In the terminology of epidemiology, vectors are organisms that transmit infections from one host to another. Most commonly known biological vectors are arthropods but many domestic animals too are important vectors or asymptomatic carriers of parasites and pathogens that attack humans or other animals. Some such pathogens and parasites are of great medical or veterinary importance. Many parasites actually are adapted to a particular vector for part of their developmental cycle, but the vector function essentially consists in transmission of the parasite to subsequent hosts.
The concept of disease vectors has some commonality with certain other concepts in medicine and veterinary science; it is worth comparing vectoring, zoonosis and carrier for perspective. Zoonosis sometimes arises from purely adventitious or non-systematic transport of the infectious agent. For example a housefly or a dog might accidentally, but routinely, carry the pathogens of typhoid or cholera in external dirt without being in any special way adapted to such a function. Technically such a process amounts to vectoring, and such vectors are important in practice, though they are logically similar to airborne disease and waterborne diseases. However, the term vector commonly, though not necessarily, is used in contexts where the parasite or pathogen is adapted to be dependent on the vector organism for the completion of its life cycle.
The ecology and principles of disease vectors vary greatly, but some themes occur frequently. For instance, in cases where the pathogen is strictly dependent on the vector (its secondary host) and gets only one chance at transmission, it commonly is adapted to avoid causing the rapid death, or often even any significant reduction of the vigour of the vector. In contrast there are plenty of examples of where the primary host is uncompromisingly sacrificed in a parasitoidal process.
The deer tick, a vector for lyme disease.
Arthropods form a major group of disease vectors with mosquitoes, flies, sand flies, lice, fleas, ticks and mites transmitting a huge number of diseases. Many such vectors are haematophagous, which feed on blood at some or all stages of their lives. When the insects blood feed, the parasite enters the blood stream of the host. This can happen in different ways.
The Anopheles mosquito, a vector for Malaria, Filariasis and various arthropod-borne-viruses (arboviruses), inserts its delicate mouthpart under the skin and feeds on its host's blood. The parasites the mosquito carries are usually located in its salivary glands (used by mosquitoes to anaesthetise the host). Therefore, the parasites are transmitted directly into the host's blood stream. Pool feeders such as the sand fly and black fly, vectors for Leishmaniasis and Onchocerciasis respectively, will chew a well in the host's skin, forming a small pool of blood from which they feed. Leishmania parasites then infect the host through the saliva of the sand fly. Onchocerca force their own way out of the insect's head into the pool of blood.
Triatomine bugs are responsible for the transmission of a trypanosome, Trypanosoma cruzi, which causes Chagas Disease. The Triatomine bugs defecate during feeding and the excrement contains the parasites which are accidentally smeared into the open wound by the host responding to pain and irritation from the bite.
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