Optimal virulence
Encyclopedia
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.
, virulence increases with horizontal transmission
(between non-relatives) and decreases with vertical transmission
(from parent to child).http://links.jstor.org/sici?sici=0003-0147(199808)152%3A2%3C165%3ATBVAVT%3E2.0.CO%3B2-U
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 mosquito
s. 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.
, nutrition
and sanitation
are all effective strategies against virulence.
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
of hosts and parasites. One definition of virulence
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
is the host's parasite-induced loss of fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
. 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
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
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
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
. 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.
Mode of transmission
According to evolutionary medicineEvolutionary medicine
Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. It provides a complementary scientific approach to the present mechanistic explanations that dominate medical science, and particularly modern medical education...
, virulence increases with horizontal transmission
Horizontal transmission
Horizontal transmission is the transmission of a bacterial, fungal, or viral infection between members of the same species that are not in a parent-child relationship....
(between non-relatives) and decreases with vertical transmission
Vertical transmission
Vertical transmission, also known as mother-to-child transmission, is the transmission of an infection or other disease from mother to child immediately before and after birth during the perinatal period. A pathogen's transmissibility refers to its capacity for vertical transmission...
(from parent to child).http://links.jstor.org/sici?sici=0003-0147(199808)152%3A2%3C165%3ATBVAVT%3E2.0.CO%3B2-U
Paul W. Ewald
Paul W. Ewald
Paul W. Ewald is an evolutionary biologist, specializing in the evolution of infectious disease. He received his B.Sc. in 1975 from the University of California, Irvine, in Biological Sciences and his Ph.D. in 1980 from the University of Washington, in Zoology, with specialization in Ecology and...
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
Cholera
Cholera is an infection of the small intestine that is caused by the bacterium Vibrio cholerae. The main symptoms are profuse watery diarrhea and vomiting. Transmission occurs primarily by drinking or eating water or food that has been contaminated by the diarrhea of an infected person or the feces...
and Dengue. Cholera is spread through sewage and Dengue through mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
s. 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
Trench warfare
Trench warfare is a form of occupied fighting lines, consisting largely of trenches, in which troops are largely immune to the enemy's small arms fire and are substantially sheltered from artillery...
provided an easy route to transmission that explains the virulence of the 1918 influenza pandemic
Spanish flu
The 1918 flu pandemic was an influenza pandemic, and the first of the two pandemics involving H1N1 influenza virus . It was an unusually severe and deadly pandemic that spread across the world. Historical and epidemiological data are inadequate to identify the geographic origin...
. 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
Urbanization
Urbanization, urbanisation or urban drift is the physical growth of urban areas as a result of global change. The United Nations projected that half of the world's population would live in urban areas at the end of 2008....
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 hygieneHygiene
Hygiene refers to the set of practices perceived by a community to be associated with the preservation of health and healthy living. While in modern medical sciences there is a set of standards of hygiene recommended for different situations, what is considered hygienic or not can vary between...
, nutrition
Nutrition
Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....
and sanitation
Sanitation
Sanitation is the hygienic means of promoting health through prevention of human contact with the hazards of wastes. Hazards can be either physical, microbiological, biological or chemical agents of disease. Wastes that can cause health problems are human and animal feces, solid wastes, domestic...
are all effective strategies against virulence.
External links
- Empirical Support for Optimal Virulence in a Castrating Parasite
- Evolution of Virulence
- Adaptive Dynamics of Infectious Diseases: In Pursuit of Virulence ...
- Integrating across levels Interesting discussion of the complexity of optimal virulence theory
- `Small worlds' and the evolution of virulence: infection occurs ...
- Pathogen Virulence: The Evolution of Sickness - A Review from the Science Creative Quarterly