Production, parasitological and carcase evaluation studies in steers exposed to trichostrongyle infection and treated with a morantel bolus or fenbendazole in two consecutive grazing seasons

Friesian steers were exposed to natural trichostrongyle infection and left untreated (controls) or treated with either a morantel sustained release bolus or fenbendazole at two week intervals (clean) in each of two consecutive grazing seasons. In the second grazing season the mean liveweight gains of the clean, morantel treated and control groups of cattle over 152 days were 105, 131 and 109 kg respectively (morantel treated greater than clean, control P less than 0.02). The cattle were slaughtered following an indoor fattening period. The dressed carcase weight, killing out percentage and related carcase measurements were superior in the treated groups. Eighty per cent of the clean and morantel treated cattle graded satisfactorily and qualified for subsidy payment whereas less than 30 per cent of the control cattle were successful. Rib joint analyses showed significantly lower total weight, eye muscle weight and area together with a higher bone content in control cattle.

Metabolism and growth in housed calves given a morantel sustained release bolus and exposed to natural trichostrongyle infection

Three groups of calves, previously grazed on permanent pastures contaminated with bovine trichostrongyle (Ostertagia and Cooperia species) infective larvae, were housed from October to May. During the grazing season one group had received fenbendazole at fortnightly intervals to suppress trichostrongyle infections, one received a morantel sustained release bolus before grazing to limit trichostrongyle contamination of the pasture, and the control group was only medicated when heavy infections caused clinical type 1 ostertagiasis. Digestibility of the whole diet was poorer in the control cattle, particularly for dry matter, crude protein and energy fractions. Balance studies conducted throughout the winter housed period showed that both increased faecal and urinary nitrogen outputs in the control animals contributed to a significantly reduced overall nitrogen retention. The effects were most apparent during clinical type 2 ostertagiasis, which occurred in March in the control group.

Effectiveness of a slow-release, morantel tartrate anthelmintic bolus for controlling parasitic gastroenteritis in replacement dairy heifers

Two experiments were done to evaluate the effectiveness of a slow-release, morantel tartrate, anthelmintic bolus for controlling parasitic gastroenteritis in replacement dairy heifers. In experiment 1, good control of nematode contamination of pasture was indicated by lower worm burdens in tracer calves that grazed with 15 heifers treated orally with a single bolus and by significantly decreased numbers of nematode eggs in the feces of the 15 treated heifers, as compared with numbers of eggs in the feces of 15 nontreated control calves. Significant differences in weight gain were not seen between treated and nontreated heifers, probably due to low amounts of larval exposure, the small number of heifers evaluated, and parasitic resistance in the heifers, which may have developed as a result of prestudy exposure to contaminated pasture. In experiment 2, done 1 year after the 1st experiment, using the same pastures that were used in experiment 1, young heifers were used. Many of the heifers were parasite naive at the beginning of the experiment (ie, nematode eggs were not found before the experiment). Treatment with the bolus was effective in controlling parasitic gastroenteritis in the heifers and in controlling nematode contamination of pasture. Compared with nontreated heifers, treated heifers had significantly higher cumulative weight gains, a significant decrease in plasma pepsinogen concentrations, and a significant decrease in worm egg excretion. The number of worms acquired by tracer calves that were grazed with treated heifers was significantly less than for tracer calves that were grazed with nontreated heifers.

Trichostrongyle larval populations on herbage from grazed and ungrazed cattle pastures in south west Scotland

In the course of studies on the control of bovine parasitic gastroenteritis in south west Scotland using a morantel sustained release bolus the populations of trichostrongyle infective larvae on herbage were monitored over a two-year period. From the results of the herbage analysis and the worm burdens in parasite naive tracer calves introduced at the end of 18 months, it was clear that considerable numbers of infective larvae of Ostertagia ostertagi and Cooperia oncophora survived for at least 18 months on pastures not grazed at all. The results suggest that in south west Scotland after one, or even two, seasons with no further contamination of grazing, permanent cattle pastures cannot be assumed to be helminthologically 'safe' and that this should be taken into consideration when determining prophylactic strategies.

Field efficacy of a morantel sustained release bolus for control of gastrointestinal nematodes in yearling steers

The efficacy of a morantel sustained release bolus (MSRB) for control of gastrointestinal nematodes in yearling steers was evaluated over a 6-month grazing period commencing on 26 March 1982. Three groups of 15 steers were allotted to the following treatments: Group 1 -- one MSRB at start of trial; Group 2 -- one therapeutic dose of thiabendazole at start of trial; Group 3 -- untreated control. The treatment groups were grazed separately. Parasite egg counts (EPG), herbage larval counts, pepsinogen levels and weight gains were monitored. Every other month, sets of 2 parasite-free tracer calves were placed in the pasture grazed by each treatment group and allowed to graze for 3 weeks before being subsequently necropsied for worm counts. At the end of the trial, 6 animals from each group were also necropsied for worm counts. The MSRB treatment resulted in significantly lower egg counts, fewer infective larvae on pasture, lower pepsinogen levels and lower worm burdens in tracer calves than was the case for the untreated group, but generally the levels were not significantly different from those associated with the thiabendazole treatment. The mean weight gain for the MSRB treated steers showed a significant advantage (70.9 lb) over the untreated animals, but was not significantly different from those which received thiabendazole. Total worm counts at the end of the trial were not different from any treatment group.

Effect of the morantel sustained-release bolus, used during one grazing season, on the sensitivity of Ostertagia and Cooperia to morantel tartrate in calves

Infective 3rd-stage larvae of Ostertagia and Cooperia, obtained from the feces of nonmedicated and morantel sustained-release bolus (MSRB)-treated calves, were orally administered to 2 groups of parasite-free calves. After a 42-day maturation period, a therapeutic dose of morantel tartrate was administered to half of the calves from each group. All calves were necropsied 7 days after treatment. After comparing the nematode counts for the nonmedicated and morantel-treated calves of each group, morantel tartrate was demonstrated to be equally effective against the nonmedicated-derived and MSRB-derived nematode populations. The sensitivity of Ostertagia spp and Cooperia spp to morantel tartrate, therefore, was not diminished after use of the MSRB for a single grazing season.

Controlled field trial on the anthelmintic effectiveness of the morantel sustained-release bolus in grazing calves

The direct and indirect anthelmintic efficacies of the morantel sustained-release bolus given to calves were assessed in a 154-day controlled field trial. A permanent calf pasture (divided into 2 lots) and naturally parasitized calves were used. The medicated calves were given the bolus at the time they were placed on the pastures. Control calves did not receive anthelmintic therapy. The effectiveness of the bolus to control parasitic gastroenteritis was determined by monitoring various parasitologic determinants. The treated calves had significantly (P less than 0.01) reduced numbers of fecal nematode eggs for every posttreatment sampling period when compared with the control calves. Tracer calves, used periodically during the study to indicate pasture larval infectivities, had equivalent worm burdens at the beginning of the trial (treated vs control pasture). Tracer calves, added later in the study to the lot with treated calves, harbored 83% to 94% fewer nematodes than did their counterparts in the lot with the controls. Plasma pepsinogen concentrations, reflective of abomasal worm burden size and/or activity, were significantly higher (P less than 0.01) in the control than in the treated calves from day 54 until trial termination. At trial termination, the treated calves weighed an average of 27.8 kg more and harbored 80.9% fewer nematodes than the control calves. The morantel sustained-release bolus is an anthelmintic delivery device that has therapeutic and prophylactic antinematode activities. To achieve its optimum performance, the bolus must be used so that the epizootiologic patterns of the predominate parasitic nematodes are effectively disrupted. Generally, internal nematode parasitisms in the calf flourish during the animal's first springtime grazing period.(ABSTRACT TRUNCATED AT 250 WORDS)

Potential problems associated with the controlled release of anthelmintics in grazing animals

A high, constant and sustained release of drug appears to be a major requirement to avoid a late season rise in pasture infectivity that may result in production losses in calves in autumn or Type II osteragiasis in the following spring. The timing of administration is of crucial importance and will vary in regions of different epidemiology, such as between northern and southern United States. Lack of a standard and reliable technique for pasture larval counts has resulted in some negative or erratic results which are open to question. Although controlled release anthelmintics offer advantages of convenience, a comparison of the economic benefits at present favours prophylactic treatment of dairy heifers with conventional anthelmintics 3 and 6 weeks after spring turnout in northern regions of the Northern Hemisphere. There is a serious risk that boluses based on controlled release by diffusion will behave like slow decaying insecticides and select strongly for drug resistance, especially if farmers administer them in succession throughout the grazing season. There are, however, 2 features in the design of a controlled release device that in theory may minimize the risk of rapid selection for resistance: a high and constant release of anthelmintic followed by a rapid decline to zero as the device becomes exhausted. Under these conditions, the device may even prolong the useful life of an anthelmintic to which resistance has already developed. This paper was presented at Pfizer Symposium on The Application of Sustained Release Anthelmintic Dosage Forms in the Control of Parasites in Grazing Animals at the World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) 10th International Conference, 18-20 August 1983, Perth, W.A., Australia.

Grazing study in Ireland using the morantel sustained release bolus for controlling nematodiasis in calves

The morantel sustained release bolus was administered at turnout to first-season grazing calves in order to assess its efficacy in the seasonal control of infection by nematode parasites in Ireland. The pastures grazed by control calves showed a marked increase in gastrointestinal trichostrongylid infective larvae by September, while numbers of infective larvae on pasture grazed by bolus-treated calves remained at a low level throughout the grazing season. In consequence, the controls showed significantly higher worm egg counts in late season and significantly higher worm burdens (mainly Ostertagia spp) at necropsy carried out in November on representative number of principal animals selected from each group. These reduced worm burdens were attributed to the suppression of egg output during the early part of the season as a result of treatment with the morantel sustained release bolus at turnout in the spring. Pasture contamination with Dictyocaulus viviparus larvae was present on all treatment pastures. The bolus-treated calves however were subjected to an increase in D. viviparus infection which occurred on their pasture in late season after the active life of the bolus had expired. It was concluded that bolus treatment delayed (rather than prevented) the buildup of D. viviparus infection on the pasture by 60-90 days.

The use of the morantel sustained release bolus under special farm practices (deprime system) in France

The efficacy of the morantel sustained release bolus in controlling gastrointestinal and lungworm parasites when used in first-season grazing animals which followed older animals onto spring pasture (deprime system) was assessed in three trials conducted in Normandy, France. In each trial first-season grazing calves were equally allocated onto two separate but equivalent paddocks where they remained throughout the grazing season. A morantel sustained release bolus was administered to one group of animals at turnout, the other group remained as controls. The effect of the treatment on contamination of pasture (herbage larval counts and tracer worm counts), on faecal worm egg and lungworm larval counts, and on weight gain performance of the principal animals was assessed. In all three trials, worm egg output in the bolus-treated animals was substantially lower throughout the season compared with the control animals. Worm burdens of tracer calves grazing pastures of the treated cattle were also reduced compared with tracer calves grazing control pastures. Clinical parasitic gastroenteritis occurred in the control animals but not in the bolus-treated animals in one trial. Overall the bolus-treated animals outperformed the controls by a mean weight gain advantage of 10.5 kg (P less than 0.01).

Morantel sustained release bolus: a new approach for the control of trichostrongylosis in Austrian cattle

The efficacy of a morantel sustained release bolus to control gastrointestinal parasitism during two consecutive grazing seasons was assessed in replacement dairy heifers raised under field conditions in Austria. Thirty calves were randomly allotted into two equal groups and maintained throughout the 1979 summer pasturing season on adjacent pastures of equal size with identical grazing history. Twenty-four of the thirty calves used in 1979 were monitored throughout 1980, each animal remained in the same treatment group as the previous year and grazed the same pastures as the previous year. In both 1979 and 1980 the bolus was administered to each animal in the treated group one week before turnout onto spring pastures, while the control animals received no anthelmintic treatment during either year. The efficacy of the bolus was assessed by comparison of faecal worm egg counts, herbage larval counts, worm counts from tracer and principal trial animals, weight gains and by the incidence of clinical disease found in both tracer and trial animals throughout the grazing season. Milk production was also monitored for all animals in both treatment groups during the first lactation. The system of treatment used in the present study with the morantel sustained release bolus was highly effective in preventing a significant buildup of infective larvae on pasture to the end of the grazing season in both years. In 1979 worm counts of tracer calves grazed on the treated pastures in mid-August were similar to those recorded at the beginning of the season, while worm counts in the control tracers were five times the total found at the beginning of the season. In 1979 the bolus-treated calves outperformed the control group by 34.8 kg (P less than 0.001) over the 168-day grazing period. These results demonstrated that administration of the bolus to calves at the time of turnout onto spring pastures was most effective both in controlling parasites within the cattle themselves and in reducing the level of parasite contamination. During the second grazing season (1980) no beneficial effect in weight gain was observed due to the second bolus treatment. Weight gains realized during the first year, however, largely remained through the second year and the bolus-treated heifers reached breeding weight on average 19 days earlier. No difference in milk yield was observed between the treated and control animals during thier first year of lactation.

The morantel sustained release bolus in the control of gastro-intestinal nematodiasis of cattle in Switzerland (preliminary results)

The efficacy of the morantel sustained release bolus was evaluated in 20 first-season grazing calves and 92 cattle in two separate trials. All animals grazed contaminated pastures and were exposed at the time of spring turnout to a risk of infection from gastrointestinal nematodes. In the first study 45 bolus-treated and 47 nontreated control (second or third season) cattle grazed similar but separate communal pastures, while in the second study nine bolus-treated and 11 nontreated control cattle grazed together on a single pasture. Efficacy determinations were conducted by frequent observations throughout the grazing season on faecal worm egg output, serum pepsinogen levels and liveweight gain. In the older group of animals (Trial 1), small differences were recorded between treated and control animals in faecal worm egg output and levels of serum pepsinogen as monitored throughout the grazing season. A significant mean liveweight advantage of 20.27 kg (P less than 0.001) was observed in the bolus-treated group. However, unrecorded pregnancy could not be excluded with certainty as a factor responsible for weight differences. The use of the morantel sustained release bolus provided an adequate protection against parasitic gastroenteritis in the first season grazing calves (Trial 2) despite the fact that bolus-treated animals were co-mingled with control animals. Both the worm egg counts and serum pepsinogen values were lower in the bolus-treated group. The morantel sustained release bolus was well tolerated in both trials.

The effects of the use of the morantel sustained release bolus system on calves grazing a highly contaminated pasture in The Netherlands

The efficacy of the morantel sustained release bolus system for the control of parasitic gastroenteritis in calves was evaluated in the Netherlands. The effect of bolus administration to calves, either on May 1 at the beginning of the summer grazing season or on June 12 after the pasture was mown for silage, was determined in respect of liveweight gain, faecal worm egg output, serum pepsinogen values and worm burdens. In addition, pairs of tracer calves were turned out with each group every four weeks and sacrificed for worm counts in order to monitor pasture larval contamination throughout the grazing period. Compared with nontreated control calves, the worm egg outputs of the bolus-treated calves turned out (Nematodirus helvetianus), while the bolus-treated calves turned out on June 12 had a corresponding reduction of 95 and 100 percent. Over the grazing season the average weight gain of bolus-treated calves turned out on May 1 was 34 kg more than nontreated controls turned out at the same time. Difference in weight gain of the treated and control calves turned out late in the season was 10.5 kg in favour of the treated group. The postmortem worm counts were highest in the control calves turned out early and lowest in the bolus-treated calves turned out late. All tracer calves placed on the paddock grazed by control calves (turned out early) died from parasitic gastroenteritis from July onwards while those from the paddock grazed by the control calves (turned out late) died from parasitic gastroenteritis from September onwards. All the tracer calves placed on the paddocks grazed by bolus-treated calves survived until the intended slaughter date.

The control of parasitic gastroenteritis of grazing cattle in Normandy, France using the morantel sustained release bolus

The efficacy of the morantel sustained release bolus in controlling parasitic gastroenteritis in 153 first-season grazing cattle was assessed in three separate field trials conducted in Normandy, France. In each trial, comparisons were made on weight gain performance and parasitology data (faecal worm egg counts, herbage larval counts and- in two of the trials- worm counts from principal animals sacrificed at the end of the grazing season) when bolus treatment was given either at spring turnout or in mid-season in order to determine the optimum time for bolus administration. Cattle were allocated into three groups, each group maintained on a separate but equivalent paddock constructed from the division of a larger pasture. A morantel sustained release bolus was administered to one group of animals at the time of turnout and to a second group of animals in midsummer. The third group of animals in each trial remained nontreated. The effect of the treatment on the contamination of pasture, and parasite levels and weight gain of the principal trial animals was assessed. Similar results were observed in all three trials. Faecal worm egg counts were reduced during the first part of the grazing season in animals receiving the bolus at turnout compared with mid-season treated animals where egg counts followed a pattern similar to the controls until bolus treatment at which time counts abruptly dropped to a low level. Likewise, levels of infective larvae on pastures grazed by control and mid-season treated animals followed similar patterns, increasing to a high level in late summer, while larval levels on pastures grazed by early-season treated animals remained at low levels throughout most of the season. Serum pepsinogen levels, worm counts and weight gain reflected the results from faecal worm egg and herbage larval counts indicating that early-season treatment with the bolus provided the most efficient treatment time for controlling parasitic gastroenteritis throughout the grazing season. The overall mean weight gain advantage of the early-season bolus-treated animals over the controls was 37.2 kg (P less than 0.01) while the advantage of the mid-season treated animals over controls was 13.7 kg.

The use of the morantel sustained release bolus in second-season grazing cattle

The efficacy of the morantel sustained release bolus in controlling gastrointestinal helminth infection in second-season grazing cattle was investigated on a total of seven farms in southeast England. On each farm only animals which had grazing experience from the previous summer pasturing season were used. The animals on each farm were allotted into two equal groups and turned out onto divided pastures of equal size. All the cattle in one group received a bolus at turnout while the cattle in the other group remained as nontreated controls. Assessment of the efficacy of treatment was based on comparisons of faecal worm egg output, herbage larval counts, weight gain, and the incidence of clinical parasitic gastroenteritis during the grazing season. Use of the morantel sustained release bolus led to the cessation of worm egg excretion in bolus-treated cattle during the early part of the grazing season thereby preventing a subsequent buildup of infective larvae on pasture. Although the overall results from worm egg and herbage larval counts demonstrated little measurable effect of treatment, clinical parasitic gastroenteritis was prevented in two trials where control animals became clinically affected by parasitism. Overall, the bolus-treated animals outperformed the controls by a mean of 15.1 kg (P less than 0.001).

Therapeutic and prophylactic efficacy of morantel when administered directly into the rumen of cattle on a continuous basis

The efficacy of morantel tartrate when administered on a continuous basis directly into the rumen/reticulum of cattle, either by a cannula or from a specially designed device, was evaluated against gastrointestinal parasites under laboratory conditions. In two experiments the continuous infusion of morantel at rates between 0.26 and 1.04 mg/kg body weight per day using a peristaltic pump was highly efficacious in both preventing the establishment of infections and removing established infections of the following species: Ostertagia ostertagi, Haemonchus contortus, Cooperia oncophora and Trichostrongylus colubriformis. The same dose rates also prevented establishment of Dictyocaulus viviparus infection but were ineffective against established infections of this parasite. In another two experiments the continuous release of morantel tartrate for at least 60 days (at rates between 0.65 and 1.11 mg/kg body weight per day) from a specially designed intraruminal sustained release bolus (secured at each end by a polyethylene impregnated semi-permeable membrane) was highly efficacious in both preventing the establishment of infections and removing established infections of the following species: O. ostertagi, T. axei, T. colubriformis and C. oncophora.

The use of a morantel sustained release bolus in the seasonal control of parasitic gastroenteritis in second-season cattle

The efficacy of a morantel sustained release bolus was evaluated during the 1980 summer grazing season in 216 yearling, second-season cattle in nine trials in West Germany. In five trials individual farms were used, in which medicated and control groups could be grazed on adjacent pastures. In four additional trials medicated and control groups were maintained on separate farms where the sites and management practice were matched as far as possible. Faecal worm egg counts, herbage larval counts and weight gain were monitored at regular intervals throughout the season. The faecal output of gastrointestinal nematode eggs was reduced by 55.8 percent in the medicated animals during the period from May to early August, while worm egg counts were equal to that of the control animals during the later part of the season. Herbage larval counts were similar on pastures grazed by medicated and nonmedicated cattle. In the five trials where individual farms were used the average daily weight gains over the entire grazing period were 900 (+/- 222) g/animal/day in the medicated group compared with 826 (+/- 263) g/animal in the nonmedicated cattle, a non-significant advantage. No advantage for the treated over the control groups was detected in the additional 4 trials but this was attributed largely to the difficulty in obtaining comparable groups of animals in comparable grazing conditions in "matching" farms, leading to inter-farm variation.

An economic and efficacy comparison between morantel (when administered from an intraruminal bolus) and conventional anthelmintic treatment in grazing cattle

A economic and efficacy comparison in the control of parasitism in grazing cattle was conducted between conventional anthelmintic treatment applied according to various recommended regimes and an anthelmintic treatment delivered intraruminally at a sustained level from a specially designed bolus. The bolus used was the morantel sustained release bolus which has been designed to prevent the establishment of parasite infections so that, when administered at turnout to all cattle intended to graze the same pasture, parasitologically "safe" pastures could be produced and maintained for an entire grazing season. The various conventional anthelmintic treatment regimes used for the study were selected from the most commonly recommended and used systems of treatment found in commercial use throughout Europe. A total of 471 first-season grazing calves received the morantel sustained release bolus at spring turnout in 38 field trials conducted in Austria, Belgium, Denmark, France, West Germany, Ireland, the Netherlands, Norway, Sweden and the United Kingdom. In each trial, equal numbers of control and bolus-treated animals were maintained on separate halves of a divided pasture. In 25 of the 38 field trials, the control animals received tactical therapy with conventional anthelmintic only when exhibiting signs of parasitic gastroenteritis, while in the remaining 13 trials all animals in each control group received strategic anthelmintic treatment at specified intervals during the grazing season. Compared with control animals, significant reductions in faecal worm egg output of bolus-treated animals was recorded. Subsequent reductions in herbage larval contamination developed on pastures grazed by bolus-treated animals compared with control pastures so that, overall, the bolus-treated animals out-performed the control animals in all 38 trials by a mean of 16.3 kg (P less than 0.05). Labour and management costs (for animal treatment and handling) were substantially reduced in bolus-treated animals compared with animals receiving either tactical or strategic anthelmintic treatment.

A two-year study on the anthelmintic effect of a pregrazing treatment with the morantel sustained release bolus in first season grazing cattle in Sweden

During two consecutive grazing seasons, the strategic use of the morantel sustained release bolus (MSRB) was monitored each year in first season grazing calves (maintained both years on the same pasture). The object of the study was to determine the effects of bolus treatment in young calves during successive years on the level of pasture larval contamination available at the beginning of the second year, and on the pattern of pasture recontamination that occurred during the second year. The level of worm burdens acquired in the trial animals at the end of the grazing season was determined both years. Tracer worm counts were determined during the first year of the study, while faecal worm egg counts, herbage larval counts and weight gain were monitored both years. During the first year (1979) the selected pasture was equally divided into three separate paddocks. Two paddocks were grazed by control calves, while each animal grazing the third paddock received a bolus at the time of turnout. During the second year (1980) three groups of calves were used. One group was treated with the bolus and grazed the same paddock as calves treated with the bolus the previous year. The second group was treated with the bolus and grazed the control paddock from the previous year, while the third group of calves remained nontreated and grazed the remaining paddock grazed by control calves from the previous year. As a result of the bolus treatment during the first year, the number of overwintering nematode larvae recovered the following spring was considerably lower than the number recorded on the paddock grazed previously by control animals. Worm counts and efficacy calculations demonstrated that worm burdens accumulated during the season were considerably lower in the bolus treated cattle during the second year than during the first. These parasitological differences were reflected in the weight gain comparisons between the treated and control cattle. Although significantly improved weight gains were recorded for both groups of treated cattle compared with the control cattle during the second year, the treated cattle grazing the treated paddock had a greater improvement than the treated cattle grazing the control paddock from the previous year.

Effect of an anthelmintic program with morantel tartrate on the performance of beef cattle

A field trial was conducted to determine (1) the effectiveness of an anthelmintic program using morantel tartrate (MT) to control subclinical gastrointestinal parasitism among cattle on pasture and (2) the effects of the program on the performance of cattle. Two Hereford herds, each on a separate pasture with forage of comparable quality and quantity, were used. A stocking rate of .62 ha/cow-calf was maintained in both pastures. One herd consisted of 124 cows and 122 calves and served as a non-mediated control group. The second herd consisted of 126 cows and 124 calves. Cows and calves from the latter group were treated with a bolus formulation of MT at a dosage of 10 mg/kg body weight at the beginning of the trial and again 55 d later. Calves were treated again 42 d after the second treatment. Cattle were weighed individually each month and rectal fecal samples for nematode egg counts were obtained from 36 cows and 36 calves from each group at that time. These donor cattle were selected at random at the time of the first weighing; fecal samples were then obtained from the same animals throughout the study. Eggs of the Cooperia-Ostertagia-Trichostrongylus complex predominated in all fecal samplings. There was no difference in the number of nematode eggs from the two groups of cows. However, the medicated cows gained significantly more weight than the controls and had an average weight gain advantage of 28.8 kg. The medicated calves had significantly lower average egg counts and made higher weight gains than the unmedicated calves. The medicated calves had an average weight gain advantage over the control group of 11.1 kg.

Control of bovine parasitic gastroenteritis and parasitic bronchitis in a rotational grazing system using the morantel sustained release bolus

Sixty cattle (12 first season and 48 second season grazing animals) were allocated to three groups according to age and bodyweight. Each group was divided into "control" and "treated" subgroups. Before turnout, a morantel sustained release bolus (MSRB) was administered to each animal in the "treated" category. The groups were moved from field to field according to the farmer's normal rotational grazing policy. Each field was, however, divided into two equal halves, one of which was reserved for the MSRB treated cattle, while the other was used exclusively for the controls. Severe parasitic gastroenteritis occurred in the first season controls during early September, while milder disease affected the untreated animals in the smaller of the second season groups. No gastrointestinal disease was apparent in the corresponding MSRB treated cattle. A mild outbreak of parasitic bronchitis occurred in the first year controls during October; there was evidence of less sever lungworm infection in the matching MSRB treated animals. The larger second season group showed no signs of parasitic disease.

Milk production effect of a morantel tartrate treatment at calving in dairy cows with subclinical parasitism

A field trial involving 217 dairy cows from 13 herds located in four different areas of Quebec was conducted to determine the level of subclinical parasitism. The effect on milk production was evaluated on 116 cows following a single anthelmintic treatment with morantel tartrate administered at freshening. The results showed that this treatment increased milk production in nine herds, which was statistically significant at P </= 0.05 in one of the herds and decreased yields in four herds. Overall, this treatment increased milk production by an average of 255 kg per cow for the official 305 day production period. This increase was found to be not statistically significant (P>0.05).

Studies on the control of bovine ostertagiasis using a morantel sustained release bolus

The efficacy of a morantel sustained release bolus to control ostertagiasis in grazing calves was assessed. Administration of the bolus into the rumen of calves immediately before turnout on to spring pasture in May substantially reduced the level of pasture contamination with infective larvae later in the season. Compared with controls there was a 71 per cent reduction in worm burdens acquired over the entire grazing season with a mean improvement in weight gain of 24 kg. When administration was delayed until midsummer (July) a good anthelmintic effect was obtained but there was little reduction in pasture contamination levels. Worm burdens acquired over the grazing season were reduced by only 48 per cent with a mean weight gain advantage of only 9 kg compared with controls. The control obtained against Dictyocaulus viviparus by both treatments was not absolute, failing to prevent the development of mild parasitic bronchitis.

Studies on the anthelmintic effect of a sustained release pre-grazing treatment with morantel in grazing cattle in Sweden

The effect of a morantel sustained release bolus (MSRB) to control gastrointestinal parasitism in cattle grazing their first season was assessed under field conditions in Sweden. The MSRB was administered orally to calves immediately prior to turnout onto spring pastures in May 1978. The level of pasture contamination of infective larvae was decreased by 73 per cent in August and by 98 per cent in September. There was a 72 per cent reduction in worm burdens acquired over the entire grazing season. During the same period the mean improvement in weight gain was 21.5 kg per animal. Weight gains were also recorded during housing until slaughter in April 1979, i.e. 335 days after MRSB administration at which time the treated animals were on an average 17.1 kg heavier than controls.

Further studies on the anthelmintic effect of a sustained release pre-grazing treatment with morantel in grazing cattle in Sweden

The efficacy of a morantel sustained release bolus (MSRB) to control gastrointestinal parasitism in first season grazing cattle was assessed under field conditions in Sweden. The MSRB was administered orally to calves immediately prior to turnout onto spring pastures in May, 1979, or in mid-season (July). The level of pasture contamination with infective larvae was reduced by 99 per cent in September by treatment at turnout. The corresponding figure was 89 per cent when the MSRB was administered in July. There was a 95 per cent reduction in worm burdens acquired over the entire grazing season in cattle treated in the early season and a 44 per cent reduction in worm burdens of cattle treated in the mid-season. During the 132 days grazing period a 19.1 kg (p less than 0.01) mean weight gain advantage was achieved by the early season treated animals, while a 10.0 kg mean weight gain advantage was achieved by the mid-season treatment. The weight gain advantage at the end of the grazing season in the early treated cattle increased continually during the winter with a resultant mean difference of 44.4 kg (p less than 0.01) by April 1980. Over the same period the mean weight gain advantage of the mid-season treated animals gradually fell to a final of 5.5 kg.

Use of guniea pigs to assay anthelmintic resistance in ovine isolates of Trichostrongylus colubriformis

Guinea pigs infected with Trichostrongylus colubriformis were used to develop an assay for anthelmintic resistance by determination of worm burdens following treatment with test anthelmintics. To achieve comparable efficacy with the recommended dose of thiabendazole and levamisole in sheep, dose rates in guinea pigs had to be increased two to four fold. For example, thiabendazole at 100 mg/kg in guinea pigs was 96.6 per cent effective against a thiabendazole susceptible (GS) strain of T colubriformis, but had no effect against a thiabendazole-resistant (VRSG) strain. In sheep 50 mg/kg of thiabendazole would have a similar efficacy against each strain respectively. Morantel tartrate at 10 mg/kg in guinea pigs was 99 to 100 per cent effective against the GS strain but only 54 per cent effective against a morantel resistant (PF4) strain. A slope ratio assay was used to calculate the relative potency of anthelmintics by comparing efficacies against resistant strains with efficacy against the GS strain. Resistance of the VRSG strain to thiabendazole was confirmed with a relative potency for this drug of 0.047 in guinea pigs. The PF4 strain was resistant to both thiabendazole and levamisole which had relative potencies of 0.168 and 0.255 respectively. The advantages of this statistical treatment together with the cost and time savings of the guinea pig model over a conventional critical anthelmintic assay in sheep are discussed.

Five generations of selection with benzimidazole and non-benzimidazole anthelmintics against benzimidazole resistant strains of Haemonchus and Ostertagia spp in sheep

Benzimidazole resistant strains of Haemonchus contortus and Ostertagia spp were subjected to selection pressure over five laboratory generations with the recommended dose rates of either cambendazole, oxfendazole or morantel. A change in response, with larger residual worm burdens remaining after treatment at the fifth generation, was shown for both cambendazole and oxfendazole against both species of nematode. No change in response against either species are shown for morantel. The results are discussed in terms of the problem associated with inefficient removal of the adult parasites after treatment.

The isolation of a field strain of Haemonchus contortus in Queensland showing multiple anthelmintic resistance

Following the apparent failure of levamisole to control infections of Haemonchus contortus in sheep at Lawes in south eastern Queensland, a strain of this parasite was isolated at the Animal Research Institute, Yeerongpilly. This strain was used to infect sheep at Yeerongpilly and the Merrindale Research Station, Victoria where four experiments to classify the resistance pattern of the parasite were carried out. Resistance to thiabendazole was first suspected in 1969, and these experiments confirmed that resistance to this drug was still present. They also showed that a strong degree of resistance had been developed to both levamisole and morantel tartrate. Other benzimidazole anthelmintics and also the organophosphorus compound naphthalophos were only moderately effective against the original isolate but rafoxanide, nitroxynil and phenothiazine were almost 100% effective. Other highly effective chemicals were disophenol and closantel. After passaging the strain for four generations with both levamisole and albendazole, resistance to both naphthalophos and the newer benzimidazole anthelmintics increased dramatically. This is the first report of a field strain of H. contortus exhibiting resistance to benzimidazole, non-benzimidazole and organophosphorus anthelmintics.

Trichostrongylus colubriformis and Ostertagia circumcincta resistant to levamisole, morantel tartrate and thiabendazole: occurrence of field strains

Field strains of Trichostrongylus colubriformis and Ostertagia circumcincta, designated PF4 and PF5 respectively, were recovered from a farm on which the sole use of levamisole over a preceding 12 year period led to the development of anthelmintic resistance. The results of field observations and preliminary critical trials in both Merino and crossbred sheep showed that both species have varying degrees of resistance to three major anthelmintics; levamisole, morantel tartrate and thiabendazole. Mean worm count reductions for adult T colubriformis (PF4) for therapeutic doses of morantel tartrate, thiabendazole and levamisole in crossbreds were 45.7 per cent, 97.3 per cent and zero respectively, and for Merinos 80.7 per cent, 88.3 per cent and 92.0 per cent respectively. Against O circumcincta the corresponding reductions for crossbreds were 51.4 percent, 95.4 per cent and 20.3 per cent and for Merinos, 52.5 per cent, 73.1 per cent and 29.8 per cent. There was no statistically significant difference in the responses of both parasite species to either levamisole or morantel. This result suggests that resistance to the two chemically unrelated drugs may be co-inherited.

[Study of the effectiveness of morantel tartrate (Exhelm II) in bovine gastrointestinal strongylosis in Ivory Coast]

A study of the seasonal incidence of the strongyles of the bovine digestive tract, under natural climatic conditions (rainfall, humidity and temperature) and the use of Morantel Tartrate (Exhelm II, Pfizer, France) for its control was conducted, in the North of the Ivory Coast during 1976-1977. Young calves belonging to the local breeds Baoulé, N'Dama, and cross bred Zebus-taurins employed for this study were located in the East, West, North and Centre of the Korhogo area. On the basis of the results obtained it could be concluded that: the infestation rate calculated on the basis of egg output, increases during the rainy season, between April and October and decreases during the dry season namely between November and March/April; the animals treated twice or three times with Exhelm II expelled considerably less E.P.G., in comparison to that of the control group. It is suggested that this drug may be used at the dosage rate of 7,5 mg per kg live body weight under field condition at least twice in a year, at the beginning and at the end of the rainy season. A third treatment using this same anthelmintic during the rainy season does not seem to be more profitable than two treatments. As such this third treatment should not however be considered as being essential. N'Dama and Baoule of that region have mixt infection of strongyles comprising of Trichchostrongylus 38%, Oesophastomum 30%, Cooperia 17%, Bunostomum 16%, Haemonchus 15%, Hematodirus 2% and Ostertagia 0,2%.

Effectiveness of broad spectrum anthelmintics against selected strains of Trichostrongylus colubriformis

A field population of Trichostrongylus columbriformis was divided into 4 lines for exposure to selection in the laboratory. The first line was selected with 50 mg/kg thiabendazole, the second with 4 mg/kg morantel tartrate, the third with 50 mg/kg thiabendazole followed by 4 mg/kg morantel tartrate and the fourth line was not selected for drug resistance. Following at least 9 generations of selection there was no difference in LD50 OR LD95 between the unselected and single selected strains of worms. The strain selected by both tiabendazole and morantel tartrate had a significantly higher LD50 against thiabendazole, morantel tartrate and levamisole than did the other three strains. The single selected strains had LD95's of 172, 21.5 and 2.3 mg/kg for thiabendazole, morantel tartrate and levamisole respectively, compared with corresponding values of 111, 17.3 and 2.4 in the unselected strain and 124, 15.5 and 3.0 in the double selected strain. The estimated efficiency of the recommended dose of each anthelmintic against the unselected field strain was;-thiabendazole (44 mg/kg) 50% efficient, morantel tartrate (8.8 mg/kg) 76% efficient and levamisole (7.0 mg/kg) 99.9%efficient.

The control of adult parasitic nematodes of cattle with morantel tartrate

The anthelmintic efficacy of morantel tartrate at 5mg/kg bodymass was investigated in three separate controlled trials comprising 68 calves. High anthelmintic activity was established against adult Haemonchus placei, Ostertagia ostertagi, Cooperia spp. (C.pertinata and C. punctata), Bunostomum phlebotomum and Oesophagostomum radiatum.

Anthelmintic treatment of subclinical parasitism of feedlot cattle in Georgia

Forty heifer calves, 27 yearling heifers, and 64 yearling steers with naturally occurring infections of nematode parasites were treated with levamisole HCl or morantel tartrate or were not treated. Although heifer calves had much larger worm egg counts before treatment than yearling heifers, necropsies showed a smaller average number of nematodes in the calves. Both anthelmintics significantly reduced the egg counts. At the end of the experiments, 98 to 240 days after treatments, there was no significant difference among groups of cattle in respect to necropsy worm counts. An advantage of 5% in rate of gain in body weight was shown in each experiment or phase of an experiment in favor of the treated groups of cattle. Overall, an advantage of 6% in feed efficiency was calculated for the treated groups of cattle.