Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “parasite”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 703 records · Page 39Linked to original sources
PubMed1990

Virulence of lizard malaria: the evolutionary ecology of an ancient parasite-host association.

The negative consequences of parasitic infection (virulence) were examined for two lizard malaria parasite-host associations: Plasmodium agamae and P. giganteum, parasites of the rainbow lizard, Agama agama, in Sierra Leone, West Africa; and P. mexicanum in the western fence lizard, Sceloporus occidentalis, in northern California. These malaria species vary greatly in their reproductive characteristics: P. agamae produces only 8 merozoites per schizont, P. giganteum yields over 100, and P. mexicanum an intermediate number. All three parasites appear to have had an ancient association with their host. In fence lizards, infection with malaria is associated with increased numbers of immature erythrocytes, decreased haemoglobin levels, decreased maximal oxygen consumption, and decreased running stamina. Not affected were numbers of erythrocytes, resting metabolic rate, and sprint running speed which is supported by anaerobic means in lizards. Infected male fence lizards had smaller testes, stored less fat in preparation for winter dormancy, were more often socially submissive and, unexpectedly, were more extravagantly coloured on the ventral surface (a sexually dimorphic trait) than non-infected males. Females also stored less fat and produced smaller clutches of eggs, a directly observed reduction in fitness. Infected fence lizards do not develop behavioural fevers. P. mexicanum appears to have broad thermal buffering abilities and thermal tolerance; the parasite's population growth was unaffected by experimental alterations in the lizard's body temperature. The data are less complete for A. agama, but infected lizards suffered similar haematological and physiological effects. Infected animals may be socially submissive because they appear to gather less insect prey, possibly a result of being forced into inferior territories. Infection does not reduce clutch size in rainbow lizards, but may lengthen the time between clutches. These results are compared with predictions emerging from several models of the evolution of parasite virulence. The lack of behavioural fevers in fence lizards may represent a physiological constraint by the lizards in evolving a thermal tolerance large enough to allow elimination of the parasite via fever. Such constraints may be important in determining the outcome of parasite-host coevolution. Some theory predicts low virulence in old parasite-host systems and higher virulence in parasites with greater reproductive output. However, in conflict with this argument, all three malarial species exhibited similar high costs to their hosts.

Animals↗
PubMed1994

The effect of Toxoplasma gondii and other parasites on activity levels in wild and hybrid Rattus norvegicus.

Using both correlational and experimental evidence, the relationship between parasite load and host activity was assessed in brown rats, Rattus norvegicus. Two hypotheses were tested--(1) that parasites with indirect life-cycles, involving transmission between a prey and its predator, will alter the activity of the intermediate host so as to increase its susceptibility to predation by the definitive host and (2) that activity levels in parasitized rats would be increased rather than decreased. Four groups of rats (n = 140) were examined. One group (n = 50) were wild brown rats trapped from 3 UK farmsteads, with naturally occurring parasites. The others were purpose-bred wild/laboratory hybrid rats with experimentally induced parasitic infections of either (n = 15) adult-acquired or (n = 15) congenitally-acquired Toxoplasma gondii (an indirect life-cycle parasite), or (n = 15) Syphacia muris (a direct life-cycle parasite). Uninfected hybrid rats ( n = 45), matched for sex, age and weight, served as controls. Rats were housed individually in outdoor cages, and their activities were recorded on video-tapes for 6 non-consecutive 10 h nights. Exercise wheels were also available for the hybrid rats. Out of 6 parasite species detected in the wild rats, T. gondii was the only one which required predation by a definitive host to complete its life-cycle, and was also the only parasite to be associated with higher activity levels in infected than uninfected rats. Hybrid rats infected with T. gondii were also more active than those uninfected, whereas there were no differences in activity levels between S. muris infected and uninfected rats. This study shows that the indirect life-cycle parasite T. gondii can influence the activity of its intermediate host the rat. I suggest that this may facilitate its transmission to the cat definitive host.

Animals↗
PubMed1994

Dynamics of the interaction between the parasitic isopod, Anilocra pomacentri, and the coral reef fish, Chromis nitida.

Field and laboratory investigations into the effect of the parasitic isopod Anilocra pomacentri (Cymothoidae) on the population dynamics of the reef fish Chromis nitida (Pomacentridae) were carried out at Heron Island, Great Barrier Reef, Australia. Fish carried a single adult parasite just posterior and dorsal to the eye either to the right or to the left of the midline. The adult parasite was overdispersed among fish on patch reefs (dispersion factor, k = 0.69). Sequential field observations on a single cohort of fish showed that parasites significantly depressed growth, reproduction, and survivorship. The von Bertalanffy growth coefficients (a measure of somatic growth) were 0.10 for parasitized fish compared with 0.17 for non-parasitized fish. Female fish carrying the parasite produced only 12% of the number of eggs produced by non-parasitized fish of the same size. In the field, the mortality of infected juvenile C. nitida (LCF 15-30 mm) was estimated to be at least 88% in the first 70 days after recruitment of the fish. The mortality of uninfected recruits over the same period was 66%. In laboratory trials, the mortality associated with the infection of juvenile fish by larval parasites ranged from 78% for small fish (mean LCF 15.0 mm) to 28% for larger fish (mean LCF 24.9 mm) within 4 days of experimental infection. This is one of the few studies that evaluates the effect of a parasite on a population of fish in the field.

Animals↗
PubMed1998

Coevolutionary interactions between host life histories and parasite life cycles.

Several recent studies have discussed the interaction of host life-history traits and parasite life cycles. It has been observed that the life-history of a host often changes after infection by a parasite. In some cases, changes of host life-history traits reduce the costs of parasitism and can be interpreted as a form of resistance against the parasite. In other cases, changes of host life-history traits increase the parasite's transmission and can be interpreted as manipulation by the parasite. Alternatively, changes of host's life-history traits can also induce responses in the parasite's life cycle traits. After a brief review of recent studies, we treat in more detail the interaction between the microsporidian parasite Edhazardia aedis and its host, the mosquito Aedes aegypti. We consider the interactions between the host's life-history and parasite's life cycle that help shape the evolutionary ecology of their relationship. In particular, these interactions determine whether the parasite is benign and transmits vertically or is virulent and transmits horizontally.

Aedes↗
PubMed1994

The population dynamics of parasitic helminth communities.

This paper describes a mathematical model which allows us to compare the data collected from short-term cross-sectional surveys with the population dynamics of host and parasite populations over longer periods of time. The model extends an earlier framework for two parasite species in one host, to one for an arbitrary number of parasite species. We show that the conditions necessary for the coexistence of two parasite species extend to expressions for the coexistence of three or more parasite species. Furthermore, the model suggests that those species which form the 'core' of the parasite community are those whose high fecundity and transmission efficiency permit them to colonize hosts readily. In contrast, those species which are classified as 'satellite' species of the community are either species with low fecundity, or low transmission efficiencies. This work confirms earlier studies that suggest that increasing degrees of aggregation are crucial in allowing several species of parasites to coexist in the same species of hosts. The properties of the model are compared with patterns observed in data collected for helminth parasites of Anolis lizards, wood mice and eels. This combined theoretical and empirical approach confirms the importance of the life history strategies of the parasite in determining the abundance of each species in the community. It suggests that studies of parasite community structure have to pay more attention to the strategies pursued by each individual species before interactions between species are considered.

Animals↗
PubMed1998

Within-host transmission strategies of transovarial, feminizing parasites of Gammarus duebeni.

The amphipod Gammarus duebeni harbours several species of vertically transmitted microsporidian parasites. G. Duebeni were collected from 3 localities in the UK. Animals from Budle Bay, Northumberland, were infected with Octosporea effeminans, and those from Millport, Isle of Cumbrae and Fenham Flats, Northumberland were infected with microsporidia of the genus Nosema. We derived expected distributions of parasites per host embryonic cell by modelling parasite transmission as a multitype, Galton-Watson branching process. Parasite prevalence (proportion of females infected) was significantly heterogeneous among localities. Parasite burden in zygotes was much higher for females infected with Nosema than in animals infected with O. effeminans. There was no significant difference between localities in the number of Nosema in the zygotes. Comparison of models and data from 64-cell host embryos showed that the distributions of parasites per cell were consistent with the hypothesis that sorting of parasites into daughter cells is biased for at least 1 cell lineage. Host embryos infected with O. effeminans could expect to contain a growing number of parasites in each cell generation within such biased cell lineages; similar estimates for Nosema predict a decline in the number of parasites per cell within a biased lineage. We discuss the possibility that the 2 species of parasite may be employing different strategies in order to ensure transmission to the next host generation.

Animals↗
PubMed1997

Epidemiologic features of intestinal parasitic infections in Italian mental institutions.

To determine the prevalence of intestinal parasitic infections in the residents of four Italian psychiatric institutions, we examined the stool specimens collected in triplicate from 238 residents, enrolled between May 1995 and May 1996. Besides, physician and staff nurses provided data about each resident by standardized questionnaires. Parasites were detected in the fecal samples from 128 patients (53.8%). However, in the stool specimens from 106 residents only non-pathogenic protozoa were found (82.8%). Trichuris trichiura ova, Giardia lamblia cysts and trophozoites, Cryptosporidium parvum oocysts, and Balantidium coli cysts were found in the fecal samples from 22 residents (9.2%). B. hominis was the most prevalent parasite. It was detected in the fecal specimens from 97 residents (40.8%). The so-called nonpathogenic amebae were detected in the fecal specimens from 65 residents, though, at the same time, there was no evidence of Entamoeba histolytica infection. Twelve residents (5.0%) showed intestinal colonization by nonpathogenic flagellates. All the subjects with T. trichiura infection were housed in the facility of Ancona. Parasites were found in fecal samples from all the 11 residents with behavioural aberrations, but only three of those suffering from intestinal pathogen infection associated to diarrhea. Statistical analyses revealed that the presence of pathogenic parasites in fecal specimens was significantly associated with diarrhea, nausea, vomiting, abdominal pain, fever, behavioural aberrations and nonpathogenic protozoa (p < 0.01), but did not demonstrate any other significant associations between these parasites and the other variables, such as pruritus, mucus or blood in the stools and presence of fecal leukocytes. On the other hand, the presence of nonpathogenic protozoa was significantly related to aberrations such as pica, geophagia, phytophagy, coprophagy, coprophilia and pathogenic parasites (p < 0.01). Data analyses revealed that both pathogenic and nonpathogenic parasites were significantly more common in institutionalized patients than in controls. The rare presence of clinical signs and symptoms in colonized patients represents an important public health problem, since the presence of asymptomatic carriers among residents with low hygienic conditions, raises concern of transmission of parasitic infections to professional staff and other residents. Since the eradication of parasitic colonization in residential facilities is hard to reach, an effective prevention is the only measure to deal with this public health problem.

Adult↗
PubMed2006

Parasites dominate food web links.

Parasitism is the most common animal lifestyle, yet food webs rarely include parasites. The few earlier studies have indicated that including parasites leads to obvious increases in species richness, number of links, and food chain length. A less obvious result was that adding parasites slightly reduced connectance, a key metric considered to affect food web stability. However, reported reductions in connectance after the addition of parasites resulted from an inappropriate calculation. Two alternative corrective approaches applied to four published studies yield an opposite result: parasites increase connectance, sometimes dramatically. In addition, we find that parasites can greatly affect other food web statistics, such as nestedness (asymmetry of interactions), chain length, and linkage density. Furthermore, whereas most food webs find that top trophic levels are least vulnerable to natural enemies, the inclusion of parasites revealed that mid-trophic levels, not low trophic levels, suffered the highest vulnerability to natural enemies. These results show that food webs are very incomplete without parasites. Most notably, recognition of parasite links may have important consequences for ecosystem stability because they can increase connectance and nestedness.

Animals↗
PubMed2000

Epidemiology of infections with intestinal parasites and human immunodeficiency virus (HIV) among sugar-estate residents in Ethiopia.

Intestinal parasitic infections could play an important role in the progression of infection with human immunodeficiency virus (HIV), by further disturbing the immune system whilst it is already engaged in the fight against HIV. HIV and intestinal parasitic infections were investigated in 1239, randomly selected individuals, aged 15-54 years, living on a sugar estate in central Ethiopia. Intestinal parasites were identified in faecal samples (one/subject) using direct, concentration, and (for Strongyloides stercoralis larvae) Baermann methods. HIV serological status was determined using ELISA, with ELISA-positive samples confirmed as positive by western blotting. Most (70.1%) of the subjects were infected with at least one intestinal parasite and 3.1% were seropositive (but asymptomatic) for HIV. The intestinal parasites identified in the study population were amoebic parasites (Entamoeba histolytica/Enta. dispar) (24.6%), hookworms (23.8%), Ascaris lumbricoides (22.2%), Trichuris trichiura (19.5%), S. stercoralis (13.0%), Taenia saginata (4.5%), Giardia lamblia (3.0%), and Enterobius vermicularis (1.3%). Overall, the HIV-positives were no more or less likely to carry intestinal parasites than the HIV-negatives (76.2% v. 69.9%; P > 0.05). However, when each parasite was considered separately, amoebic parasites were found to be more common in the HIV-positives than the HIV-negatives (43.7% v. 24.0%; P < 0.05). This difference remained significant in a multivariate analysis, after controlling for the socio-demographic characteristics of the study participants. In conclusion, there was moderate interaction between intestinal parasites and HIV at the asymptomatic stage of HIV infection. The observed association between amoebic and HIV infections requires confirmation in a prospective study, allowing for the analysis of biological mechanisms involved in the association.

Adolescent↗
PubMed2002

Parasite polymorphism may serve to enhance fitness in different host environments.

Phenotypic expression of trans-sialidase (TS) by Trypanosoma cruzi (Silvio strain) is restricted to a subpopulation (20-30%) of broad trypomastigotes (TS+ parasites), while the larger subpopulation of slender trypomastigotes lacks TS expression (TS- parasites). Rhodnius prolixus nymphs were infected with fractionated T. cruzi parasite populations (Silvio strain). Bugs ingesting TS- parasites produced significantly higher parasite loads than bugs ingesting either TS+ parasites or unfractionated populations. The addition of 1.0 microg of TS monoclonal antibody (TCN-2)/ml of blood meal to TS+ populations increased parasite populations to levels comparable to those of TS- populations. In contrast, the addition of exogenous TS to TS- parasite blood meals significantly reduced parasite loads to levels comparable to bugs ingesting TS+ parasites. These results suggest that T. cruzi trypomastigote polymorphism may serve to enhance survival in different host environments.

Animals↗
PubMed2002

Host range and local parasite adaptation.

Parasites may be expected to become locally adapted to their hosts. However, while many empirical studies have demonstrated local parasite adaptation, others have failed to demonstrate it, or have shown local parasite maladaptation. Researchers have suggested that gene flow can swamp local parasite-host dynamics and produce local adaptation only at certain geographical scales; others have argued that evolutionary lags can account for both null and maladaptive results. In this paper, we use item response theory (IRT) to test whether host range influences the likelihood of parasites locally adapting to their hosts. We collated 32 independent experiments testing for local adaptation, where parasites could be assigned as having either broad or narrow host ranges (BHR and NHR, respectively). Twenty-five tests based on BHR parasites had a significantly lower average effect size than seven NHR tests, indicating that studies based on BHR parasites are less likely to demonstrate local parasite adaptation. We argue that this may relate to evolutionary lags during diffuse coevolution of BHR parasites with their hosts, rather than differences in experimental approaches or other confounds between BHR and NHR studies.

Adaptation, Physiological↗
PubMed2004

Temporal stability of insular avian malarial parasite communities.

Avian malaria is caused by a diverse community of genetically differentiated parasites of the genera Plasmodium and Haemoproteus. Rapid seasonal and annual antigenic allele turnover resulting from selection by host immune systems, as observed in some parasite populations infecting humans, may extend analogously to dynamic species compositions within communities of avian malarial parasites. To address this issue, we examined the stability of avian malarial parasite lineages across multiple time-scales within two insular host communities. Parasite communities in Puerto Rico and St Lucia included 20 and 14 genetically distinct parasite lineages, respectively. Lineage composition of the parasite community in Puerto Rico did not vary seasonally or over a 1 year interval. However, over intervals approaching a decade, the avian communities of both islands experienced an apparent loss or gain of one malarial parasite lineage, indicating the potential for relatively frequent lineage turnover. Patterns of temporal variation of parasite lineages in this study suggest periodic colonization and extinction events driven by a combination of host-specific immune responses, competition between lineages and drift. However, the occasional and ecologically dynamic lineage turnover exhibited by insular avian parasite communities is not as rapid as antigenic allele turnover within populations of human malaria.

Animals↗
PubMed2005

Do malaria ookinete surface proteins P25 and P28 mediate parasite entry into mosquito midgut epithelial cells?

BACKGROUND: P25 and P28 are related ookinete surface proteins highly conserved throughout the Plasmodium genus that are under consideration as candidates for inclusion in transmission-blocking vaccines. Previous research using transgenic rodent malaria parasites lacking P25 and P28 has demonstrated that these proteins have multiple partially redundant functions during parasite infection of the mosquito vector, including an undefined role in ookinete traversal of the mosquito midgut epithelium, and it has been suggested that, unlike wild-type parasites, Dko P25/P28 parasites migrate across the midgut epithelium via an intercellular, rather than intracellular, route. PRESENTATION OF THE HYPOTHESIS: This paper presents an alternative interpretation for the previous observations of Dko P25/P28 parasites, based upon a recently published model of the route of ookinete invasion across the midgut epithelium. This model claims ookinete invasion is intracellular, with entry occurring through the lateral apical plasma membrane of midgut epithelial cells, and is associated with significant invagination of the midgut epithelium localised at the site of parasite penetration. Following this model, it is hypothesized that: (1) a sub-population of Dko P25/P28 ookinetes invaginate, but do not penetrate, the apical surface of the midgut epithelium and thus remain within the midgut lumen; and (2) another sub-population of Dko P25/P28 parasites successfully enters and migrates across the midgut epithelium via an intracellular route similar to wild-type parasites and subsequently develops into oocysts. TESTING THE HYPOTHESIS: These hypotheses are tested by showing how they can account for previously published observations and incorporate them into a coherent and consistent explanatory framework. Based upon these hypotheses, several quantitative predictions are made, which can be experimentally tested, about the relationship between the densities of invading Dko P25/P28 ookinetes in different regions of the midgut epithelium and the number of oocyst stage parasites to which these mutant ookinetes give rise. IMPLICATIONS OF THE HYPOTHESIS: The recently published model of ookinete invasion implies that Dko P25/P28 parasites are greatly, although not completely, impaired in their ability to enter the midgut epithelium. Therefore, P25 and/or P28 have a novel, previously unrecognized, function in mediating ookinete entry into midgut epithelial cells, suggesting that one mode of action of transmission-blocking antibodies to these ookinete surface proteins is to inhibit this function.

Animals↗
PubMed2002

Community ecology of the metazoan parasites of pink cusk-eel, Genypterus brasiliensis (Osteichthyes: Ophidiidae), from the coastal zone of the State of Rio de Janeiro, Brazil.

Fifty-five specimens of pink cusk-eel, Genypterus brasiliensis Regan, 1903 (Osteichthyes: Ophidiidae) collected from the coastal zone of the State of Rio de Janeiro, Brazil (approx. 21-23 degrees S, 41-45 degrees W), from September 2000 to January 2001, were necropsied to study their parasites. All fish were parasitized by one or more metazoan. Fourteen species of parasites were collected. G. brasiliensis is a new host record for nine parasite species. The larval stages of cestodes and the nematodes were the majority of the parasite specimens collected, with 38.4% and 36.5%, respectively. Cucullanus genypteri was the dominant species with highest prevalence and/or abundance. The parasites of G. brasiliensis showed the typical overdispersed pattern of distribution. Six parasite species showed correlation between the host's total body length and prevalence and abundance. Host sex did not influence prevalence and parasite abundance of any parasite species. The mean diversity in the infracommunities of G. brasiliensis was H= 0.364 +/- 0.103, with correlation with the host's total length and without differences in relation to sex of the host. One pair of adult endoparasites (C. genypteri and A. brasiliensis) showed positive covariations between their abundances. Negative association or covariation was not found. Differences between the qualitative and quantitative aspects of the parasite community of G. brasiliensis from Rio de Janeiro and Argentina suggest the existence of two population stocks of pink cusk-eel in the South America Atlantic Ocean.

Animals↗
PubMed2000

In vitro studies on optimal requirements for the growth of Spironucleus vortens, an intestinal parasite of the freshwater angelfish.

Spironucleus vortens were cultivated in either an artificial medium at different temperatures, or in medium at various pH conditions or supplemented with different bile concentrations at 25 degrees C. Temperature, pH and bile requirements for the optimal growth of the parasite were determined. Parasites multiplied quickly at 28 and 31 degrees C and reached maximum numbers on Day 4 of cultivation, whereafter they did not survive. At 25 degrees C, parasites survived longer than those at 28 and 31 degrees C with no difference in multiplication rate during the exponential phase. The longest survival period was seen at 22 degrees C, although the growth rate of the parasite was not as high as those at 25 degrees C. At a higher temperature of 37 degrees C, no parasites were observed alive after the second day of cultivation. Optimal pH range for the parasite's growth was 6.5 to 7.5, with the highest cell number at pH 7.5. Parasites survived longest (15 d) at pH 6.0, although the maximum number of cells was lower than those at the optimal pH. Parasites were dead within 24 h at pH levels above 8.5 or below 5.5. All cultures supplemented with either bovine or fish bile yielded numbers of parasites lower than cultures with no bile. In addition, parasite growth was significantly suppressed in medium supplemented with higher concentrations of bile. These results indicate that the optimal condition for the in vitro cultivation of S. vortens is 25 degrees C and pH 6.5 to 7.5 without supplementation with bile.

Animals↗
PubMed2000

A role for parasite-induced PGE2 in IL-10-mediated host immunoregulation by skin stage schistosomula of Schistosoma mansoni.

Significant quantities of PGE(2) were produced by cercariae of Schistosoma mansoni following incubation with linoleic acid, a free fatty acid found on the surface of the skin. Cyclooxygenase (COX) 2 inhibitors failed to block this PGE(2) production, suggesting that a different biochemical pathway may be involved in the production of PGE(2) by the parasite. In addition, the parasites were also able to induce PGE(2) and IL-10 from human and mouse keratinocytes. Analysis of mouse skin during skin migratory phases of infection confirmed these in vitro observations. COX2 inhibitors blocked the parasite-induced PGE(2) and IL-10 from keratinocytes. Further analysis of the parasite secretions showed that the PGE(2)/IL-10-inducing effect was associated with a fraction <30 kDa molecular size. Addition of this fraction or parasite-stimulated keratinocyte culture supernatant to Con A-stimulated spleen cells resulted in the suppression of cell proliferation. This effect could be blocked by anti-IL-10 treatment. In sharp contrast, attenuation of the parasites with gamma-irradiation significantly abrogated their ability to induce PGE(2) or IL-10 from skin cells. Significance of IL-10 in host immunoregulation by skin stage schistosomula of S. mansoni was further confirmed by using IL-10-deficient mice. In these mice the normal subdued cutaneous reaction to the parasite was absent. Instead, a prominent cellular reaction occurred around the parasite, and there was considerable delay in parasitic migration through the skin. Thus these results suggest a key role for parasite-induced PGE(2) in IL-10-dependent down-regulation of host immune responses in the skin.

Animals↗
PubMed1999

Parasite adaptation to extreme conditions in a desert environment.

Deserts represent universally recognized extreme environments for animal life. This paper documents the highly specialized adaptations of Pseudodiplorchis americanus, a monogenean parasite of the desert toad, Scaphiopus couchii. Building on a long-term record of parasite population ecology (continuing since the early 1980s), field studies focus on the effects of severe drought in the Sonoran Desert, Arizona, in the mid 1990s. This provides a test of the ability of the host-parasite system to tolerate exceptional perturbation. The analysis provides new insight into parasite infection dynamics in a natural wildlife system through integration of host and parasite population age structure. The environmental check interrupted host recruitment in 1993-95 and parasite recruitment in 1995-97. This produced an imprint in age structure and infection levels recognizable over several years: parasite recruitment failure reduced transmission 2-3 years later. The host (maximum life span 17 years) tolerated the disruption but the impact was more serious for the parasite (life span 3 years) leading to extinction of some previously stable populations. Despite this demonstration of a rare event exacerbating external environmental constraints, experimental studies suggest that the internal (host) environment normally creates the most severe conditions affecting P. americanus. Only about 3% of parasites survive from invasion until first reproduction. Post-invasion factors including host immunity, characteristic of most parasite life cycles, constitute a greater constraint upon survival than external conditions, even in a desert environment.

Adaptation, Physiological↗
PubMedDate not supplied

[Specificity of host-parasite relations between arthropods and terrestrial vertebrates].

Specificity of partners in host-parasite system is one of its main characteristics. Unfortunately this term has different senses in scientific literature. In everyday practice one judges an extent of host specificity of a parasite mainly by indices of its occurrence and abundance on different host species. An occurrence of parasites in nature reflects general result of complex eco-physiological interrelationships between partners in hostparasite system. Specificity of parasites in a choice of hosts may depend on a belonging of the latter to certain taxa (phylogenetic specificity), or on biotic and abiotic factors (ecological specificity). In arthropods, the phylogentic specificity and coevolution are characteristic to a greater extent for permanent hosts (lice, Mallophaga, cheyletoid and feather mites). A coevolutionaryphylogenesis is disturbed by transfers of parasites onto new hosts, by different rates of speciation in filial lines or by an extinction of several parasite taxa. In temporary parasites different forms of ecological specificity are prevalent. A host specificity is expressed to the lesser extent in mosquitoes, horseflies and in other blood-sucking Diptera. In temporary parasites with a long-term feeding (ticks) coevolutionary sequences are relatively rare, because this parasites had to adapt not only to a life on host, but also to a lesser stable environment. In some nest-burrow bloodsuckers (fleas, gamasid mites and argasid ticks) the ecological specificity is shown no by their relations with certain host species, but by an associations with habitats occupied by hosts (burrow, nests, caves). In relation with a high dynamics of host-parasite system, a specificity of its partners is comparative and it is kept up only under specific ecological conditions.

Animals↗