Disease vector

In the field of epidemiology, a disease vector is any living agent that carries and transmits an infectious pathogen. These pathogens can be microbes, such as bacteria and viruses, or larger parasites like nematodes. Vectors are a major global health challenge because they facilitate the spread of illness across populations. The World Health Organization reports that vector-borne illnesses make up over 17% of all infectious diseases worldwide. These diseases are responsible for hundreds of thousands of deaths every year.

Most vectors are hematophagous arthropods, which means they feed on blood at some or all stages of their lives. The mechanism of transmission often involves a complex biological process. When a vector like a mosquito takes a blood meal from an infected host, the pathogen enters its gut. If the pathogen is capable of growing within the insect, it will multiply inside the vector's body. These pathogens eventually move to the salivary glands. When the vector bites a new host to feed again, the pathogen is transferred from the salivary gland into the new host's bloodstream.

Different species of mosquitoes act as vectors for different specific diseases. The Anopheles mosquito is known to transmit malaria, which is caused by Plasmodium parasites. It also carries lymphatic filariasis and the O'nyong'nyong virus. The Aedes mosquito genus transmits several different viruses, including dengue, yellow fever, Zika, and chikungunya. Meanwhile, Culex mosquitoes serve as vectors for West Nile fever and Japanese encephalitis.

Other arthropods besides mosquitoes play significant roles in disease transmission. Ticks are highly diverse vectors and can carry over one hundred different pathogens. These include viruses, bacteria, and protozoans found across Europe, Asia, and North America. Ticks transmit diseases such as Lyme disease, which is caused by the bacteria Borrelia burgdorferi. Other tick-borne illnesses include relapsing fever, rickettsial diseases, and tularemia.

Many other insects and small creatures function as vectors in specific environments. For example, the tsetse fly transmits the protozoan parasite that causes sleeping sickness. Triatome bugs spread Chagas disease by defecating during a blood meal, allowing parasites to enter the host's wound. Blackflies, or Simulium rasyani, are the vectors for onchocerciasis, also known as river blindness. In urban environments, biting midges are the main vectors for the oropouche virus. Even fleas play a role by spreading the plague bacteria, Yersinia pestis, between humans and small mammals.

Vectors are not limited to animals; plants and fungi can also act as vectors. Fungal vectors, such as species in the Chytridiomycota division, can carry plant viruses through the soil. For instance, the fungus Olpidium brassicae uses motile zoospores to attach to plant roots and release virus particles. Some parasitic plants, like the Cuscuta vine, act as vectors by forming haustoria. These structures connect the vascular systems of multiple host plants, allowing viruses and phytoplasmas to move through shared tissues.

Our understanding of these biological processes has changed significantly due to scientific discovery. The first major breakthrough occurred in 1897 when Ronald Ross discovered the malaria pathogen. He achieved this by dissecting the stomach tissue of a mosquito. This discovery helped establish the concept of vector incrimination, which is the process of proving a specific vector is responsible for transmitting a pathogen. Understanding these links is vital as environmental shifts occur. Changes in climate, such as warmer temperatures and altered rainfall, can expand the geographic reach of vectors. Additionally, international trade and human movement can rapidly transport vectors to naive populations, which are groups with no prior immunity to a specific condition.