Vector (disease)

Vector is a phrase used in parasitology and entomology to describe a special type of intermediate host for parasites. A vector is not only required as part of the parasite's development, but it also delivers the parasite directly to subsequent hosts, avoiding free living stages such as those observed in Schistosoma which infects snails before having a brief free living stage that actively infects their next host. The phrase also does not apply where the disease is transmitted passively, whereby the intermediate host is consumed by the next host, resulting in infection, such as the beetle Tenebrio molitor which passively transmits the rodent tapeworm Hymenolepis diminuta to rats. The phrase, however, is often used to describe intermediate hosts, but is technically inaccurate. Another type of "non-true" vector host are so-called mechanical vectors. These vectors may pick up a parasite or bacteria and carry them to food or hots. For example, the bacteria shigella may be carried on the feet of a fly to food which may be consumed by the host, or to sensitive areas of the host such as the eyes. The parasites do not require the fly for their development, meaning the latter is not a true host.

True disease vectors are typically haematophagous, meaning they feed on blood at some or all stages of their lives. When the insects blood feed, the parasite then enters the blood stream of the host. This can happen in different ways. The most prolific of parasite vectors, Anopheles mosquitoes, vectors for malaria Filariasis and various arthropod-borne-viruses (arboviruses), insert their delicate mouthparts under the skin and feed on blood. Parasites are usually located in the salivary glands which mosquitoes use to anaesthetise the host, thus directly transmitting parasites into the blood stream. Pool feeders such as the sand fly and black fly, vectors for Leishmaniasis and Onchocerciasis respectively, will chew a well in the skin, forming a small pool of blood from which they feed. Leishmania parasites then infect 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, actually defecate during a blood meal in order to take up as much blood as possible. The excrement contains the parasites and are often accidentally smeared into the open wound by the host responding to pain and irritation from the bite.

Examples of Vectors

A few have been mentioned in the text above, but this is a more comprehensive list. Bubonic Plague - Fleas, typically human fleas such as Pulex and Xenopsylla but there is some evidence that other fleas can transmit the bacteria.

Malaria - Mosquitoes of the Anopheles genus, although avian malaria parasites can be transmitted by Aedes mosquitoes.

Human African trypanosomiasis (sleeping sickness) - Tsetse flies, from a wide range of genera. The parasites may also cause a disease in agricultural animals known as Nagana

Chagas disease - Triatomine bugs

Lyme disease - Ticks of the genus Ixodes

Some arboviruses can be very specific to their invertebrate hosts. The Dengue virus for example has only been observed in a few species within the Aedes genus. West Nile virus however has been observed in more than 70 species of mosquito, in 8 genera and is also transmitted by several species of tick.

References