Genetic resistance of hosts and its influence on epidemiology

Ovine PAPPA2 gene coding variants are linked to decreased fecal egg shedding in native Turkish sheep naturally infected with gastrointestinal nematodes

In this study, the association between PAPPA2 coding variants and gastrointestinal (GI) nematode fecal egg count (FEC) score in adult Turkish sheep was investigated. For this purpose, the FEC score was determined in adult sheep from six breeds: Karacabey Merino (n = 137), Kivircik (n = 116), Cine capari (n = 109), Karakacan (n = 102), Imroz (n = 73), and Chios (n = 50). Sheep were classified as shedders or non-shedders within breeds and flocks. The first group was the fecal egg shedders (> 50 per gram of feces), and the second group was the no fecal egg shedders (≤ 50 per gram of feces). The exon 1, exon 2, exon 5, exon 7, and a part of 5'UTR of the ovine PAPPA2 gene were genotyped by Sanger sequencing of these two groups. Fourteen synonymous and three non-synonymous single-nucleotide polymorphisms (SNPs) were found. The non-synonymous SNPs, D109N, D391H, and L409R variants, are reported for the first time. Two haplotype blocks were constructed on exon 2 and exon 7. The specific haplotype, C₃₉₁G₄₂₄G₄₄₉T₄₇₃C₅₁₅A₅₄₂ on the exon 2 that carries the 391H variant, was tested against four other common haplotypes. Our results indicate that C₃₉₁G₄₂₄G₄₄₉T₄₇₃C₅₁₅A₅₄₂ haplotype was significantly associated with fecal egg shedding status in adult Turkish sheep (p-value, 0.044).

Meta-analysis of the parasitic phase traits of Haemonchus contortus infection in sheep

BACKGROUND

The parasitic nematode Haemonchus contortus shows highly variable life history traits. This highlights the need to have an average estimate and a quantification of the variation around it to calibrate epidemiological models.

METHODS

This paper aimed to quantify the main life history traits of H. contortus and to identify explanatory factors affecting these traits using a powerful method based on a systematic review and meta-analysis of current literature. The life history traits considered are: (i) the establishment rate of ingested larvae; (ii) the adult mortality rate; (iii) the fertility (i.e. the number of eggs laid/female/day); and (iv) fecundity of female worms (i.e. the number of eggs per gram of faeces).

RESULTS

A total of 37 papers that report single experimental infection with H. contortus in sheep and published from 1960 to 2015, were reviewed and collated in this meta-analysis. This encompassed 115 experiments on 982 animals. Each trait was analysed using a linear model weighted by its inverse variance. The average (± SE) larval establishment rate was 0.24 ± 0.02, which decreased as a function of the infection dose and host age. An average adult mortality rate of 0.021 ± 0.002) was estimated from the literature. This trait varied as a function of animal age, breed and protective response due to prior exposure to the parasite. Average female fertility was 1295.9 ± 280.4 eggs/female/day and decreased in resistant breeds and previously infected hosts. Average faecal egg count at necropsy was 908.5 ± 487.1 eggs per gram of faeces and varied as a function of infection duration and host resistance. The average sex ratio of H. contortus was 0.51 ± 0.006.

CONCLUSION

This work is the first systematic review to summarise the available information on the parasitic phase of H. contortus in sheep. The results of the meta-analysis provide robust estimates of life history traits for parametrization of epidemiological models, their expected variation according to experimental factors, and provides correlations between these.

Influence of management and biological factors on the parasitic invasions in the wild-spread of blood-sucking nematode Ashworthius sidemi in European bison (Bison bonasus)

The full course of new parasite introductions in wild animals is difficult to accurately trace. We documented and analysed the invasive blood-sucking nematode Ashworthius sidemi (Trichostrongylidae) introduction and spread in European bison (Bison bonasus) from the initial phase of its progression. In the Polish part of the Białowieża Primeval Forest (BPF) the parasite was first found in 2000. From 2002 to 2015, 165 culled bison were investigated. The prevalence and intensity of A. sidemi Schulz, 1933 infection increased over the following years, reaching 100% of investigated bison four years after introduction and a maximal median intensity of 8200 nematodes per animal in the winter of 2008/2009. Afterwards, a significant decline of median infection intensity was observed to the minimum value of 410 nematodes per animal. Between 2011 and 2014 prevalence varied from 89 to 100%. Among the factors analysed, the number of years since introduction, herd size, age and sex proved to significantly influence infection intensity. A higher infection intensity was recorded in sub-adults compared to juveniles and adults. Males had significantly lower infection intensity than females, but this was the case for adults only. The highest infection intensities were recorded in the biggest bison herds, where the winter supplementary feeding of bison is intense. Moreover, the longer the parasite was present in the host population, the more important herd size became as a factor. Our study indicates that it is not solely biological factors that determine the spread of a newly detected parasite in wildlife, but that management practices can also have a strong influence. This is especially important in endangered species under intensive human care as the management practices may pose a threat to the species.

Genetic parameters for resistance to nematode infections in Texel lambs and their utility in breeding programmes

This paper addresses the inheritance of host resistance to gastro-intestinal nematode parasite infections in commercial Texel lambs, and the rôle of resistance to parasites in breeding programmes. In two flocks of Texel sheep, faecal egg counts following natural parasite challenge were measured on up to three occasions post weaning per lamb over a 4-year period, with 1385 and 287 lambs measured on the two farms. Live weight, and ultrasonically measured fat and muscle depth at weaning were available for each lamb, as were deep pedigrees. Egg counts were moderately to strongly heritable on all occasions, with Nematodirus egg counts more heritable than the Strongyle egg counts. Weighted average heritabilities for Strongyle and Nematodirus egg counts were 0.26 and 0.38, respectively. Within the same category of parasite, genetic correlations across time were positive and strong but somewhat less than unity, as were the correlations between Strongyle and Nematodirus egg counts measured at the same time. Genetic correlations between performance traits and Strongyle egg counts were usually favourable (i.e. negative) but weak, whereas those with Nematodirus egg counts were generally neutral or slightly positive. Whilst Nematodirus resistance may not necessarily be included in a breeding goal, the results suggest that Nematodirus egg counts can be used as an additional genetic indicator of Strongyle egg counts, at little extra cost. Including the epidemiological consequences of decreasing Strongyle egg counts in benefits of increasing parasite resistance, it is suggested that under UK conditions selection goals that place equal emphasis on live weight and log-transformed egg counts will be a robust means of improving growth rate and decreasing parasite larval challenge.

A discrete-time epidemiological model to quantify selection for disease resistance

A genetic epidemiological model (GEM) for investigating the effect of selection for disease resistance on the epidemiology of infectious diseases is presented and applied to a pig breeding scenario. Fundamental to the model is R0, the basic reproductive ratio. R0 is the expected number of secondary infections caused by a single infection. If R0 is greater than 1, there will be an epidemic. The aim of the model is to quantify the effect of selection on R0 and the consequences this has on disease epidemiology. Two implementations are presented: selection for reduced susceptibility/infectivity to a disease and introgression of a major resistance gene. The results suggest that the effects of selection for reduced susceptibility I infectivity are critically dependent on the infectiousness of the disease. Under the assumptions made in the model, for a disease with a low infection level, it takes approximately 15 years of selection until R0 is less than 1 and the population is safe from epidemics should the infectious agent be present. For a highly infectious disease, this time may be as long as 100 years. For gene introgression, the population is expected to be free from epidemics within 5 years and the time to reduce R0 to less than 1 is largely independent of the disease being considered. With gene introgression, the proportion of the population which needs to be resistant to ensure that R0 is less than one is shown to be a function of the initial R0 for the disease. Although selection, as modelled, results in a linear decline in R0, the reduction in the proportion of animals infected during an epidemic is non-linear. The selection process reduces the amount of infectious material that is in the environment when an infection occurs and this decreases the force of infection on unselected animals. This phenomenon results in a marked interaction between host genotype and disease epidemiology. Thus, the results of the model show that altering the genetics of individual animals affects the epidemiology of the disease at the population level. The model can be applied to any farm structure and any microparasitic infectious disease.

Monkeys in the middle: parasite transmission through the social network of a wild primate

In wildlife populations, group-living is thought to increase the probability of parasite transmission because contact rates increase at high host densities. Physical contact, such as social grooming, is an important component of group structure, but it can also increase the risk of exposure to infection for individuals because it provides a mechanism for transmission of potentially pathogenic organisms. Living in groups can also create variation in susceptibility to infection among individuals because circulating levels of immunosuppressive hormones like glucocorticoids often depend on an individual's position within the group's social structure. Yet, little is known about the relative roles of socially mediated exposure versus susceptibility in parasite transmission among free-living animal groups. To address this issue, we investigate the relationship between host dominance hierarchy and nematode parasite transmission among females in a wild group of Japanese macaques (Macaca fuscata yakui). We use social network analysis to describe each individual female's position within the grooming network in relation to dominance rank and relative levels of infection. Our results suggest that the number of directly-transmitted parasite species infecting each female, and the relative amount of transmission stages that one of these species sheds in faeces, both increase with dominance rank. Female centrality within the network, which shows positive associations with dominance hierarchy, is also positively associated with infection by certain parasite species, suggesting that the measured rank-bias in transmission may reflect variation in exposure rather than susceptibility. This is supported by the lack of a clear relationship between rank and faecal cortisol, as an indicator of stress, in a subset of these females. Thus, socially mediated exposure appears to be important for direct transmission of nematode parasites, lending support to the idea that a classical fitness trade-off inherent to living in groups can exist.

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.

Systems genetics approach reveals candidate genes for parasite resistance from quantitative trait loci studies in agricultural species

A systems genetics approach combining pathway analysis of quantitative trait loci (QTL) and gene expression information has provided strong evidence for common pathways associated with genetic resistance to internal parasites. Gene data, collected from published QTL regions in sheep, cattle, mice, rats and humans, and microarray data from sheep, were converted to human Entrez Gene IDs and compared to the KEGG pathway database. Selection of pathways from QTL data was based on a selection index that ensured that the selected pathways were in all species and the majority of the projects overall and within species. Pathways with either up- and down-regulated genes, primarily up-regulated genes or primarily down-regulated genes, were selected from gene expression data. After comparing the data sets independently, the pathways from each data set were compared and the common set of pathways and genes was identified. Comparisons within data sets identified 21 pathways from QTL data and 66 pathways from gene expression data. Both selected sets were enriched with pathways involved in immune functions, disease and cell responses to signals. The analysis identified 14 pathways that were common between QTL and gene expression data, and four directly associated with IFNγ or MHCII, with 31 common genes, including three MHCII genes. In conclusion, a systems genetics approach combining data from multiple QTL and gene expression projects led to the discovery of common pathways associated with genetic resistance to internal parasites. This systems genetics approach may prove significant for the discovery of candidate genes for many other multifactorial, economically important traits.

Management practices to control gastrointestinal parasites in dairy and beef goats in Minas Gerais; Brazil

Parasitic infection is recognized worldwide as a limiting factor in the production of goats, and various control methods are used to reduce economic losses, often without considering the epidemiology of the parasites. This has led to the development of highly tolerant parasite populations and the presence of chemical residues in the beef and milk. The objective of this study was to determine the level of knowledge of goat farmers about parasitic diseases and to correlate this with the epidemiology of endoparasites and parasite control practices in goat farms in the state of Minas Gerais, Brazil. The analysis was based on a questionnaire applied by trained veterinarians. The sample was homogeneous throughout the state, covering 18.4% (157/853) of municipalities. Eighty-four dairy goat farms in 81 municipalities and 200 properties with beef goats in 76 municipalities were evaluated. The herd size per goat farm ranged from 4 to 57 (average 24) for beef herds and from 2 to 308 (average 63) for dairy farms. The majority of the beef herd production was extensive and semi-extensive (98.5%), while the dairy herds were maintained under intensive farming (98.8%). The mixed production of goats and sheep was reported by 36.5% of beef goat farmers and by 20.2% of dairy goat farmers. Among the beef goats farms on which the technological level was determined, 2.0% were categorized as having high technological level, 34.5% as medium, and 63.5% as low. Of the 84 dairy farms, 30% operated at a high, 47% at a medium, and 23% at a low technological level. The adoption of practices to reduce parasitism, such as the quarantine of animals, treatment of newly arrived animals, regular cleaning of the floor, and technical assistance, was significantly higher on dairy farms than on beef farms. Although 85.7% of dairy farmers and 83% of beef farmers medicate their animals, the treatments were performed without technical criteria, and deworming intervals ranged from 30 to 120 days or more. The average interval between treatments was significantly longer in dairy goat herds (4.8 months) than in the beef herds (3.6 months). The most commonly used drugs were macrocyclic lactones (37.7% in dairy and 39.5% in beef herds) and benzimidazoles (48.9% in dairy and 31.5% in beef herds). Goat production in Minas Gerais is still in its infancy, and even though using a control program associated with other health practices, producers still rely heavily on chemicals to get satisfactory results.

Modelling responses to selection for resistance to gastro-intestinal parasites in sheep

This paper describes a general framework which enables responses to selection for resistance to gastro-intestinal parasites in sheep to be stochastically modelled. The model incorporates between-animal variation for pasture intake, the proportion of larvae ingested from the pasture which survive to become adults, the fecundity of the mature worm, along with density-dependent control of this trait and the mortality rate of the worms. The between-animal variation for each component is partitioned into genetic, permanent and temporary environmental components which vary with age. Infection rates are estimated from existing pasture larval contamination and new contamination from infected animals. Using this framework, selection for reduced mean faecal egg count was practised, in silico, for a period of 10 years. Several general patterns emerged. First, a curvilinear response to selection was observed, with responses initially being large then declining over time. Mean faecal egg count declined from approximately 500 to 140 eggs per gin 10 years and worm burdens and pasture larval contamination showed similar patterns of response. The initial responses to selection were approximately 1·7 times that predicted by quantitative genetic theory because the epidemiology of the disease changed as the animals' genetic resistance improved. A method of partitioning selection responses into components due to the altered genotypes of the animals and components due to altered disease epidemiology is outlined. Secondly, the faecal egg count distribution became more aggregated, or skewed, as selection progressed. Thirdly, correlating pasture contamination levels across years (carry-over effects) resulted in even greater apparent responses to selection. Finally, regular anihelmintic treatment reduced mean faecal egg counts but did not alter the patterns of response to selection, indicating that selective breeding should be feasible under a variety of anthelmintic regimes.

Haemonchus contortus and Trichostrongylus colubriformis did not adapt to long-term exposure to sheep that were genetically resistant or susceptible to nematode infections

We tested the hypothesis that Haemonchus contortus and Trichostrongylus colubriformis would adapt to long-term exposure to sheep that were either genetically resistant or susceptible to H. contortus. Sheep genotypes were from lines with 10 years prior selection for low (resistant, R) or high (susceptible, S) faecal worm egg count (WEC) following H. contortus infection. Long-term exposure of H. contortus and T.colubriformis to R or S genotypes was achieved using serial passage for up to 30 nematode generations. Thus, we generated four nematode strains; one strain of each species solely exposed to R sheep and one strain of each species solely exposed to S sheep. Considerable host genotype differences in mean WEC during serial passage confirmed adequate nematode selection pressure for both H. contortus (R 4900 eggs per gram (epg), S 19,900 epg) and T. colubriformis (R 5300 epg, S 13,500 epg). Adaptation of nematode strain to host genotype was tested using seven cross-classified tests for H. contortus, and two cross-classified and one outbred genotype test for T. colubriformis. In the cross-classified design, where each strain infects groups of R, S or randomly bred control sheep, parasite adaptation would be indicated by a significant host genotype by nematode strain interaction for traits indicating parasite reproductive success; specifically WEC and, for H. contortus strains, packed cell volume. We found no significant evidence of parasite adaptation to host genotype (P>0.05) for either the H. contortus or T. colubriformis strains. Therefore, we argue that nematodes will not adapt quickly to sheep bred for nematode resistance, where selection is based on low WEC, although selecting sheep using a subset of immune functions may increase adaptation risk. Our results support the hypothesis that nematode resistance is determined by many genes each with relatively small effect. In conclusion, selection of sheep for nematode resistance using WEC should be sustainable in the medium to long-term.

Immunological responses and cytokine gene expression analysis to Cooperia punctata infections in resistant and susceptible Nelore cattle

Cellular and humoral immune response, as well as cytokine gene expression, was assessed in Nelore cattle with different degrees of resistance to Cooperia punctata natural infection. One hundred cattle (male, weaned, 11-12 months old), kept together on pasture, were evaluated. Faecal and blood samples were collected for parasitological and immunological assays. Based on nematode faecal egg counts (FEC) and worm burden, the seven most resistant and the eight most susceptible animals were selected. Tissue samples of the small intestine were collected for histological quantification of inflammatory cells and analysis of cytokine gene expression (IL-2, IL-4, IL-8, IL-12p35, IL-13, TNF-alpha, IFN-gamma, MCP-1, MCP-2, and MUC-1) using real-time RT-PCR. Mucus samples were also collected for IgA levels determination. Serum IgG1 mean levels against C. punctata antigens were higher in the resistant group, but significant differences between groups were only observed 14 days after the beginning of the experiment against infective larvae (L3) and 14 and 84 days against adult antigens. The resistant group also presented higher IgA levels against C. punctata (L3 and adult) antigens with significant difference 14 days after the beginning of the trial (P<0.05). In the small-intestine mucosa, levels of IgA anti-L3 and anti-adult C. punctata were higher in the resistant group, compared with the susceptible group (P<0.05). Gene expression of both T(H)2 cytokines (IL-4 and IL-13) in the resistant group and T(H)1 cytokines (IL-2, IL-12p35, IFN-gamma and MCP-1) in the susceptible group was up-regulated. Such results suggested that immune response to C. punctata was probably mediated by T(H)2 cytokines in the resistant group and by T(H)1 cytokines in the susceptible group.

Feeding practices and effects of gastrointestinal parasite infections on live weight gain of small ruminants in smallholder mixed farms in Kenya

The objective of this study was to quantify the effects of gastrointestinal nematodosis on live weight gain (LWG) of sheep and goats kept in smallholder farms in Kenya. A total of 307 sheep and goats from smallholder farms were sampled using a 2-stage cluster and systematic random approach. Sampled farms were visited once a month for nine months during which a health and production survey questionnaire was administered, animals weighed and fecal samples taken for fecal egg count. Descriptive statistics and a generalized linear mixed model were performed in SAS. The mean LWG of suckling kids and lambs was low (mean=46 g/day). High fecal egg count and lack of feed supplementation were identified as the main factors limiting growth. Improved helminth control and nutrition are required to optimize production.

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.

Risk factors of gastrointestinal nematode parasite infections in small ruminants kept in smallholder mixed farms in Kenya

BACKGROUND

Helminth infections in small ruminants are serious problems in the developing world, particularly where nutrition and sanitation are poor. This study investigated the burden and risk factors of gastrointestinal nematode parasite infections in sheep and goats kept in smallholder mixed farms in the Kenyan Central Highlands. Three hundred and seven small ruminants were sampled from 66 smallholder mixed farms in agro-ecological zones 1 (humid) and 3 (semi-humid) in the Kenyan Central highlands. The farms were visited once a month for eight months during which a health and production survey questionnaire was administered. Fecal samples were collected at each visit from each animal. Fecal egg counts (FEC) were performed using the modified McMaster technique. Associations between potential risk factors and FEC were assessed using 3-level Poisson models fit in SAS using GLIMMIX macro. Correlations among repeated observations were adjusted for using three different correlation structures.

RESULTS

A rise in FEC was observed two months after the onset of rains. Farmer education, age category, de-worming during the preceding month and grazing system were significant predictors of FEC. Additionally, there were significant interactions between grazing system and both de-worming and age category implying that the effect of grazing system is dependent on both de-worming status and age category; and that the effect of de-worming depends on the grazing system. The most important predictors of FEC in the study area were grazing system, de-worming status and education of the farmers.

CONCLUSION

Since several factors were important predictors of FEC, controlling gastrointestinal helminths of small ruminants in these resource-poor smallholder mixed farms requires a sustainable integrated helminth control strategy that includes adoption of zero-grazing and more farmer education probably through extension services. Achieving improved helminth controls in these resource-poor farming systems offers an opportunity to increase small ruminant productivity and hence has a potential of improving the livelihood of the resource-poor farmers.

Towards the implementation of the “basket of options” approach to helminth parasite control of livestock: Emphasis on the tropics/subtropics

The virtual reliance on anthelmintic drugs alone to control internal parasites of livestock is inappropriate and ultimately unsustainable. In the tropics and subtropics, widespread and high levels of anthelmintic resistance, particularly in nematode parasites of small ruminants, is rife. But more to the point, many farmers in these regions of the world are resource poor and cannot afford, or are reluctant to purchase drugs that may also be of dubious quality. As it is with any intervention, the benefits must outweigh the costs. This is not only in terms of conventional parameters such as reduced mortality and increasing productivity (meat, milk, fibre and traction power) of livestock, but also within the broad framework of helminths of veterinary/human importance, the aim should be a positive impact on reducing the threat of helminth zoonoses. However, understanding the issues involved and education of the end-users (farmers) is of fundamental importance, before any internal parasite control program should be promoted. Within the above context, we provide examples of how the "basket of options" approach could be adopted for the control of three quite disparate helminth problems in the tropics and subtropics, viz.: strongyle nematode infections of donkeys, the Taenia solium cysticercosis/taeniosis problem of pig and man and Haemonchus contortus infections in small ruminants. The "best practice" approaches can be defined as those "basket of options" that are practical, affordable, available and appropriate, whether to the commercial producer, or to the resource-poor farmer. Constraints that may restrict applying such options are accessibility to, and affordability of, suitable remedies and above all, the availability of information needed to make informed decisions in this regard.

Texel sheep are more resistant to natural nematode challenge than Suffolk sheep based on faecal egg count and nematode burden

Limited information is available on differences between sheep breeds with respect to helminth resistance under temperate conditions. The present study was designed to confirm and extend preliminary findings on observed breed differences in resistance to naturally acquired gastrointestinal nematodes in Suffolk and Texel sheep. Three trials were carried out. In trial 1 (1999-2003) lambs co-grazed from birth were faecal sampled at various time points up to 17 weeks of age. Worm burden was assessed at 17 weeks of age from a minimum of six lambs per breed in each of the 3 years. In trial 2, faecal egg count (FEC) was determined on six farms with co-grazed Suffolk and Texel purebred lambs. In trial 3 (2001-2003), ewes were faecal sampled at winter housing. In all three trials, an influence of breed on resistance to naturally acquired trichostrongyle infection was demonstrated. In trial 1, significantly higher FEC and worm burden was observed in Suffolk compared with Texel lambs following natural challenge. In trial 2, FEC recorded in lambs from six farms confirmed the breed differences previously observed. A breed difference in resistance to GI parasites was also observed in older ewes. In both breeds, an age effect on the FEC was observed with younger ewes having greater FEC than older ewes.

The relationship between responsiveness against gastrointestinal nematodes in lambs and the numbers of circulating IgE-bearing cells

The objective of this study was to determine if any relationship exists between responsiveness against gastrointestinal nematodes (GI) in lambs and circulating IgE-bearing cells. Scottish Blackface lambs that were grazing on pasture infected with Teladorsagia circumcincta (T. circumcincta) were ranked in order of their cumulative resistance to nematode infection as determined by their faecal egg counts (FECs) from material taken throughout the grazing season. The 10 lambs that had the lowest egg count rankings termed "responders" and the 10 lambs that had the highest egg count rankings termed "non-responders", were selected using data from the whole season or the middle or end of the grazing season. The number of circulating IgE-bearing cells present in the selected responder and non-responder lambs was investigated. The study was followed for two grazing seasons. Results showed that there were higher numbers of circulating IgE-bearing cells in the blood of responder lambs when compared to non-responders in the lambs selected from the middle of the season during both grazing seasons. A significant difference in the number of these IgE-bearing cells in responder and non-responder lambs was also demonstrated at the last two time points (p<0.05) of the initial grazing season. FACS analysis confirmed that IgE-bearing cells were found predominantly among lymphocytes/monocytes. The present study provides further evidence for an active role for IgE antibody in nematode immunity, and suggests that these circulating IgE-bearing cells might serve as additional markers for selecting animals that are responsive against T. circumcincta infections in lambs.

Frequency of bovine Nramp1 (Slc11a1) alleles in Holstein and Zebu breeds

Natural resistance against brucellosis in cattle is linked to the Nramp1 gene, which encodes a divalent cation transporter that localizes in the phagolysosome membrane in macrophages. Nramp1 gene in mouse plays a critical role in innate immunity favoring bacterial killing by macrophages in addition to its influence on adaptative immunity. Polymorphisms at the bovine Nramp1 3' untranslated region (3'UTR), detectable by Single Strand Conformational Analysis (SSCA), are associated with natural resistance against brucellosis. Such polymorphisms are associated with variation in the number of GT repeats. This study compared the frequency of Nramp1 3'UTR polymorphisms between Zebu and European bovine breeds. Eighty-one Holsteins (Bos taurus taurus) and 167 Zebu (Bos taurus indicus), including the following breeds: Nelore (n=95), Guzerá (n=37), and Gir (n=35), totaling 248 pure breed cattle studied. DNA extraction was performed using the guanidium protocol and genotyping was performed by SSCA. DNA from cattle considered genotypically resistant to brucellosis resulted in a single band (homozygous) with 175bp, corresponding to the 3'UTR with 13 GT pairs (GT13), whereas DNA from genotypically susceptible cattle generated one single band with 177bp (homozygous GT14) or double bands with both 175 and 177bp, or 175 and 179bp (heterozygous GT13/GT14 or GT13/GT15, respectively). A marked difference in the frequency of alleles was detected between the Zebu and Holstein cattle. Holsteins had an extremely homogeneous genotype, with 100% of the individuals with a GT13 genotype. In sharp contrast the Nelore breed had the most heterogeneous genotype with four allelic combinations, namely, homozygous GT13, homozygous GT14, heterozygous GT13/GT14, and heterozygous GT13/GT15. When the Zebu breeds were compared to each other, the only significant difference observed was the frequencies of the genotypes GT13 and GT14 between the Nelore and Guzerá breeds. The knowledge of allelic frequencies in different breeds of cattle may prove to be very useful in the future for planning breeding strategies for selection of resistant cattle.

Resistance of Santa Ines, Suffolk and Ile de France sheep to naturally acquired gastrointestinal nematode infections

A study was conducted to assess the breed resistance against nematode infections in Santa Ines, Ile de France and Suffolk male lambs over a 9-month period in São Paulo state, Brazil. Lambs were born during the winter (year 2000) and were weaned at 2 months of age. The animals were then housed and treated with anthelmintics to eliminate natural infections by gastrointestinal nematodes. In late October 2000, lambs were placed in a paddock, where they stayed until August of the following year. Fecal and blood samples were taken from each animal every 2 weeks. On the same day, a pasture sample was collected to determine the number of infective larvae on the herbage. To prevent deaths, individual treatment with anthelmintics was provided to lambs with fecal egg counts (FEC) higher than 4000 eggs per gram (EPG) or with a packed cell volume (PCV) lower than 21%. In August 2001, all animals were slaughtered and the worms present in samples of the gastrointestinal contents were identified and counted. Most of the Suffolk and Ile de France sheep received three to six anthelmintic treatments over a period of 7 months, while most of the Santa Ines were not treated. Reductions in PCV and plasma protein values associated with high FEC and worm burdens were recorded, particularly, in Suffolk and Ile de France lambs. Haemonchus contortus and Oesophagostomum columbianum burdens and number of nodular lesions caused in the large intestine by O. columbianum larvae were significantly lower in Santa Ines sheep. All three breeds showed similar Trichostrongylus colubriformis worm burdens. The relative resistance of Santa Ines young male sheep was superior to that of Suffolk and Ile de France sheep.

Modeling of host genetics and resistance to infectious diseases: understanding and controlling nematode infections

This paper considers approaches to modeling the dynamics of infectious disease and the application of such models to nematode parasite infections in ruminants. Particularly, these models are developed to account for host genetics and may be used to assess the effects of using genetics to control nematode infections. Three main issues are critically examined: the infection transmission cycle from pasture to host to pasture, the expected genetic relationships between resistance and performance, and the risks of parasite evolution in response to genetic changes in the host. To obtain answers that are realistic and of practical use, the modeling approaches require a solid grounding in biology. This biology is formalized and described using mathematical techniques, with the models parameterized using experimental or field data. Transmission dynamics have been quantified by modeling and are backed by strong experimental data. Selection for resistance will be successful in reducing egg output, pasture larval contamination and hence subsequent larval challenge. Modeling frameworks have been developed to predict genetic relationships between resistance to infectious disease and performance in general, and genetic correlations predicted for nematode resistance are close to mean published values. These predicted correlations strengthen as the larval challenge increases and the dietary (protein) adequacy decreases, however modeling challenges remain. Lastly, although convincing experimental data is not yet available, arguments based on modeling suggest that the risks of parasite evolution in response to genetic changes in the host should be less than the risks arising from other control strategies, such as anthelmintics. Thus, modeling techniques predict that selective breeding for resistance should be an effective and sustainable complementary control measure.

Heritability of gastrointestinal nematode faecal egg counts in West African village N'Dama cattle and its relation to age

Offspring-dam regression was used to estimate the heritability of strongyle faecal egg counts (FEC) of traditionally raised West African N'Dama cattle in the Central River Division in The Gambia. Faecal samples were taken monthly from June-October 1992, and again from July-October 1993, including 179-463 dams and their calves sampled on each occasion. The only proven genetic relationship was the dam-offspring relationship. Gastrointestinal strongyle FEC was expressed as epg (eggs per gram faeces). Regression of offspring FEC on dam FEC, showed a heritability (h(2)) of 0.18 (95% Confidence Limits 0.10, 0.25). Heritabilities were higher at the beginning and end of the rainy season than during the months of the peak rainy season. This is in line with earlier suggestions that genetic control of faecal egg counts is most effective during periods of low parasite transmission. There was a significant (p<0.001) increase in heritability of 0.086+/-0.018 with each year of age of the corresponding offspring. In view of the virtual absence of national cattle breeding systems in West Africa, which are a precondition for exploitation of heritable traits in cattle, integrated control using improved management and strategic prophylaxis remain the methods of choice to control gastrointestinal nematodes in the given conditions.

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.

Sustainable helminth control of ruminants in developing countries

Widespread anthelmintic resistance, at least amongst the important nematode parasites of small ruminants, threatens the sustainability of these livestock industries throughout both the developed and developing world. The exacerbation of this problem over the last decade or so, has provided the impetus for research into non-chemotherapeutic parasite control alternatives, such as host genetic resistance, grazing management, worm vaccines and biological control. Although some of these options provide practical benefits if currently adopted, or exciting prospects for the future, collectively they are unlikely to dispense with the need for the timely intervention of effective anthelmintic treatment. The issue of sustainability of helminth control practices therefore rests with the preservation of anthelmintic effectiveness through the implementation of principles of integrated pest management. Herein lies the difficulty-putting the principles into practice. Much of the research into sustainable nematode parasite control of ruminants has been done in the developed rather than the developing world, and the efforts to transfer this information to livestock owners has also been commensurately greater in the former. However the need for research and technology transfer is much more urgent in the developing world because of the lack of scientific and financial resources, the greater dependence on livestock industries and the much greater severity of the problem of anthelmintic resistance. This will require a major philosophical change in the affluent western world to the funding of national and international aid organisations who are largely responsible for these activities.

Epidemiology of nematodosis in Romney lambs selectively bred for resistance or susceptibility to nematode infection

Field trials were undertaken to compare nematode population dynamics, lamb productivity and levels of breech soiling in experimental flocks of Romney lambs selectively bred for increased resistance or susceptibility to nematode infection. In each year of the 2 year study, spring-born ewe lambs derived from Wallaceville Animal Research Centre's divergent nematode-resistant and nematode-susceptible breeding lines were grazed as separate flocks on matched farmlets from weaning (at 3 months old) until they were approximately 10-11 months old. Allocation of farmlets was reversed between Years 1 and 2 of the study to account for any possible paddock-related effects. Within each year both flocks were subjected to identical management conditions, including anthelmintic treatment (which was administered only when the overall mean faecal worm egg count measured across both genotypes reached 1500 eggs g-1). In both years, by mid-autumn (April) nematode larval infestation levels on pasture were approximately 5-6-fold greater on the farmlet grazed by susceptible (S) genotype lambs than on that grazed by their resistant (R) counterparts (Year 1: 2506 cf. 544 larvae kg-1 herbage; Year 2: 431 cf. 74 larvae kg-1 herbage). This led to 51-fold and 56-fold differences in faecal egg count between R and S lambs by late autumn (May) and winter (July) in Years 1 and 2, respectively. Although mean growth rates were similar in the R and S lambs over summer (while pasture infestation levels on the farmlets were still in the process of diverging), significantly higher growth rates occurred in the R than in the S lambs over autumn-winter in both years of the study (P < 0.01). In contrast, no significant differences in growth rate occurred in either year between male lambs derived from the nematode-resistant and nematode-susceptible breeding lines which were grazed together on another area of the Wallaceville farm from weaning until late autumn. Despite the substantially lower pasture infestation levels encountered by the R ewe lambs, they nevertheless temporarily suffered more breech soiling (dags) than their S counterparts (P < 0.01) in both years. Yearling fleece-weights of the R and S genotypes did not differ significantly in either year. Although the results of our study confirmed that there are potentially significant epidemiological benefits to be derived from breeding sheep for resistance to nematode infection, these benefits did not appear to be associated with large advantages in animal performance. Further work is needed to establish how these results should be interpreted with respect to anthelmintic drench requirements of genetically resistant animals.

Prospects for integration of novel parasite control options into grazing systems

Novel parasite control methods are considered under criteria of efficacy, cost, ease of implementation and sustainability, together with a subjective assessment of when they are likely to become available commercially. It is concluded that vaccines, genetically resistant hosts, biological control, anthelmintics directed against free-living stages, and grazing management will probably satisfy all of these criteria, but that most of these methods are at least 5-10 years away from commercial exploitation. The exception is grazing management, which can be used immediately and at low cost, to minimise current use of anthelmintics. Research is required to assess the possibility of control programs based on grazing management that avoid anthelmintic treatment altogether. Extension of such control programs, in a context of declining government enthusiasm for provision of agricultural extension services, may be their greatest barrier to widespread adoption.

Prospects of breeding small ruminants for resistance to internal parasites

Resistance to nematode parasites can be improved by selection, but efforts to include appropriate traits in commercial livestock breeding programs are only a recent development. Procedures for including resistance in breeding programs are similar to those involving other traits. The steps are described with special reference to sheep, and areas are highlighted where particular considerations exist. Three approaches are described and contrasted: breeding for resistance (reduced parasite numbers, as determined by faecal worm egg count); resilience (production during parasitism); or number of treatments required during parasitism. It is necessary, but difficult, to assess the economic benefits of improving resistance relative to other traits. Disease costs vary widely depending on the prevalence of the disease and on the availability, effectiveness and sustainability of alternative control measures. Costs of treatment and control are relatively simple to estimate for a given situation, but production losses are more difficult. Methods of dealing with this problem are discussed. Breeding for disease resistance usually requires that either selection candidates, or their relatives, are exposed to the pathogen so that resistance levels can be compared. Parasitic diseases generally create no special ethical problems in a breeding program unless natural challenge levels are insufficient to enable discrimination between hosts in their susceptibility. In the longer term, it is desirable that selection criteria for all major diseases be developed that will be informative in healthy animals. Molecular genetic markers offer promise, but simple genetic markers have so far been as elusive as physiological traits to predict resistance in undiseased animals. In the longer term, useful genetic markers will be found and techniques for combining these with phenotypic information need to be developed. Commercial breeding programs for sheep which include resistance to gastrointestinal roundworms are now operating in Australia and New Zealand, and issues related to breeding in the tropics are discussed.

Evidence for multiple anthelmintic resistance in sheep and goats reared under the same management in coastal Kenya

Four experiments, two with sheep and two with goats, were carried out to determine the efficacy of ivermectin, fenbendazole, levamisole, closantel and some of their combinations by faecal egg count reduction tests. In the first experiment, injectable ivermectin, oral ivermectin, fenbendazole and levamisole were tested in 6-month-old lambs, and their reduction percentages were 77%, 13%, 42% and 92%, respectively. In the second experiment, with yearling sheep, the reduction percentages were 35% for injectable ivermectin, 32% for fenbendazole, 99% for levamisole, 48% for closantel, 92% for injectable ivermectin combined with fenbendazole, 99% for injectable ivermectin combined with levamisole, and 100% for fenbendazole combined with levamisole. In the study with 18-month-old goats given the same dose rates as those recommended for sheep, the reduction percentages were 73% for injectable ivermectin, 25% for fenbendazole, and 78% for levamisole. Another group of 14-month-old goats was treated with dose rates 1.5 times those recommended for sheep and the reduction percentages were 93% for levamisole, 92% for injectable ivermectin, and 97% for a combination of levamisole and ivermectin. In all experiments with sheep and goats the gastrointestinal nematode parasites identified by larval cultures were Haemonchus contortus, Trichostrongylus spp. and Oesophagostomum spp. The gastrointestinal nematodes of both sheep and goats on this farm are resistant to ivermectin and fenbendazole, whereas levamisole is still effective in sheep, but not in goats. The results are discussed in relation to the farm as a source of breeding stock to smallholder farmers and its potential to spread anthelmintic resistance.

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.

Parasites, animal production and sustainable development

Ecologically sustainable development is aimed at reducing environmental degradation while enabling economic development with equity between the developed and developing worlds and between generations. Parasite control in livestock can both contribute to, and take advantage of, sustainable agriculture. This will tend towards less intensive, lower input, diversified crop and animal production with less risk of parasite-induced losses and greater opportunities for integrated control including the exploitation of grazing management. Chemotherapy will continue to play a part but the most serious problem is resistance in the target species. Except for a few isolated issues, currently used parasiticides are relatively minor contaminants of the food supply or the environment. Nevertheless, the compounds of the future will need to be narrow-spectrum, non-persistent and rapidly degraded, with convenience in the hands of the user reduced in importance. Environmentally friendly alternatives to chemotherapy, including genetic resistance of hosts, vaccines, and biological control, show considerable promise and must be pursued. Sustainable systems pose optimisation problems and more attention must be given to systems research, models and products to aid decisions. If governments are serious about sustainable development, greater support will be needed for longer-term patient, multi-disciplinary research.

The distribution of nematode egg counts and larval counts in grazing sheep and their implications for parasite control

The frequency distribution of gastrointestinal nematode egg and lungworm larval counts was examined in 101 and 87 naturally infected ewes on two farms. The egg and larval outputs of the two flocks followed the negative binomial pattern of distribution (with k values below unity) at each time of sampling, which suggests highly overdispersed worm burdens. The results of the statistical analysis indicated that a relatively small part of the two flocks was responsible for the excretion of the majority of both gastrointestinal nematode eggs and Dictyocaulus filaria larvae. It is concluded that by eliminating "wormy" individuals of flocks either by selective breeding or by their selective anthelmintic treatment, effective control of parasite transmission can be achieved. Because of the phenomenon of nematode "clumping" it seems necessary to expand new methods for estimation of flock productivity caused by nematodes in livestock and to incorporate negative binomial parameter (k) in mathematical models of nematode population dynamics.

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.

Control of gastrointestinal nematodes in Australia in the 21st century

Because the Australian livestock industries rely overwhelmingly on year-round grazing of pastures, nematode parasites are of major concern. Nematode parasitism is recognized as the cause of the most economically important diseases of sheep, while in cattle, the relative importance of nematode parasitism will probably increase following the eradication of diseases such as brucellosis and tuberculosis. The extraordinarily high prevalence of resistance to benzimidazoles, levamisole and morantel in nematodes parasitising sheep has stimulated the formulation and, to a surprisingly large extent, adoption by farmers of strategic control programmes aimed at preserving the useful life of ivermectin. Even so, it is highly likely that we will enter the 21st century with ivermectin resistance. Current research aimed at control in the next century is progressing in three broad directions. First, towards better control using existing anthelmintics through modifications to pharmacokinetics, together with integrated management aided by computer simulation models of nematode population biology. Secondly, towards immunological control arising from a better understanding of the ruminant immune response to helminths, particularly in young animals, and finally, a coordinated investigation of genetic resistance, within breeds, of sheep to nematode parasitism. The appearance in the 21st century of novel anthelmintic compounds, most probably as a result of research by the international pharmaceutical industry, may well reduce the sense of urgency driving current Australian research. Nevertheless, success in one or more of these three lines of endeavour will mean a longer and more profitable life for the next generation of anthelmintics marketed in Australia.

Epidemiology of Ostertagia ostertagi in weaner-yearling cattle

Epidemiologic events in the life cycle of Ostertagia ostertagi are best known in the weaner-yearling phase of cattle development throughout the concentrated cattle-rising areas of the world. Animal and pasture management demands placed on this age class are greater than for suckling calves and adult stock in either beef or dairy breeds. This fact alone would likely account for a higher prevalence of clinical and subclinical disease in weaner-yearlings. Additionally, the developing immune response provides relatively early protection against intestinal genera such as Cooperia and Oesophagostomum, but is delayed against Ostertagia ostertagi and Trichostrongylus axei. Both Type I and Type II disease may occur within the weaner-yearling stage. Factors affecting population changes of Ostertagia ostertagi have been described as extrinsic, i.e. weather-climate and grazing management, and intrinsic or host factors, i.e. age, sex, immune status, heredity and reproductive state. Immune status, particularly in weaner-yearlings, may be of primary importance, as affected by host and extrinsic factors. With slow development of protective immunity against Ostertagia ostertagi in calves, the possible role of immunity in both induction of inhibition and larval maturation, the potential immunopathologic involvement in pathogenesis of Type II disease, hypersensitivity to larval intake in resistant adult cows, and the reported delay of a protective response following anthelmintic prophylaxis in younger cattle, the immune response may have profound influence on epidemiologic variation through age classes. Although continual epidemiological observations from birth to early adulthood in the same cattle have not been undertaken, some notable studies in the UK, the Netherlands, and Denmark have closely examined epidemiological events through first and second grazing seasons.

Selection of Merino sheep for increased and decreased resistance to Haemonchus contortus: peri-parturient effects on faecal egg counts

Peri-parturient worm egg counts were compared in 395 Merino ewes bred for either increased (IRH) or decreased (DRH) resistance to Haemonchus contortus, or from an unselected control flock (CH). Following a 10 month period with no anthelmintic treatment, a rise in egg counts began about 4 weeks before parturition and continued into lactation. At all stages, egg counts were significantly lower in IRH ewes, even those failing to conceive. Before lambing, IRH ewes averaged 263 epg, compared with 1113 epg in CH and 1618 epg in DRH ewes. After 2 weeks of lambing, lactating ewes averaged 1050, 2173 and 3708 epg, respectively, in the three lines. At the end of lambing, egg counts had increased to 1645, 3959 and 4124 epg, respectively. Counts appeared to have peaked in IRH ewes but not in DRH ewes. Ewes with twins had higher counts than those with singles, and ewes suckling lambs had higher counts than those losing lambs. No significant variation was associated with age of ewe (2-7 years). Although there were several nematode species present, the results suggest that, in comparison with DRH ewes, IRH ewes were particularly effective in reducing pasture contamination with H. contortus, both before and during the peri-parturient period.

Breeding cattle and sheep for resistance to gastrointestinal nematodes

Gastrointestinal nematodes are an important cause of reduced production of meat, milk and wool in domestic livestock. It is generally believed that problems caused by these parasites have increased owing to the intensification of animal husbandry(1-3) of resistance to anthelmintics, current research is focussed on alternative control strategies that do not rely on anthelmintics. Here, Bram Kloosterman, Henk Parmentier and Harm Ploeger review work on the genetic resistance of domestic ruminants to these nematodes and discuss the practicality of breeding programmes.

Parasitological and immunological responses of genetically resistant Merino sheep on pastures contaminated with parasitic nematodes

One hundred and twenty lambs were grazed continuously from weaning until 9 months of age on 12 plots contaminated with larvae of three nematode species (Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta). The lambs were sired by either a genetically resistant ram or susceptible rams (determined by the response of previous progeny to artificial H. contortus infection). Half the resistant and half the susceptible lambs were given strategic anthelmintic treatment and the remainder remained untreated. Faecal egg counts and blood packed cell volume were measured frequently in all animals. One and 5 months after weaning, two lambs from each plot were slaughtered, and worm burdens and larval establishment rates of the three species of nematode were estimated. At the second slaughter, leukotriene levels and larval migration inhibitory (LMI) activity were measured in mucus collected from the small intestine. The dominant species in all faecal samples and the gastrointestinal tract was T. colubriformis. Lambs of the resistant genotype had lower faecal worm egg counts, lower worm burdens and higher levels of resistance to larval establishment. There were no differences in larval migration inhibition (LMI) activity, but resistant lambs had higher levels of the leukotriene LTC4/D4/E4. Further, the resistant genotype, identified on responsiveness to artificial infections with H. contortus, was more resistant to infections of three important species acquired naturally from contaminated pastures. All these genetic differences were maintained while the lambs were subject to strategic anthelmintic treatment.

Population dynamics of Trichostrongylus colubriformis in sheep: computer model to simulate grazing systems and the evolution of anthelmintic resistance

A computer model was developed to simulate Trichostrongylus colubriformis populations, their level of resistance to the common anthelmintics, host mortalities and acquired immunity. Predictions were based on sheep management practices such as lambing, weaning, sheep/paddock rotation, anthelmintic treatment, the use of controlled release devices (capsules) for anthelmintic delivery and daily meteorological records to determine the development and survival of infective larvae (L3) on pasture. Evolution of drug resistance was determined by a simple genetic system which allowed for up to three genes, each with two alleles, to give a maximum of 27 genotypes associated with one drug or three genotypes for each of three drugs. The model was validated against egg counts, L3 counts on pasture and host mortalities observed in a grazing trial, however, aspects of the model such as the development of drug resistance and use of the model in a variety of climatic zones have yet to be tested against field observations. The model was used to examine the impact of grazing management and capsule use on anthelmintic resistance and sheep production over 20 years using historical weather data. Predictions indicated that grazing management can play a dominant role in parasite control and that capsule use will reduce sheep mortalities and production losses, and in some circumstances will not cause a substantial increase in anthelmintic resistance for up to 5 years.

Is there a relationship between haemoglobin genotype and the innate resistance to experimental Haemonchus contortus infection in Merino lambs?

Responses to a single or repeated infection with 7000 infective larvae of Haemonchus contortus were studied in an experiment using a total of 106 3-month-old lambs with AA, AB or BB haemoglobin (Hb) genotypes. Results were assessed by faecal egg counts, adult worm counts, haematocrit values, haemoglobin concentrations, total serum protein and serum antibody IgG1 and IgA ELISA titres. None of these parameters showed a strong relationship to the Hb type. The prevalence of low responder (greater than 500 worms) and of high responder (less than 50 worms) animals in groups AA, AB and BB Hb types was 3.8 and 34.6, 20.6 and 35.2, 28.1 and 43.7%, respectively, suggesting that the responsiveness to nematode infection is under the control of gene(s) not closely linked with those determining the Hb genotype. Worm counts of a primary infection are more subject to variation than those of a secondary infection. There is a strong relationship between adult worm counts and faecal egg counts taken close to the time of slaughter. In living animals low and high responder discrimination can be based on individual faecal egg counts around 50 days after a secondary infection. Haematocrit values proved to be of little value in the low and high responder selection. In this regard neither Hb concentration nor total serum protein values are of practical significance. In 3-month-old lambs primary infection induced partial immunity which could prevent the establishment of a part of the secondary infection, irrespective of the presence or absence of the primary worm population. The development of immunity was not associated with an increase of serum IgG1 and IgA antibody levels. Specific antibody production was not influenced by Hb types. Mean antibody levels of low responder lambs showed no difference from those of high responders. Thus, serum IgG1 and IgA levels are of no predictive value in identifying lambs which are genetically resistant to Haemonchus infection.