Threshold host density

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Threshold host density (NT), in the context of wildlife disease ecology, refers to 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. [1]

Wildlife, 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 changes further enhances the interface between wildlife, domestic animals, and humans, thereby facilitating additional infectious disease emergence. The wildlife component of this triad has received inadequate focus in the past to effectively protect human health as evidenced by such contemporary diseases as SARS, Lyme disease, West Nile Fever, and a host of other emerging diseases. Further, habitat loss and other factors associated with human-induced landscape changes have reduced past ability for many wildlife populations to overcome losses due to various causes. This disease emergence and resurgence has reached unprecedented importance for the sustainability of desired population levels for many wildlife populations and for the long-term survival of some species.

Ecology Scientific study of the relationships between living organisms and their environment

Ecology is the branch of biology which studies the interactions among organisms and their environment. Objects of study include interactions of organisms with each other and with abiotic components of their environment. Topics of interest include the biodiversity, distribution, biomass, and populations of organisms, as well as cooperation and competition within and between species. Ecosystems are dynamically interacting systems of organisms, the communities they make up, and the non-living components of their environment. Ecosystem processes, such as primary production, pedogenesis, nutrient cycling, and niche construction, regulate the flux of energy and matter through an environment. These processes are sustained by organisms with specific life history traits. Biodiversity means the varieties of species, genes, and ecosystems, enhances certain ecosystem services.

Population density A measurement of population numbers per unit area or volume

Population density is a measurement of population per unit area or unit volume; it is a quantity of type number density. It is frequently applied to living organisms, and most of the time to humans. It is a key geographical term. In simple terms population density refers to the number of people living in an area per kilometer square.


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, [2] 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 Human disease

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.

<i>Echinococcus multilocularis</i> species of worm

Echinococcus multilocularis is a small cyclophyllid tapeworm found extensively in the northern hemisphere. E. multilocularis, along with other members of the Echinococcus genus, produce diseases known as echinococcosis. Unlike E. granulosus,E. multilocularis produces many small cysts that spread throughout the internal organs of the infected animal. The resultant disease is called Alveolar echinococcosis, and is caused by ingesting the eggs of E. multilocularis.

Leptospirosis human disease

Leptospirosis is an infection caused by corkscrew-shaped bacteria called Leptospira. Signs and symptoms can range from none to mild such as headaches, muscle pains, and fevers; to severe with bleeding from the lungs or meningitis. If the infection causes the person to turn yellow, have kidney failure and bleeding, it is then known as Weil's disease. If it also causes bleeding into the lungs then it is known as severe pulmonary hemorrhage syndrome.

Host density-independent diseases

Simian immunodeficiency viruses (SIVs) are retroviruses that cause persistent infections in at least 45 species of African non-human primates. Based on analysis of strains found in four species of monkeys from Bioko Island, which was isolated from the mainland by rising sea levels about 11,000 years ago, it has been concluded that SIV has been present in monkeys and apes for at least 32,000 years, and probably much longer.

Contact between individuals within a population as it relates to density in host density-dependent disease

Densitydependentdiseasegraph.png 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. [4]

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.

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Infection invasion of a host by disease-causing organisms

Infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. Infectious disease, also known as transmissible disease or communicable disease, is illness resulting from an infection.

Parasitism Interaction between two organisms living together in more or less intimate association in a relationship in which association is disadvantageous or destructive to one of the organisms

In evolutionary biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one". Parasites include protozoans such as the agents of malaria, sleeping sickness, and amoebic dysentery; animals such as hookworms, lice, mosquitoes, and vampire bats; fungi such as honey fungus and the agents of ringworm; and plants such as mistletoe, dodder, and the broomrapes. There are six major parasitic strategies of exploitation of animal hosts, namely parasitic castration, directly transmitted parasitism, trophically transmitted parasitism, vector-transmitted parasitism, parasitoidism, and micropredation.

Epidemic rapid spread of infectious disease to a large number of people in a given population within a short period of time

An epidemic is the rapid spread of infectious disease to a large number of people in a given population within a short period of time, usually two weeks or less. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.

In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected.

In epidemiology, the basic reproduction number of an infection can be thought of as the number of cases one case generates on average over the course of its infectious period, in an otherwise uninfected population.

Mathematical models can project how infectious diseases progress to show the likely outcome of an epidemic and help inform public health interventions. Models use some basic assumptions and mathematics to find parameters for various infectious diseases and use those parameters to calculate the effects of different interventions, like mass vaccination programmes. The modelling can help in deciding which intervention/s to avoid and which to trial.

Natural reservoir long-term host of the pathogen of an infectious disease

In infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally lives and reproduces, or upon which the pathogen primarily depends for its survival. A reservoir is usually a living host of a certain species, such as an animal or a plant, inside of which a pathogen survives, often without causing disease for the reservoir itself. By some definitions a reservoir may also be an environment external to an organism, such as a volume of contaminated air or water.

In epidemiology, contact tracing is the identification and diagnosis of people who may have come into contact with an infected person. For sexually transmitted diseases, this is generally limited to sexual partners and can fall under the heading of partner services. But, for highly virulent diseases such as Ebola and tuberculosis, a thorough contact tracing would require information regarding casual contacts.

<i>Babesia</i> genus of protozoan parasites

Babesia, also called Nuttallia, is an Apicomplexan parasite that infects red blood cells, transmitted by ticks. Originally discovered by the Romanian bacteriologist Victor Babeș, over 100 species of Babesia have since been identified.

Avian malaria birds disease

Avian malaria is a parasitic disease of birds, caused by parasite species belonging to the genera Plasmodium and Hemoproteus. The disease is transmitted by a dipteran vector including mosquitoes in the case of Plasmodium parasites and biting midges for Hemoproteus. The range of symptoms and effects of the parasite on its bird hosts is very wide, from asymptomatic cases to drastic population declines due to the disease, as is the case of the Hawaiian honeycreepers. The diversity of parasites is large, as it is estimated that there are approximately as many parasites as there are species of hosts. Co-speciation and host switching events have contributed to the broad range of hosts that these parasites can infect, causing avian malaria to be a widespread global disease, found everywhere except Antarctica.

In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population. This article will focus on density-dependence in the context of macroparasite life cycles.

Cassava mosaic virus

African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), and South African cassava mosaic virus (SACMV) are distinct species of circular single-stranded DNA viruses that are whitefly-transmitted and primarily infect cassava plants. These have thus far only been reported from Africa; related species of viruses are found in India and neighbouring islands, though cassava is cultivated in Latin America as well as South East Asia. Nine species of cassava-infecting geminiviruses have been identified between Africa and India based on genomic sequencing and phylogenetic analysis. This number will probably grow due to a high rate of natural transformation associated with CMV.

Soybean mosaic virus (SMV) is a member of the plant virus genus Potyvirus. It infects mainly plants belonging to the family Fabacea but has also been found infecting other economically important crops. SMV is the cause of soybean mosaic disease that occurs in all the soybean productions areas of the word. Soybean is one of the most important sources of edible oil and proteins and pathogenic infections are responsible for annual yield losses of about $4 billion dollars in the United States. Among these pathogens, SMV is the most important and prevalent viral pathogen in soybean production worldwide. It causes yield reductions of about 8% to 35% but losses as high as 94% have been reported.

<i>Evolution of Infectious Disease</i> book by Paul W. Ewald

Evolution of Infectious Disease is a 1993 book by the evolutionary biologist Paul Ewald. In this book Ewald contests the traditional view that parasites should evolve toward benign coexistence with their hosts. He draws on various studies which contradict this dogma and asserts his own theory that is based on fundamental evolutionary principles. This book provides one of the first in-depth presentations of insights from evolutionary biology on various fields in health science, including epidemiology and medicine.

In population ecology delayed density dependence describes a situation where population growth is controlled by negative feedback operating with a time lag.

Economic epidemiology is a field at the intersection of epidemiology and economics. Its premise is to incorporate incentives for healthy behavior and their attendant behavioral responses into an epidemiological context to better understand how diseases are transmitted. This framework should help improve policy responses to epidemic diseases by giving policymakers and health-care providers clear tools for thinking about how certain actions can influence the spread of disease transmission.

Sexually transmitted infection Infection transmitted through human sexual behavior

Sexually transmitted infections (STI), also referred to as sexually transmitted diseases (STD), are infections that are commonly spread by sexual activity, especially vaginal intercourse, anal sex and oral sex. Many times STIs initially do not cause symptoms. This results in a greater risk of passing the disease on to others. Symptoms and signs of disease may include vaginal discharge, penile discharge, ulcers on or around the genitals, and pelvic pain. STIs can be transmitted to an infant before or during childbirth and may result in poor outcomes for the baby. Some STIs may cause problems with the ability to get pregnant.

Mosquito-borne disease

Mosquito-borne diseases or mosquito-borne illnesses are diseases caused by bacteria, viruses or parasites transmitted by mosquitoes. They can transmit disease without being affected themselves. Nearly 700 million people get a mosquito-borne illness each year resulting in over one million deaths.

In biology, a pathogen, in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.


A super-spreader is a host—an organism infected with a disease—that infects, disproportionately, more secondary contacts than other hosts who are, also, infected with the same disease. A sick human can be a super-spreader; they would be more likely to infect others than most people with the disease. Super-spreaders are thus of high concern in epidemiology.


  1. Begon, M.; Harper, J.L.; Townsend, C.R. (2009). "Glossary: H". In Striano, T.; Reid, V. Essentials Of Ecology (3rd ed.). Wiley. ISBN   1444305344.
  2. Holmes, E., Kareiva, P., Sabo, J. (2004). "Efficacy of Simple Viability Models in Ecological Risk Assessment: Does Density Dependence Matter?". Ecology. 85 (2): 328–341. CiteSeerX Lock-green.svg. doi:10.1890/03-0035. JSTOR   3450199.
  3. Wobeser, Gary A. (2005). Essentials of Disease in Wild Animals. Wiley. ISBN   978-0-8138-0589-4.
  4. Cabaret, J.; Hugonnet, L. (1987). "Infection of roe-deer in France by the lung nematode, Dictyocaulus eckerti Skrjabin, 1931 (Trichostrongyloidea): influence of environmental factors and host density". J. Wildl. Dis. 23 (1): 109–12. doi:10.7589/0090-3558-23.1.109. PMID   2950246.

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