Vector (epidemiology)

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A mosquito shortly after obtaining blood from a human (note the droplet of blood being expelled as a surplus). Mosquitos are a vector for several diseases, most notably malaria.

In epidemiology, a vector is any agent (person, animal or microorganism) that carries and transmits an infectious pathogen into another living organism.[1][2][3]

Basic concepts[edit]

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.


The Oriental rat flea (Xenopsylla cheopis) engorged with blood after a blood meal. This species of flea is the primary vector for the transmission of Yersinia pestis the organism responsible for bubonic plague in most plague epidemics in Asia, Africa, and South America. Both male and female fleas feed on blood and can transmit the infection.

See also[edit]

Further reading[edit]


  1. ^ WordNet Search
  2. ^ Last, James, ed. (2001). A Dictionary of Epidemiology. New York: Oxford University Press. p. 185. ISBN 978-0-19-514169-6. OCLC 207797812. 
  3. ^ Roberts, Larry S.; John, Janovy; Gerald D., Schmidt (2008). Foundations of Parasitology. McGraw Hill. ISBN 978-0-07-302827-9. OCLC 226356765. 
  4. ^ CDC: Aedes albopictus
  5. ^
  6. ^ Halpin, K.; Young, P. L.; Field, H. E.; MacKenzie, J. S. (2000). "Isolation of Hendra virus from pteropid bats: A natural reservoir of Hendra virus". The Journal of general virology 81 (Pt 8): 1927–1932. PMID 10900029.  edit
  7. ^ Li, W.; Shi, Z.; Yu, M.; Ren, W.; Smith, C.; Epstein, J. H.; Wang, H.; Crameri, G.; Hu, Z.; Zhang, H.; Zhang, J.; McEachern, J.; Field, H.; Daszak, P.; Eaton, B. T.; Zhang, S.; Wang, L. F. (2005). "Bats Are Natural Reservoirs of SARS-Like Coronaviruses". Science 310 (5748): 676–679. doi:10.1126/science.1118391. PMID 16195424.  edit
  8. ^ McColl, K. A.; Tordo, N.; Aguilar Setién, A. A. (2000). "Bat lyssavirus infections". Revue scientifique et technique (International Office of Epizootics) 19 (1): 177–196. PMID 11189715.  edit
  9. ^ Arellano-Sota, C. (1988). "Vampire bat-transmitted rabies in cattle". Reviews of infectious diseases. 10 Suppl 4: S707–S709. doi:10.1093/clinids/10.Supplement_4.S707. PMID 3206085.  edit
  10. ^ Yelifari, L.; Frempong, E.; Olsen, A. (1997). "The intermediate hosts of Dracunculus medinensis in northern region, Ghana". Annals of tropical medicine and parasitology 91 (4): 403–409. doi:10.1080/00034989761021. PMID 9290847.  edit
  11. ^ Ryan KJ, Ray CG (eds) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 722–727. ISBN 0-8385-8529-9. 
  12. ^ Vivan, A. L.; Caceres, R. A.; Basso, L. A.; Santos, D. S.; de Azevedo Jr., W. F. (2009). "Structural studies of PNP from Toxoplasma gondii". International journal of bioinformatics research and applications 5 (2): 154–162. doi:10.1504/IJBRA.2009.024095. PMID 19324601.  edit
  13. ^ Taking the Lead: Fleas
  14. ^ Vector Transmission of Xylella fastidiosa
  15. ^ MicrobiologyBytes: Malaria
  16. ^ Dedet, J. P. (2005). "Stages in the identification of phlebotomine sandflies as vectors of leishmaniases and other tropical diseases". Parassitologia 47 (3–4): 291–295. PMID 16866034.  edit
  17. ^ tick
  18. ^ Online Textbook of Bacteriology
  19. ^ WHO Chagas disease (American trypanosomiasis)
  20. ^ WHO African trypanosomiasis (sleeping sickness)
  21. ^ Pathogens from Humans and Cats Kill Seals and Dolphin; "The pathogens of land animals are spreading to the oceans, threatening otters, seals, whales, coral and other sea creatures" May 5, 2013 Scientific American
  22. ^ Planet of the Ape: ‘Between Man and Beast,’ April 4, 2013 New York Times