]