Читать книгу Basic Virology - Martinez J. Hewlett - Страница 32

The source of the infectious virus is termed the reservoir, and virus entry into the host generally follows a specific pattern leading to its introduction at a specific site or region of the body. Epidemiologists working with human, animal, and plant diseases often use special terms to describe parts of this process. The actual means of infection between individuals is termed the vector of transmission or, more simply, the vector. This term is often used when referring to another organism, such as an arthropod, that serves as an intermediary in the spread of disease.

Many viruses must continually replicate to maintain themselves – this is especially true for viruses that are sensitive to desiccation and are spread between terrestrial organisms. For this reason, many virus reservoirs will be essentially dynamic; that is, the virus constantly must be replicating actively somewhere. In an infection with a virus with broad species specificity, the external reservoir could be a different population of animals. In some cases, the vector and the reservoir are the same – for example, in the transmission of rabies via the bite of a rabid animal. Also, some arthropod‐borne viruses can replicate in the arthropod vector as well as in their primary vertebrate reservoir. In such a case the vector serves as a secondary reservoir, and this second round of virus multiplication increases the amount of pathogen available for spread into the next host.

Some reservoirs are not entirely dynamic. For example, some algal viruses exist in high levels in many bodies of freshwater. It has been reported that levels of some viruses can approach 10⁷ per milliliter of seawater. Further, the only evidence for the presence of living organisms in some bodies of water in Antarctica is the presence of viruses in that water. Still, ultimately all viruses must be produced by an active infection somewhere, so in the end all reservoirs are, in some sense at least, dynamic.

Viruses (or their genomes) enter cells via the cooperative interaction between the host cell and the virus – this interaction requires a hydrated cell surface. Thus, initial virus infection and entry into the host cell must take place at locations where such cell surfaces are available, not, for example, at the desiccated surface layer of keratinized, dead epithelial cells of an animal's skin, or at the dry, waxy surface of a plant. In other words, virus must enter the organism at a site that is “wet” as a consequence of its anatomical function or must enter through a trauma‐induced break in the surface. Figure 2.4 is a schematic representation of some modes of virus entry leading to human infection.