We first review some of the most common uses of the term (summarized in figure 1 a and table 1), a number of which we immediately dismiss, either because we believe they are too broad or too narrow to be of practical use. It is therefore of interest to explore the factors that have gone into defining one or other species as a vector, why such a distinction has proved useful, and conversely, if there are dangers involved in pursuing these definitions. Nevertheless, the designation of one or the other host as a vector is commonplace in the literature on infectious diseases.
VECTOR BORNE TRANSMISSION HTLV1 FULL
In principle, a full understanding of the dynamics of the system requires knowledge of the contributions and feedbacks involving all participants, and the outcome will be independent of what designations are given to them. If the pathogen is present in each of the two species of hosts, and transmission between those species is required to maintain the pathogen in the system, there is no inherent theoretical reason why one or other species should have the designation of ‘host’ or ‘vector’. This is perhaps most easily understood by considering the simplest canonical case, namely a one pathogen, two host species system. Clearly a large number of definitions of ‘vector’ are currently being used, and the question in any multi-host system should be to ask when and why a particular host in that system warrants designation as a ‘vector’. For instance, the term is universally applied to haematophagous arthropods, such as Ixodes ticks that transmit Borrelia burgdorferi or Aedes mosquitoes that transmit dengue virus, but the term ‘vector’ has also been applied to badgers transmitting Mycobacterium bovis, dogs transmitting rabies virus, snails transmitting Schistosoma flatworms and rodents transmitting hantaviruses. However, different definitions of a vector are used in different fields. Almost 20% of human deaths are caused by infectious diseases that are described as vector-borne, chiefly malaria, yellow fever, leishmaniosis, trypanosomiasis, Chagas' disease and Japanese encephalitis, and such diseases are predicted to present a growing threat in the near future. These include emerging parasites and pathogens such as dengue virus throughout the tropical world, West Nile virus in North America and Europe, Crimean–Congo haemorrhagic fever virus in Turkey, hantavirus in Europe, bluetongue virus in Europe, zika virus in South America, Lyme borreliosis in Europe and chikungunya virus in the Caribbean. Many parasites and pathogens responsible for some of the most important diseases in humans, agriculture and nature are routinely described as ‘vector-borne’. This article is part of the themed issue ‘Opening the black box: re-examining the ecology and evolution of parasite transmission’. However, we explicitly recognize that the value of a definition always depends on the research question under study. From a population dynamics and evolutionary perspective, we suggest that a combination of the ‘micropredator’ and ‘sequential’ definition is most appropriate because it captures the key aspects of transmission biology and fitness consequences for the pathogen and vector itself.
We conclude that from a medical and veterinary perspective, a combination of the ‘haematophagous arthropod’ and ‘mobility’ definitions is most useful because it offers important insights into contact structure and control and emphasizes the opportunities for pathogen shifts among taxonomically similar species with similar feeding modes and internal environments.
We then consider how the use of designations can afford insights into our understanding of epidemiological and evolutionary processes that are not otherwise apparent.
In this perspective, we briefly review some common definitions, identify the strengths and weaknesses of each and consider the functional differences between vectors and other hosts from a range of ecological, evolutionary and public health perspectives. However, the term ‘vector’ has been applied to diverse agents in a broad range of epidemiological systems. Many important and rapidly emerging pathogens of humans, livestock and wildlife are ‘vector-borne’.
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