The parasitic phase of Ostertagia ostertagi: quantification of the main life history traits through systematic review and meta-analysis

Efficacy in US cattle of a novel fixed-dose combination injectable (0.2 mg/kg doramectin + 6.0 mg/kg levamisole hydrochloride) against naturally acquired gastrointestinal nematode infections

Reports of macrocyclic lactone (ML) loss of efficacy suggest ML resistance in gastrointestinal nematodes (GINs) is a growing problem in the US cattle industry. Empirical and modeling data support combining an ML and second anthelmintic from a different drug class to help ML resistance development while effectively treating existing resistant parasite populations. Here, we present a novel fixed-dose combination injectable (FDCI) solution for cattle that delivers 0.2 mg of doramectin and 6.0 mg of levamisole hydrochloride (HCl) per kg of body weight. Field studies were conducted at six sites across the Midwest United States to investigate the efficacy of a single administration of the FDCI in treating common cattle GINs. Cattle (n = 425) with GIN infections confirmed by fecal egg count (FEC) on Day -10( ± 2) were randomly allocated to the control (saline) or treatment (FDCI) group. On Day 0, pre-treatment fecal samples were collected, and cattle were administered a single subcutaneous injection of saline (n = 106) or FDCI (n = 319). Post-treatment fecal samples were collected on Day 14. Fecal egg count reduction tests (FECRTs) were conducted using Day 0 and Day 14 FECs. Efficacy was evaluated using Day 14 FECs (FDCI-treated versus saline-treated). Within treatment, samples collected on Days 0 and 14 with ≥ 20 eggs per gram (EPG) were cultured for nematode larvae recovery and identification. Day -10 FECs for enrolled animals were similar between treatment groups. Coprocultures from cattle with FEC ≥ 20 EPG (n = 68 saline; n = 211, FDCI) on Day 0 showed the presence of Cooperia punctata, Ostertagia spp., Haemonchus spp., C. oncophora, Oesophagostomum spp. and Trichostrongylus spp. Day 14 FECs for FDCI-treated cattle were significantly reduced (0-3 EPG) compared to saline-treated cattle (0-1252 EPG) (p ≤ 0.0042). At all study sites, the efficacy of the new FDCI was ≥ 99.4% and the FECR was 0.99 or 1.00. Day 14 coprocultures from control cattle showed infections of common GIN genera, confirming the efficacy of the FDCI against GINs in the field. A single administration of the doramectin + levamisole HCl combination injectable effectively treats common and economically important cattle GINs.

Laboratory diagnosis of gastrointestinal nematodes in first-grazing-season cattle

This focus article has been prepared by Natalie Jewell of the APHA Cattle Expert Group with the assistance of Vanessa Swinson (veterinary lead of the APHA Cattle Expert Group), Claire Hayman, Lucy Martindale and Anna Brzozowska (Surveillance Intelligence Unit), and Siân Mitchell (formerly the APHA's parasitology discipline champion).

Novel epidemiological model of gastrointestinal nematode infection to assess grazing cattle resilience by integrating host growth, parasite, grass and environmental dynamics

Gastrointestinal nematode (GIN) infections are ubiquitous and often cause morbidity and reduced performance in livestock. Emerging anthelmintic resistance and increasing change in climate patterns require evaluation of alternatives to traditional treatment and management practices. Mathematical models of parasite transmission between hosts and the environment have contributed towards the design of appropriate control strategies in ruminants, but have yet to account for relationships between climate, infection pressure, immunity, resources, and growth. Here, we develop a new epidemiological model of GIN transmission in a herd of grazing cattle, including host tolerance (body weight and feed intake), parasite burden and acquisition of immunity, together with weather-dependent development of parasite free-living stages, and the influence of grass availability on parasite transmission. Dynamic host, parasite and environmental factors drive a variable rate of transmission. Using literature sources, the model was parametrised for Ostertagia ostertagi, the prevailing pathogenic GIN in grazing cattle populations in temperate climates. Model outputs were validated on published empirical studies from first season grazing cattle in northern Europe. These results show satisfactory qualitative and quantitative performance of the model; they also indicate the model may approximate the dynamics of grazing systems under co-infection by O. ostertagi and Cooperia oncophora, a second GIN species common in cattle. In addition, model behaviour was explored under illustrative anthelmintic treatment strategies, considering impacts on parasitological and performance variables. The model has potential for extension to explore altered infection dynamics as a result of management and climate change, and to optimise treatment strategies accordingly. As the first known mechanistic model to combine parasitic and free-living stages of GIN with host feed-intake and growth, it is well suited to predict complex system responses under non-stationary conditions. We discuss the implications, limitations and extensions of the model, and its potential to assist in the development of sustainable parasite control strategies.

Practical guide to the diagnostics of ruminant gastrointestinal nematodes, liver fluke and lungworm infection: interpretation and usability of results

The diagnostics of ruminant parasites remains one of the cornerstones for parasite control best practices. Field veterinarians have several techniques at their disposal (fecal egg count, coproculture, FAMACHA®, plasma pepsinogen, ELISA-Ostertagia, ELISA-Fasciola, Baermann and ELISA-Lungworm) for the identification and/or quantification of gastrointestinal nematodes, lungworms and liver fluke infecting small ruminants and cattle. Each of these diagnostic tools has its own strengths and weaknesses and is more appropriate for a specific production operation and/or age of the animal (young and adults). This review focuses on the usability and interpretation of the results of these diagnostic tools. The most advanced technical information on sampling, storage, advantages and limitations of each tool for different types of production operations and animal categories is provided.

Sublethal effects of parasitism on ruminants can have cascading consequences for ecosystems

Parasitic infections are common, but how they shape ecosystem-level processes is understudied. Using a mathematical model and meta-analysis, we explored the potential for helminth parasites to trigger trophic cascades through lethal and sublethal effects imposed on herbivorous ruminant hosts after infection. First, using the model, we linked negative effects of parasitic infection on host survival, fecundity, and feeding rate to host and producer biomass. Our model, parameterized with data from a well-documented producer–caribou–helminth system, reveals that even moderate impacts of parasites on host survival, fecundity, or feeding rate can have cascading effects on ruminant host and producer biomass. Second, using meta-analysis, we investigated the links between helminth infections and traits of free-living ruminant hosts in nature. We found that helminth infections tend to exert negative but sublethal effects on ruminant hosts. Specifically, infection significantly reduces host feeding rates, body mass, and body condition but has weak and highly variable effects on survival and fecundity. Together, these findings suggest that while helminth parasites can trigger trophic cascades through multiple mechanisms, overlooked sublethal effects on nonreproductive traits likely dominate their impacts on ecosystems. In particular, by reducing ruminant herbivory, pervasive helminth infections may contribute to a greener world.

Confounding factors affecting faecal egg count reduction as a measure of anthelmintic efficacy

Increasing anthelmintic resistance (AR) in livestock has stimulated growing efforts to monitor anthelmintic effectiveness (AE) on livestock farms. On-farm assessment of AE relies on measuring the reduction in faecal egg count (FEC) following treatment; and if conducted rigorously, qualifies as a formal FEC reduction test (FECRT) for AR. Substantial research effort has been devoted to designing robust protocols for the FECRT and its statistical interpretation; however, a wide range of factors other than AR can affect FEC reduction on farms. These are not always possible to control, and can affect the outcome and repeatability of AE measurements and confound the on-farm classification of AR using FECRT. This review considers confounders of FEC reduction, focusing on gastrointestinal nematodes of ruminants, including host and parasite physiology and demography; pharmacokinetic variation between drugs, parasites and hosts; and technical performance. Drug formulation and delivery, host condition and diet, and seasonal variation in parasite species composition, can all affect AE and hence observed FEC reduction. Causes of variation in FEC reduction should be attenuated, but this is not always possible. Regular monitoring of AE can indicate a need to improve anthelmintic administration practices, and detect AR early in its progression. Careful interpretation of FEC reduction, however, taking into account possible confounders, is essential before attributing reduced FEC reduction to AR. Understanding of confounders of FEC reduction will complement advances in FECRT design and interpretation to provide measures of anthelmintic efficacy that are both rigorous and accessible.

Seasonal epidemiology of gastrointestinal nematodes of cattle in the northern continental climate zone of western Canada as revealed by internal transcribed spacer-2 ribosomal DNA nemabiome barcoding

Background

Gastrointestinal nematode (GIN) epidemiology is changing in many regions of the world due to factors such as global warming and emerging anthelmintic resistance. However, the dynamics of these changes in northern continental climate zones are poorly understood due to a lack of empirical data.

Methods

We studied the accumulation on pasture of free-living infective third-stage larvae (L3) of different GIN species from fecal pats deposited by naturally infected grazing cattle. The field study was conducted on three organic farms in Alberta, western Canada. Grass samples adjacent to 24 fecal pats were collected from each of three different pastures on each farm. Internal transcribed spacer-2 nemabiome metabarcoding was used to determine the GIN species composition of the harvested larvae. The rotational grazing patterns of the cattle ensured that each pasture was contaminated only once by fecal pat deposition. This design allowed us to monitor the accumulation of L3 of specific GIN species on pastures under natural climatic conditions without the confounding effects of pasture recontamination or anthelmintic treatments.

Results

In seven out of the nine pastures, grass L3 counts peaked approximately 9 weeks after fecal deposition and then gradually declined. However, a relatively large number of L3 remained in the fecal pats at the end of the grazing season. Nemabiome metabarcoding revealed that Cooperia oncophora and Ostertagia ostertagi were the two most abundant species on all of the pastures and that the dynamics of larval accumulation on grass were similar for both species. Daily precipitation and temperature across the whole sampling period were similar for most of the pastures, and multiple linear regression showed that accumulated rainfall 1 week prior to sample collection had a significant impact on the pasture L3 population, but accumulated rainfall 3 weeks prior to sample collection did not.

Conclusions

The results suggest that the pasture L3 population was altered by short-term microclimatic conditions conducive for horizontal migration onto grass. Overall, the results show the importance of the fecal pat as a refuge and reservoir for L3 of cattle GIN on western Canadian pastures, and provide an evidence base for the risk assessment of rotational grazing management in the region.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-05101-w.

Biology and Epidemiology of Gastrointestinal Nematodes in Cattle

This article reviews the basics of gastrointestinal nematode biology and pathophysiology in cattle and describes how gastrointestinal nematode epidemiology is driven by environmental, host, and farm economic determinants. Adverse effects from gastrointestinal nematodes on their hosts are caused by tissue damage, nutrient absorption, immunopathologic effects, and reduced food intake induced by hormonal changes. Weather and microenvironmental factors influence the development and survival of free-living parasitic stages. A holistic control approach entails the consideration of environmental, immunologic, and socioeconomic aspects of nematode epidemiology and is key for the development and communication of sustainable control strategies.

GLOWORM-PARA: a flexible framework to simulate the population dynamics of the parasitic phase of gastrointestinal nematodes infecting grazing livestock

Gastrointestinal nematodes are a significant threat to the economic and environmental sustainability of keeping livestock, as adequate control becomes increasingly difficult due to the development of anthelmintic resistance in some systems and climate-driven changes to infection dynamics. To mitigate any negative impacts of climate on gastrointestinal nematode epidemiology and slow anthelmintic resistance development, there is a need to develop effective, targeted control strategies that minimise the unnecessary use of anthelmintic drugs and incorporate alternative strategies such as vaccination and evasive grazing. However, the impacts climate and gastrointestinal nematode epidemiology may have on the optimal control strategy are generally not considered, due to lack of available evidence to drive recommendations. Parasite transmission models can support control strategy evaluation to target field trials, thus reducing the resources and lead-time required to develop evidence-based control recommendations incorporating climate stochasticity. Gastrointestinal nematode population dynamics arising from natural infections have been difficult to replicate and model applications have often focussed on the free-living stages. A flexible framework is presented for the parasitic phase of gastrointestinal nematodes, GLOWORM-PARA, which complements an existing model of the free-living stages, GLOWORM-FL. Longitudinal parasitological data for two species that are of major economic importance in cattle, Ostertagia ostertagi and Cooperia oncophora, were obtained from seven cattle farms in Belgium for model validation. The framework replicated the observed seasonal dynamics of infection in cattle on these farms and overall, there was no evidence of systematic under- or over-prediction of faecal egg counts. However, the model under-predicted the faecal egg counts observed on one farm with very young calves, highlighting potential areas of uncertainty that may need further investigation if the model is to be applied to young livestock. The model could be used to drive further research into alternative parasite control strategies such as vaccine development and novel treatment approaches, and to understand gastrointestinal nematode epidemiology under changing climate and host management.

Rapamycin exposure to host and to adult worms affects life history traits of Heligmosomoides bakeri

Parasite life history can be affected by conditions of the host and of the external environment. Rapamycin, a known immunosuppressant of mammals, was fed to laboratory mice that were then infected with the Trichostrongylid nematode Heligmosomoides bakeri to determine if host rapamycin exposure would affect parasite survival, growth, and reproduction. In addition, adult worms from control fed mice were directly exposed to rapamycin to assess if rapamycin would affect worm viability and ex vivo reproduction. We found that host ingestion of rapamycin did not affect H. bakeri survival or growth for male or female worms, but female worms had increased reproduction both in vivo and when removed from the host and cultured ex vivo. After direct rapamycin exposure, motility of female worms was greater at low levels of rapamycin compared to high levels of rapamycin or high levels of DMSO (the vehicle used to solubilize rapamycin) in control media, but was similar to females in low levels of DMSO in control media. Male motility was not affected by the presence of rapamycin or DMSO in the media. Ex vivo egg deposition was higher when exposed to rapamycin than when cultured in control media that contained DMSO, regardless of DMSO dose. Overall, we conclude that host ingestion of rapamycin or direct exposure to rapamycin was generally favorable or neutral for parasite life history traits.

Farmer Behavior and Gastrointestinal Nematodes in Ruminant Livestock-Uptake of Sustainable Control Approaches

Gastrointestinal nematode (GIN) infections are a common constraint in pasture-based herds and cause a decrease in animal health, productivity and farm profitability. Current control practices to prevent production losses of GIN infections in livestock depend largely on the use of anthelmintic drugs. However, due to the continued use of these drugs over more than three decades, the industry is now increasingly confronted with nematode populations resistant to the available anthelmintics. This emerging anthelmintic resistance (AR) in cattle nematodes emphasizes the need for a change toward more sustainable control approaches that limit, prevent or reverse the development of AR. The uptake of diagnostic methods for sustainable control could enable more informed treatment decisions and reduce excessive anthelmintic use. Different diagnostic and targeted or targeted selective anthelmintic control approaches that slow down the selection pressure for anthelmintic resistance have been developed and evaluated recently. Now it is time to transform these insights into guidelines for sustainable control and communicate them across the farmer community. This article reviews the current uptake of such sustainable practices with a focus on farmer's socio-psychological factors affecting this uptake. We investigate communication as a possible tool to change current behavior and successfully implement more sustainable anthelmintic treatment strategies.

Significance of anti-CarLA salivary IgA antibody in first grazing season cattle naturally infected with gastrointestinal nematodes

A carbohydrate larval surface antigen (CarLA) present on infective larvae of all trichostrongylid nematodes is a target antigen for host immunoglobulins (Ig). Levels of anti-CarLA salivary IgA antibody (CarLA-IgA) have been shown to be correlated to the level of protective immunity to GIN in sheep and deer but no information is available in cattle. The first objective of this study was to assess the pattern of CarLA-IgA response in 7 groups (G1-G7) of first grazing season cattle (FGSC) naturally infected with gastrointestinal nematodes. The second objective was to assess the phenotypic correlations between CarLA-IgA level, 3 parasitological indicators (faecal egg count-FEC, pepsinogen level, serum anti-O. ostertagi IgG antibody level-OstertagiaIgG), a clinical indicator (diarrhea score) and average daily weight gain (ADWG). Overall, CarLA-IgA response gradually increased over grazing season and showed large variations in speed and magnitude both between and within groups. Based on the mean group CarLA-IgA response pattern, the 7 groups could be allocated to 3 different classes: (i) 'Late High' class characterized by a high response at housing (G1 and G2); (ii) 'Low' class with a low response over time (G3, G4 and G5); and (iii) 'Early' class with an high initial then stable response (G6 and G7). This classification was consistent with the grazing management practices. In the 'Late High' class, the mean CarLA-IgA at housing was 6.05units/mL and negatively correlated with FEC while no correlation was seen with the other indicators nor ADWG. In the 'Low' class, CarLA response at housing was low (1.95units/mL) and mainly positively correlated with OstertagiaIgG. In the 'Early' class, mean CarLA-IgA ranged from 1.32 to 1.86units/mL during the grazing season and positive correlations were seen with parasitological and clinical indicators. These results suggest that, according to the intensity of larval challenge occurring during the first grazing season, CarLA-IgA response in cattle could be either an indicator of the early manifestation of immunity (FEC decreases) or the reflection of exposure to GIN.

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.

Modelling the consequences of targeted selective treatment strategies on performance and emergence of anthelmintic resistance amongst grazing calves

The development of anthelmintic resistance by helminths can be slowed by maintaining refugia on pasture or in untreated hosts. Targeted selective treatments (TST) may achieve this through the treatment only of individuals that would benefit most from anthelmintic, according to certain criteria. However TST consequences on cattle are uncertain, mainly due to difficulties of comparison between alternative strategies. We developed a mathematical model to compare: 1) the most 'beneficial' indicator for treatment selection and 2) the method of selection of calves exposed to Ostertagia ostertagi, i.e. treating a fixed percentage of the population with the lowest (or highest) indicator values versus treating individuals who exceed (or are below) a given indicator threshold. The indicators evaluated were average daily gain (ADG), faecal egg counts (FEC), plasma pepsinogen, combined FEC and plasma pepsinogen, versus random selection of individuals. Treatment success was assessed in terms of benefit per R (BPR), the ratio of average benefit in weight gain to change in frequency of resistance alleles R (relative to an untreated population). The optimal indicator in terms of BPR for fixed percentages of calves treated was plasma pepsinogen and the worst ADG; in the latter case treatment was applied to some individuals who were not in need of treatment. The reverse was found when calves were treated according to threshold criteria, with ADG being the best target indicator for treatment. This was also the most beneficial strategy overall, with a significantly higher BPR value than any other strategy, but its degree of success depended on the chosen threshold of the indicator. The study shows strong support for TST, with all strategies showing improvements on calves treated selectively, compared with whole-herd treatment at 3, 8, 13 weeks post-turnout. The developed model appeared capable of assessing the consequences of other TST strategies on calf populations.

A stochastic model to investigate the effects of control strategies on calves exposed to Ostertagia ostertagi

Predicting the effectiveness of parasite control strategies requires accounting for the responses of individual hosts and the epidemiology of parasite supra- and infra-populations. The first objective was to develop a stochastic model that predicted the parasitological interactions within a group of first season grazing calves challenged by Ostertagia ostertagi, by considering phenotypic variation amongst the calves and variation in parasite infra-population. Model behaviour was assessed using variations in parasite supra-population and calf stocking rate. The model showed the initial pasture infection level to have little impact on parasitological output traits, such as worm burdens and FEC, or overall performance of calves, whereas increasing stocking rate had a disproportionately large effect on both parasitological and performance traits. Model predictions were compared with published data taken from experiments on common control strategies, such as reducing stocking rates, the ‘dose and move’ strategy and strategic treatment with anthelmintic at specific times. Model predictions showed in most cases reasonable agreement with observations, supporting model robustness. The stochastic model developed is flexible, with the potential to predict the consequences of other nematode control strategies, such as targeted selective treatments on groups of grazing calves.

Experimental insight into the process of parasite community assembly

Community assembly is a fundamental process that has long been a central focus in ecology. Extending community assembly theory to communities of co-infecting parasites, we used a gastrointestinal nematode removal experiment in free-ranging African buffalo to examine the community assembly patterns and processes. We first asked whether reassembled communities differ from undisturbed communities by comparing anthelmintic-treated and control hosts. Next, we examined the temporal dynamics of assembly using a cross-section of communities that reassembled for different periods of time since last experimental removal. Next, we tested for evidence of assembly processes that might drive such reassembly patterns: environmental filtering based on host traits (i.e. habitat patches), interspecific interactions, priority effects and chance dispersal from the environmental pool of infective stages (i.e. the regional species pool). On average, reassembled parasite communities had lower abundance, but were more diverse and even, and these patterns varied tightly with reassembly time. Over time, the communities within treated hosts progressively resembled controls as diversity and evenness decreased, while total abundance increased. Notably, experimental removal allowed us to attribute observed differences in abundance, diversity and evenness to the process of community assembly. During early reassembly, parasite accumulation was biased towards a subordinate species and, by excluding stochastic assembly processes (i.e. chance dispersal and priority effects), we were able to determine that early assembly is deterministic. Later in the reassembly process, we established that host traits, as well as stochastic dispersal from the environmental pool of infective stages, can affect the community composition. Overall, our results suggest that there is a high degree of resiliency and environmental dependence to the worm communities of buffalo. More generally, our data show that both deterministic and stochastic processes may play a role in the assembly of parasite communities of wild hosts, but their relative importance may vary temporally. Consequently, the best strategy for managing reassembling parasite communities may also need to shift over time.

Prevalence of Ostertagia ostertagi lesions in slaughtered dairy cattle from São Miguel Island, Azores, Portugal

Ostertagiosis caused by Ostertagia ostertagi is considered the most important parasitosis in dairy cattle raised in temperate areas. The aim of this investigation was to determinate the prevalence of ostertagiosis in cattle slaughtered in São Miguel Island (SMI), Azores, the geographical distribution of positive cases, its economic impact and farmers awareness regarding preventive and control measurements against this parasitosis. From the 2000 animals sampled, 1282 presented lesions compatible with ostertagiosis representing a prevalence of 49.0% in young animals and 75.7% in adults. Positive animals had significantly worse carcass rating when slaughtered and the fat rating of positive uncastrated males older than 24months was also significantly lower. The presence of lesions compatible with ostertagiosis resulted in a total weight loss of 2.810.89kg (2.19kg mean weight loss per animal - 1.07% carcass weight drop), which extrapolating to the total of animals slaughtered in SMI in 2014 corresponds to a value of losses of roughly about 91.000.00€. Our data also indicate that ostertagiosis has a widespread distribution throughout the Island and that 85.7% of the farmers dewormed calves and 42.9% dewormed cows (only one administration was provided for each deworming). Data shows that ostertagiosis represents an important economical lost in dairy cattle from SMI and a significant lack of knowledge by farmers about the benefits of deworming and preventive management practices.

A simulation model to investigate interactions between first season grazing calves and Ostertagia ostertagi

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A deterministic model to address calf—O. ostertagi interactions was developed.

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The model predicts performance and FEC for different infection intensities.

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It performs well when validated against published data.

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It does not account for calf genotypic variation.

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A future aim is to develop a stochastic model to account for between host variation.

A dynamic, deterministic model was developed to investigate the consequences of parasitism with Ostertagia ostertagi, the most prevalent and economically important gastrointestinal parasite of cattle in temperate regions. Interactions between host and parasite were considered to predict the level of parasitism and performance of an infected calf. Key model inputs included calf intrinsic growth rate, feed quality and mode and level of infection. The effects of these varied inputs were simulated on a daily basis for key parasitological (worm burden, total egg output and faecal egg count) and performance outputs (feed intake and bodyweight) over a 6 month grazing period. Data from published literature were used to parameterise the model and its sensitivity was tested for uncertain parameters by a Latin hypercube sensitivity design. For the latter each parameter tested was subject to a 20% coefficient of variation. The model parasitological outputs were most sensitive to the immune rate parameters that affected overall worm burdens. The model predicted the expected larger worm burdens along with disproportionately greater body weight losses with increasing daily infection levels. The model was validated against published literature using graphical and statistical comparisons. Its predictions were quantitatively consistent with the parasitological outputs of published experiments in which calves were subjected to different infection levels. The consequences of model weaknesses are discussed and point towards model improvements. Future work should focus on developing a stochastic model to account for calf variation in performance and immune response; this will ultimately be used to test the effectiveness of different parasite control strategies in naturally infected calf populations.

ECONOHEALTH: Placing helminth infections of livestock in an economic and social context

Livestock farming is central to global food security and to the sustainability of rural communities throughout Europe. Animal health management has a major impact on farming efficiency. Although animal health research has provided effective prevention strategies for the major endemic diseases of livestock, these strategies typically provide solutions for single infectious diseases and they are often not adequately implemented due to farm-specific constraints. We propose a concept termed "ECONOHEALTH" which aims at including the economic and social context in our understanding of the factors that drive animal health. The concept is elaborated on using the example of the major helminthic diseases of cattle in temperate climate regions (gastrointestinal nematodes, liver fluke and lungworm). By considering major diseases simultaneously and placing disease-complexes in an economic and a social context, we believe that insights will be generated upon which more integrated, situation-adapted and thus more effective prevention strategies can be devised.