Monthly fluctuations of worm burdens and hypobiosis of gastrointestinal nematodes of calves in extensive management systems in the Pyrenees (Spain)

Cyclospora cayetanensis: A Perspective (2020-2023) with Emphasis on Epidemiology and Detection Methods

Cyclospora cayetanensis infections are prevalent worldwide, and the parasite has become a major public health and food safety concern. Although important efforts have been dedicated to advance toward preventing and reducing incidences of cyclosporiasis, there are still several knowledge gaps that hamper the implementation of effective measures to prevent the contamination of produce and water with Cyclospora oocysts. Some of these data gaps can be attributed to the fact that access to oocysts is a limiting factor in C. cayetanensis research. There are no animal models or in vivo or in vitro culture systems to propagate the oocysts needed to facilitate C. cayetanensis research. Thus, researchers must rely upon limited supplies of oocysts obtained from naturally infected human patients considerably restricting what can be learnt about this parasite. Despite the limited supply of C. cayetanensis oocysts, several important advances have happened in the past 3 years. Great progress has been made in the Cyclospora field in the areas of molecular characterization of strains and species, generation of genomes, and development of novel detection methods. This comprehensive perspective summarizes research published from 2020 to 2023 and evaluates what we have learnt and identifies those aspects in which further research is needed.

Integration of ITS-2 rDNA nemabiome metabarcoding with Fecal Egg Count Reduction Testing (FECRT) reveals ivermectin resistance in multiple gastrointestinal nematode species, including hypobiotic Ostertagia ostertagi, in western Canadian beef cattle

A large-scale Fecal Egg Count Reduction Test (FECRT) was integrated with ITS-2 rDNA nemabiome metabarcoding to investigate anthelmintic resistance in gastrointestinal nematode (GIN) parasites in western Canadian beef cattle. The study was designed to detect anthelmintic resistance with the low fecal egg counts that typically occur in cattle in northern temperate regions. Two hundred and thirty-four auction market-derived, fall-weaned steer calves coming off pasture were randomized into three groups in feedlot pens: an untreated control group, an injectable ivermectin treatment group, and an injectable ivermectin/oral fenbendazole combination treatment group. Each group was divided into six replicate pens with 13 calves per pen. Individual fecal samples were taken pre-treatment, day 14 post-treatment, and at monthly intervals for six months for strongyle egg counting and metabarcoding. Ivermectin treatment resulted in an 82.4% mean strongyle-type fecal egg count reduction (95% CI 67.8-90.4) at 14 days post-treatment, while the combination treatment was 100% effective, confirming the existence of ivermectin-resistant GIN. Nemabiome metabarcoding of third-stage larvae from coprocultures revealed an increase in the relative abundance of Cooperia oncophora, Cooperia punctata, and Haemonchus placei at 14 days post-ivermectin treatment indicating ivermectin resistance in adult worms. In contrast, Ostertagia ostertagi third-stage larvae were almost completely absent from day 14 coprocultures, indicating that adult worms of this species were not ivermectin resistant. However, there was a recrudescence of O. ostertagi third stage larvae in coprocultures at three to six months post-ivermectin treatment, which indicated ivermectin resistance in hypobiotic larvae. The calves were recruited from the auction market and, therefore, derived from multiple sources in western Canada, suggesting that ivermectin-resistant parasites, including hypobiotic O. ostertagi larvae, are likely widespread in western Canadian beef herds. This work demonstrates the value of integrating ITS-2 rDNA metabarcoding with the FECRT to enhance anthelmintic resistance detection and provide GIN species- and stage-specific information.

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.

Genetic parameters for resistance to trichostrongylid infection in dairy sheep

In sheep, the traditional chemical control of gastrointestinal nematode (GIN) parasites with anthelmintics has led to the widespread development of anthelmintic resistance. The selection of sheep with enhanced resistance to GIN parasites has been suggested as an alternative strategy to develop sustainable control of parasite infections. Most of the estimations of the genetic parameters for sheep resistance to GIN parasites have been obtained from young animals belonging to meat- and/or wool-specialised breeds. We present here the estimated genetic parameters for four parasite resistance traits studied in a commercial population of adult Spanish Churra dairy ewes. These involved two faecal egg counts (FECs) (LFEC0 and LFEC1) and two serum indicator traits, the anti-Teladorsagia circumcincta fourth stage larvae IgA (IgA) and the pepsinogen (Peps) levels. In addition, this study has allowed us to identify the environmental factors influencing parasite resistance in naturally infected Spanish Churra sheep and to quantify the genetic component of this complex phenotype. The heritabilities estimated for the two FECs analysed (0.12 for LFEC0 and 0.09 for LFEC1) were lower than those obtained for the examined serum indicators (0.19 for IgA and 0.21 for Peps). The genetic correlations between the traits ranged from 0.43 (Peps-IgA) to 0.82 (LFEC0-LFEC1) and were higher than their phenotypic counterparts, which ranged between 0.07 and 0.10. The heritabilities estimated for the studied traits were lower than previously reported in lambs. This may be due to the differences in the immune mechanisms controlling the infection in young (antibody reactions) and adult (hypersensitivity reactions) animals/sheep. In summary, this study demonstrates the presence of heritable variation in parasite resistance indicator traits in the Churra population studied, which suggests that genetic improvement is feasible for this complex trait in this population. However, further studies in which the experimental variables are controlled as much as possible are needed to identify the best trait that could be measured routinely in adult sheep as an indicator of parasite resistance.

Obligate larval inhibition of Ostertagia gruehneri in Rangifer tarandus? Causes and consequences in an Arctic system

Larval inhibition is a common strategy of Trichostrongylidae nematodes that may increase survival of larvae during unfavourable periods and concentrate egg production when conditions are favourable for development and transmission. We investigated the propensity for larval inhibition in a population of Ostertagia gruehneri, the most common gastrointestinal Trichostrongylidae nematode of Rangifer tarandus. Initial experimental infections of 4 reindeer with O. gruehneri sourced from the Bathurst caribou herd in Arctic Canada suggested that the propensity for larval inhibition was 100%. In the summer of 2009 we infected 12 additional reindeer with the F1 and F2 generations of O. gruehneri sourced from the previously infected reindeer to further investigate the propensity of larval inhibition. The reindeer were divided into 2 groups and half were infected before the summer solstice (17 June) and half were infected after the solstice (16 July). Reindeer did not shed eggs until March 2010, i.e. 8 and 9 months post-infection. These results suggest obligate larval inhibition for at least 1 population of O. gruehneri, a phenomenon that has not been conclusively shown for any other trichostrongylid species. Obligate inhibition is likely to be an adaptation to both the Arctic environment and to a migratory host and may influence the ability of O. gruehneri to adapt to climate change.

Management and environmental factors related to benzimidazole resistance in sheep nematodes in Northeast Spain

A survey to determine the level of parasite resistance to benzimidazoles (BZ) under field conditions was performed on 107 commercial sheep farms located in the Aragon region of northeast Spain. Resistance was measured using the discriminant dose, a simplified form of the in vitro egg hatch assay (EHA). Taking into account the spatial structure of the data, a multivariate approach was applied to management and environmental variables as well as to their relationships with BZ resistance levels compiled from each flock. Results estimated that 11% of flocks had resistant parasite populations, although we suspected the presence of BZ-resistant parasite strains in 98% of the sample. Resistance levels were more similar among the nearest flocks, suggesting a contagious spatial distribution of resistance (i.e., resistance at neighbouring farms was not independent from one another). Management variables such as frequency of deworming, grazing in private pastures and underdosing were positively related to resistance levels, whereas only the use of BZ was negatively related to resistance levels, likely because BZ were replaced by other anthelmintics in flocks where reduced BZ efficacy was suspected. In addition to climatic conditions and seasonality, land use was an environmental variable associated with observed BZ resistance levels. Generally, resistance was highest in cooler and wetter areas but was lower in flocks sampled during January-March compared to flocks sampled in April-June or October-December. Variation partitioning procedures showed that the variation of resistance explained by the effect of environmental variables was higher than management variables. The effects of both variable groups, however, highly overlapped with the spatial structure of resistant levels, which suggests that a considerable amount of the effects attributable to both variable groups may be actually due to the spatial distribution of resistance. The resistance variation explained by the spatial component suggested that other uncontrolled factors acting at short spatial scale (e.g., common management and environmental variables; the importation of resistant strains and their posterior spread across neighbouring flocks; the selection history of the worms carried out by historical management events previous to this survey; and genetic, physiological or both types of parasite population variation) could yield this contagious spatial structure of BZ resistance. Although further research is needed, both seasonal variation and the dependence of resistance levels among neighbouring flocks should be taken into account in the design of future research or observational resistance programmes to minimise spatial and temporal pseudo-replication. Thus, research would avoid biased estimations of resistance prevalence or of its relationship with putative factors.

A 3-year field evaluation of pasture rotation and supplementary feeding to control parasite infection in first-season grazing cattle—Dynamics of pasture infectivity

A 3-year grazing trial (2002-2004) was conducted on a commercial beef cattle farm in south-central Sweden to assess different methods of parasite control. This paper focuses on the dynamics of the free-living larval stages, whereas data on performance and within-host parasitological variables are presented in a complementary paper. Each year in May, 4 groups of 10 first-season grazing (FSG) steers were turned out on to separate 2ha paddocks and subjected to the following strategies: (1) spring turn-out on to pasture which had been grazed the previous year by second-season grazing (SSG) steers (paddock RT), followed by a move to aftermath (paddock AM) after 10 weeks (mid-July), (2) supplementary feeding with concentrate and hay for 4 weeks following turn-out (paddock FD), set stocked, (3) untreated control (paddock UT), set stocked and (4) anthelmintic treated control (paddock DO), set stocked. All paddocks were assigned a new set of FSG cattle each year whereas the treatments remained the same. Pasture infectivity were monitored partly by two tracer calves that grazed each paddock along with the FSG calves for 3 weeks after turn-out and prior to housing, partly by analysis of herbage samples for infective larvae (L3) that were collected from each paddock at monthly intervals between April and October. The predominant genera found were Cooperia and Ostertagia. Tracers grazing paddock RT overall harboured less worms, and in particular less Ostertagia spp., and tracers grazing paddock AM in mid-July harboured insignificant numbers of nematodes compared to tracers on the FD and UT paddocks. Although total worm counts varied between groups, smaller numbers were generally observed early in the grazing-season (May), compared to close to housing (September) when inhibited early L4 larvae were almost exclusively found. Results observed from herbage samples showed high numbers of L3 in spring before the time of turn-out, compared to around housing. In conclusion, the rotation control strategy showed promising results and provided a turn-out pasture that was 'nematode safe' to FSG cattle the following spring, whereas the feeding strategy failed as applied in this experiment.

Field efficacy of minidosed eprinomectin against Hypoderma spp. in dairy cattle

A chemoprophylactic field trial was conducted to assess the efficacy of pour-on eprinomectin applied at the approximate dose of 50 mcg/kg to dairy cattle with naturally occurring hypodermosis. Two-hundred-eleven cattle, selected from two herds with a high prevalence of Hypoderma spp. infestation, were divided in three groups: Group A (N = 71) was treated with pour-on eprinomectin at the recommended dosage of 500 mcg/kg, Group B (N = 64) at the lower dose of 50 mcg/kg, a third group (Group C, N = 76) served as untreated control group. Treatments were performed in November-December 2002 and the animals were examined for the presence of warbles in the following April and June. No larvae emerged in the treated groups, whereas a variable number of warbles (ranging from 1 to 28) were found in control animals. Adverse reactions were not observed in any animal, and only minor side effects were observed. A larger field trial carried out in the following year (1064 treated and 131 untreated control cattle) confirmed the chemoprophylactic efficacy of minidosed eprinomectin against Hypoderma spp. Administration of eprinomectin minidoses in dairy cattle is interesting because of the low costs involved and no need for milk withdrawal.

Arrested development of Ostertagia ostertagi: effect of the exposure of infective larvae to natural spring conditions of the Humid Pampa (Argentina)

The aim of this study was to determine the effect of environmental conditions and the time of exposure to the conditions required for Ostertagia ostertagi to become inhibited in development at the early fourth larval stage in the host. Two comparable experiments were conducted from September to January, experiment I in 1997-1998 and experiment II in 1999-2000. Twenty-thousand third-stage larvae (L3), freshly obtained from coprocultures, were spread in different parasite-free grass plots at the beginning of September, October and November in each experiment and exposed to environmental conditions throughout spring and early summer. Duplicate plots for each exposure period were grazed for 3 days by two dewormed tracer calves after 1, 2, 3, 4 weeks of exposure during the corresponding month, and the remaining plots were grazed for 3 days at monthly intervals until the end of the experimental period. For each month in both experiments, control animals were inoculated orally with 20,000 L3 newly recovered from coprocultures (week 0 animals; infection controls). The control and tracer calves were sacrificed and their parasite burdens analysed. The time required to obtain greater than 50% inhibited larvae (IeL4) in the tracer animals during September and October was 3 weeks, whereas during November around 60% of the parasites were inhibited after one week of exposure. During the period tested, greater than 50% inhibition was found in concurrence with a photoperiod ranging between 13 and 14 h. The highest proportion of IeL4 (75% average) in the animals was found concomitant with a 14 h 43 min photoperiod. A high correlation between the percentage of inhibition and day length was established (0.870 p < 0.001 and 0.815 p < 0.001 for experiment I and II, respectively). In both years, the capacity for developmental arrest was lost by the end of December, when the photoperiod begins to decrease, suggesting either a disappearance of the induction stimulus, or that an excess of the stimulus could block the mechanism of inhibition. The induction time was extended 2 weeks in all months tested when the coprocultures were maintained in the dark (experiment II), suggesting that accumulation of the light stimulus contributes to shortening of the induction time. The data presented here would suggest that photoperiod is a key environmental factor for the induction of hypobiosis.

Dynamics of pasture contamination by gastrointestinal nematodes of cattle under extensive management systems: proposal for strategic control

An epidemiological study of gastrointestinal nematode parasitism in beef cattle in mountainous areas of Spain was performed. The dynamics of contamination with gastrointestinal nematode larvae of Pyrenean pastures was studied over four years at five areas at different altitudes (900 m to 2100 m), grazed by animals according to traditional systems of beef cattle in mountainous areas. Grass samples were taken every two weeks and larval differentiation was performed. Worm egg counts of grazing animals were assessed in cows, heifers and calves. A consistent seasonal pattern of infective larvae on pasture through the study was observed. In hay meadows, located below 1000 m, infective larvae were found from the end of October until June of the following year. At higher altitudes (1200-2100 m), a bimodal pattern of pasture larvae contamination was observed with increases in late spring (March-June) and in late autumn (September-November). Ostertagia spp., Cooperia spp., Trichostrongylus spp., Oesophagostomum spp., and Nematodirus spp. were found, with Ostertagia spp. being the most frequently found, followed by Cooperia spp. The highest increase of larval contamination in autumn coincided with the grazing of animals in hay meadows. This elevated autumn larval population had a very important epidemiological role because these larvae remained as overwintered larvae until the following grazing season, starting the cycle of contamination of the animals.