Experimental Ostertagia spp. infection of sheep: development of worm populations and lesions resulting from different dose-levels of larvae

Worm-free sheep were dosed with 8 000, 64 000 or 512 000 infective Ostertagia spp. larvae and killed 4, 7, 10, 13, 18, 24 or 66 days later. Up to day 10 the worm counts ranged from 52 to 86% of the infecting dose and results suggested no substantial reduction either in rate of development or in percentage establishment with increasing dose level. However, a reduction of worm counts, which was substantial at the higher dose-rates but comparatively small at the low rate, occurred after day 10.Gross lesions in the abomasum ranged from little more than the development of a few nodules in the low-dose sheep, to a roughened congested surface with diptheritic membranes and severe oedema in the high-dose animals at days 10–24. These effects had subsided by day 66. Histopathological changes are described.The severity and extent of the reaction was related to the parasite dose-level, but not necessarily to the numbers of worms recovered post-mortem. The rate of return to normality was quicker at the lower dose rates.     

Goats and sheep as hosts for some common cattle trichostrongylids

Recently weaned kid goats, lambs and calves, raised under worm-free conditions, were dosed with infective larvae of Ostertagia spp. and Cooperia spp. derived from naturally infected cattle. Each animal received a similar mixed dose of 60 000 infective larvae. Postmortem examination four weeks after infection revealed that the overall establishment of Ostertagia spp. in the kids was markedly less than in either the calves or the lambs, while the establishment in the lambs was marginally less than in the calves. Species composition of the Ostertagia burdens at necropsy differed widely between the host species. In the calves, O. ostertagi was the dominant species (57.8–73.4%), whereas in the lambs it was less successful (12.3–51.4%), and in the goats it was present in only very small numbers (0.0–3.8%). The proportion of O. crimensis present in the Ostertagia burden of each of the lost species was inversely related to the level of O. ostertagi present. A similar level of establishment of Cooperia spp. occurred in both lambs and calves, but no Cooperia established in the goats.     

Multigeneric resistance to benzimidazole anthelmintics in four sheep flocks

Clinical parasitism associated with multigeneric resistance to benzimidazole anthelmintics was investigated in four sheep flocks during February and March 1989. The genera considered resistant were Haemonchus and Nematodirus spp. on all four farms and Trichostrongylus and Ostertagia spp. on three of the four farms. It is suggested that when investigating cases of suspected drench resistance, postdrenching worm counts are useful to help identify the resistant worm species present.     

Gastrointestinal Nematode Populations in Stabled Ewes of Rimouski Region

Nematode populations is stabled ewes of the Rimouski region were studied by means of fecal worm egg counts, fecal culture of larvae, and worm counts at necropsy. It was found that during the winter strongyle egg counts were low, Trichostrongylus eggs being most numerous, The stronglye egg counts increased following lambing and reached peak in June. Ostertagia spp was the principal contributor to this “spring-rise”, with substantial contribution from Trichostrongylus and Haemonchus contortus. The bulk of adult worm populations in winter, however, was made up of Trichostrongylus, whereas the great majority of the populations of Ostertagia spp, H. contortus and Nematodirus spp were inhibited in development at the fourth larval stage. All the worms recovered at necropsy in spring were adults, coinciding with the “spring-rise”.     

Strategic anthelmintic treatment of young cattle during summer in a Mediterranean-type climatic environment

A worm-control program utilising treatment of young grazing cattle with fenbendazole on two occasions during summer was tested in the Mediterranean-type climatic environment of south-west Western Australia. The grazing system aimed to produce steers by introducing three-month-old weaned calves to pasture in mid-winter until they were sold in early summer. Comparisons were made of the numbers of worm eggs passed on to plots by treated and untreated animals during autumn, the performances of treated and untreated cattle and the performances of calves introduced to the plots in mid-winter. The “tracer” calf technique was used to determine the availability of infective larvae on one untreated and one treated plot for each of the two years of the experiment.Treated animals deposited less Ostertagia spp. eggs on to pasture during autumn than did untreated animals in one of the two years. In both years they deposited less eggs of worm species other than Ostertagia spp. Less intestinal worms were acquired by “tracer” calves grazing treatment plots than those grazing no-treatment plots in both years, but there were no differences in the number of abomasal worms acquired.The reduction in availability of infective larvae of intestinal worms was insufficient to prevent the occurrence of parasitic gastroenteritis in calves introduced to the plots in mid-winter. The fenbendazole treatments did not confer any immediate body-weight advantage on treated animals.On both treatment and no-treatment plots, there were few infective larvae available to grazing cattle during early autumn, there was a rapid attainment of peak availability in winter and then a decline to low availability by mid-spring. In one year, infective larvae of intestinal worms (almost exclusively Cooperia spp.) increased in availability again in late spring and early summer. A high proportion of retarded worms was never a feature of the worm counts of “tracer” calves.It was concluded that the treatments may have been more effective had they been given during autumn.     

The availability of ovine infective trichostrongyle larvae on forested paddocks

A study was made to compare the numbers of infective ovine nematode parasite larvae on an open pastured block and an adjacent block forested with Pinus radiate at 200 stems per hectare. Each block consisted of two paddocks which had similar nematode parasite egg contamination. Infective larvae of six genera were recovered over a 12 month period viz., Trichostrongylus spp., Ostertagia spp., Cooperia spp., Nematodirus spp., Chabertia/Oesophagostomum spp. and Haemonchus sp. A significantly higher recovery of Trichostrongylus spp. (p<0.01) and Ostertagia spp. (p<0.05) from the pine forested block was probably due to the more favourable climatic environment under the pines.

The lower mean body weights of sheep grazing the pine block was considered to be due to the greater exposure to infective larvae and marginal undernutrition.     

Seasonal Fluctuations of Nematode Populations in Breeding Ewes and Lambs

The seasonal changes in nematode populations of a flock of sheep in the Montreal area were determined using serial fecal egg counts, fecal culture of larvae and necropsy worm counts. It was found that Ostertagia spp.,Nematodirus spp., Trichostrongylus agei, Trichostrongylus spp. and Chabertia ovina over-wintered on pasture and could initiate patent infections the following spring. The development of populations of H. contortus was typical of that seen with most of the other species and was characterized by the following series of events. In early winter when the study was started with stabled pregnant ewes, most of the populations were immature and the egg counts were low and remained so throughout the entire winter. However, in the spring, following lambing, large numbers of adult worms were seen with a consequent decrease in immatures and a sudden increase in egg counts. When the ewes and lambs were pastured together, the egg counts in ewes dropped consequent to “self-cure”, the “spring-rise” providing the major source of overwhelming infections for lambs with deaths by the end of July. As the season progressed larvae taken in by both ewes and lambs did not mature, and by early fall, most of the worm population consisted of immature forms. It appeared that H. contortus could not have more than two generations in ewes or lambs in a single grazing season.     

EFFECTS OF A PREVENTIVE AND SUPPRESSIVE CONTROL SCHEME ON THE DEVELOPMENT OF THIABENDAZOLE-RESISTANCE IN OSTERTAGIA SPP

SUMMARY Three years after the start of an experiment to assess the merits of thiabendazole (TBZ) treatment of trichostrongylid parasites in weaner sheep, field isolates of Ostertagia spp and Trichostrongylus spp were made from weaner sheep treated under one of three treatment schemes. Treatment frequencies were “nil”, “planned” (5 or 6/year) and “regular” (every 3 weeks). In addition an isolate was taken from a group of “tracer” sheep drenched with TBZ every 10 days. Resistance to TBZ was assessed using an in vitro egg hatch assay, pre- and post-treatment faecal egg counts and a controlled anthelmintic efficiency test. Pre- and post-treatment egg counts revealed the presence of TBZ-resistance in field isolates of mixed species. Egg hatch assays indicated a level of resistance for Ostertagia spp which was proportional to the frequency of TBZ treatment. The “planned”, “regular” and “tracer” strains of Ostertagia spp had resistance ratios for eggs of 4, 13 and 15 respectively when compared to the “nil” strain. In the anthelmintic efficiency assay treatment with 44 mg kg-1 and 88 mg kg-1 of TBZ removed 82 and 96% respectively of the total Ostertagia burden (adults and larvae) from the “nil” strain and 30 and 75% respectively from the “planned” strain. The same dose rates against the “regular” and “tracer” strains and additional rates of 132 or 176 mg kg-1 against the “tracer” strain failed to reduce the Ostertagia burden significantly. Intestinal Trichostrongylus spp from all isolates were fully susceptible to TBZ at 44 mg kg-1. Levamisole at 7.0 mg kg-1 was highly effective (99% reduction) against the “tracer” strain of Ostertagia.     

Egg output of Ostertagia circumcincta in sheep given single infections of varying size

The faecal egg counts of groups of sheep given 5 000, 20 000 or 40 000 infective larvae of Ostertagia circumcincta reached a peak at about 28 days after infection; the height of the peak varying directly with the number of larvae given. At 35 days the ratios of total worm burdens and the total eggs in all females amongst the groups were similar. Between 35 and 70 days post infection the faecal egg counts fell to a low level which was roughly the same regardless of the number of larvae administered. At 70 days the two groups of lambs given the two higher doses of larvae had similar worm burdens, but those given 5 000 larvae harboured about half as many. At this time the total eggs in all females was approximately the same in each group. Whatever factor, in addition to worm numbers, which may have limited egg output appeared to be ineffective until some time after the peak egg count was reached.     

CRITICAL AND CONTROLLED LABORATORY TRIALS IN SHEEP WITH TETRAMISOLE

Critical and controlled anthelmintic tests were carried out using tetramisole hydrochloride per os against adult and immature gastro-entestinal nematodes of sheep. The drug was highly effective against adult Haemonchus contortus, Ostertagia spp, Trichostrongylus axei, intestinal Trichostrongylus spp, Nematodirus spp, Oesophagostomum columbi-anum and Chabertia ovina, and against 5th stage worms of the two latter species, at 10 mg/kg bodyweight and above. Efficiency against Trichuris ovis ranged from 67 to 100% at 20 mg/kg. A dose rate of 10 mg/kg was 98 to 100% effective against 14-day old H. contortus and T. axei, and against both 7-day and 14-day old intestinal Trichostrongylus spp infections, but a rate of 15 mg/kg was required for high activity against 14-day old Ostertagia spp. Seven-day old infections of H. contortus, T. axei and particularly Ostertagia spp were more resistant, and drug activity was variable at a dose rate of 15 mg/kg. Tetramisole would appear to have good activity against 7-day and 14-day old larvae of O. columbianum at 15 mg/kg in lambs in which the development of this species tended to be retarded.     

The distribution of Nematodirus spp. occurring in sheep in New Zealand

Extract

Most outbreaks of parasitic gastro-enteritis in sheep in New Zealand have been ascribed to the presence of nematodes of the family Trichostrongylidae, and in particular the genera, Haemonchus, Ostertagia, and Trichostrongylus. However, during the period 1956–58, a number of outbreaks of disease occurred in lambs and the condition was associated with heavy Neniatodirus infestation. Although Neinatodirus spp. are commonly foundin sheep in this country, only moderate numbers of worms are usually present in any individual and the worm has been regarded as non-pathogenic. Similar views were held in the British Isles until about 1951, when an acute disease condition of lambs, associated with heavy Neniatodirus burdens, caused serious losses.     

Trichostrongylid infections in sheep after rainfall during summer in southern Australia

OBJECTIVE: To relate trichostrongylid infections acquired by sheep during summer to prevailing weather conditions. PROCEDURE: Groups of worm-free 'tracer' sheep were put onto pastures, previously contaminated with trichostrongylid eggs, for successive periods of 2 weeks from December to March. After grazing the sheep were housed for 6 weeks. Weekly worm egg counts and worm counts were used to estimate the numbers of worms acquired and related to weather conditions during the grazing period. RESULTS: No worm eggs were detected in the faeces of sheep that grazed at the end of January when only 7 mm of rainfall was recorded. At other times rainfall between 12 and 24 mm occurred and strongyle egg counts were generally either < 50 or > 150 eggs per g (epg). Mean counts of 1,100 Ostertagia and Trichostrongylus adults gave rise to mean counts of about 350 epg whereas about 6,000 Nematodirus spp were associated with mean egg counts of about 200 Nematodirus spp epg. CONCLUSIONS: Rainfall events during summer determine the numbers of trichostrongylid larvae acquired by sheep in summer but further studies are necessary before the implications for strategic control programs in southern Australia can be fully assessed.     

Genetic variation among calves in resistance to nematode parasites

Dutch Friesian male calves, which received a dose of 100 000 Cooperia spp. larvae at an age of 3 months, displayed a great variability in reaction, measured by number and length of worms, egg output, serum antibodies and liveweight gain. In an experiment with half-sib groups from randomly chosen sires, only the antibody response was significantly different between sire-groups. In a further experiment with half-sibs from two selected sires, egg output, worm numbers, worm length and antibody response differed. A third trial with half-sib groups of the same two sires was carried out under natural conditions of grazing. It is concluded that, within this breed, genetical variation exists in resistance to Cooperia spp.     

The efficacy of eprinomectin extended-release injection against naturally acquired nematode parasites of cattle,with special regard to inhibited fourth-stage Ostertagia larvae

The efficacy of eprinomectin in an extended-release injection (ERI) formulation in the treatment of cattle harboring naturally acquired nematode populations (including inhibited nematodes) was evaluated. Five studies were conducted under a similar protocol in the USA, the UK, and in Germany. All study animals were infected by grazing naturally contaminated pastures. The adequacy of pasture infectivity was confirmed by examining tracer calves prior to allocation and treatment of the study animals. The cattle were of various breeds or crosses, weighing 79–491 kg, and aged approximately 6–15 months. In each study, 20 animals were infected by grazing, and then removed from pasture and housed in a manner to preclude further nematode infections for 8–16 days until treatment. Animals were blocked based on descending pre-treatment body weight and randomly allocated to one of two treatments: ERI vehicle (control) at 1 mL/50 kg body weight or eprinomectin 5% (w/v) ERI at 1 mL/50 kg body weight (1.0 mg eprinomectin/kg). Treatments were administered once on Day 0 by subcutaneous injection in front of the shoulder. For parasite recovery and count, all study animals were humanely euthanized 14/15 days after treatment. Cattle treated with eprinomectin ERI had significantly (p < 0.05) fewer of the following nematodes than the controls with overall reduction of parasite counts of ≥94%: adult Dictyocaulus viviparus, Capillaria spp., Cooperia oncophora, Cooperia pectinata, Cooperia punctata, Cooperia surnabada, Haemonchus placei, Nematodirus helvetianus, Oesophagostomum radiatum, Ostertagia lyrata, Ostertagia ostertagi, Trichostrongylus axei, Trichostrongylus colubriformis, Trichuris discolor, Trichuris skrjabini, and Trichuris spp.; developing fourth-stage larvae of Ostertagia spp. and Trichostrongylus spp.; and inhibited fourth-stage larvae of Cooperia spp., Haemonchus spp., Nematodirus spp., Oesophagostomum spp., Ostertagia spp., and Trichostrongylus spp.     

Observations on the post-parturient rise of nematode egg-output in cattle

The nematode egg-output from 20 young cows with a grazing history of two seasons and all calving for the first time, was followed from 3 weeks before calving until 14 weeks afterwards. The egg-output was measured as the number of larvae cultured per gram of faeces (L.P.G.) accompanied by larval differentiation.In the first 2 weeks after parturition there was a marked rise in parasite egg-output mainly caused by Ostertagia spp., Trichstronglus spp., and Cooperia oncophora.A second peak of C. oncophora occured 4 weeks later. Peaks of Oesophagostomum spp., Haemonchus spp. and Bunostomum spp. were also later.Epidemiolgical consequences with respect to pasture contamination due to the post-parturient rise are possible, particularly when calving takes place in spring or early summer and the cows are kept indoors for only a few days.     

Pilot project to investigate over-wintering of free-living gastrointestinal nematode larvae of sheep in Ontario,Canada

This study investigated the overwintering survival and infectivity of free-living gastrointestinal nematode (GIN) stages on pasture. The presence of GIN larvae was assessed on 3 sheep farms in Ontario with a reported history of clinical haemonchosis, by collecting monthly pasture samples over the winter months of 2009/2010. The infectivity of GIN larvae on spring pastures was evaluated using 16 tracer lambs. Air and soil temperature and moisture were recorded hourly. Free-living stages of Trichostrongylus spp. and Nematodirus spp. were isolated from herbage samples. Gastrointestinal nematodes were recovered from all tracer lambs on all farms; Teladorsagia sp. was the predominant species. Very low levels of Haemonchus contortus were recovered from 1 animal on 1 farm. The results suggest that Haemonchus larvae do not survive well on pasture, while Teladorsagia sp., Trichostrongylus spp. and Nematodirus spp. are able to overwinter on pasture in Ontario and are still infective for sheep in the spring.     

Field studies investigating anthelmintic resistance in young cattle on five farms in New Zealand

AIM: To investigate the efficacy of pour-on anthelmintics against field strains of parasitic nematodes in young cattle on five farms in New Zealand.

METHODS: Faecal nematode egg count (FEC) reduction (FECR) tests were carried out on five calf-rearing farms using pour-on formulations of levamisole, ivermectin, eprinomectin, and the simultaneous administration of levamisole and ivermec- tin. Faecal samples were collected per rectum before treatment and about 7, 14, 21 and 28 days after treatment, for FEC and faecal nematode larval culture.

RESULTS: Resistance (i.e. <95% reduction in FEC) of Cooperia oncophora to ivermectin and eprinomectin was identified on all five farms. There was limited evidence of possible emerging resistance in Ostertagia spp to ivermectin but not eprinomectin, in short-tailed larvae of Cooperia spp to ivermectin and eprinomec- tin, and in Trichostrongylus spp to ivermectin, eprinomectin and levamisole used separately. Levamisole was effective against C. oncophora, but had variable efficacy against Ostertagia spp in the calves in this study. Simultaneous treatment with levamisole and ivermectin pour-on formulations were effective against all genera on all farms.

CONCLUSIONS: To effectively manage roundworm parasites in their calves farmers need to be aware of the resistance status of the parasites on their farms. Levamisole is likely to be an effective anthelmintic on most farms at times of the year when the impact of Ostertagia spp is not high. Simultaneous administration of levamisole and ivermectin pour-on anthelmintics to cattle is likely to control both ML-resistant C. oncophora and stages of Ostertagia spp that are not controlled by levamisole alone.     

Multigeneric Resistance to Benzimidazole Anthelmintics in Sheep

Benzimidazole resistance involving several gastrointestinal nematode genera on a single sheep farm in New Zealand is reported for the first time. A controlled slaughter trial showed that at the recommended dose rate of 12.5 mg/kg, mebendazole had efficacies of 0, 60, 66, 90, 54 and 38% against Haemonchus contortus, Ostertagia spp., Nematodirus spp., intestinal Trichostrongylus spp., Strongyloides spp. and Oesophagostomum venulosum, respectively. The relevance of such multigeneric resistance to possible future options for controlling anthelmintic resistant sheep trichostrongylids is briefly discussed.     

Dose confirmation studies for monepantel, an amino-acetonitrile derivative, against fourth stage gastro-intestinal nematode larvae infecting sheep

Monepantel is the first compound from the recently discovered amino-acetonitrile derivative (AAD) class of anthelmintics to be developed for use in sheep. Nine dose confirmation studies were conducted in Australia, New Zealand and Switzerland to confirm the minimum therapeutic oral dose of monepantel to control fourth stage (L4) gastro-intestinal nematode larvae in sheep (target species were Haemonchus contortus, Teladorsagia (Ostertagia) circumcincta, Teladorsagia trifurcata, Trichostrongylus axei, Trichostrongylus colubriformis, Trichostrongylus vitrinus, Cooperia curticei, Cooperia oncophora, Nematodirus battus, Nematodirus filicollis, Nematodirus spathiger, Chabertia ovina and Oesophagostomum venulosum). In each study, sheep infected with a defined selection of the target nematodes were treated with 2.5 mg monepantel/kg liveweight. Following euthanasia and worm counting, efficacy was calculated against worm counts from untreated control groups. The results demonstrate high (95 < 100%) efficacy of monepantel when administered orally to sheep at 2.5 mg/kg for most species tested. Efficacy levels against N. spathiger and O. venulosum were variable and failed to meet the required regulatory standard (≥90%) in some studies. Efficacy was demonstrated against L4 stages of nematodes known to be resistant to either benzimidazole and/or levamisole anthelmintics (macrocyclic lactone resistant isolates were not available for testing). The broad-spectrum activity of monepantel against L4 larvae of common gastro-intestinal nematodes in sheep and its favorable safety profile represents a significant advance in the treatment of parasitic gastro-enteritis in this animal species. No adverse effects related to treatment with monepantel were observed.     

The epidemiology and pathogenic significance of haemonchosis in a merino flock in East Africa

The epidemiology of H. contortus was studied for 21 months in 80 set-stocked Merino ewes and their grazing in an endemic area in Kenya. Observations included faecal egg counts (weekly), worm burdens (monthly), haematological indices and clinical signs. The levels of infective larvae on the pasture were estimated using tracer sheep.The classical acute form of haemonchosis was seen with equal intensity in both ewes and lambs during periods of high rainfall and self-cure was confirmed as a flock phenomenon which occured simultaneously on one occasion in sheep of all ages.Not previously recognised were the uniformity of the faecal egg counts in both ewes and lambs over long periods when the worm burdens fluctuated greatly, the marked decrease in worm burdens without the occurence of classical self-cure during periods when the intake of infective larvae was low, and the inverse relationship between the individual worm-weight and the total worm population. It was also concluded that one of the most significant features of haemonchosis is not the acute syndrome so characteristically associated with the disease, but the fact that a moderate infection of a few hundred worms persisting over a period of several months will produce chronic anaemia and, ultimately, severe loss of bodily condition and deathes in ewes and lambs grazing on poor quality pasture.