Correlations between numbers of enteric nematode parasites in grazing lambs

An examination was made of correlations between counts of the enteric nematodes Haemonchus, Ostertagia, Trichostrongylus and Nematodirus from 104 lambs killed over a 3-week period in 1 year. Logarithmically transformed numbers of all genera were positively correlated, which supports the hypothesis that host-mediated common factors influence numbers of all these genera. It was found that the correlations between genera parasitising the same gastrointestinal organ were significantly stronger than the correlations between genera parasitising different gastrointestinal organs, suggesting the existence of regulatory common factors that are organ-mediated. Self-cure and cross-protection may be examples of such organ-mediated factors.     

Effects of dung beetle activity on the number of nematode parasites acquired by grazing cattle.

Parasitized steers were used to contaminate 3 pastures containing dung beetle populations of different densities. Six parasite-free calves were then allowed to graze the pastures for 43 to 45 days. Two calves that grazed a pasture with dung beetle population rendered lower than normal by screening and trapping acquired 9 times more parasites than 2 calves that grazed a pasture which captured beetles were released. Two calves that grazed a pasture with a natural dung beetle population acquired 4 times more worms than the calves on pasture with the increased beetle population. The results indicated that the 2 major genera of nematodes present (Ostertagia and Cooperia) were significantly reduced in number by dung beetle activity.     

Gastrointestinal nematode parasites of sheep: a dynamic model for their effect on liveweight gain

This paper presents an individual-based model for gastrointestinal nematode parasites of sheep and includes the effect of these parasites on the liveweight performance of young sheep. Parasitism is known to affect the host animal in at least two ways. The first induces a loss of appetite in the host, which reduces pasture consumption compared with the parasite-free animal. This effect is examined in the first part of the study. The second major effect of parasitism is a reduction in the metabolic efficiency of the host which decreases nutrients available for maintenance and growth. The latter part of the paper examines the consequences of incorporating this effect on the liveweight changes in individuals in a group of sheep. Previous models addressing this issue have only given mean liveweight and worm burden changes.     

Plant bioactives for ruminant health and productivity

Plants have been used throughout history for their medicinal properties. This use has often focused on human health but plants have also been, and still are, applied in ethnoveterinary practice and animal health management. In recent times, the use of synthetic chemicals has become prevalent. Public awareness of the potential environmental and health risks associated with heavy chemical use has also increased. This has put pressure on regulatory bodies to reduce the use of chemicals in agriculture. The most striking example is the 2006 banning of antibiotics in animal feed by the European Union. Moves such as this have increased the drive to find alternatives to synthetic chemicals and research has again turned to the use of plant bioactives as a means of improving animal health. Current scientific evidence suggests there is significant potential to use plants to enhance animal health in general and that of ruminants (cattle, deer, sheep, etc.) in particular. Active areas of research for plant bioactives (particularly saponin and tannin containing plants) include reproductive efficiency, milk and meat quality improvement, foam production/bloat control and methane production. Nematode control is also a significant area of research and the evidence suggests a much broader range of phytochemicals may be effective. This review presents a summary of the literature and examines international research efforts towards the development of plant bioactives for animal health.     

Soil intake of lactating dairy cows in intensive strip grazing systems

Involuntary soil intake by cows on pasture can be a potential route of entry for pollutants into the food chain. Therefore, it appears necessary to know and quantify factors affecting soil intake in order to ensure the food safety in outside rearing systems. Thus, soil intake was determined in two Latin square trials with 24 and 12 lactating dairy cows. In Trial 1, the effect of pasture allowance (20 v. 35 kg dry matter (DM) above ground level/cow daily) was studied for two sward types (pure perennial ryegrass v. mixed perennial ryegrass–white clover) in spring. In Trial 2, the effect of pasture allowance (40 v. 65 kg DM above ground level/cow daily) was studied at two supplementation levels (0 or 8 kg DM of a maize silage-based supplement) in autumn. Soil intake was determined by the method based on acid-insoluble ash used as an internal marker. The daily dry soil intake ranged, between treatments, from 0.17 to 0.83 kg per cow in Trial 1 and from 0.15 to 0.85 kg per cow in Trial 2, reaching up to 1.3 kg during some periods. In both trials, soil intake increased with decreasing pasture allowance, by 0.46 and 0.15 kg in Trials 1 and 2, respectively. In Trial 1, this pasture allowance effect was greater on mixed swards than on pure ryegrass swards (0.66 v. 0.26 kg reduction of daily soil intake between medium and low pasture allowance, respectively). In Trial 2, the pasture allowance effect was similar at both supplementation levels. In Trial 2, supplemented cows ate much less soil than unsupplemented cows (0.20 v. 0.75 kg/day, respectively). Differences in soil intake between trials and treatments can be related to grazing conditions, particularly pre-grazing and post-grazing sward height, determining at least in part the time spent grazing close to the ground. A post-grazing sward height lower than 50 mm can be considered as a critical threshold. Finally, a dietary supplement and a low grazing pressure, that is, high pasture allowance increasing post-grazing sward height, would efficiently limit the risk for high level of soil intake, especially when grazing conditions are difficult. Pre-grazing and post-grazing sward heights, as well as faecal crude ash concentration appear to be simple and practical tools for evaluating the risk for critical soil intake in grazing dairy cows.     

Vaccines against gastrointestinal nematode parasites of ruminants

A consequence of intensive livestock production is an increase in the incidence and impact of gastrointestinal (GI) parasites. Farmers have sought to redress this shift in the natural host-parasite relationship by chemotherapy. However, with the widespread development of resistance to anthelmintics and the current impetus for sustainable agricultural practices, alternatives such as vaccines are being sought to maintain animal productivity. In this article, David Emery and Barry Wagland discuss recent advances in immunity to nematode infections of ruminants and the development of vaccines made possible by the dogged persistence and ingenuity of cadres of parasitologists who have done more than 'go through the motions'.     

Cryopreservation of the first-stage larvae of trichostrongylid nematode parasites

First stage (L1) larvae of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta can be cryopreserved in the presence of DMSO using a two-step freezing protocol involving an initial period at −80°C prior to transfer to liquid nitrogen. Thawed L1 larvae continue development in vitro producing third stage (L3) larvae that are infective to sheep when dosed per os. Establishment rates for L3 larvae grown from thawed L1 larvae were 40 and 80% for H. contortus and T. colubriformis, respectively. There was no difference in survival or infectivity between benzimidazole (BZ)-susceptible and BZ-resistant H. contortus isolates and cryopreservation caused no shift in their BZ-resistance status as indicated in an in vitro larval development assay. Cryopreservation also had no effect on the sensitivity of these isolates to the avermectins or levamisole in vitro. High survival rates (60–70%), good levels of establishment and the stability of anthelmintic resistance status of isolates indicate that little if any selection occurs during the cryopreservation process. L1 larvae of all 3 species have been successfully recovered after 16 months storage in liquid nitrogen, cultured to the L3 stage and established in sheep.     

A re-evaluation of marine nematode productivity

Nematodes are the most abundant multicellular animals in marine sediments but their role in the benthos has not been properly quantified yet. In nearly all energy-flow budgets of marine systems their annual production P is given as about nine times their mean biomass B and their part in the total energy-flow is consequently estimated as anywhere between 3 and 30% of the total (carbon) input in the benthic system. Our laboratory experiments demonstrate that nematode productivity is much higher than P/B 9 per year and may reach values of over 60 for bacterial grazers. To obtain more reliable estimates for field populations we propose a regression equation relating egg-to-egg development time T_min to temperature (t) and adult female weight (W in µg wet weight):log T_min = 2.202–0.0461 t + 0.627 log W. When multiplied by the constant biomass turnover per generation (P/B)_gen = 3, development rate 1/T_min is a good predictor of daily P/B. This method was applied to two series of field data. A rather stable community from a sublittoral mud in the North Sea had an annual P/B = 20. A less stable Aufwuchs community from Sargassum in Japan had an annual P/B = 58.     

Gastrointestinal parasites of the guenons of western Uganda

From January 1998 to December 2002, we collected 293 fecal samples from free-ranging individuals of the 4 guenon species of western Uganda, i.e., redtail guenons (Cercopithecus ascanius), blue monkeys (Cercopithecus mitis), l'hoesti monkeys (Cercopithecus lhoesti), and vervet monkeys (Cercopithecus aethiops), to quantify the prevalence of gastrointestinal parasites. Helminth eggs, larvae, and protozoan cysts were isolated by sodium nitrate flotation and fecal sedimentation. Helminth parasites were identified, and infection prevalence was determined for all 4 guenon species. Coprocultures facilitated identification of strongylate nematodes. For the most common species, the redtail guenon, we documented prevalence of protozoan parasites and examined the effect of season and host sex on infection prevalence. Six nematodes (Strongyloidesfulleborni, Oesophagostomum sp., unidentified strongyle, Trichuris sp., Streptopharagus sp., and Enterobius sp.), 1 cestode (Bertiella sp.), 1 trematode (Dicrocoeliidae), and 5 protozoans (Entamoeba coli, Entamoeba histolytica, lodameoba butschlii, Giardia lamblia, and Chilomastix mesnili) were detected. Seasonal patterns of infection were not readily apparent for any parasite species infecting redtail guenons. Although prevalence never differed between male and female guenons, only adult females were infected with Oesophagostomum sp. and S. fulleborni.     

Gastrointestinal parasites of the colobus monkeys of Uganda

From August 1997 to July 2003, we collected 2,103 fecal samples from free-ranging individuals of the 3 colobus monkey species of Uganda-the endangered red colobus (Piliocolobus tephrosceles), the eastern black-and-white colobus (Colobus guereza), and the Angolan black-and-white colobus (C. angolensis)--to identify and determine the prevalence of gastrointestinal parasites. Helminth eggs, larvae, and protozoan cysts were isolated by sodium nitrate flotation and fecal sedimentation. Coprocultures facilitated identification of helminths. Seven nematodes (Strongyloides fulleborni, S. stercoralis, Oesophagostomum sp., an unidentified strongyle, Trichuris sp., Ascaris sp., and Colobenterobius sp.), 1 cestode (Bertiella sp.), 1 trematode (Dicrocoeliidae), and 3 protozoans (Entamoeba coli, E. histolytica, and Giardia lamblia) were detected. Seasonal patterns of infection were not apparent for any parasite species infecting colobus monkeys. Prevalence of S. fulleborni was higher in adult male compared to adult female red colobus, but prevalence did not differ for any other shared parasite species between age and sex classes.     

Diversity of benzimidazole-resistance alleles in populations of small ruminant parasites

The resistance of gastro-intestinal nematodes of small ruminants (sheep and goat) to benzimidazole anthelmintic drugs seems to be linked primarily to a single mutation in the isotype 1 beta-tubulin gene. This study was carried out to investigate the origin and diversity of benzimidazole-resistance alleles in trichostrongylid nematodes. We sequenced a 550 bp fragment of the isotype 1 beta-tubulin gene from several benzimidazole-resistant Teladorsagia circumcincta populations isolated from dairy goat farms in the central and south-western France. We also sequenced the same beta-tubulin fragment from Trichostrongylus colubriformis and Haemonchus contortus populations in south-western France. We found eight benzimidazole-resistance alleles in all T. circumcincta populations studied, six in H. contortus populations, and only one in T. colubriformis populations. In most cases, only one benzimidazole-resistance allele was present in T. circumcincta and H. contortus populations, but two alleles were found in a fewer number of them. Some T. circumcincta populations shared the same benzimidazole-resistance allele whereas some others had a specific benzimidazole-resistance allele. Similar findings were obtained for H. contortus. As no parasites are introduced once the flock of dairy goat farms has been constituted, these data indicate for the three studied species that rare pre-existing benzimidazole-resistance alleles already present before the isolation of populations had been selected. On the other hand, the fact that some benzimidazole-resistance alleles were specific to one population of T. circumcincta or H. contortus, seems to be in agreement with the hypothesis of the selection of spontaneous mutations. Thus, the origin of benzimidazole-resistance alleles in trichostrongylid nematodes seems to involve primarily the selection of rare alleles and possibly of spontaneous mutations.     

Mineral content of the free-living nematode Turbatrix aceti and ruminant parasite Trichostrongylus colubriformis

1. The parasitic nematode Trichostrongylus colubriformis and free-living nematode Turbatrix aceti were examined for element content. 2. T. colubriformis contained significantly less phosphorus, potassium, manganese and boron than T. aceti. 3. T. colubriformis contained significantly more nitrogen and copper than T. aceti. 4. Comparative study of the chemical composition of parasitic vs free-living nematodes may facilitate in vitro cultivation of the parasitic forms.     

Resistance in nematode parasites of livestock to the benzimidazole anthelmintics

The benzimidazoles have served humans well in the continual battle to control nematode parasites among livestock. However, the value of these drugs has been seriously undermined in various countries, particularly in the southern hemisphere, by the development of resistance in the target parasite species. Scientists in many countries are trying to come to grips with this problem, with studies both in the laboratory and the field. However, there are shortcomings with both approaches, particularly the former, which need to be recognized. In this article, Peter Waller discusses these in relation to research into the experimental and epidemiological aspects of benzimidazole resistance.     

Stochastic and spatial dynamics of nematode parasites in farmed ruminants

Host-parasite systems provide powerful opportunities for the study of spatial and stochastic effects in ecology; this has been particularly so for directly transmitted microparasites. Here, we construct a fully stochastic model of the population dynamics of a macroparasite system: trichostrongylid gastrointestinal nematode parasites of farmed ruminants. The model subsumes two implicit spatial effects: the host population size (the spatial extent of the interaction between hosts) and spatial heterogeneity ('clumping') in the infection process. This enables us to investigate the roles of several different processes in generating aggregated parasite distributions. The necessity for female worms to find a mate in order to reproduce leads to an Allee effect, which interacts nonlinearly with the stochastic population dynamics and leads to the counter-intuitive result that, when rare, epidemics can be more likely and more severe in small host populations. Clumping in the infection process reduces the strength of this Allee effect, but can hamper the spread of an epidemic by making infection events too rare. Heterogeneity in the hosts' response to infection has to be included in the model to generate aggregation at the level observed empirically.     

Prospects for vaccines of helminth parasites of grazing ruminants

Defined molecular vaccines for several ruminant heliminth parasites are being pursued at several different laboratories. The most fruitful sources of antigen have been oncosphere surface proteins, excretory/secretory products and integral gut membrane proteins. Nematode gut membrane proteins are unconventional in that they do not come into contact with the host immune response during infection, a feature which brings advantages as well as disadvantages. The genes encoding several of the protective antigens have been cloned, but only in the case of the oncosphere surface proteins has substantial protection been reported with recombinant versions. In addition to the problem of identifying suitable expression systems, issues such as choice of adjuvant and/or the possible use of a vaccine vector have to be solved before molecular vaccines for the economically important helminths can be launched. Of the latter, it seems that vaccines for Haemonchus and Fasciola are the brightest prospects.