A vector is a living organism that transmits an infectious agent from an infected animal to a human or another animal. Vectors are frequently arthropods, such as mosquitoes, ticks, flies, fleas and lice.
Vectors can transmit infectious diseases either actively or passively:
- Biological vectors, such as mosquitoes and ticks may carry pathogens that can multiply within their bodies and be delivered to new hosts, usually by biting.
- Mechanical vectors, such as flies can pick up infectious agents on the outside of their bodies and transmit them through physical contact.
Diseases transmitted by vectors are called vector-borne diseases. Many vector-borne diseases are zoonotic diseases, i.e. diseases that can be transmitted directly or indirectly between animals and humans. These include for example Lyme disease, tick-borne encephalitis, West Nile virus, Leishmaniosis and Crimean-Congo haemorrhagic fever.
Many vector-borne diseases are considered as emerging infectious diseases in the European Union:
- a disease that appears in a population for the first time, or
- that may have existed previously but is rapidly increasing in incidence or geographic range.
Some vectors are able to move considerable distances. This may affect the transmission ranges of vector-borne zoonotic diseases. Vectors can be introduced to new geographic areas for example by:
- travel of humans and international trade;
- animal movement, for instance of livestock;
- migratory birds;
- changing agricultural practices;
- or the wind.
Other factors may play a role in their establishment and persistence in new areas, including climatic conditions.