Threshold host density (N_T), in the context of wildlife disease

ecology Ecology () is the natural science of the relationships among living organisms and their Natural environment, environment. Ecology considers organisms at the individual, population, community (ecology), community, ecosystem, and biosphere lev ...

, 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 is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...

. 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 A sexually transmitted infection (STI), also referred to as a sexually transmitted disease (STD) and the older term venereal disease (VD), is an infection that is spread by sexual activity, especially vaginal intercourse, anal sex, 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 zoonosis spread primarily via ingestion of raw milk, unpasteurized milk from infected animals. It is also known as undulant fever, Malta fever, and Mediterranean fever. The bacteria causing this disease, ''Brucella'', are small ...

* Avian flu *

Echinococcus multilocularis ''Echinococcus multilocularis'', the fox tapeworm, is a small Cyclophyllidea, cyclophyllid Cestoda, tapeworm found extensively in the northern hemisphere. ''E. multilocularis,'' along with other members of the ''Echinococcus'' genus (especially ' ...

Leptospirosis Leptospirosis is a blood infection caused by the bacterium ''Leptospira'' that can infect humans, dogs, rodents and many other wild and domesticated animals. Signs and symptoms can range from none to mild (headaches, Myalgia, muscle pains, a ...

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 isolated fr ...

(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 th ...

(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