Breeding for worm resistance: a perspective

Evidence for a constitutive cost of host resistance on body fat growth in ewe lambs from lines selected for resistance or susceptibility to experimental infections with Haemonchus contortus

Although benefits of selection for host resistance to gastro-intestinal nematodes have long been recognized, its costs on production traits remain unclear. A main difficulty when studying those costs is to disentangle genetic effects due to selection from plastic responses induced by infection. Putative costs of host resistance have been extensively investigated in growing sheep. However, while most of those studies have relied on live weight to assess body growth, more comprehensive assessments accounting for body composition are advocated to detect trade-offs. In this study we used 90 female lambs from lines divergently selected on resistance to Haemonchus contortus that we experimentally infected (n = 60) or not (n = 30) under controlled conditions. As those conditions were defined to enable uninfected lambs to fully express their growth potential, we sought to precisely identify the effects of selection for host resistance on health traits and on growth traits. We assessed muscular and fat growth based on repeated measurements with dorsal ultrasonography for all lambs on farm, and with whole-body computed tomography (CT) scans for a subgroup of 18 infected lambs. Lambs achieved a high growth rate, including infected ones despite their high worm burden (confirmed at necropsy in the subgroup). As expected, lambs from the resistant (R) line were less infected than those from the susceptible (S) line. However, the clear pathogenic effects observed on muscular growth and voluntary feed intake were similar between lines. In contrast, a line difference in body fat was supported both by dorsal and volumetric CT measurements. Specifically, lower fat in the R line compared with the S line was observed equally in infected and uninfected groups, thus providing evidence for a constitutive cost of host resistance. Although this cost is not necessarily disadvantageous in nutrient-rich environments exposing animals to excess fat deposition, its consequences in nutrient-scarce environments may be important to promote sustainable breeding strategies for host resistance.

Sustainable Approaches to Parasite Control in Ruminant Livestock

It is increasingly difficult to manage and control gastrointestinal nematode parasites in pasture-based ruminant livestock operations because of the high prevalence of anthelmintic resistance. Anthelmintics should be combined with alternative forms of control. Sustainable tools include copper oxide wire particles and condensed tannin-rich plants, which target primarily Haemonchus contortus in small ruminants. Nematophagous fungi reduce larvae on pasture and target nematode larvae in feces of most livestock species. In addition, and perhaps most importantly, genetic selection focuses on parasite resistance. Producers should use as many tools as possible to minimize the need for pharmaceutical interventions and optimize animal production.

Variability of resistance in Black Bengal goats naturally infected with Haemonchus contortus

A total 290 Black Bengal goats (6 buck, 109 doe and 175 kids born from 11 sires) were studied to evaluate the variability of resistance in Black Bengal goats naturally infected with Haemonchus contortus. The variability of resistance in Black Bengal goat was studied for both genetic and non-genetic factors like village, sex, age dam, sire, dam resistance group and offspring resistance group. Male kids have slightly higher resistance than female kids although it was not significant. Resistance of kids was increased as age increases and kid population showed significantly different resistance status among the offspring resistant groups. The doe population showed significantly different LEPG as per the resistance group in all the collections. The present study found that the resistance of kids under sire were varied significantly and observed that the kids under sire 1, 6-8 were significantly more resistant than the kids of the sire 2, 5 and 11 in 3rd collection and it is also noticed that maternal genetic effect has a very little impact on resistance of kids. Males (buck) were most resistant and the kids were least resistant and the resistance of dam was in between the male and kids population.

Patterns and processes in parasite co-infection

Co-infection of individual hosts by multiple parasite species is a pattern that is very commonly observed in natural populations. Understanding the processes that generate these patterns poses a challenge. For example, it is difficult to discern the relative roles of exposure and susceptibility in generating the mixture and density of parasites within hosts. Yet discern them we must, if we are to design and deliver successful medical interventions for co-infected populations. Here, we synthesise an emergent understanding of how processes operate and interact to generate patterns of co-infection. We consider within-host communities (or infracommunities) generally, that is including not only classical parasites but also the microbiota that are so abundant on mucosal surfaces and which are increasingly understood to be so influential on host biology. We focus on communities that include a helminth, but we expect similar inferences to pertain to other taxa. We suggest that, thanks to recent research at both the within-host (e.g. immunological) and between-host (e.g. epidemiological) scales, researchers are poised to reveal the processes that generate the observed distribution of parasite communities among hosts. Progress will be facilitated by using new technologies as well as statistical and experimental tools to test competing hypotheses about processes that might generate patterns in co-infection data. By understanding the multiple interactions that underlie patterns of co-infection, we will be able to understand and intelligently predict how a suite of co-infections (and thus the host that bears them) will together respond to medical interventions as well as other environmental changes. The challenge for us all is to become scholars of co-infections.

The immunology and genetics of resistance of sheep to Teladorsagia circumcincta

Teladorsagia circumcincta is one of the most economically important gastrointestinal nematode parasites of sheep in cool temperate regions, to which sheep show genetically-varying resistance to infection. This is a very common parasite and viable sheep production requires the extensive use of anthelmintic drugs. However, the emergence of drug-resistant parasites has stimulated the search for alternative control strategies to curb production losses. Lambs become infected soon after weaning and begin to control parasite burden within 8-10 weeks of continual infection. This control is an acquired characteristic mediated by the development of parasite-specific antibodies. This paper describes the immunology associated with resistance and susceptibility, focussing on differential T cell activation that regulates the production of specific effector mechanisms. It continues by summarizing the methods used to identify genes that could be exploited as molecular markers of selection for resistance. In particular it focusses on the link between understanding the molecular immunology of infection and the identification of candidate genes for selection.

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.

Association between major histocompatibility complex microsatellites, fecal egg count, blood packed cell volume and blood eosinophilia in Pelibuey sheep infected with Haemonchus contortus

The objective of this study was to assess the correlation among traits associated with resistance or susceptibility to Haemonchus contortus infestation and to evaluate the participation of the ovine major histocompatibility complex (MHC) in Pelibuey sheep, a prevalent breed in tropical and sub-tropical regions in Mexico and elsewhere. Association among the fecal egg count (FEC), blood packed cell volume (PCV), antibody (AB) levels, serum proteins (SP) and blood eosinophil count (EOS) was assessed in 52 lambs experimentally infected with H. contortus, and the participation of the MHC was evaluated using polymorphisms in three microsatellites, located at the class I (OMHC1) and class II (OLADRB1, OLADRB2) regions of the MHC. Spearman correlation analysis among the traits showed a negative association (P<0.01) between FEC and PCV (-0.35), EOS (-0.50), SP (-0.30) and AB (-0.57), and a positive correlation of antibodies with EOS (0.50). The homozygotes for the OMHC1-188 and OLADRB2-282 alleles were associated with a reduction in FEC (-813 and -551, respectively). Conversely, the OMHC1-200 and OMHC1-206 alleles were associated with an increase in FEC (1704 and 1008, respectively). Furthermore, the OLADRB1-482 allele was associated with an increase of 163 EOS by allele copy, while the OMHC1-200 allele showed a reduction of 95 EOS in homozygotes. The associations among microsatellite MHC loci and the remaining variables were not significant. These results reinforce the evidence that MHC polymorphisms have an important role in parasite resistance or susceptibility in Pelibuey sheep and could be used as genetic markers to assist selection and improve parasite resistance to H. contortus.

Quantitative trait loci for resistance to Heligmosomoides bakeri and associated immunological and pathological traits in mice: comparison of loci on chromosomes 5, 8 and 11 in F2 and F6/7 inter-cross lines of mice

A comparison of F2 and F6/7 inter-cross lines of mice, derived from CBA and SWR parental strains, has provided strong evidence for several previously undetected quantitative trait loci (QTL) for resistance to Heligmosomoides bakeri. Five QTL affecting average faecal egg counts and/or worm burdens in week 6 were detected on mouse chromosomes 5 (Hbnr9 and Hbnr10), 8 (Hbnr11) and 11 (Hbnr13 and Hbnr14). Three QTL for faecal egg counts in weeks 4 and 6 were found on both chromosomes 5 (Hbnr9) and 11 (Hbnr13 and Hbnr14). Two QTL for the mucosal mast cell protease 1 (MCPT1) response were located on chromosomes 8 (Hbnr11) and 11 (Hbnr13), two for the IgG1 antibody response to adult worms on chromosomes 5 (Hbnr10) and 8 (Hbnr11), two for PCV in week 6 on chromosomes 5 (Hbnr9) and 11 (Hbnr13), and two for the granulomatous response on chromosome 8 (Hbnr12) and 11 (Hbnr15). Our data emphasize that the control of resistance to H. bakeri is multigenic, and regulated by genes within QTL regions that have a complex range of hierarchical relationships.

Alternative approaches to control--quo vadit?

The increasing prevalence of anthelmintic resistance has provided a spur for research into 'alternative/novel' approaches to the control of helminthoses that are intended to reduce our reliance upon using chemoprophylaxis. The different approaches either target the parasite population in the host or on pasture, but the goal of all of them is to restrict host parasite contact to levels which minimise the impact of helminths on host welfare and/or performance. Infrapopulation regulation can be achieved through methods that enhance immunity such as optimised nutrition (immunonutrition), genetic selection and vaccination, or by an 'anthelmintic' route using bioactive forages, copper oxide wire particles, or use of targeted selective treatment strategies such as FAMACHA, which reduce the selection pressure for the development of resistance by maintaining a population in refugia. Suprapopulation control can be achieved through grazing management, or by using predacious fungi such as Duddingtonia flagrans. All of these approaches have been developed beyond the proof of concept stage and some are capable of being employed currently. However, some still require knowledge transfer, or commercialisation before they can be tested and widely applied in the field. All of the different approaches present unique challenges to the researchers engaged in developing them, and in comparison to simple prescriptive anthelmintic treatments, their use appears complex and requires some expertise on behalf of the advisor and/or end user. At present, most of our data are derived from trials using single approaches, but it is apparent that we need to move towards integrating some of these technologies which again represents a further challenge to the extension/advisory services. Progress in establishing different approaches requires not only the funding to support their scientific development but also to support the development of computer based models which can be used to highlight deficiencies in our understanding of the control mechanisms and to identify impediments to their introduction. It is inevitable that some of the approaches currently under investigation will fail to become widely applied for a variety of reasons that are not solely financial. These include issues concerned with practicability/applicability, affordability/appropriateness, availability/deliverability and above all, the failure to provide a consistent, reliable effect when used under commercial farming conditions.

Gene expression profiling of naïve sheep genetically resistant and susceptible to gastrointestinal nematodes

BACKGROUND

Gastrointestinal nematodes constitute a major cause of morbidity and mortality in grazing ruminants. Individual animals or breeds, however, are known to differ in their resistance to infection. Gene expression profiling allows us to examine large numbers of transcripts simultaneously in order to identify those transcripts that contribute to an animal's susceptibility or resistance.

RESULTS

With the goal of identifying genes with a differential pattern of expression between sheep genetically resistant and susceptible to gastrointestinal nematodes, a 20,000 spot ovine cDNA microarray was constructed. This array was used to interrogate the expression of 9,238 known genes in duodenum tissue of four resistant and four susceptible female lambs. Naïve animals were used in order to look at genes that were differentially expressed in the absence of infection with gastrointestinal nematodes. Forty one unique known genes were identified that were differentially expressed between the resistant and susceptible animals. Northern blotting of a selection of the genes confirmed differential expression. The differentially expressed genes had a variety of functions, although many genes relating to the stress response and response to stimulus were more highly expressed in the susceptible animals.

CONCLUSION

We have constructed the first reported ovine microarray and used this array to examine gene expression in lambs genetically resistant and susceptible to gastrointestinal nematode infection. This study indicates that susceptible animals appear to be generating a hyper-sensitive immune response to non-nematode challenges. The gastrointestinal tract of susceptible animals is therefore under stress and compromised even in the absence of gastrointestinal nematodes. These factors may contribute to the genetic susceptibility of these animals.

Microarray analysis of selection lines from outbred populations to identify genes involved with nematode parasite resistance in sheep

Gastrointestinal nematodes infect sheep grazing contaminated pastures. Traditionally, these have been controlled with anthelmintic drenching. The selection of animals resistant to nematodes is an alternative to complete reliance on drugs, but the genetic basis of host resistance is poorly understood. Using a 10,204 bovine cDNA microarray, we have examined differences in gene expression between genetically resistant and susceptible lambs previously field challenged with larval nematodes. Northern blot analysis for a selection of genes validated the data obtained from the microarrays. The results identified over one hundred genes that were differentially expressed based on conservative criteria. The microarray results were further analyzed to identify promoter motifs common to the differentially expressed genes. Motifs identified in upregulated gene promoters were primarily restricted to those promoters; however, motifs identified in downregulated gene promoters were also found in the promoters of upregulated genes but not in the promoters of genes whose expression was unaltered. Protein Annotators’ Assistant was used for lexical analysis of the differentially expressed genes, and Gene Ontology was used to look for metabolic and cell signaling pathways associated with parasite resistance. Two pathways represented by genes differentially expressed in resistant animals were those involved with the development of an acquired immune response and those related to the structure of the intestine smooth muscle. Genes involved in these processes appear from our analysis to be key genetic determinants of parasite resistance.

Chasing the genes that control resistance to gastrointestinal nematodes

The host-protective immune response to infection with gastrointestinal (GI) nematodes involves a range of interacting processes that begin with recognition of the parasite's antigens and culminate in an inflammatory reaction in the intestinal mucosa. Precisely which immune effectors are responsible for the loss of specific worms is still not known although many candidate effectors have been proposed. However, it is now clear that many different genes regulate the response and that differences between hosts (fast or strong versus slow or weak responses) can be explained by allelic variation in crucial genes associated with the gene cascade that accompanies the immune response and/or genes encoding constitutively expressed receptor/signalling molecules. Major histocompatibility complex (MHC) genes have been recognized for some time as decisive in controlling immunity, and evidence that non-MHC genes are equally, if not more important in this respect has also been available for two decades. Nevertheless, whilst the former have been mapped in mice, only two candidate loci have been proposed for non-MHC genes and relatively little is known about their roles. Now, with the availability of microsatellite markers, it is possible to exploit linkage mapping techniques to identify quantitative trait loci (QTL) responsible for resistance to GI nematodes. Four QTL for resistance to Heligmosomoides polygyrus, and additional QTL affecting faecal egg production by the worms and the accompanying immune responses, have been identified. Fine mapping and eventually the identification of the genes (and their alleles) underlying QTL for resistance/susceptibility will permit informed searches for homologues in domestic animals, and human beings, through comparative genomic maps. This information in turn will facilitate targeted breeding to improve resistance in domestic animals and, in human beings, focused application of treatment and control strategies for GI nematodes.

Evolution of nematode community in grazing sheep selected for resistance and susceptibility to Teladorsagia circumcincta and Trichostrongylus colubriformis: a 4-year experiment

Field trials were undertaken to compare nematode population dynamics in two experimental flocks of rams selected for high and for low faecal egg counts (FECs) following two doses with 20000 infective larvae, 12000 Teladorsagia circumcincta (Tcirc) and 8000 Trichostrongylus colubriformis (Tcol) separated by a treatment. The selection was done by dosing 200 ram lambs (INRA 401 breed) and FECs proceeded with egg identification on days 28 and 35 post-each-infection. The 30 lambs with the lowest FECs constituted the group of permanent resistant rams (R), the 30 with the highest values the group of susceptible ones (S). Each group grazed separated pastures from April to November, this during 4 consecutive years. The contamination (number of eggs deposited) was estimated every 3 weeks on each pasture. The larval populations was measured by successive groups of tracer lambs. The infection of the permanent rams was done by slaughtering eight R and S rams on mid and end of each grazing season. Each autumn, new R and S ram lambs were selected to complete the R and S groups before turning out in spring. On third year, a third species (Haemonchus contortus, Hcon) was added to compare its behaviour with the two others. The regulation of parasite populations were studied by comparing what happens in R rams and pasture to S ones. The selected R ram lambs had a FEC of Tcirc 50% lower than S ones. This induced a similar lowered pasture contamination, a 25% decrease of infective larval population in years 2 and 3, and the worm burdens in R rams were the half of that in S rams after 2 years. The FEC of Tcol was 75% lower at the selection and induced similar difference in pasture contamination, but 65 and 96% reduction in L3 population, respectively, after 1 and 2 years. The worm burden was reduced by 99%. Adding Hcon species during the third year, in year 4, the L3 population was half in R pasture compared with S one, and R rams had 60% less worms of this species than S ones. This selection without any drench prevent outbreaks observed in young tracer lambs on S pasture. The comparison of selected resistant rams to selected susceptible ones and not to unselected controls gave more rapid information about the tendency of the regulation of the communities of nematode parasites.

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.

The response of breeding doses to nematodiasis: segregation into "responders" and "non-responders"

Responder and non-responder does were identified from a flock of 95 Scottish cashmere 2-6 year-old does exposed to natural nematode infection over a 12-month period. Every 5 weeks, the does were faecal sampled for worm-egg counts prior to anthelmintic treatment. Responsive and non-responsive individuals were identified on the basis of their cumulative faecal egg count (FEC) rankings: the 8 lowest and 8 highest rankings were deemed to be responders and non-responders, respectively. Retrospective analysis showed that the mean egg count of the 8 responders was significantly lower than that of the 8 non-responders. The selected responders and non-responders were subsequently housed together with 8 randomly selected does from a control line, and given a mixed trickle challenge with Teladorsagia circumcincta and Trichostrongylus vitrinus larvae (L3). Mean responders FEC was significantly lower following artificial infection than that of non-responder and unselected does. Peripheral eosinophilia was significantly greater in responders in the first 3 weeks of this infection. On day 60, the infection was terminated with anthelmintic and 7 days later the goats were given a single challenge of 50,000 T. circumcincta L3. The mean responder worm burden was lower, and exhibited greater evidence of retardation of worm development, than those of non-responder and unselected does. Responders had significantly more mast cells and globule leukocytes post-challenge than did the other 2 groups. These results suggest that under the conditions encountered in this experiment, it is possible to segregate goats into responders and non-responders using simple parasitological criteria, as individual responsiveness is a relatively repeatable phenomenon.

Comparison of resistance of four genotypes of rams to experimental infection with Haemonchus contortus

Fifty-five rams aged about 18-24 months weighing 30-42 kg were used in this study. Ten rams of each of four genotypes, S (Sumatra), H1 (50% Sumatra-50% Virgin Island), B1 (50% Sumatra-50% Barbados Blackbelly), E1 (50% Sumatra-50% Java Fat-tail), were infected orally with a newly isolated strain of Haemonchus contortus. Each animal received 2000 infective larvae 3 times week-1 for 3 weeks, with a total of 18,000 larvae. Fifteen rams belonging to Sumatra and its crosses with Virgin Island were used as uninfected controls. Peak egg counts were observed on Day 35 for genotype B1 and on Day 42 for genotype H1 and S. In genotype E1 a slow but consistent increase in EPG continued until Day 49 when the experiments terminated. Overall faecal egg counts at all sampling dates were not statistically different between genotypes (P > 0.05). There was a large variation in the EPG of individual rams within a genotype. The overall average EPG (geometric means) of individual rams within a genotype ranged from 3 to 1028 for B1, 4 to 261 for E1, 7 to 3119 for H1 and 9 to 506 for S. The analysis of packed cell volume (PCV) for four genotypes and controls from all sampling times showed significant differences (P < 0.05). The overall mean PCV was highest in S (31.1) and lowest in H1 (28.4). The ranking of four genotypes for PCV was S > E1 > B1 > H1. The decrease of PCV during the course of infection was highly significant for all genotypes of infected rams (B1, E1, H1, S) (P < 0.01). PCV of the control group did not exhibit much change during the course of the experimental period. Weight gain of infected rams was lower than those of uninfected controls (P < 0.5) but there was no significant difference between the four genotypes of infected rams. Individual variation within genotype in susceptibility to infections was generally more important than between genotype differences. Two major conclusions of the present study are: (1) The imported breeds with higher body weight, namely the Barbados Blackbelly and Virgin Island, may be used in cross breeding to increase the body size of local Sumatra sheep. (2) Based on the faecal egg counts it is possible to identify the animals for use in selective breeding programmes for higher resistance to H. contortus.

An ovine major histocompatibility complex DRB1 allele is associated with low faecal egg counts following natural, predominantly Ostertagia circumcincta infection

Infection with Ostertagia circumcincta is a major constraint on sheep production in temperate areas of the world. A potential control strategy is the use of genetically resistant sheep. Therefore we examined the association between MHC-DRB1 alleles and faecal egg counts following natural, predominately O. circumcincta infection in a flock of Scottish Blackface sheep. Nineteen DRB1 alleles were identified by a combination of variation in the length of simple repetitive sequences within the intron between exons 2 and 3 and hybridisation of selected oligonucleotides to polymorphisms within exon 2. Faecal samples were taken from 200 lambs from one to six months of age at intervals of 4 weeks. Genetic effects were strongest at 6 months of age. Least-squares analysis indicated that substitution of the most common allele (I) by allele G2 would result in a 58-fold reduction in faecal egg counts in 6-month-old lambs and a 22-fold reduction in 5-month-old lambs. These results suggest that the major histocompatibility complex plays an important role in the development of resistance to O. circumcincta.

The heritability and specificity of responsiveness to infection with Haemonchus contortus in sheep

High responder (HR) and low responder (LR) lines of sheep of the Hungarian Merino breed were selected by using faecal egg counts (epg) as parameter of responsiveness after double artificial Haemonchus contortus test infections. A total of 81 lambs born to assortative matings of HR rams with HR ewes and LR rams with LR ewes, as well as 28 control lambs from unselected parent sheep, were studied to estimate the heritability of responsiveness. A relatively high level of heritability (h2 = 0.49 +/- 0.17) was observed. A comparative assessment of responder classification carried out by two subsequent and distinct H. contortus and Trichostrongylus colubriformis infections revealed a remarkable degree of agreement: identical results in 69.7% and marginal differences in 23.2% of 99 animals. These results suggest that by selecting sheep for high responsiveness to a certain species of nematodes, one can also achieve a substantial improvement of resistance against a wide range of other nematode species.

The effect of age on the response of Romney sheep to gastrointestinal nematodes during grazing

Three groups of Romney sheep, reared worm-free in pens from birth until 4, 16 or 28 months of age, were permitted to graze nematode-infected pasture for 4 weeks after which they were returned to pens and slaughtered 4 weeks later. Three groups of three Romney sheep reared and maintained worm-free for similar periods served as uninfected control groups. Faecal egg counts (FECs) 8 weeks after grazing commenced were significantly greater in 4-month-old sheep than the 28-month-old animals. At slaughter, no difference in nematode burdens among these three age groups was found. Histological examination of the abomasal and small intestine mucosa, taken from sheep at slaughter, revealed significantly greater numbers of globule leukocytes/mucosal mast cells (GL/MMC) bur fewer connective tissue type mast cells (CTMC) in sheep aged 16 and 28 months than in those aged 4 months. The numbers of eosinophils in the gastrointestinal mucosal tissues of the three groups of infected sheep were not significantly different. In uninfected sheep the numbers of eosinophils, GL/MMC and CTMC in the mucosal tissues did not differ between the three age groups and the numbers of GL/MMC and CTMC were similar to those in 4-month-old infected sheep. Eosinophil numbers in the mucosal tissue of the infected groups were significantly higher than in the uninfected groups. In the infected groups, FECs but not worm burdens were significantly negatively correlated with GL/MMC numbers.(ABSTRACT TRUNCATED AT 250 WORDS)

Repeatability of ovine faecal oocyst counts in natural infections with Eimeria spp

The repeatability of ovine faecal oocyst counts was studied in Merino of Arles and Romanov sheep over short (4 days) or longer periods (1 month). The repeatabilities over short periods ranged from 0.27 to 0.67 for total oocyst output, from 0.06 to 0.54 for Eimeria ovinoidalis and from 0.29 to 0.52 for E. parva counts. E. ovinoidalis had lower and more irregular repeatabilities than E. parva. Their repeatabilities ranged, respectively from 0.18 (E. ovinoidalis) to 0.22 (E. parva) for longer periods. The genetic share in the determination of magnitude of oocyst counts seems similar to that recorded for trichostrongylid egg counts.

Response to repeated inoculations with Ascaris suum eggs in pigs during the fattening period. I. Studies on worm population kinetics

This experimental study on pigs was designed to simulate natural, long-term exposure to Ascaris suum under modern management conditions. Parasite kinetics were followed in pigs receiving A. suum eggs as repeated trickle inoculations at two dose levels beginning at a body weight of 25 kg until their slaughter at 90 kg (baconers). In pigs inoculated twice weekly with 500 eggs, there was an initial marked rise in the numbers of hepatic milk spots, but as early as around week 6 after the start of inoculations and until week 16, at which time the last pigs were slaughtered, the numbers of spots diminished drastically. In pigs receiving only 25 eggs twice weekly, low and moderately fluctuating numbers of spots were seen throughout the experiment. Larvae recoverable from the livers and lungs were observed mainly during the beginning of the experiment. Before patency, immature intestinal worms were found in moderate numbers that showed a rough positive correlation with the dose levels, but at the time at which adult worms started to appear, immature parasites could practically no longer be found. In all, only 10 of 40 pigs harbored adults, and 4 of these 10 pigs harbored 80% of the total worm population. The results show that acquired dose-dependent host responses to A. suum play an important role in regulating the worm population along the migratory route of the parasite and that the final burden of worms in the small intestine is dose-dependent and highly variable.

Nutritional and dietary interrelationships with diseases of grazing beef cattle

This article describes current methods of controlling acute bovine pulmonary emphysema (ABPEE) and edema, lungworm and gastrointestinal nematodes, and bloat in grazing beef cattle. Success in handling outbreaks of these conditions and in their prevention depends on an understanding of their epidemiology and pathogenesis. Supplementation with ionophore antibiotics is effective in prevention of ABPEE and bloat and may also prevent other diseases of cattle that graze lush pastures.

Genetic resistance of hosts and its influence on epidemiology

Evidence for the genetic control of resistance of sheep to gastrointestinal nematode infections was reviewed and compared with that of cattle to infestation with the cattle tick. Variation within sheep breeds in resistance to nematode infection is as great as that between breeds, is moderately heritable, and selection of lines of resistant sheep should result in animals carrying worm burdens around 10%-20% of those of unselected animals. The consequences of genetically resistant sheep for the epidemiology of Trichostrongylus colubriformis infection were examined with the aid of a simulation model. Predicted results were similar to those observed in cattle herds resistant to Boophilus microplus; seasonal peaks in parasite burdens were greatly reduced, and larval numbers on pasture were reduced even further. The use of genetically resistant hosts should permit a reduced frequency of anthelmintic treatment, and hence reduce the rate of development of anthelmintic resistance in the parasite.