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.
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.
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.
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.
An 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. An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection.
Parasitism is a symbiotic relationship between species, where one organism, the parasite, lives on or inside 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.
An epidemic is the rapid spread of disease to a large number of people in a given population within a short period of time. 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.
Herd immunity is a form of indirect protection from infectious disease that occurs when a sufficient percentage of a population has become immune to an infection, whether through vaccination or previous infections, thereby reducing the likelihood of infection for individuals who lack immunity. Immune individuals are unlikely to contribute to disease transmission, disrupting chains of infection, which stops or slows the spread of disease. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual.
Human papillomavirus infection is an infection caused by human papillomavirus (HPV), a DNA virus from the Papillomaviridae family. About 90% of HPV infections cause no symptoms and resolve spontaneously within two years. However, in some cases, an HPV infection persists and results in either warts or precancerous lesions. These lesions, depending on the site affected, increase the risk of cancer of the cervix, vulva, vagina, penis, anus, mouth, or throat. Nearly all cervical cancer is due to HPV; two strains, HPV16 and HPV18, account for 70% of cases. Between 60% and 90% of the other cancers listed above are also linked to HPV. HPV6 and HPV11 are common causes of genital warts and laryngeal papillomatosis.
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist guest (symbiont). The guest is typically provided with nourishment and shelter. Examples include animals playing host to parasitic worms, cells harbouring pathogenic (disease-causing) viruses, a bean plant hosting mutualistic (helpful) nitrogen-fixing bacteria. More specifically in botany, a host plant supplies food resources to micropredators, which have an evolutionarily stable relationship with their hosts similar to ectoparasitism. The host range is the collection of hosts that an organism can use as a partner.
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.
Mathematical models can project how infectious diseases progress to show the likely outcome of an epidemic and help inform public health interventions. Models use basic assumptions or collected statistics along with 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 decide which intervention(s) to avoid and which to trial, or can predict future growth patterns, etc.
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.
An emergent virus is a virus that is either newly appeared, notably increasing in incidence/geographic range or has the potential to increase in the near future. Emergent viruses are a leading cause of emerging infectious diseases and raise public health challenges globally, given their potential to cause outbreaks of disease which can lead to epidemics and pandemics. As well as causing disease, emergent viruses can also have severe economic implications. Recent examples include the SARS-related coronaviruses, which have caused the 2002-2004 outbreak of SARS (SARS-CoV-1) and the 2019–20 pandemic of COVID-19 (SARS-CoV-2). Other examples include the human immunodeficiency virus which causes HIV/AIDS; the viruses responsible for Ebola; the H5N1 influenza virus responsible for avian flu; and H1N1/09, which caused the 2009 swine flu pandemic. Viral emergence in humans is often a consequence of zoonosis, which involves a cross-species jump of a viral disease into humans from other animals. As zoonotic viruses exist in animal reservoirs, they are much more difficult to eradicate and can therefore establish persistent infections in human populations.
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 is the common name used to refer to any of eleven different species of plant pathogenic virus in the genus Begomovirus. 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 which are transmitted by whiteflies 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 Southeast Asia. Nine species of cassava-infecting geminiviruses have been identified between Africa and India based on genomic sequencing and phylogenetic analysis. This number is likely to 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 Fabaceae 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 world. 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 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.
Evolution of Infectious Disease is a 1993 book by the evolutionary biologist Paul W. 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 infections (STIs), also referred to as sexually transmitted diseases (STDs), are infections that are commonly spread by sexual activity, especially vaginal intercourse, anal sex and oral sex. STIs often do not initially cause symptoms, which results in a greater risk of passing the disease on to others. Symptoms and signs of STIs 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, which may result in poor outcomes for the infant. Some STIs can cause infertility.
Mosquito-borne diseases or mosquito-borne illnesses are diseases caused by bacteria, viruses or parasites transmitted by mosquitoes. Nearly 700 million people get a mosquito-borne illness each year resulting in over one million deaths.
Host–parasite coevolution is a special case of coevolution, the reciprocal adaptive genetic change of a host and a parasite through reciprocal selective pressures.