Density dependence

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In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population. [1] This article will focus on density dependence in the context of macroparasite life cycles.

Contents

Positive density-dependence

Positive density-dependence, density-dependent facilitation, or the Allee effect describes a situation in which population growth is facilitated by increased population density.[ citation needed ]

Examples

dioecious (separate sex) obligatory parasites, mated female worms are required to complete a transmission cycle. At low parasite densities, the probability of a female worm encountering a male worm and forming a mating pair can become so low that reproduction is restricted due to single sex infections. At higher parasite densities, the probability of mating pairs forming and successful reproduction increases. This has been observed in the population dynamics of Schistosomes. [2]

Positive density-dependence processes occur in macroparasite life cycles that rely on vectors with a cibarial armature, such as Anopheles or Culex mosquitoes. For Wuchereria bancrofti , a filarial nematode, well-developed cibarial armatures in vectors can damage ingested microfilariae and impede the development of infective L3 larvae. At low microfilariae densities, most microfilariae can be ruptured by teeth, preventing successful development of infective L3 larvae. As more larvae are ingested, the ones that become entangled in the teeth may protect the remaining larvae, which are then left undamaged during ingestion. [3]

Positive density-dependence processes may also occur in macroparasite infections that lead to immunosuppression. Onchocerca volvulus infection promotes immunosuppressive processes within the human host that suppress immunity against incoming infective L3 larvae. This suppression of anti-parasite immunity causes parasite establishment rates to increase with higher parasite burden. [4]

Negative density-dependence

Negative density-dependence, or density-dependent restriction, describes a situation in which population growth is curtailed by crowding, predators and competition.[ citation needed ]

In cell biology, it describes the reduction in cell division. When a cell population reaches a certain density, the amount of required growth factors and nutrients available to each cell becomes insufficient to allow continued cell growth.[ citation needed ]

This is also true for other organisms because an increased density means an increase in intraspecific competition. Greater competition means an individual has a decreased contribution to the next generation i.e. offspring. Density-dependent mortality can be overcompensating, undercompensating or exactly compensating.[ citation needed ]

There also exists density-independent inhibition, where other factors such as weather or environmental conditions and disturbances may affect a population's carrying capacity.[ citation needed ]

An example of a density-dependent variable is crowding and competition.

Examples

Density-dependent fecundity Generalized fecundity graph.png
Density-dependent fecundity

Density-dependent fecundity exists, where the birth rate falls as competition increases. In the context of gastrointestinal nematodes, the weight of female Ascaris lumbricoides and its rates of egg production decrease as host infection intensity increases. Thus, the per-capita contribution of each worm to transmission decreases as a function of infection intensity. [5]

Parasite-induced vector mortality Blackfly life expectancy.png
Parasite-induced vector mortality

Parasite-induced vector mortality is a form of negative density-dependence. The Onchocerciasis life cycle involves transmission via a black fly vector. In this life-cycle, the life expectancy of the black fly vector decreases as the worm load ingested by the vector increases. Because O. volvulus microfilariae require at least seven days to mature into infective L3 larvae in the black fly, the worm load is restricted to levels that allow the black fly to survive for long enough to pass infective L3 larvae onto humans. [6]

In macroparasite life cycles

Density-dependence processes (red) in filariasis life cycle Density-dependence in filariasis.jpg
Density-dependence processes (red) in filariasis life cycle

In macroparasite life cycles, density-dependent processes can influence parasite fecundity, survival, and establishment. Density-dependent processes can act across multiple points of the macroparasite life cycle. For filarial worms, density-dependent processes can act at the host/vector interface or within the host/vector life-cycle stages. At the host/vector interface, density-dependence may influence the input of L3 larvae into the host's skin and the ingestion of microfilariae by the vector. Within the life-cycle stages taking place in the vector, density-dependence may influence the development of L3 larvae in vectors and vector life expectancy. Within the life-cycle stages taking place in the host, density-dependence may influence the development of microfilariae and host life expectancy. [7]

In reality, combinations of negative (restriction) and positive (facilitation) density-dependent processes occur in the life cycles of parasites. However, the extent to which one process predominates over the other vary widely according to the parasite, vector, and host involved. This is illustrated by the W. bancrofti life cycle. In Culex mosquitoes, which lack a well-developed cibarial armature, restriction processes predominate. Thus, the number of L3 larvae per mosquito declines as the number of ingested microfilariae increases. Conversely, in Aedes and Anopheles mosquitoes, which have well-developed cibarial armatures, facilitation processes predominate. Consequently, the number of L3 larvae per mosquito increases as the number of ingested microfilariae increases. [3]

Implications for parasite persistence and control

Negative density-dependent (restriction) processes contribute to the resilience of macroparasite populations. At high parasite populations, restriction processes tend to restrict population growth rates and contribute to the stability of these populations. Interventions that lead to a reduction in parasite populations will cause a relaxation of density-dependent restrictions, increasing per-capita rates of reproduction or survival, thereby contributing to population persistence and resilience. [7]

Contrariwise, positive density-dependent or facilitation processes make elimination of a parasite population more likely. Facilitation processes cause the reproductive success of the parasite to decrease with lower worm burden. Thus, control measures that reduce parasite burden will automatically reduce per-capita reproductive success and increase the likelihood of elimination when facilitation processes predominate. [8]

Extinction threshold

The extinction threshold refers to minimum parasite density level for the parasite to persist in a population. Interventions that reduce parasite density to a level below this threshold will ultimately lead to the extinction of that parasite in that population. Facilitation processes increase the extinction threshold, making it easier to achieve using parasite control interventions. Conversely, restriction processes complicates control measures by decreasing the extinction threshold. [8]

Implications for parasite distribution

Anderson and Gordon (1982) propose that the distribution of macroparasites in a host population is regulated by a combination of positive and negative density-dependent processes. In overdispersed distributions, a small proportion of hosts harbour most of the parasite population. Positive density-dependent processes contribute to overdispersion of parasite populations, whereas negative density-dependent processes contribute to underdispersion of parasite populations. As mean parasite burden increases, negative density-dependent processes become more prominent and the distribution of the parasite population tends to become less overdispersed. [9]

Consequently, interventions that lead to a reduction in parasite burden will tend to cause the parasite distribution to become overdispersed. For instance, time-series data for Onchocerciasis infection demonstrates that 10 years of vector control lead to reduced parasite burden with a more overdispersed distribution. [10]

See also

Related Research Articles

<i>Loa loa</i> filariasis Medical condition

Loa loa filariasis is a skin and eye disease caused by the nematode worm Loa loa. Humans contract this disease through the bite of a deer fly or mango fly, the vectors for Loa loa. The adult Loa loa filarial worm migrates throughout the subcutaneous tissues of humans, occasionally crossing into subconjunctival tissues of the eye where it can be easily observed. Loa loa does not normally affect one's vision but can be painful when moving about the eyeball or across the bridge of the nose. The disease can cause red itchy swellings below the skin called "Calabar swellings". The disease is treated with the drug diethylcarbamazine (DEC), and when appropriate, surgical methods may be employed to remove adult worms from the conjunctiva. Loiasis belongs to the so-called neglected diseases.

<i>Loa loa</i> Species of roundworm

Loa loa is a filarial (arthropod-borne) nematode (roundworm) that causes Loa loa filariasis. Loa loa actually means "worm worm", but is commonly known as the "eye worm", as it localizes to the conjunctiva of the eye. Loa loa is commonly found in Africa. It mainly inhabits rain forests in West Africa and has native origins in Ethiopia. The disease caused by Loa loa is called loiasis and is one of the neglected tropical diseases.

<span class="mw-page-title-main">Onchocerciasis</span> Human helminthiasis (infection by parasite)

Onchocerciasis, also known as river blindness, is a disease caused by infection with the parasitic worm Onchocerca volvulus. Symptoms include severe itching, bumps under the skin, and blindness. It is the second-most common cause of blindness due to infection, after trachoma.

<span class="mw-page-title-main">Filariasis</span> Parasitic disease caused by a family of nematode worms

Filariasis is a parasitic disease caused by an infection with roundworms of the Filarioidea type. These are spread by blood-feeding insects such as black flies and mosquitoes. They belong to the group of diseases called helminthiases.

<i>Wuchereria bancrofti</i> Species of parasitic worm

Wuchereria bancrofti is a filarial (arthropod-borne) nematode (roundworm) that is the major cause of lymphatic filariasis. It is one of the three parasitic worms, together with Brugia malayi and B. timori, that infect the lymphatic system to cause lymphatic filariasis. These filarial worms are spread by a variety of mosquito vector species. W. bancrofti is the most prevalent of the three and affects over 120 million people, primarily in Central Africa and the Nile delta, South and Central America, the tropical regions of Asia including southern China, and the Pacific islands. If left untreated, the infection can develop into lymphatic filariasis. In rare conditions, it also causes tropical pulmonary eosinophilia. No vaccine is commercially available, but high rates of cure have been achieved with various antifilarial regimens, and lymphatic filariasis is the target of the World Health Organization Global Program to Eliminate Lymphatic Filariasis with the aim to eradicate the disease as a public-health problem by 2020. However, this goal was not met by 2020.

<i>Brugia malayi</i> Medical condition

Brugia malayi is a filarial (arthropod-borne) nematode (roundworm), one of the three causative agents of lymphatic filariasis in humans. Lymphatic filariasis, also known as elephantiasis, is a condition characterized by swelling of the lower limbs. The two other filarial causes of lymphatic filariasis are Wuchereria bancrofti and Brugia timori, which both differ from B. malayi morphologically, symptomatically, and in geographical extent.

<i>Dirofilaria immitis</i> Species of worm that causes parasitic disease in animals

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<i>Onchocerca volvulus</i> Nematode

Onchocerca volvulus is a filarial (arthropod-borne) nematode (roundworm) that causes onchocerciasis, and is the second-leading cause of blindness due to infection worldwide after trachoma. It is one of the 20 neglected tropical diseases listed by the World Health Organization, with elimination from certain countries expected by 2025.

Acanthocheilonema is a genus within the family Onchocercidae which comprises mainly tropical parasitic worms. Cobbold created the genus Acanthocheilonema with only one species, Acanthocheilonema dracunculoides, which was collected from aardwolf in the region of South Africa in the nineteenth century. These parasites have a wide range of mammalian species as hosts, including members of Carnivora, Macroscelidea, Rodentia, Pholidota, Edentata, and Marsupialia. Many species among several genera of filarioids exhibit a high degree of endemicity in studies done on mammalian species in Japan. However, no concrete evidence has confirmed any endemic species in the genus Acanthocheilonema.

<span class="mw-page-title-main">Lymphatic filariasis</span> Medical condition

Lymphatic filariasis is a human disease caused by parasitic worms known as filarial worms. Usually acquired in childhood, it is a leading cause of permanent disability worldwide, impacting over a hundred million people and manifesting itself in a variety of severe clinical pathologies While most cases have no symptoms, some people develop a syndrome called elephantiasis, which is marked by severe swelling in the arms, legs, breasts, or genitals. The skin may become thicker as well, and the condition may become painful. Affected people are often unable to work and are often shunned or rejected by others because of their disfigurement and disability.

<i>Mansonella perstans</i> Species of roundworm

Mansonella perstans is a filarial (arthropod-borne) nematode (roundworm), transmitted by tiny blood-sucking flies called midges. Mansonella perstans is one of two filarial nematodes that causes serous cavity filariasis in humans. The other filarial nematode is Mansonella ozzardi. M. perstans is widespread in many parts of sub-Saharan Africa, parts of Central and South America, and the Caribbean.

Mansonelliasis is the condition of infection by the nematode Mansonella. The disease exists in Africa and tropical Americas, spread by biting midges or blackflies. It is usually asymptomatic.

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<span class="mw-page-title-main">Microfilaria</span> Early stage in the life cycle of certain parasitic nematodes in the family Onchocercidae

The microfilaria is an early stage in the life cycle of certain parasitic nematodes in the family Onchocercidae. In these species, the adults live in a tissue or the circulatory system of vertebrates. They release microfilariae into the bloodstream of the vertebrate host. The microfilariae are taken up by blood-feeding arthropod vectors. In the intermediate host the microfilariae develop into infective larvae that can be transmitted to a new vertebrate host.

Mansonella ozzardi is a filarial (arthropod-borne) nematode (roundworm). This filarial nematode is one of two that causes serous cavity filariasis in humans. The other filarial nematode that causes it in humans is Mansonella perstans. M. ozzardi is an endoparasite that inhabits the serous cavity of the abdomen in the human host. It lives within the mesenteries, peritoneum, and in the subcutaneous tissue.

<i>Dirofilaria repens</i> Species of roundworm

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<span class="mw-page-title-main">Filarioidea</span> Superfamily of roundworms

The Filarioidea are a superfamily of highly specialised parasitic nematodes. Species within this superfamily are known as filarial worms or filariae. Infections with parasitic filarial worms cause disease conditions generically known as filariasis. Drugs against these worms are known as filaricides.

Mansonella streptocerca,, is a filarial (arthropod-borne) nematode (roundworm) causing the disease streptocerciasis. It is a common parasite in the skin of humans in the rain forests of Africa, where it is thought to be a parasite of chimpanzees, as well.

Setaria cervi is a species of parasitic roundworms belonging to the genus Setaria. It infects cattle, bison, yak, reindeer, buffalo, moose, and sheep all over the world. It is most prevalent in Europe and Asia. Different species of Aedes mosquito can transmit the filarial worm. Stable fly Haematobia stimulans is the major vector. The mature roundworms are primarily present in the abdominal (peritoneal) cavity, but are capable of migrating to central nervous system causing serious neurological disease.

<span class="mw-page-title-main">Nematode infection in dogs</span> Threadworm infections of dogs are frequent

Nematode infection in dogs - the infection of dogs with parasitic nemamotodes - are, along with tapeworm infections and infections with protozoa, frequent parasitoses in veterinary practice. Nematodes, as so-called endoparasites, colonize various internal organs - most of them the digestive tract - and the skin. To date, about 30 different species of nematode have been identified in domestic dogs; they are essentially also found in wild dog species. However, the majority of them often cause no or only minor symptoms of disease in adult animals. The infection therefore does not necessarily have to manifest itself in a worm disease (helminthosis). For most nematodes, an infection can be detected by examining the feces for eggs or larvae. Roundworm infection in dogs and the hookworm in dogs is of particular health significance in Central Europe, as they can also be transmitted to humans (zoonosis). Regular deworming can significantly reduce the frequency of infection and thus the risk of infection for humans and dogs.

References

  1. ↑ Hixon, M (2009), "Density Dependence and Independence", Encyclopedia of Life Sciences, Chichester: John Wiley & Sons Ltd, doi:10.1002/9780470015902.a0021219, ISBN   978-0470016176
  2. ↑ May, R.M. (1977). "Togetherness among Schistosomes: its effects on the dynamics of the infection". Mathematical Biosciences. 35 (3–4): 301–343. doi:10.1016/0025-5564(77)90030-X.
  3. 1 2 Snow, L.C. (2006). "Transmission dynamics of lymphatic filariasis: vector-specific density dependence in the development of Wuchereria bancrofti infective larvae in mosquitoes". Medical and Veterinary Entomology. 20 (3): 261–272. doi:10.1111/j.1365-2915.2006.00629.x. PMID   17044876. S2CID   23152390.
  4. ↑ Duerr, H.P. (2003). "Density-dependent parasite establishment suggests infection-associated immunosuppression as an important mechanism for parasite density regulation in onchocerciasis". Transactions of the Royal Society of Tropical Medicine and Hygiene. 97 (2): 242–250. doi:10.1016/S0035-9203(03)90132-5. PMID   14584385.
  5. ↑ Walker, M (2009). "Density-dependent effects on the weight of female Ascaris lumbricoides infections of humans and its impact on patterns of egg production". Parasites & Vectors. 2 (1): 11. doi: 10.1186/1756-3305-2-11 . PMC   2672930 . PMID   19208229.
  6. ↑ Basanez, M.G. (1996). "Density-dependent processes in the onchocerciasis: relationship between microfilarial intake and mortality of the simuliid vector". Parasitology. Cambridge University Press. 113 (4): 331–355. doi:10.1017/S003118200006649X. PMID   8873475. S2CID   42555004.
  7. 1 2 Churcher, T.S. (2006). "Density dependence and the control of helminth parasites". Journal of Animal Ecology. 75 (6): 1313–1320. doi:10.1111/j.1365-2656.2006.01154.x. PMID   17032363.
  8. 1 2 Duerr, H.P. (2005). "Determinants of the eradicability of filarial infections: a conceptual approach". Trends in Parasitology. 21 (2): 88–96. doi:10.1016/j.pt.2004.11.011. PMID   15664532.
  9. ↑ Anderson, R.M. (1982). "Processes influencing the distribution of parasite numbers within host populations with special emphasis on parasite-induced host mortalities". Parasitology. 85 (2): 373–398. doi:10.1017/S0031182000055347. PMID   7145478. S2CID   19196511.
  10. ↑ Plaisier, A.P. (1996). Modelling Onchocerciasis Transmission and Control. Rotterdam: Erasmus University. ISBN   90-72245-68-7.
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