What are vectors and vectorborne diseases?

Vector-borne diseases are infections transmitted by the bite of infected arthropod species, such as mosquitoes, ticks, triatomine bugs, sandflies, and blackflies (1). Arthropod vectors are cold-blooded (ectothermic) and thus especially sensitive to climatic factors. Weather influences survival and reproduction rates of vectors (2), in turn influencing habitat suitability, distribution and abundance; intensity and temporal pattern of vector activity (particularly biting rates) throughout the year; and rates of development, survival and reproduction of pathogens within vectors. However, ECDC states that climate is only one of many factors influencing vector distribution, such as habitat destruction, land use, pesticide application, and host density. Vector-borne diseases are widespread in Europe and are the best studied diseases associated with climate change (3).

Disease vectors in Europe

Together with the network of vectorborne disease experts, ECDC monitors occurrence of disease vectors in the European Union. Geographical surveillance maps for mosquitoes, ticks, and phlebotomine sandflies are continuously updated and published online.

Mosquito map Europe

Example of ECDC vector surveillance maps for mosquitoes, January 2018.

Studying the epidemiology of infectious diseases that are transmitted to humans via vectors, we need to know more about the relationship between the reservoir of the infectious agent and the vector that is needed to transmit the disease. The fact that a vector is required between the reservoir and the host will make the spread of the disease within a population more complex to study and predict.

The basis of vector borne disease epidemiology is the triangle between pathogen, vector and hosts. As with other type of infectious diseases, the pathogens (virus, parasites, bacteria) cause disease, yet they depend on the vector to be transmitted to the hosts. The natural (or primary) host of a vector-borne disease is part of the reservoir that maintains the pathogen in natural cycles of infection and transmission by vectors to other susceptible natural hosts. For example, in West Nile Virus, the primary transmission cycle takes place among various bird species and a number of mosquito species. In most birds in Europe, Africa and Asia, fatal outcome is rare when infected with West Nile Virus, in contrast to birds in the Americas (especially the family of crows). In this particular example of West Nile Virus, humans and horses are incidental (dead - end) hosts; this means that they do not contribute to the further spread of the disease.

Fig 1. Example of vector borne disease transmission cycle

Fig 1. Example of vector borne disease transmission: the West Nile Virus cycle

Modes of transmission of vector borne diseases

Vertical transmission from vector (e.g. mosquito) to progeny may occur via transoverial passage of the infectious agent.

Horizontal transmission occurs when infected mosquitoes transfer the agent to vertebrate hosts. This can be mechanical (e.g. when the agent is transferred by the vector via the mouth parts, without multiplication in the vector) or biological (where the agent multiplies in the vector).

Surveillance of vector borne diseases

Surveillance can target the vector:

  • measuring abundance and spread
  • testing vectors for infection (if tests exist for such investigation)
  • calculate vector infection rates
Surveillance can target hosts:
  • Sentinel animals, via periodic bleeding and testing for infection
  • Via notification of animal or human infections / disease.

References

  1. Confalonieri U, Menne B, Akhtar R, Ebi KL, Hauengue M, Kovats RS, Revich B, Woodward A. Human Health. In: Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hansson CE (eds). Cambridge University Press, Cambridge, U.K., 2007: 391-431
  2. Rogers DJ, Randolph SE. Climate change and vector-borne diseases. Adv Parasitol. 2006;62:345-81.
  3. Semenza JC, Menne B. Climate Change and Infectious Diseases in Europe. Lancet ID. 2009;9:365-75.