Optimal virulence


Optimal virulence

Optimal virulence is a concept relating to the ecology of hosts and parasites. One definition of virulence is the host's parasite-induced loss of fitness. The parasite's fitness is determined by its success in transmitting offsprings to other hosts. At one time, the consensus was that over time, virulence moderated and parasitic relationships evolved toward symbiosis. This view has been challenged. A pathogen that is too restrained will lose out in competition to a more aggressive strain that diverts more host resources to its own reproduction. However, the host, being the parasite's resource and habitat in a way, suffers from this higher virulence. This might induce faster host death, and act against the parasite's fitness by reducing probability to encounter another host (killing the host too fast to allow for transmission). Thus, there is a natural force providing pressure on the parasite to "self-limit" virulence. The idea is, then, that there exists an equilibrium point of virulence, where parasite's fitness is highest. Any movement on the virulence axis, towards higher or lower virulence, will result in lower fitness for the parasite, and thus will be selected against.

Contents

Mode of transmission

According to evolutionary medicine, virulence increases with horizontal transmission (between non-relatives) and decreases with vertical transmission (from parent to child).[1]

Paul W. Ewald has explored the relationship between virulence and mode of transmission. He came to the conclusion that virulence tends to remain especially high in waterborne and vector-borne infections, such as cholera and Dengue. Cholera is spread through sewage and Dengue through mosquitos. In the case of respiratory infections, the pathogen depends on an ambulatory host to survive. It must spare the host long enough to find a new host. Water- or vector-borne transmission circumvents the need for a mobile host. Ewald is convinced that the crowding of trench warfare provided an easy route to transmission that explains the virulence of the 1918 influenza pandemic. In crowded conditions the time to find a new host is minimal.

Other epidemiologists have expanded on the idea of a tradeoff between costs and benefits of virulence. One factor is the time or distance between potential hosts. Airplane travel, crowded factory farms and urbanization have all been suggested as possible sources of virulence. Another factor is the presence of multiple infections in a single host leading to increased competition among pathogens. In this scenario, the host can survive only as long as it resists the most virulent strains. The advantage of a low virulence strategy becomes moot. Multiple infections can also result in gene swapping among pathogens, increasing the likelihood of lethal combinations.

Expansion into new environments

A potential for virulence exists whenever a pathogen invades a new environment, host or tissue. The new host is likely to be poorly adapted to the intruder, either because it has not built up an immunological defense or because of a fortuitous vulnerability. In times of change, natural selection favors mutations that exploit the new host more effectively than the founder strain, providing an opportunity for virulence to erupt.

Host susceptibility

Host susceptibility contributes to virulence. Once transmission occurs, the 10 pathogen must establish an infection to continue. The more competent the host immune system, the less chance there is for the parasite to survive. It may require multiple transmission events to find a suitably vulnerable host. During this time, the invader is dependent upon the survival of its current host. For this reason virulence thrives in a community with prevalent immune dysfunction and poor nutrition. Virulence weakens in a healthy population and as hosts acquire resistance. Good hygiene, nutrition and sanitation are all effective strategies against virulence.

External links


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Virulence — (also called pestiferousness) refers to the degree of pathogenicity of a microbe, or in other words the relative ability of a microbe to cause disease.The word virulent , which is the adjective for virulence, derives from the Latin word… …   Wikipedia

  • Evolutionary medicine — Charles Darwin’s daughter Anne, “the joy of the household” died aged 10 due to tuberculosis …   Wikipedia

  • human disease — Introduction       an impairment of the normal state of a human being that interrupts or modifies its vital functions. health versus disease       Before human disease can be discussed, the meanings of the terms health, physical fitness, illness …   Universalium

  • Sigma 38 — Sigma factors are bacterial proteins that help control how genes are used (expressed). In particular, sigma factors are used when genes are turned on , eg if a bacterium is growing in a liquid (growth medium) that is poor in food, and a rich food …   Wikipedia

  • Erwinia chrysanthemi — Erwinia Chrysanthemi …   Wikipédia en Français

  • Bacteria — Taxobox color = lightgrey name = Bacteria fossil range = Archean or earlier Recent image width = 210px image caption = Escherichia coli image is 8 micrometres wide. domain = Bacteria subdivision ranks = Phyla [cite web… …   Wikipedia

  • bacteria — bacterial, adj. bacterially, adv. /bak tear ee euh/, n.pl., sing. bacterium / tear ee euhm/. ubiquitous one celled organisms, spherical, spiral, or rod shaped and appearing singly or in chains, comprising the Schizomycota, a phylum of the kingdom …   Universalium

  • Design of experiments — In general usage, design of experiments (DOE) or experimental design is the design of any information gathering exercises where variation is present, whether under the full control of the experimenter or not. However, in statistics, these terms… …   Wikipedia

  • Methicillin-resistant Staphylococcus aureus — MRSA redirects here; you may be looking for Metrolina Regional Scholars Academy Methicillin resistant Staphylococcus aureus SEM micrograph of MRSA Scientific classification …   Wikipedia

  • Dickeya dadantii — Erwinia chrysanthemi Erwinia Chrysanthemi …   Wikipédia en Français


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.