Vector (epidemiology) - Basic Concepts

Basic Concepts

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.