Threshold host density
Encyclopedia
Threshold host density, in the context of wildlife disease ecology
, refers the concentration of a population of a particular organism as it relates to disease. Specifically, the threshold host density (NT) of a species refers to the minimum concentration of individuals necessary to sustain a given disease within a population.
Threshold host density (NT) only applies to density dependent diseases, where there is an "aggregation of risk" to the host in either high host density or low host density patches. When low host density causes an increase in incidence of parasitism or disease, this is known as inverse host density dependence, whereas when incidence of parasitism or disease is elevated in high host density conditions, it is known as direct host density dependence.
Host density independent diseases show no correlation between the concentration of a given host population and the incidence of a particular disease. Some examples of host density independent diseases are sexually transmitted diseases in both humans and other animals. This is due to the constant incidence of interaction observed in sexually transmitted diseases—even if there are only 20 individuals left of a given population, survival of the species requires sexual contact, and continued spread of the disease.
Density dependent diseases are significantly less likely to cause extinction of a population, as the natural course of disease will bring down the density, and thus the propinquity of individuals in the population. In other words, less individuals—as caused by disease—means lower infection rates and a population equilibrium.
Avian Flu
Echinococcus multilocularis
Leptospirosis
Simian Immunodeficiency Virus
(SIV
)
Human Immunodeficiency Virus (HIV
)
This graph shows the direct relationship between disease spread through contact and population density. As the population density increases, so do transmission events between individuals.
There is a rapid initial increase in disease transmission as the population increases from zero, and then the plateau of transmission throughout most of the graph. As sexual contact is required in nearly all sexually reproducing species, transmission is not very host density dependent. It is only in cases of near-extinction where sexually transmitted diseases show any dependence on host density. It is for this reason that sexually transmitted diseases are more likely than density dependent diseases to cause extinction.
Ebert, D. "Epidemiology -- Ecology, Epidemiology, and Evolution of Parasitism in Daphnia -- NCBI Bookshelf." NCBI HomePage. 2005. Web. 27 Sept. 2009..
Jaffe, B., Phillips, R., Muldoon, A., Mangel, M. “Density-Dependent Host-Pathogen Dynamics in Soil Microcosms.” Ecology. 73:2 (1992): 495-506. Ecological Society of America, 2001..
Lentz, Amanda Jean. “The Effect of Aphids in Parasitoid-Caterpillar-Plant Interactions.” Blacksburg, Va: University Libraries, Virginia Polytechnic Institute and State University, 2007..
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...
, refers the concentration of a population of a particular organism as it relates to disease. Specifically, the threshold host density (NT) of a species refers to the minimum concentration of individuals necessary to sustain a given disease within a population.
Threshold host density (NT) only applies to density dependent diseases, where there is an "aggregation of risk" to the host in either high host density or low host density patches. When low host density causes an increase in incidence of parasitism or disease, this is known as inverse host density dependence, whereas when incidence of parasitism or disease is elevated in high host density conditions, it is known as direct host density dependence.
Host density independent diseases show no correlation between the concentration of a given host population and the incidence of a particular disease. Some examples of host density independent diseases are sexually transmitted diseases in both humans and other animals. This is due to the constant incidence of interaction observed in sexually transmitted diseases—even if there are only 20 individuals left of a given population, survival of the species requires sexual contact, and continued spread of the disease.
Density dependent diseases are significantly less likely to cause extinction of a population, as the natural course of disease will bring down the density, and thus the propinquity of individuals in the population. In other words, less individuals—as caused by disease—means lower infection rates and a population equilibrium.
Host density dependent diseases
BrucellosisBrucellosis
Brucellosis, also called Bang's disease, Crimean fever, Gibraltar fever, Malta fever, Maltese fever, Mediterranean fever, rock fever, or undulant fever, is a highly contagious zoonosis caused by ingestion of unsterilized milk or meat from infected animals or close contact with their secretions...
Avian Flu
Avian flu
Avian influenza, sometimes avian flu, and commonly bird flu, refers to "influenza caused by viruses adapted to birds." Of the greatest concern is highly pathogenic avian influenza ....
Echinococcus multilocularis
Echinococcus multilocularis
Echinococcus multilocularis is a cyclophyllid tapeworm that, along with some other members of the Echinococcus genus , produces the disease known as echinococcosis in certain terrestrial mammals, including wolves, foxes, jackals, coyotes, domestic dogs and humans. Unlike E. granulosus, E...
Leptospirosis
Leptospirosis
Leptospirosis is caused by infection with bacteria of the genus Leptospira, and affects humans as well as other mammals, birds, amphibians, and reptiles.The...
Host density-independent diseases
Chlamydia in koalasSimian Immunodeficiency Virus
Simian immunodeficiency virus
Simian immunodeficiency virus , also known as African Green Monkey virus and also as Monkey AIDS is a retrovirus able to infect at least 33 species of African primates...
(SIV
SIV
SIV or Siv may refer to:* Simian immunodeficiency virus, a virus found in primates and related to HIV* Siv, a character in the children's fiction book series Guardians of Ga'Hoole...
)
Human Immunodeficiency Virus (HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
)
Contact between individuals within a population as it relates to density in host density dependent disease
This graph shows the direct relationship between disease spread through contact and population density. As the population density increases, so do transmission events between individuals.
Contact between individuals within a population as it relates to density in sexually transmitted infections
There is a rapid initial increase in disease transmission as the population increases from zero, and then the plateau of transmission throughout most of the graph. As sexual contact is required in nearly all sexually reproducing species, transmission is not very host density dependent. It is only in cases of near-extinction where sexually transmitted diseases show any dependence on host density. It is for this reason that sexually transmitted diseases are more likely than density dependent diseases to cause extinction.
Contact between individuals within a population as it relates to density in vector-borne disease
This graph shows the relationship between population density and the transmission of vector-borne disease. Initially, the number of contacts between individuals and vectors increases as population density increases. Eventually, however, the advantage of host density diminishes as the density becomes too great for the vector to maintain its natural ecological relationship with the host, and transmission decreases.Further reading
(scientific journal articles pertaining to host density and disease)Ebert, D. "Epidemiology -- Ecology, Epidemiology, and Evolution of Parasitism in Daphnia -- NCBI Bookshelf." NCBI HomePage. 2005. Web. 27 Sept. 2009.
Jaffe, B., Phillips, R., Muldoon, A., Mangel, M. “Density-Dependent Host-Pathogen Dynamics in Soil Microcosms.” Ecology. 73:2 (1992): 495-506. Ecological Society of America, 2001.
Lentz, Amanda Jean. “The Effect of Aphids in Parasitoid-Caterpillar-Plant Interactions.” Blacksburg, Va: University Libraries, Virginia Polytechnic Institute and State University, 2007.