Threshold host density (N_T), in the context of

wildlife disease Wild animals, domestic animals and humans share a large and increasing number of infectious diseases, known as zoonoses. The continued globalization of society, human population growth, and associated landscape change further increase the interacti ...

ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...

, refers to the concentration of a population of a particular organism as it relates to disease. Specifically, the threshold host density (N_T) of a species refers to the minimum concentration of individuals necessary to sustain a given disease within a

population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...

. Threshold host density (N_T) 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 Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal diseases, are infections that are spread by sexual activity, especially vaginal intercourse, anal sex, and oral ...

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

Brucellosis Brucellosis is a highly contagious zoonosis caused by ingestion of unpasteurized milk or undercooked meat from infected animals, or close contact with their secretions. It is also known as undulant fever, Malta fever, and Mediterranean fever. The ...

Avian flu Avian influenza, known informally as avian flu or bird flu, is a variety of influenza caused by viruses adapted to birds.

Echinococcus multilocularis ''Echinococcus multilocularis'' is a small cyclophyllid tapeworm found extensively in the northern hemisphere. ''E. multilocularis,'' along with other members of the ''Echinococcus'' genus (especially '' E. granulosus''), produce diseases known ...

* Leptospirosis

Host density-independent diseases

*Chlamydia in koalas *

Simian immunodeficiency virus ''Simian immunodeficiency virus'' (''SIV'') is a species of retrovirus that cause persistent infections in at least 45 species of non-human primates. Based on analysis of strains found in four species of monkeys from Bioko Island, which was iso ...

(SIV) *

Human immunodeficiency virus The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause AIDS, acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the ...

(HIV)

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.

References

External links

Wildlife Disease Association, WDA
{{DEFAULTSORT:Threshold Host Density Disease ecology Conservation biology Epidemiology Ecology Habitat