The prophylactic effect of ivermectin treatment of calves, three weeks after turnout, on gastro-intestinal helminthiasis

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.

Integrated control of nematode infections in cattle: a reality? A need? A future?

Helminth infections are a major cause of production loss in cattle. Great progress has been achieved in the design of control strategies for these infections. Control is based mainly on the use of anthelmintics, and these have become more potent and easier to administer. However, the most effective control is possible only through the integration of different approaches. Moreover, an increasing number of disadvantages of chemotherapy/prophylaxis--biological, economical and environmental--have been suggested. In sheep, the high incidence of anthelmintic resistance has simply forced veterinarians/producers to adopt alternative control strategies; in cattle, no real need for deviation from the actual control programmes seems to exist. Therefore, the following questions are discussed: (1) Based on the distribution of cattle worldwide, what are the target parasites? (2) Can we continue to rely on control based mainly on the use of (highly effective) anthelmintics? (3) What are the prospects for non-chemical control? (4) Who will develop and implement integrated control systems? (5) In the case of parasite control in Western Europe, has it been efficient and can/need it be changed? (6) How can we integrate helminth control in the general design of herd disease control?

The effects of early-season treatments with doramectin on set-stocked calves naturally exposed to trichostrongyles

A study was carried out to evaluate the effects of strategic early-season treatments with doramectin on first-season grazing calves exposed to trichostrongylid infection on a naturally contaminated pasture. Two groups of cross-bred Limousine/Red Danish calves were turned out in early May on two plots that were similar with respect to size and herbage infectivity. They grazed separately until housing in early October. One of these groups was given doramectin at turnout and 10 weeks later, while the other group served as untreated controls. The results showed that the treatments significantly reduced trichostrongylid loads throughout the season, as evidenced by significant reductions in both their Ostertagia ostertagi burdens and serum pepsinogen levels compared with the controls. Furthermore, the results of herbage larval counts and post-mortem worm counts in tracer animals demonstrated that the treatment had successfully suppressed herbage infectivity on the 'treated' plot.

The efficacy and persistent anthelmintic effect of ivermectin in sheep

The direct efficacy and the long-term persistent anthelmintic effect of an oral suspension and an injectable formulation of ivermectin at a dose rate of 0.2 mg kg-1 was studied in sheep. Lambs were infected experimentally with Haemonchus contortus, Trichostrongylus vitrinus and Cooperia curticei. After 3 weeks they were treated with an oral suspension or with an injection, and reinfected with the same dose of larvae 3, 6 or 10 days after treatment. Post-mortem worm counts showed no persistent effect of the oral suspension. The injectable formulation showed an excellent persistent effect for up to 10 days against H. contortus. Reinfection with C. curticei 3 days after treatment resulted in a 64% reduction of the worm burden, but reinfection after 6 and 10 days was 100% reduced. The reduction of T. vitrinus was 46% after 3 days and 92% after 6 and 10 days.

Economic benefits of parasite control in cattle

The economic losses often associated with parasitism in cattle are universally accepted. The degree of economic significance related to these losses, the burdens of parasites required to cause such losses, and the specific control measures needed to avoid these losses are, however, topics of serious debate and outright disagreement among parasitologists and veterinarians. This is because most cattle have parasite burdens that are truly subclinical, with no obvious signs of parasitism but significant losses in potential production. These losses are often very insidious in nature and often difficult to prove statistically in a consistent manner. This discussion focuses on the need to: (1) define the benefits related to parasite control in cattle; (2) develop clear and effective parasite control recommendations that will allow the producer to realize these benefits; (3) effectively transfer this information to the cattleman.

Persistent anthelmintic activity of abamectin injection in cattle

The duration of the anthelmintic activity of abamectin administered by subcutaneous injection at 200 microg/kg was determined using induced infections of Haemonchus placei, Ostertagia ostertagi, Cooperia spp., and Oesophagostomum radiatum in cattle. Twenty-four Friesian calves, raised under worm-free conditions, were assigned to four groups of six animals using a randomised block design based on bodyweight. One group acted as untreated controls and the other three were treated with abamectin 14, 10 or 7 days before infection. At slaughter, 50-52 days after treatment (36-38 days after infection), efficacy was greater than 98-99% against all the parasite species for the groups challenged 10 and 7 days after treatment, whereas no significant activity could be demonstrated 14 days after treatment.

Dictyocaulus viviparus in calves: effect of rotational grazing on the development of infections

A study was made of the possibility of reducing lungworm infections in young grazing calves by rotational grazing for weekly periods on six paddocks. For this purpose three groups of four calves each were grazed on separate pastures in 1989, whereas a fourth group served as a permanently housed control group. Two groups of calves were infected experimentally with six doses of 10 larvae of Dictyocaulus viviparus during the first 3 weeks on pasture. In the third group, low natural infections with overwintered larvae occurred. One of the experimentally infected groups was rotationally grazed for weekly periods on six small plots while both other groups were set-stocked. Faecal larval counts and worm counts in tracer calves demonstrated lower lungworm infections in the rotationally grazed group than in both set-stocked groups. However, the numbers of worms found after challenge infection and subsequent necropsy were relatively high in the rotationally grazed group, indicating that development of immunity was less than in both other groups. Owing to the dry weather conditions in the summer of 1989, no serious clinical signs of husk developed in any of the three groups. These dry conditions, however, did not prevent the build-up of heavy pasture infectivity with gastrointestinal nematodes resulting in heavy worm burdens and serious clinical signs in tracer calves grazing for 4 days in August and September-October, respectively. This implies that rotational grazing did not have a clear effect on build-up of gastrointestinal nematode infections.

Three treatments with ivermectin in year-long control of gastrointestinal nematode parasites of weaner-yearling beef cattle

Four groups of 17 crossbred beef weaners were used in an experiment which extended from 14 November 1985 to 8 October 1986 (328 days). All groups began grazing on separate, contaminated pastures at a stocking rate of 5.3 cattle ha-1 and the different treatments were: Group 1, ivermectin (IVM) injectable X 1 (200 micrograms kg-1) on 14 November only, with provision for individual salvage treatment; Group 2, IVM X 3 on 14 November, 4 February and 2 July; Group 3, IVM X 2 on 14 November and 2 July; Group 4, fenbendazole (FBZ) paste X 2 (5 mg kg-1) on 14 November and 2 July. Pairs of parasite-free tracer calves were grazed on all group pastures for 1 month at the beginning of the experiment (13 November-12 December and in spring (1 April-1 May). Yearling cattle from each group were randomly selected and removed from pasture during spring (n = 2 per group, 3 April) and at the end of the experiment (n = 3 per group, 8 October) for slaughter analysis of worm population characteristics and observation of gross pathology in the abomasum and intestinal tract. At monthly intervals, all cattle were weighed and fecal egg counts, pasture larval counts and plasma pepsinogen values were determined. The results of this investigation demonstrated that three IVM treatments of weaner-yearling beef cattle during year-long grazing, were more effective than a single IVM treatment or two treatments with IVM or FBZ in the enhancement of productivity and protection from the effects of infection with nematode parasites.

Utilisation of the persistent anthelmintic activity of ivermectin in a preventive drenching programme for dairy calves

A study was undertaken to assess whether the persistent anthelmintic effect of ivermectin in cattle would allow an extension of the drenching interval in the currently recommended preventive drenching programme for the control of gastrointestinal nematode infection in dairy calves. A comparison was made of the parasitological and production responses of treatment groups of calves, grazing replicated plots, receiving either six drenches of oxfendazole at four-weekly intervals or four subcutaneous treatments with ivermectin at six-weekly intervals. Compared with the levels of infective larvae on pasture grazed by untreated control calves, mean larval numbers on pasture grazed by ivermectin and oxfendazole treated calves were 94.3% and 98.3% lower, respectively, during the period of maximum larval availability (March-May). Mean liveweight gains (December-August) of the treated groups (101.4kg and 110.2kg respectively) were not significantly different, but both were highly significantly different from that of the untreated controls (57.4kg). Mean plasma pepsinogen levels for the ivermectin, oxfendazole and control groups over the period of maximum values (June-August) were 1.92 i.u., 1.72 i.u. and 5.53 i.u., respectively. The difference between the treated groups was not statistically significant but both were highly significant different from the control group. The present results indicate that four treatments with ivermectin (subcutaneously) at six-weekly intervals achieved a similar level of prophylactic control to that effected by six treatments with oxfendazole at four-weekly intervals.

Epidemiological patterns of gastrointestinal and lung helminth infections in grazing calves in The Netherlands

The patterns of gastrointestinal and lung helminth infections in grazing calves were studied in two small grazing experiments carried out between May and October in 1985 and 1986, respectively, at Utrecht State University. For lungworm a very rapid translation of infective larvae was seen in both experiments, resulting in a rapid build-up of the worm burdens, and in July the second generation of Dictyocaulus caused husk. After the end of patency for lungworm, in the beginning of August, a rapid decrease in pasture infectivity, measured by pasture larval counts and worm counts of tracer calves, was observed. For the gastrointestinal nematodes the translation of infective larvae was much slower as a distinct 'midsummer increase' of pasture infectivity was not seen before the end of July. The worm counts of the tracer calves and the pasture larval counts showed that these larvae remained on pasture much longer than the lungworm larvae. As in Dictyocaulus, patency stopped in August-September in Nematodirus but not in Ostertagia and Cooperia, indicating a slower rate of development of immunity in both latter genera. In contrast to lungworm, the results indicate a clear and predictable seasonal pattern for gastrointestinal helminth infections.

The prophylactic effect of ivermectin treatments on gastrointestinal helminthiasis of calves turned out early on pasture or late on mown pasture

A study was undertaken to ascertain the prophylactic effect on gastrointestinal helminthiasis of (1) a single ivermectin treatment of calves 3 weeks after a late turnout on mown pasture and (2) two ivermectin treatments of calves 3 and 8 weeks after an early turnout. The single ivermectin treatment after a late turnout on mown pasture appeared to be an effective control measure for infections of Cooperia and, in particular, Ostertagia. The two treatments after an early turnout were also effective, although less so than the single treatment combined with the late turnout on mown pasture. Ivermectin treatment had no effect on the faecal egg output or the pasture larval counts of Nematodirus helvetianus. The proportion of inhibited early fourth stage larvae of Ostertagia was much higher in the control group than in either of the experimental groups.