Genetic control of susceptibility and resistance to parasitic infection

Resistance to natural tick infestation varies with age and coat and hair traits in a tropically adapted beef cattle breed

Rhipicephalus (Boophilus) microplus causes considerable livestock production losses. Knowledge of the traits that influence tick resistance contributes to the development of breeding strategies designed to improve herd productivity. Within this context, this study evaluated the resistance of Caracu, a tropically adapted cattle breed, to R. microplus. Tick count, hair length, coat thickness, and coat color were evaluated in 202 naturally tick-infested females (cows and heifers) over a period of 18 months. Blood samples were collected from all animals during the winter season for hematological analysis. Data were analyzed using Pearson correlations, generalized linear models, and principal component analysis. Correlation coefficients of tick count with coat color, coat thickness, and hair length were estimated within each season. Hematological parameters were only included in the winter season analysis and were analyzed by the restricted maximum likelihood method using log-transformed data. No differences in blood parameters were observed between animals with and without ticks. However, tick count was negatively correlated with erythrocytes (-0.29) and hematocrit (-0.24) and positively correlated with mean corpuscular hemoglobin (0.21) and mean corpuscular hemoglobin concentration (0.25). These findings suggest that higher tick counts lead to a decrease in erythrocytes but also to an increase in the amount of hemoglobin per erythrocyte, which could reduce the damage caused by low erythrocyte levels due to tick hematophagy, delaying or preventing anemia. Although tick infestation on pasture was demonstrated by the infestation of all staff members during herd management, none of the animals exhibited high tick counts, providing evidence of resistance of Caracu animals to R. microplus. Tick infestation was influenced by age class (cows > heifers), season (spring and summer > fall and winter), coat thickness (>1.5 mm > <1.5 mm), and hair length (>6 mm > <6 mm). Three components were extracted by principal component analysis, which accounted for 69.46% of data variance. The findings of this study will contribute to the development of efficient strategies aimed at reducing economic losses due to tick infestation and could be applied in animal breeding to select for tick resistance traits, reducing chemical control strategies and consequently improving sustainable livestock production.

Heritability and preadult survivorship costs of ectoparasite resistance in the naturally occurring Drosophila-Gamasodes mite system

Our understanding of the evolutionary significance of ectoparasites in natural communities is limited by a paucity of information concerning the mechanisms and heritability of resistance to this ubiquitous group of organisms. Here, we report the results of artificial selection for increasing ectoparasite resistance in replicate lines of Drosophila melanogaster derived from a field-fresh population. Resistance, as ability to avoid infestation by naturally co-occurring Gamasodes queenslandicus mites, increased significantly in response to selection and realized heritability (SE) was estimated to be 0.11 (0.0090). Deployment of energetically expensive bursts of flight from the substrate was a main mechanism of host resistance that responded to selection, aligning with previously documented metabolic costs of fly behavioral defenses. Host body size, which affects parasitism rate in some fly-mite systems, was not shifted by selection. In contrast, resistant lines expressed significant reductions in larva-to-adult survivorship with increasing toxic (ammonia) stress, identifying an environmentally modulated preadult cost of resistance. Flies selected for resistance to G. queenslandicus were also more resistant to a different mite, Macrocheles subbadius, suggesting that we documented genetic variation and a pleiotropic cost of broad-spectrum behavioral immunity against ectoparasites. The results demonstrate significant evolutionary potential of resistance to an ecologically important class of parasites.

[Opportunities and risks of the use of genetic resistances to infectious diseases in pigs - an overview]

Demands for health, performance and welfare in pigs, as well as the desire for consumer protection and reduced antibiotic use, require optimal measures in advance of disease development. This includes, in principle, the use of genetically more resistant lines and breeding animals, whose existence has been proven for a wide range of pathogen-host interactions. In addition, attempts are being made to identify the gene variants responsible for disease resistance in order to force the selection of suitable populations, also using modern biotechnical technics. The present work is intended to provide an overview of the research status achieved in this context and to highlight opportunities and risks for the future.The evaluation of the international literature shows that genetic disease resistance exist in many areas of swine diseases. However, polygenic inheritance, lack of animal models and the influence of environmental factors during evaluation render their implementation in practical breeding programs demanding. This is where modern molecular genetic methods, such as Gene Editing, come into play. Both approaches possess their pros and cons, which are discussed in this paper. The most important infectious diseases in pigs, including general diseases and epizootics, diseases of the respiratory and digestive tract and diseases of the immune system are taken into account.

How much epigenetics and quantitative trait loci (QTL) mapping tell us about parasitism maintenance and resistance/susceptibility to hosts

Interactions between the environment, parasites, vectors, and/or intermediate hosts are complex and involve several factors that define the success or failure of an infection. Among these interactions that can affect infections by a parasite, it is possible to highlight the genetic and epigenetic mechanisms in hosts and parasites. The interaction between genetics, epigenetics, infection, and the host's internal and external environment is decisive and dictates the outcome of a parasitic infection and the resistance, susceptibility, and transmission of this parasite. Epigenetic changes become important mediators in the regulation of gene expression, allowing the evasion of the parasite to immune host barriers, its transmission to new hosts, and the end of its development cycle. Epigenetics is a new frontier in the understanding of the interaction mechanisms between parasite and host that, along with information from the gene regions associated with complex phenotypic variations, the Quantitative Trait Loci, brings new possibilities to investigate more modern and efficient approaches to the treatment, control, and eradication of parasitic diseases. In this brief review, a general overview of the use of epigenetic information and mapping of Quantitative Trait Loci was summarized, both in genes of parasites and hosts, for understanding the mechanisms of resistance and/or susceptibility in parasitic relationships; also, the main search platforms were quantitatively compared, aiming to facilitate access data produced over a period of twenty years.

Host Susceptibility Modulates Escovopsis Pathogenic Potential in the Fungiculture of Higher Attine Ants

Health and disease emerge from intricate interactions between genotypes, phenotypes, and environmental features. The outcomes of such interactions are context-dependent, existing as a dynamic continuum ranging from benefits to damage. In host-microbial interactions, both the host and environmental conditions modulate the pathogenic potential of a microorganism. Microbial interactions are the core of the agricultural systems of ants in the subtribe Attina, which cultivate basidiomycete fungi for food. The fungiculture environment harbors a diverse microbial community, including fungi in the genus Escovopsis that has been studied as damage-causing agent. Here, we consider the ant colony as a host and investigate to what extent its health impacts the dynamics and outcomes of host-Escovopsis interactions. We found that different ant fungal cultivars vary in susceptibility to the same Escovopsis strains in plate-assays interactions. In subcolony-Escovopsis interactions, while healthy subcolonies gradually recover from infection with different concentrations of Escovopsis conidia, insecticide-treated subcolonies evidenced traits of infection and died within 7 days. The opportunistic nature of Escovopsis infections indicates that diseases in attine fungiculture are a consequence of host susceptibility, rather than the effect of a single microbial agent. By addressing the host susceptibility as a major modulator of Escovopsis pathogenesis, our findings expand the understanding of disease dynamics within attine colonies.

Workflow of Lotmaria passim isolation: Experimental infection with a low-passage strain causes higher honeybee mortality rates than the PRA-403 reference strain

The impact of trypanosomatid parasites on honeybee health may represent a major threat to bee colonies worldwide. However, few axenic isolates have been generated to date and with no details on cell culture passages, a parameter that could influence parasite virulence. To address this question, a trypanosomatid isolation protocol was developed and a new strain was obtained, named L. passim C1. Using experimental infection of worker honeybees, we compared the virulence and mortality rates of the ATCC PRA-403 reference strain and C1 strain, the latter showing higher virulence from 10 days post-infection onward. This study highlights the impact of cell culture passages on the pathogenicity of L. passim in honeybees, providing new evidence of its negative effects on honeybee health.

Interactions with ectoparasitic mites induce host metabolic and immune responses in flies at the expense of reproduction-associated factors

Parasites cause harm to their hosts and represent pervasive causal agents of natural selection. Understanding host proximate responses during interactions with parasites can help predict which genes and molecular pathways are targets of this selection. In the current study, we examined transcriptional changes arising from interactions between Drosophila melanogaster and their naturally occurring ectoparasitic mite, Gamasodes queenslandicus. Shifts in host transcript levels associated with behavioural avoidance revealed the involvement of genes underlying nutrient metabolism. These genetic responses were reflected in altered body lipid and glycogen levels in the flies. Mite infestation triggered a striking immune response, while male accessory gland protein transcript levels were simultaneously reduced, suggesting a trade-off between host immune responses to parasite challenge and reproduction. Comparison of transcriptional analyses during mite infestation to those during nematode and parasitoid attack identified host genes similarly expressed in flies during these interactions. Validation of the involvement of specific genes with RNA interference lines revealed candidates that may directly mediate fly-ectoparasite interactions. Our physiological and molecular characterization of the Drosophila-Gamasodes interface reveals new proximate mechanisms underlying host-parasite interactions, specifically host transcriptional shifts associated with behavioural avoidance and infestation. The results identify potential general mechanisms underlying host resistance and evolutionarily relevant trade-offs.

Copper oxide and closantel prevent alterations in hepatic energetic metabolism and reduce inflammation in Haemonchus contortus infection

The aims of this study were to evaluate if the use of copper oxide wire particles, isolated or in association with closantel, in lambs infected with Haemonchus contortus enhances the anthelmintic efficacy of closantel, as well as to evaluate the effects of treatment in hepatic energy metabolism, inflammatory markers and hematological and biochemical tests. The lambs were randomly divided into five groups (6 animals each), as follows: uninfected animals (Control); animals infected with H. contortus (HC); infected and treated with closantel (HC + CL); infected and treated with copper oxide wire particles (HC + Cu); and infected and treated with closantel plus copper oxide wire particles (HC + CL + Cu). The animals of infected groups were infected orally with H. contortus (5,000 L3 -larvae) and on day 14 post infection (p.i) the treatments were initiated. The egg per gram of feces (EPG), butyrylcholinesterase (BuChE), myeloperoxidase (MPO), adenylate kinase (AK) and pyruvate kinase (PK) activities and hematological and biochemical tests were evaluated. Treatments with copper oxide (isolated and associated) were able to reduce the EPG count on days 28, 35, 42 and 49 p.i when compared to HC group, while closantel was able to reduce EPG only from day 35 p.i. Moreover, treatment with closantel (isolated or associated) was able to prevent the inhibition of hepatic AK and PK activities caused by H. contortus infection, which may contribute to efficient intracellular energetic communication in order to maintain the balance between cellular ATP consumption and production. Butyrylcholinesterase and MPO activities were higher in infected lambs compared to uninfected, while treated groups showed lower enzymatic activity compared to the group HC. The use of all therapeutic protocols was able to reduce the EPG count. Based on these evidences, the use of copper oxide plus closantel may be considered an alternative to treat lambs infected by H. contortus.

Increased Parasitic Load in Captive-Released European Bison (Bison bonasus) has Important Implications for Reintroduction Programs

Captive-bred animals, widely used in reintroduction programmes, are often immunologically naïve and more susceptible to pathogens. We analysed infection of invasive blood-sucking nematode Ashworthius sidemi in captive-bred European bison (Bison bonasus) released to the wild in the Białowieża Forest (Poland). Mean A. sidemi infection intensity of released bison (29,137 nematodes) was over threefold higher than in wild bison (8756). It indicates a rapid acquisition and increase in the infection intensity in previously dewormed bison released from captivity. Thus, reintroduction programmes should consider the impact of pathogens and involve controlled exposure of captive animals to specific parasites prior to release.

EFFECTS OF A HAEMATOPHAGOUS MITE ON THE BARN SWALLOW (HIRUNDO RUSTICA): A TEST OF THE HAMILTON AND ZUK HYPOTHESIS

I tested three assumptions of the Hamilton and Zuk hypothesis (1982), which suggests that the extravagant male plumage of many bird species allows females to choose mates that are resistant to the parasites exploiting the host population at a given time. By choosing such males as mates, females will rear offspring carrying the genes for resistance. I tested three necessary conditions for the Hamilton and Zuk model: (1) whether parasites affect the fitness of their hosts; (2) whether there is heritable variation in parasite resistance, and (3) whether the expression of the sexual ornament varies with parasite burden. The haematophagous mite Ornithonyssus bursa (Macronyssidae, Gamasida) sucks blood from their Barn Swallow (Hirundo rustica) hosts. Experimental manipulation of mite loads and partial cross-fostering experiments on Barn Swallows, where half of the nestlings in the brood were exchanged with nestlings from another nest, shows that parasite burdens and origin, but not rearing conditions, of Bam Swallow nestlings, affected their adult tarsus length and maximum body weight shortly before fledging. Mite loads of adult Barn Swallows at spring arrival were more similar to mite loads of their own offspring, whether reared in their own or in foster nests inoculated with mites, than to loads of foster offspring. Parent Barn Swallows with long tail ornaments had offspring with smaller mite loads in the partial cross-fostering experiments. The amount of increase in male tail ornaments from one year to another was negatively related to experimentally manipulated mite loads of Barn Swallow nests during the preceding breeding season. In conclusion, the three assumptions of the hypothesis were supported by the experimental tests.

THE EFFECTS OF HOST GENOTYPE AND SPATIAL DISTRIBUTION ON TREMATODE PARASITISM IN A BIVALVE POPULATION

A basic assumption underlying models of host-parasite coevolution is the existence of additive genetic variation among hosts for resistance to parasites. However, estimates of additive genetic variation are lacking for natural populations of invertebrates. Testing this assumption is especially important in view of current models that suggest parasites may be responsible for the evolution of sex, such as the Red Queen hypothesis. This hypothesis suggests that the twofold reproductive disadvantage of sex relative to parthenogenesis can be overcome by the more rapid production of rare genotypes resistant to parasites. Here I present evidence of significant levels of additive genetic variance in parasite resistance for an invertebrate host-parasite system in nature. Using families of the bivalve mollusc, Transennella tantilla, cultured in the laboratory, then exposed to parasites in the field, I quantified heritable variation in parasite resistance under natural conditions. The spatial distribution of outplanted hosts was also varied to determine environmental contributions to levels of parasite infection and to estimate potential interactions of host genotype with environment. The results show moderate but significant levels of heritability for resistance to parasites (h² = 0.36). The spatial distribution of hosts also significantly influenced parasite prevalence such that increased host aggregation resulted in decreased levels of parasite infection. Family mean correlations across environments were positive, indicating no genotype-environment interaction. Therefore, these results provide support for important assumptions underlying coevolutionary models of host-parasite systems.

Fine-Scale Spatial Covariation between Infection Prevalence and Susceptibility in a Natural Population

The prevalence of infection varies dramatically on a fine spatial scale. Many evolutionary hypotheses are founded on the assumption that this variation is due to host genetics, such that sites with a high frequency of alleles conferring susceptibility are associated with higher infection prevalence. This assumption is largely untested and may be compromised at finer spatial scales where gene flow between sites is high. We put this assumption to the test in a natural snail-trematode interaction in which host susceptibility is known to have a strong genetic basis. A decade of field sampling revealed substantial spatial variation in infection prevalence between 13 sites around a small lake. Laboratory assays replicated over 3 years demonstrate striking variation in host susceptibility among sites in spite of high levels of gene flow between sites. We find that mean susceptibility can explain more than one-third of the observed variation in mean infection prevalence among sites. We estimate that variation in susceptibility and exposure together can explain the majority of variation in prevalence. Overall, our findings in this natural host-parasite system argue that spatial variation in infection prevalence covaries strongly with variation in the distribution of genetically based susceptibility, even at a fine spatial scale.

Tolerance and resistance to a nematode challenge are not always mutually exclusive

The relationship between the manifestations of tolerance (a host's ability to reduce the impact of a given level of pathogens) and resistance (a host's ability to clear pathogens) has been assumed to be an antagonistic one. Here we tested the hypothesis that mice from strains more resistant to intestinal nematodes will experience reduced tolerance compared with less resistant mice. Three inbred strains of mice were used: C57BL/6 mice have been characterised as susceptible, whereas BALB/c and NIH mice have been characterised as resistant to Heligmosomoides bakeri infection. Mice of each strain were either parasitised with a single dose of 250 L3H. bakeri (n=10) in water or were sham-infected with water (n=10). Body weight, food intake and worm egg output were recorded regularly throughout the experiment. Forty-two days p.i. mice were euthanised and organ weights, eggs in colon and worm counts were determined. C57BL/6 mice showed significantly greater worm egg output (P<0.001), eggs in colon (P<0.05) and female worm fecundity (P<0.05) compared with NIH and BALB/c mice. Parasitised BALB/c mice grew more whilst parasitised C57BL/6 mice grew less than their sham-infected counterparts during the first 2 weeks post-challenge (P=0.05). Parasitism significantly increased liver, spleen, small intestine and caecum weights (P<0.001) but reduced carcass weight (P<0.01). Average daily weight gain and worm numbers were positively correlated in NIH mice (P=0.05); however, the relationship was reversed when carcass weight was used as a measure for tolerance. BALB/c mice did not appear to suffer from the consequences of parasitism, with carcass weight similar in all animals. Our hypothesis that strains more resistant to the H. bakeri infection are less tolerant compared with less resistant strains is rejected, as the two resistant strains showed variable tolerance. Thus, tolerance and resistance to an intestinal nematode infection are not always mutually exclusive.

Prevalence of Batrachochytrium dendrobatidis infection is extremely low in direct-developing Australian microhylids

The emerging infectious disease chytridiomycosis has been implicated in declines and disappearances of amphibian populations around the world. However, susceptibility to infection and the extent of pathological effects of infection vary among hosts, and species with life histories that include parental care of direct-developing terrestrial eggs may tend to be less susceptible. We examined samples from a total of 595 individuals of 9 species of direct-developing Australian frogs in the family Microhylidae for the presence of infection by Batrachochytrium dendrobatidis (Bd). Between 1995 and 2004, 336 samples were collected; 102 of these were analysed histologically and 234 were tissues stored in alcohol, which were examined using diagnostic quantitative PCR (qPCR). Swab samples were collected from 259 frogs from 2005 to 2008 and were examined using qPCR. None of the 595 samples showed evidence of infection by Bd. If these data are regarded as a single sample representative of Australian microhylids, the upper 95% binomial confidence limit for the prevalence of infection in frogs of this family is 0.0062 (<1%). Even if only the data from the more powerful diagnostic qPCR tests are used, the upper 95% confidence limit for prevalence is 0.0075 (<1%). Our data suggest that Australian microhylids have a very low prevalence of infection by Bd in nature, and thus are either not susceptible, or are only slightly susceptible, to chytridiomycosis. This could be due solely to, or in combination with, low rates of transmission and to factors that promote resistance to infection, including ecological or behavioural characteristics, innate immune functions such as antimicrobial skin peptides, or antimicrobial symbionts in skin flora.

Small ruminant resistance against gastrointestinal nematodes: a case of Haemonchus contortus

Gastrointestinal nematode (GIN) infections are a common constraint to small ruminant industry throughout the world, and among those, haemonchosis has its own significance. Control of GIN primarily relies on the use of anthelmintics, but this approach has become less reliable due to the development of resistance in GINs against commonly used anthelmintics and an increased consumer demand for environmentally friendly animal products. These issues have stimulated investigations to find alternative sustainable control strategies, which are less reliant on anthelmintic input. One of such strategies is breeding of small ruminants for their resistance to the GINs. The susceptibility and resistance of animals to GIN infections varies within and between breeds. Various parasitological, biochemical and immunological parameters are employed to evaluate natural resistance status of animals both in natural pasture and artificial infections. The immune mechanisms responsible for resistance are not completely understood, but it has a significant effect in inherited resistance. Relatively resistant or tolerant animals show better local and generalised immune response as compared to susceptible. Immune response against GINs is influenced by many physiological factors. Determination of specific genes linked with host resistance will provide a valuable approach to find out the molecular mechanism of host resistance to GINs. Resistance has been reported to reduce pasture contamination, which in turn reduces re-infection and thus the requirement of the frequent anthelmintic treatments. The efficiency of control can be increased through objective and accurate identification of genetically tolerant individuals by natural and artificial infections with GINs. Complete resistance is the ultimate solution, but this has generally been ignored as a commercial reality. This paper reviews the published reports on natural resistance in small ruminants and discusses the prospects of developing small ruminants, which could be resistant to GINs.

Investigating the underlying mechanism of resistance to Ascaris infection

The generative mechanism(s) of predisposition to Ascaris infection are currently unknown. While many factors play a role in interindividual infection intensity, much focus has been placed on the host's immunological response to infection and the underlying genetics. The present review describes the research conducted that has examined various immunological parameters and genetic factors that may play a role in resistance to ascariasis. We also discuss the contribution that animal models have made to our understanding of resistance to the parasitic roundworm and their role in possible future work.

Heligmosomoides bakeri: a model for exploring the biology and genetics of resistance to chronic gastrointestinal nematode infections

The intestinal nematode Heligmosomoides bakeri has undergone 2 name changes during the last 4 decades. Originally, the name conferred on the organism in the early 20th century was Nematospiroides dubius, but this was dropped in favour of Heligmosomoides polygyrus, and then more recently H. bakeri, to distinguish it from a closely related parasite commonly found in wood mice in Europe. H. bakeri typically causes long-lasting infections in mice and in this respect it has been an invaluable laboratory model of chronic intestinal nematode infections. Resistance to H. bakeri is a dominant trait and is controlled by genes both within and outside the MHC. More recently, a significant QTL has been identified on chromosome 1, although the identity of the underlying genes is not yet known. Other QTL for resistance traits and for the accompanying immune responses were also defined, indicating that resistance to H. bakeri is a highly polygenic phenomenon. Hence marker-assisted breeding programmes aiming to improve resistance to GI nematodes in breeds of domestic livestock will need to be highly selective, focussing on genes that confer the greatest proportion of overall genetic resistance, whilst leaving livestock well-equipped genetically to cope with other types of pathogens and preserving important production traits.

Host life-history variation in response to parasitism

Over half of all living species of plants and animals are parasitic, which by definition involves intimate association with and unfavourable impact on hosts (Price, 1980). This paper will only consider parasites whose ‘unfavourable impact’ adversely affects the birth and/or mortality rates of their hosts (Anderson, 1978). Most organisms are potential hosts and must deal with the problem of parasitism. The probability of parasitic infection of a host is influenced by both environmental and genetic factors. Traditionally it was assumed that a host was either resistant or susceptible to a particular parasite and therefore the interaction between a parasite and potential host had only two possible outcomes: either the resistant host rebuffed the parasitic attack and remained uninfected or the parasite successfully invaded and significantly reduced the reproductive success of the susceptible host. This approach, however, ignored the intraspecific genetic variation present within both host and parasite populations (Wakelin, 1978). Since the outcome is determined by the interaction of a finite set of host genes and parasite genes, genetic variation in host susceptibility and parasite infectivity (Richards, 1976; Wakelin, 1978) suggests that more than two outcomes are possible. Variation in host and parasite genomes does not begin and end at the susceptibility/infectivity loci. Other genes may also influence the outcome of host–parasite interactions by altering the life-history patterns of hosts and parasites, and lead to a variety of outcomes.

Variability in the susceptibility of the fish host, Poecilia reticulata, to infection with Gyrodactylus bullatarudis (Monogenea)

Heterogeneity in susceptibility of guppies (Poecilia reticulata) to infection with Gyrodactylus bullatarudis was examined by comparing the establishment, reproduction and survival of the parasite on 4 inbred strains of the fish host following low and high levels of exposure to infection. The 4 strains fell into 2 broad categories of innately ‘resistant’ and ‘susceptible’ fish on the basis of strain differences in (a) the rates of growth and decay of the parasite populations, (b) the proportional occurrence of susceptible and resistant fish within each strain, (c) the average duration of primary infection, and (d) the duration of resistance to reinfection following recovery from a primary exposure. The results suggest genetic control of resistance/susceptibility traits and experiments with hybrids tentatively suggest dominance of resistance over susceptibility. The potential of the guppy – Gyrodactylus laboratory system for the investigation of the role of genetic factors in the study of host–parasite interactions is discussed.

Immunological aspects of nematode parasite control in sheep

Gastrointestinal nematode parasitism is arguably the most serious constraint affecting sheep production worldwide. Economic losses are caused by decreased production, the costs of prophylaxis and treatment, and the death of the infected animals. The nematode of particular concern is Haemonchus contortus, which can cause severe blood loss resulting in anemia, anorexia, depression, loss of condition, and eventual death. The control of nematode parasites traditionally relies on anthelmintic treatment. The evolution of anthelmintic resistance in nematode populations threatens the success of drug treatment programs. Alternative strategies for control of nematode infections are being developed, and one approach is to take advantage of the host's natural or acquired immune responses, which can be used in selection programs to increase the level of resistance in the population. Vaccination can also be used to stimulate or boost the host's acquired immunity. The induction of protective resistance is dependent on the pattern of cytokine gene expression induced during infection by two defined CD4+ T-helper cell subsets, which have been designated as Th1 or Th2. Intracellular parasites most often invoke a Th1-type response, and helminth parasites a Th2-type response. Breeds of sheep resistant to infection have developed resistance over a much longer term of host-parasite relationship than genetically selected resistant lines. The immune components involved in these different responses and types of host-parasite relationships will be reviewed. The potential for using vaccines has been investigated, with variable results, for several decades. The few successes and potential new antigen candidates will also be reviewed.

COSTS OF RESISTANCE IN THE DROSOPHILA?MACROCHELES SYSTEM: A NEGATIVE GENETIC CORRELATION BETWEEN ECTOPARASITE RESISTANCE AND REPRODUCTION

Genetic variation for parasite resistance occurs in most host populations. Costs of resistance, manifested as reduced fitness of resistant genotypes in the absence of parasitism, can be an important factor contributing to the maintenance of this variation. One powerful tool for detecting costs of resistance is the study of correlated responses to artificial selection. Provided that experimental lines are recently derived from large outbreeding populations, and that inbreeding is minimized during the experiment, correlated responses to selection are expected to be strong indicators of pleiotropy. We artificially selected for elevated behavioral resistance against an ectoparasitic mite (Macrocheles subbadius) in replicate populations of the fly Drosophila nigrospiracula. Resistance was modeled as a threshold trait, and the realized heritability of resistance was estimated to be 12.3% (1.4% SE) across three replicate lines recently derived from nature. We contrasted the longevity and fecundity of resistant and control (unselected) flies under a variable thermal environment. We report that reduced fecundity is a correlated response to artificial selection for increased resistance, and that the strength of this effect increases from 25 degrees to 29 degrees C. In contrast, longevity differences were not detected between resistant and control lines at either temperature. These findings are robust as they were confirmed with an independent set of experimental lines. Thus, our results identify a negative genetic correlation between ectoparasite resistance and an important life-history trait. That a correlated response was only detected for fecundity, and not longevity, suggests that the genetic correlation is attributable to pleiotropic effects with narrower effects than reallocation of a general resource pool within the organism, although other interpretations are discussed. Combined with fluctuating parasite-mediated selection and temperature, the presence of this trade-off may contribute to the maintenance of genetic variation for resistance in natural populations.

High transmission rates restore expression of genetically determined susceptibility of mice to nematode infections

This study investigated why the susceptible or resistance phenotype to the nematode Heligmosomoides polygyrus was lost when susceptible (C57BL/6) and resistant (Balb/c) strains of mice were housed together in indoor arenas with continuous transmission of the parasite larvae present in peat trays (Scott, 1991). First, both strains expressed their normal phenotype when given a controlled challenge while living in arenas, and when experimentally infected with only 5 parasite larvae. To test whether chronic exposure to peat altered the resistance phenotype, mice were given a challenge infection while living on peat. C57BL/6 mice living on peat had higher egg production and higher worm numbers than Balb/c mice, except at 2 months post-challenge. Finally, natural transmission rates were increased in arena experiments through either regular replacement of arena mice with naïve mice or direct introduction of additional larvae. A transient difference in infection levels between strains was detected in response to a modest increase in transmission whereas a 10-fold increase in transmission allowed C57BL/6 mice to exhibit the typical profile of high egg production and elevated worm numbers. These data indicate that C57BL/6 mice are less able to regulate parasite numbers at high transmission rates compared with lower transmission rates.

Immunological responses to parasitic arthropods

Parasitic arthropods are responsible for enormous economic losses to livestock producers throughout the world. These production losses may range from simple irritation caused by biting and non-biting flies to deaths and/or damage to carcass, fleece, or skin resulting from attack by myiasis flies. The estimated costs of these losses are colossal but even these usually include only direct losses and ignore those associated with pesticide application. In the USA alone (in 1976), these losses were conservatively estimated at more than 650 million US dollars. The long term use of chemical control measures for these pests has resulted in many serious problems including residues in meat and milk products, rapid development of insecticide resistance, the destruction of non-target organisms, environmental pollution, and mortality and morbidity of livestock. These concerns have prompted researchers to seek alternative methods of arthropod control, including the artificial induction of immunity. In this review, R. W. Baron and J. Weintraub discuss several examples of ectoparasites that can induce immunological resistance in the host, including Sarcoptes and Demodex mites, the sheep ked (Melophagus ovinus), Anopluran lice and myiasis-causing flies such as Hypoderma.

Spatiotemporal heterogeneity in recruitment of larval parasites to shore crab intermediate hosts: the influence of shorebird definitive hosts

Parasitism is a major biotic determinant of animal population dynamics and community structure. Temporal and spatial heterogeneity in parasitism is commonly observed in intermediate host populations. Understanding the causes of temporal and spatial variation in the recruitment of parasites is crucial if we are to manage host populations and animal communities effectively. Here, the temporal and spatial dynamics of Profilicollis antarcticus and Profilicollis novaezelandensis (Acanthocephala) infections in three species of shore crabs (Macrophthalmus hirtipes, Hemigrapsus edwardsii, and Hemigrapsus crenulatus) are examined in relation to the distribution and abundance of shorebird definitive hosts. Temporal patterns of infection were observed in M. hirtipes but not the other two species. Spatial heterogeneity in recruitment of acanthocephalan larvae to M. hirtipes and H. edwardsii populations was found both within and between locations. Weak evidence is found that infection levels in crab populations are related to the distribution and abundance of shorebird hosts both temporally and spatially. In this system, abiotic factors seem to be at least as important in determining how infection levels vary in time and space as the input of parasite eggs from bird definitive hosts.

Brugia malayi: establishment in inbred and outbred strains of mice

Immunocompetent mouse model for human filarial parasite Brugia malayi is urgently required in view of the paucity of commercial reagents for other susceptible rodent viz. mastomys and gerbil. Genes within the major histocompatibility complex have been reported to influence the susceptibility of mouse to helminth parasites. Attempts have therefore been made in the present investigation to experimentally infect various inbred strains of mice viz. NZB/BINJ, BALB/c, AKR, C(3)H, and SJL/J with H-2 haplotype (H-2: d, d, k, k, s, respectively) and outbred strains of mice viz. Parks and Swiss. Findings indicate that susceptibility of mice to B. malayi is strain associated. This is the first report on the successful completion of full developmental cycle of subperiodic B. malayi in NZB/BINJ, an immunocompetent mouse strain. In some of the other strains, partial development or low degree of establishment of worms was observed.

Heritability of resistance against ectoparasitism in the Drosophila-Macrocheles system

Ectoparasites are abundant in natural communities, can have pronounced deleterious fitness consequences to their host and are important vectors of transmissible parasitic disease. Yet very few studies have estimated the magnitude of heritable genetic variation underlying resistance against ectoparasitism, which significantly limits our ability to predict the evolution of this ecologically important character. The present paper reports results of artificial selection for increased resistance in Drosophila nigrospiracula against ectoparasitic, haematophagous mites, Macrocheles subbadius. In this system, which occurs naturally in the Sonoran Desert of North America, ectoparasitism significantly damages the expression of host fitness traits, including longevity, fecundity and male mating success. In the present study, resistance, which was modelled as a threshold trait, responded significantly to selection applied on either sex. Realized heritability, calculated as a mean across four replicates, was estimated to be 0.152 +/- 0.014 (SE). The heritability estimate from selection on males did not differ from that on females, but both estimates differed significantly from zero. This documented presence of additive genetic variation for resistance, coupled with knowledge of the fitness consequences of ectoparasitism, indicates that the host population possesses significant evolutionary potential. Selection was applied on the pre-attachment phase, thereby targeting behavioural forms of defence. This study therefore establishes parallels between insects and other animals in their ability to protect themselves and evolve behavioural defences against ectoparasites.

Pattern of Eimeria oocyst output and repeatability in naturally infected suckling Rhön lambs

The faeces of 14 Rhön lambs were examined every second day between 7 and 99 days of age for the presence of Eimeria oocysts. Eimeria absata, E. bakuensis, E. faurei, E. granulosa, E. intricata, E. ovinoidalis, E. pallida, E. parva and E. crandallis/weybridgensis were identified. The predominant species were E. ovinoidalis, E. parva, E. crandallis/weybridgensis and E. bakuensis. Using a statistical model, the oocyst excretion rate was described as a sequence of periods with decreasing levels and varying length ('excretion periods') interrupted by intervals with no or very low oocyst counts. Several variables could be deduced from these two parameters, including the length of an excretion period and the maximum output during an excretion period. Thc estimated repeatability for oocyst counts for the different species ranged from 0.05 to 0.41. This result provides a starting point for possible genetic selection based on faecal oocyst counts of Rhön sheep for resistance to Eimeria infections.

The sustainability, feasibility and desirability of breeding livestock for disease resistance

Selective breeding for disease resistance utilises proven animal breeding methods to improve animal health, welfare and productivity. Unsurprisingly, it is receiving more and more attention from livestock breeders. However, there are a number of largely theoretical arguments that have been raised as potential problems in selection for disease resistance. These can be classified under sustainability, feasibility and desirability. This review considers each of these areas in turn. Several examples show that enhanced resistance to disease is stable under natural selection and therefore deliberate selection for disease resistance should also be stable and sustainable. The feasibility of selective breeding depends in part upon the heritability of the trait or traits used to measure disease resistance, as well as the amount of variation among animals. The heritability of traits associated with resistance to many important diseases is often high and considerable variation among animals exists. Consequently, selective breeding for enhanced disease resistance is certainly feasible. The desirability of breeding for disease resistance depends upon whether there are trade-offs with other economically important traits. By and large these remain to be defined. However, even if unfavourable associations exist, breeders can create selection indices that include traits with unfavourable associations and maximise the desired responses while attempting to minimise undesirable effects. In conclusion, so long as one or more diseases exert a significant influence on livestock production, selective breeding will be a useful tool to assist in disease control.

Dietary differences between phenotypes with symmetrical and asymmetrical pelvis in the stickleback Gasterosteus aculeatus

Differential parasitism among phenotypes within populations can result from intrinsic factors such as immunocompetence or extrinsic factors such as ecological overlap with pathogens. In a recent study of a population of sticklebacks (Gasterosteus aculeatus) from Boulton Lake, British Columbia, relative cestode infections were related to pelvic-girdle symmetry. Adult sticklebacks with an asymmetrical pelvis had a higher incidence of infections than fish with a symmetrical pelvis, yet the pattern was reversed among yearlings. In the current study we test whether this unexpected result for yearlings, which is inconsistent with general theory coupling asymmetry with reduced immuno competence, might be due to ecological factors rather than to differences in immunocompetence. We analyze the diet of 9089 uninfected sticklebacks collected during a 15-year study and show that male and female yearling sticklebacks with an asymmetrical pelvis are more benthic than symmetrical phenotypes. This could reduce the probability of exposure to pelagic copepods, the primary hosts of the cestode Schistocephalus solidus, and thereby lead to lower infection rates relative to pelagic fish. This finding would account for the unusual reversal in relative parasitism in this population and raises the possibility of finding further diet-induced causes of differential parasitism in studies of asymmetry.

Heritable basis for choice of group size in a colonial bird

Sizes of most kinds of animal groups vary considerably within a population, with group size often causing direct effects on the fitness of group members. Although the consequences of varying group size have been well studied, the causes of variation in group size remain poorly known for most animals. Groups might vary in size because different individuals perform better in differently sized groups and thus have genetic predispositions to choose large or small groups. We examined whether heritable variation for choice of group size exists in the cliff swallow (Petrochelidon pyrrhonota), a colonial bird that nests in colonies ranging from 2 to 3,700 nests. Parent-offspring regressions showed significant heritabilities for choice of colony size under natural conditions. Partial cross-fostering experiments showed that individuals reared in colonies of sizes different from those of their birth returned to breed the next year in colonies that matched their birth colony in size and actively avoided those similar to their rearing colony, suggesting that choice of colony size is genetically based. Common environmental effects, maternal effects, and philopatry did not explain these results. Variation in group size probably results in part from a polymorphism in genetic preferences within the population, and the range in colony sizes is maintained by natural selection on the type of bird occupying each site.

Development of the one-on-one quality assessment assay for entomopathogenic nematodes

The one-on-one bioassay was developed using Steinernema carpocapsae (All) nematodes against the wax moth larva, Galleria mellonella. The assay was used to develop and compare virulence profiles of both in vitro- and in vivo-produced nematodes and to provide a quality assessment 'standard' for in vitro-produced nematodes. The bioassay was subsequently used to develop virulence profiles for Steinernema carpocapsae (UK), S. feltiae (UK), S. feltiae (R1.5), S. feltiae (SN), S. glaseri (NJ-43), and S. riobrave (RGV). These profiles are unique for each species and isolate and are used as a standard of virulence in routine quality assessment of nematodes produced in liquid fermentation.

Epidemiology of gastrointestinal nematode parasitism in Suffolk and Gulf Coast Native sheep with special emphasis on relative susceptibility to Haemonchus contortus infection

An eight-year study was conducted to define the epidemiology of gastrointestinal nematode infection in Suffolk and Gulf Coast Native (Native) breeds of sheep, and to determine if the Native sheep is more resistant to infection. For the initial three years, each breed grazed separate pastures where anthelmintic treatments were administered to individual animals on a salvage basis. For the last five years, both breeds grazed concurrently; anthelmintic treatments were administered to individual animals on a salvage basis for the first three years, and to all animals, when treatment criteria were met, for the last two years. The fecal egg count (FEC) and blood packed cell volume (PCV) were monitored, and tracer lamb nematode burdens were determined. Overall, FEC for both breeds increased in the spring (periparturient rise) for most years and in the summer for all years. Under separate grazing conditions, Native ewes and lambs had consistently lower infection levels than Suffolk ewes and lambs. During the haemonchosis season (June-September) each year, Suffolk ewe and lamb PCV decreased, and Native ewe and lamb PCV remained relatively stable. The salvage treatment protocol resulted in 27 treatments for Suffolk and one for Native ewes; similarly for lambs, 13 for Suffolk and zero for Native. Tracer lambs grazed with their respective breed, and the FEC and mean total nematode burden corresponded with the pattern of infection for their respective breed. The predominant nematodes found in Suffolk and Native tracer lambs were Haemonchus contortus and Trichostrongylus spp., respectively. Under concurrent grazing conditions, the same seasonal repeatable pattern of infection was present and was exhibited by both breeds, with the Native ewes and lambs being consistently and significantly (p < or = 0.05) lower for FEC and higher for PCV. The salvage treatment protocol resulted in 57 and zero treatments for Suffolk and Native ewes, respectively; for lambs, 46 and 11. Tracer lamb nematode burdens again corresponded to their respective breed pattern of infection, with H. contortus and Trichostrongylus spp. being predominant in Suffolk and Native lambs, respectively. Data from all tracer lambs showed a relatively low level of hypobiosis (H. contortus only), and, although there was no consistent hypobiosis season, the tendency was for a higher level to occur in the fall. These results showed that the classic repeatable seasonal pattern of gastrointestinal nematode infection occurred in both breeds of sheep, and that Native sheep were more resistant to infection (specifically H. contortus) than Suffolk sheep.

Expulsion of Trichostrongylus colubriformis by high and low responder guinea-pigs

Guinea-pigs with genetically determined susceptibility to infection with Trichostrongylus colubriformis (or low responders) rejected both primary and secondary infections with this parasite more slowly than resistant animals (high responders). Low responders were not protected with a vaccination procedure which was highly effective in outbred animals. The relatively poor protective immune responses that develop in low responder guinea-pigs are evocative of the responses of the natural host to infection with this parasite and suggest that low responder guinea-pigs have potential for the study of T. colubriformis protective antigens and for the development of adjuvants to enhance antiparasitic effector responses in vaccinated hosts.

The ecology of host-finding behaviour and parasite transmission: past and future perspectives

Host location by parasites can be achieved by either active or passive mechanisms. In spite of their significance, the efficacy of these methods has been little researched. High fecundity in parasites is discussed in terms of the role it plays in dispersal and transmission. Some concepts developed by mainstream behavioural ecologists are outlined and their relevance to parasitology is indicated. 'Reproductive value' is recommended as an appropriate measure of the costs and benefits of behavioural cts. Although costs of reproduction have been rarely studied in parasites, they are likely to occur in cosexual insects, nematodes and crustaceans. Experiments using captive hosts and/or in vitro cultivation could help in the construction of realistic optimality models. We suggest that r- and K-selection theory could assist in the study of the evolution of parasite behaviour. We discuss how parasite populations are dispersed and controlled and consider the implications of overdispersion. We outline three sources of signals to which parasites may respond and suggest that understanding evolutionary mechanisms and community organisation of parasites and hosts requires evaluation of fundamental behavioural responses to environmental signals. The study of closely related groups of parasites and their hosts may advance our knowledge of the evolution of parasite life cycles and the evolutionary costs and benefits of behavioural acts.

Host genes, parasites and parasitic infections

Resistance to infection of mammalian hosts by parasites is under genetic control at many different levels: between species, between races, breeds and lines of single species and between individuals. These genetic effects have been described in many host-parasite systems. Here we review the interaction between three elements: host genes, parasites and the environment in which parasitic infections develop. Already livestock industries exploit genetic variation between breeds, particularly for the control of trypanosomiasis and tick infestation in cattle. In most populations, and to many diseases, resistance is heritable and selective breeding for resistance in commercial livestock species has been successful experimentally. Attempts at utilizing genetic variation are placed in the broad context of the coevolution of host and parasite, the limited knowledge we have of the mode of action of resistance genes and our ability to use genetic information to predict resistance to parasites.

Niche breadth in parasites: an evolutionarily stable strategy model, with special reference to the protozoan parasite Leishmania

A parasite's host range essentially defines its niche breadth, which, as foraging theory predicts, is influenced by resource availability. For parasites, the interaction of infection and transmission characteristics with host population dynamics determines host availability. An epidemiological model, involving two host types and describing competition between a "generalist" parasite strain and a related "specialist" strain, is used to examine the interplay among host range, relative host availabilities, and adaptational compromises engendered by increased host range. Results show that the generalist can predominate even when it cannot maintain itself in either host alone, but that the specialist can persist if its reproductive rate attains some threshold relative to either of the generalist's respective rates in its two hosts. The model is in rough, qualitative agreement with observed dynamics of two Leishmania parasite-host systems, and overall results suggest that infection of two species with a common parasite can lead to complex, indirect coevolutionary dynamics.

Susceptibility of adult Heligmosomoides polygyrus to intestinal inflammatory responses induced by heterologous infection

Adult H. polygyrus are capable of surviving for many months after primary exposure of mice to infective larvae, raising the possibility that worms of this species have inherent resistance to intestinal immune responses. Accordingly experiments were carried out to determine whether H. polygyrus are resistant to the inflammatory changes elicited during the acute phase of the intestinal response to Trichinella spiralis. Adult worms were expelled from mice when their presence coincided with the most intense phase of inflammation elicited by T. spiralis. The effect was dose-dependent with more intense T. spiralis challenge resulting in a correspondingly greater loss of H. polygyrus. Even the less pathogenic species T. pseudospiralis elicited a response of sufficient intensity in NIH mice to cause the expulsion of H. polygyrus from concurrently infected animals. Tissue larval stages of H. polygyrus were protected from expulsion by their location deep in the intestinal walls and the maximum detrimental effect against H. polygyrus was observed during the adult phase or during the establishment of L3 larvae. Acceleration of the response to T. spiralis in immune challenged mice resulted in earlier loss of H. polygyrus. When the expulsion of T. spiralis was delayed (e.g. from slow responder C57BL/10 mice) the loss of H. polygyrus took place correspondingly later. These experiments demonstrate unequivocally that mouse strains which normally tolerate chronic infections with H. polygyrus have the capacity to mount intestinal inflammatory responses of sufficient vigour to remove the worms but that this potential is not normally realized. However, the observation that some H. polygyrus always survived even when the response induced by T. spiralis was of the rapid secondary type suggests that the parasites are resilient in the face of the inflammatory response capable of removing most of the worms. It is suggested that in addition to the immunomodulatory strategy employed by adult worms to prevent the intestinal response being elicited, the worms have a second line of defence which is reflected in their resilience to responses which they have been unable to prevent.

Heligmosomoides polygyrus (Nematoda): susceptible and resistant strains of mice are indistinguishable following natural infection

BALB/c mice were characterized as more resistant to infection with Heligmosomoides polygyrus (Nematoda) than C57BL/6 mice based on lower establishment and survival during a primary infection and stronger protection induced by an immunizing regime. It was hypothesized, therefore, that C57BL/6 mice would be more heavily infected than BALB/c mice when they lived together as a single population in a large indoor arena where transmission occurred through contact between the mice and damp peat trays where parasite eggs developed into larvae. Fifty female mice (including 5 infected mice) of each strain were placed in a 3.2 x 0.8 m arena. Net egg production and numbers of larvae acquired by sentinel mice of each strain were monitored every two weeks. The experiment was replicated twice. The results did not support the hypothesis. No difference was detected between strains of mice in the number of larvae acquired by sentinel mice during 24 h exposure periods, or in the numbers of worms present after 12 or 23 weeks. Net egg production was also comparable between strains. A hypothesis that the unexpected similarity of infection in the two strains was related to differences in rates of contact with the peat trays was not supported by preliminary data on mouse behaviour that revealed equal frequency of contact with peat trays between strains. A second hypothesis that continuous exposure to larvae led to similar infection levels in the two strains (in contrast to the controlled characterization experiments) was also unsupported. Mice were infected weekly with 10, 50 or 100 larvae for 5 or 10 weeks. Net egg production and numbers of worms were consistently higher in C57BL/6 than BALB/c mice. At this time it is not clear why infection in the two strains was virtually identical in the large arenas but clearly distinct in all controlled infection experiments.