Parasite load

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Parasite load is a measure of the number and virulence of the parasites that a host organism harbours. Quantitative parasitology deals with measures to quantify parasite loads in samples of hosts and to make statistical comparisons of parasitism across host samples.

Contents

In evolutionary biology, parasite load has important implications for sexual selection and the evolution of sex, as well as openness to experience. [1]

High parasite load. Lappet moth caterpillar parasited by braconid wasps. Lappet moth caterpillar parasited by braconid wasps (Apanteles sp.) (5050724084).jpg
High parasite load. Lappet moth caterpillar parasited by braconid wasps.
Enumeration of Plasmodium parasites (blue) in human red blood cells (pink) to quantify relative parasite load. Plasmodium.jpg
Enumeration of Plasmodium parasites (blue) in human red blood cells (pink) to quantify relative parasite load.

Infection and distribution

A single parasite species usually has an aggregated distribution across host individuals, which means that most hosts harbor few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology: use of parametric statistics should be avoided. Log-transformation of data before the application of parametric test, or the use of non-parametric statistics is often recommended. However, this can give rise to further problems. Therefore, modern day quantitative parasitology is based on more advanced biostatistical methods.

In vertebrates, males frequently carry higher parasite loads than females. [2] Differences in movement patterns, habitat choice, diet, body size, and ornamentation are all thought to contribute to this sex bias observed in parasite loads. Often males have larger habitat ranges and thus are likely to encounter more parasite-dense areas than female conspecifics. Whenever sexual dimorphism is exhibited in species, the larger sex is thought to tolerate higher parasite loads.

In insects, susceptibility to parasite load has been linked to genetic variation in the insect colony. [3] In colonies of Hymenoptera (ants, bees and wasps), colonies with high genetic variation that were exposed to parasites experienced lesser parasite loads than colonies that are more genetically similar.

Methods of quantifying

Depending on the parasitic species in question, various methods of quantification allow scientists to measure the numbers of parasites present and determine the parasite load of an organism. Quantifying the parasite depends on what type of parasite is in question as well as where it resides in the host body. For example, intracellular parasites such as the protozoan genus Plasmodium which causes Malaria in humans, are quantified through performing a blood smear and counting the number of white blood cells infected by viewing the smear through a microscope. [4] Other parasites residing in the blood of a host could be similarly counted on a blood smear using specific staining methods to better visualize the cells. As technology advances, more modernized methods of parasite quantification are emerging such as hand held automated cell counters, in order to efficiently count parasites such as Plasmodium in blood smears.

Quantifying intestinal parasites, such as nematodes present in an individual, often it requires dissection of the animal, extraction and counting of the parasites. Other techniques to determine intestinal parasites exist which do not require dissection; such as detection of parasitic infections by fecal examination. This is a common practice in veterinary medicine and is used to calculate parasite load in domestic animals, such as cats and dogs. Methods of fecal examination include fecal smears and flotation methods. Fecal floats can detect reproductive means of endoparasitic (see endoparasite) organisms (eggs, larvae, oocysts, and cysts) that are passed through the digestive system and are therefore present in the feces. [5]

For analytical statistical methods used to study the extent and intensity of parasitic infection see Quantitative parasitology.

Effects

Sexual selection

Parasite load has been known to effect sexual selection in various species. Hamilton and Zuk (1982) suggested that females of species could base their choice of mates on heritable resistance to parasites. [6] This hypothesis proposes that the expression of secondary sex characteristics depends on the hosts overall health. Hosts coevolve with parasites and thus generate heritable resistance to parasites, which have a net negative effect on host viability. Therefore, females will select males with few or no parasites by basing their choice on whether or not the male has fully expressed secondary sexual, otherwise known as 'healthy' characteristics.

One study found that parasite load predicts mate choice in guppies. [7] When controlling for other variables, females were shown to prefer males with relatively few parasites with this preference being associated with higher display rates that occur in less parasitized males. This phenomenon has also been observed in other species.

Behaviour

Parasite load has also been shown to affect the behavior of the infected individual. Numerous studies have been done looking at the effects of number of parasites present in a host and how this correlates with behaviors such as foraging, migration, and competitive behavior. In a study performed at the University of Georgia, it was found that beetles with higher parasite loads won more fights than those with lower parasitic loads. [8] When put up against beetles with no parasites present, the parasite-laden beetles lost the fights.

Bird species have also exhibited behavioural effects in relation to parasite load. In passerine songbirds, high parasite load results in reduced song outputs, affecting the output of secondary sexual characteristics that influence mate selection. [9] Similar effects have been observed in other bird species.

In medicine

Parasite load has been shown to affect the spread of infectious diseases. For example, parasitologists at the Universidade de São Paulo researched the effect of Chaga's disease on the immune system. They found that individuals who survived the acute phase of infection develop parasite-specific immune response that reduces parasite levels in tissues and blood. [10] This research aims to discover if the parasite load during the acute stage of infection affects if the host will eventually have a positive immune response. The research was conducted on mice, with the intention of eventually using the information gleaned from the experiments to assist humans who have contracted Chaga's disease. Marinho et al. found that parasite loads in the acute phase of infection correlates at the late chronic stage of the disease, with the intensity of the activation and response of the immune system of the host. This research could lead to new discoveries in parasitology. This could potentially prevent the spread of parasites and therefore diseases linked to parasite infection within a given population.

Host stress

Host stress causes conditions within the host to be less than ideal for parasites, leading to and causing parasite load. Malnutrition has been shown to suppress the immune system, leading to higher parasite loads within a population and increased transmission rates throughout the population. [11] It has been shown that malnutrition, and putrefaction can lead to illness within a population and therefore increase the amount of parasites within a population. Those individuals that are malnourished and stressed exhibit the highest numbers of parasite load. This implies that these individuals have a higher likelihood of dying due to the environmental factors, as well as parasite infection, likely killing the population of parasites within that specific host. This would then limit the propagation of the parasites within the population.

In the experiment conducted by Pulkkinen et al. [12] it was found that when food was limited in a population of crabs infected with daphnia, there were mortalities among the infected population of crabs. This was due to stress within environment, as well as stress within the host (crab body) from parasite infection. Pulkkinen et al. also found that after a period of time there was a corresponding reduction in average size of crabs, and therefore the mortality rate due to malnutrition and environmental stress was reduced. This increased the parasite load within the population. Parasite load is a complex ecological phenomenon, often exhibiting a negative feedback loop, as it is within the interest of the parasite population for the host to survive infection.

Related Research Articles

Chagas disease Human parasitic disease

Chagas disease, also known as American trypanosomiasis, is a tropical parasitic disease caused by Trypanosoma cruzi. It is spread mostly by insects in the subfamily Triatominae, known as "kissing bugs". The symptoms change over the course of the infection. In the early stage, symptoms are typically either not present or mild, and may include fever, swollen lymph nodes, headaches, or swelling at the site of the bite. After four to eight weeks, untreated individuals enter the chronic phase of disease, which in most cases does not result in further symptoms. Up to 45% of people with chronic infections develop heart disease 10–30 years after the initial illness, which can lead to heart failure. Digestive complications, including an enlarged esophagus or an enlarged colon, may also occur in up to 21% of people, and up to 10% of people may experience nerve damage.

A human pathogen is a pathogen that causes disease in humans.

Parasitism Relationship between species where one organism lives on or in another organism, causing it harm

Parasitism is a close 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 single-celled 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.

<i>Strongyloides stercoralis</i> Species of worm

Strongyloides stercoralis is a human pathogenic parasitic roundworm causing the disease strongyloidiasis. Its common name in the US is threadworm. In the UK and Australia, however, the term threadworm can also refer to nematodes of the genus Enterobius, otherwise known as pinworms.

<i>Toxoplasma gondii</i> Type of protozoan parasite

Toxoplasma gondii is an obligate intracellular parasitic protozoan that causes toxoplasmosis. Found worldwide, T. gondii is capable of infecting virtually all warm-blooded animals, but felids, such as domestic cats, are the only known definitive hosts in which the parasite may undergo sexual reproduction.

Parasitology Study of parasites, their hosts, and the relationship between them

Parasitology is the study of parasites, their hosts, and the relationship between them. As a biological discipline, the scope of parasitology is not determined by the organism or environment in question but by their way of life. This means it forms a synthesis of other disciplines, and draws on techniques from fields such as cell biology, bioinformatics, biochemistry, molecular biology, immunology, genetics, evolution and ecology.

Veterinary parasitology is the study of animal parasites, especially relationships between parasites and animal hosts. Parasites of domestic animals,, as well as wildlife animals are considered. Veterinary parasitologists study the genesis and development of parasitoses in animal hosts, as well as the taxonomy and systematics of parasites, including the morphology, life cycles, and living needs of parasites in the environment and in animal hosts. Using a variety of research methods, they diagnose, treat, and prevent animal parasitoses. Data obtained from parasitological research in animals helps in veterinary practice and improves animal breeding. The major goal of veterinary parasitology is to protect animals and improve their health, but because a number of animal parasites are transmitted to humans, veterinary parasitology is also important for public health.

Parasitemia is the quantitative content of parasites in the blood. It is used as a measurement of parasite load in the organism and an indication of the degree of an active parasitic infection. Systematic measurement of parasitemia is important in many phases of the assessment of disease, such as in diagnosis and in the follow-up of therapy, particularly in the chronic phase, when cure depends on ascertaining a parasitemia of zero.

Schistosoma japonicum is an important parasite and one of the major infectious agents of schistosomiasis. This parasite has a very wide host range, infecting at least 31 species of wild mammals, including 9 carnivores, 16 rodents, one primate (human), two insectivores and three artiodactyls and therefore it can be considered a true zoonosis. Travelers should be well-aware of where this parasite might be a problem and how to prevent the infection. S. japonicum occurs in the Far East, such as China, the Philippines, Indonesia and Southeast Asia.

Parasitic worm Large type of parasitic organism

Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as schistosomes reside in blood vessels.

An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, which can act as a parasite but does not rely on its host to continue its life-cycle. Obligate parasites have evolved a variety of parasitic strategies to exploit their hosts. Holoparasites and some hemiparasites are obligate.

<i>Trypanosoma cruzi</i> Species of parasitic euglenoids (protozoans)

Trypanosoma cruzi is a species of parasitic euglenoids. Among the protozoa, the trypanosomes characteristically bore tissue in another organism and feed on blood (primarily) and also lymph. This behaviour causes disease or the likelihood of disease that varies with the organism: Chagas disease in humans, dourine and surra in horses, and a brucellosis-like disease in cattle. Parasites need a host body and the haematophagous insect triatomine is the major vector in accord with a mechanism of infection. The triatomine likes the nests of vertebrate animals for shelter, where it bites and sucks blood for food. Individual triatomines infected with protozoa from other contact with animals transmit trypanosomes when the triatomine deposits its faeces on the host's skin surface and then bites. Penetration of the infected faeces is further facilitated by the scratching of the bite area by the human or animal host.

<i>Anguillicoloides crassus</i> Species of roundworm

Anguillicoloides crassus is a parasitic nematode worm that lives in the swimbladders of eels and appears to spread easily among eel populations after introduction to a body of water. It is considered to be one of the threats to the sustainability of populations of European eel. It was introduced to the European continent in the 1980s, where it was reported independently from Germany and Italy in 1982, having probably been introduced from Taiwan. It is thought to have reached England in 1987 from continental Europe. It is a natural parasite of the Japanese eel in its native range.

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

Babesia, also called Nuttallia, is an apicomplexan parasite that infects red blood cells and is transmitted by ticks. Originally discovered by the Romanian bacteriologist Victor Babeș in 1888, over 100 species of Babesia have since been identified.

Fish disease and parasites Disease that afflicts fish

Like humans and other animals, fish suffer from diseases and parasites. Fish defences against disease are specific and non-specific. Non-specific defences include skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth. If pathogens breach these defences, fish can develop inflammatory responses that increase the flow of blood to infected areas and deliver white blood cells that attempt to destroy the pathogens.

Hematodinium is a genus of dinoflagellates. Species in this genus, such as Hematodinium perezi, the type species, are internal parasites of the hemolymph of crustaceans such as the Atlantic blue crab and Norway lobster. Species in the genus are economically damaging to commercial crab fisheries, including causing bitter crab disease in the large Tanner or snow crab fisheries of the Bering Sea.

Ecoimmunology or Ecological Immunology is the study of the causes and consequences of variation in immunity. The field of ecoimmunology seeks to give an ultimate perspective for proximate mechanisms of immunology. This approach places immunology in evolutionary and ecological contexts across all levels of biological organization.

Behavior-altering parasites are parasites with two or more hosts, capable of causing changes in the behavior of one of their hosts to enhance their transmission, sometimes directly affecting the hosts' decision-making and behavior control mechanisms. They do this by making the intermediate host, where they may reproduce asexually, more likely to be eaten by a predator at a higher trophic level which becomes the definitive host where the parasite reproduces sexually; the mechanism is therefore sometimes called parasite increased trophic facilitation or parasite increased trophic transmission. Examples can be found in bacteria, protozoa, viruses, and animals. Parasites may also alter the host behaviour to increase protection of the parasites or their offspring. The term bodyguard manipulation is used for such mechanisms.

Eustrongylidosis is a parasitic disease that mainly affects wading birds worldwide; however, the parasite's complex, indirect lifecycle involves other species, such as aquatic worms and fish. Moreover, this disease is zoonotic, which means the parasite can transmit disease from animals to humans. Eustrongylidosis is named after the causative agent Eustrongylides, and typically occurs in eutrophicated waters where concentrations of nutrients and minerals are high enough to provide ideal conditions for the parasite to thrive and persist. Because eutrophication has become a common issue due to agricultural runoff and urban development, cases of eustrongylidosis are becoming prevalent and hard to control. Eustrongylidosis can be diagnosed before or after death by observing behavior and clinical signs, and performing fecal flotations and necropsies. Methods to control it include preventing eutrophication and providing hosts with uninfected food sources in aquaculture farms. Parasites are known to be indicators of environmental health and stability, so should be studied further to better understand the parasite's lifecycle and how it affects predator-prey interactions and improve conservation efforts.

<i>Cystoisospora belli</i> Species of single-celled organism

Cystoisospora belli, previously known as Isospora belli, is a parasite that causes an intestinal disease known as cystoisosporiasis. This protozoan parasite is opportunistic in immune suppressed human hosts. It primarily exists in the epithelial cells of the small intestine, and develops in the cell cytoplasm. The distribution of this coccidian parasite is cosmopolitan, but is mainly found in tropical and subtropical areas of the world such as the Caribbean, Central and S. America, India, Africa, and S.E. Asia. In the U.S., it is usually associated with HIV infection and institutional living.

References

  1. ↑ Thornhill, Randy et al. Zoonotic and Non-zoonotic Diseases in Relation to Human Personality and Societal Values: Support for the Parasite-Stress Model. Evolutionary Psychology, 2010. 8(2): 151-169
  2. ↑ Hillegass, M.A., Waterman, J.M., Roth, J.D. (2008) "The influence of sex and sociality on parasite laods in an African ground squirrel". Behavioral Ecology. Vol 19 (5) pp. 1006-1011
  3. ↑ Liersch, S., Schmid-Hempel, P. (1998) "Genetic Variation Within Social Insect Colonies Reduces Parasite Load". Proceedings of the Royal Society. Vol 265 (1392)
  4. ↑ Prudhomme O'Meara W, Remich S, Ogutu B, et al. Systematic comparison of two methods to measure parasite density from malaria blood smears. Parasitology research. 2006;99(4):500-504
  5. ↑ Detection of Parasitic Infections by Fecal Examination (2009). Diagnostic Clinical Parasitology Service Laboratory, University of Tennessee College of Veterinary Medicine Knoxville, Tennessee
  6. ↑ Hamilton W. D., Zuk M. (1982). "Heritable true fitness and bright birds: A role for parasites?". Science 218 (4570): 384–387
  7. ↑ Kennedy, C. E. J., Endler, J. A., Poynton, S. L. (1987) "Parasitic Load Predicts Mate Choice in Guppies". Behavioral Ecology and Sociobiology. Vol 21(5) pp. 291
  8. ↑ "Fighting while Parasitized: Can Nematode Infections Affect the Outcome of Staged Combat in Beetles?" PLoS ONE 10(4)
  9. ↑ Moller, A. P. (1991) "Parasite Load Reduces Song Output in a Passerine Bird". Animal Behaviour, Vol 41 (4) pp. 723-730
  10. ↑ Marinho, C.R.F., Lima, M.R.D., Grisotto, M.G., Alvarez, J.M. (1999). "Influence of Acute-Phase Parasite Load on Pathology, Parasitism, and Activation of the Immune System at the Late Chronic Phase of Chaga's Disease". Infection and Immunology. Vol 67 (1) pp. 308-318
  11. ↑ Pelletier, F., Festa-Bianchet, M. (2004). "Effects of Body Mass, Age, Dominance and Parasite load on Foraging Time of Bighorn Rams, Ovis canadensis". Behavioral Ecology and Sociobiology. Vol 56 (6) pp 546-551
  12. ↑ Pulkkinen, K., Ebert, D. (2004) "Host Starvation Decreases Parasite Load and Mean Host Size in Experimental Populations". Ecology, Vol 85 (3) pp. 823-833
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