Efficacy of a morantel sustained-release bolus for control of gastrointestinal parasites in winter-grazed steers in Mississippi

From Nov 22, 1983 through May 15, 1984, 36 crossbred steers were allotted into 3 treatment groups (12/group) and were grazed on separate 3.4-hectare pastures. On Nov 22, 1983, the steers were administered a single morantel sustained-release bolus (MSRB), orally (group 1), or a single dose of thiabendazole (TBZ; 66 mg/kg of body weight, orally; group 2), or were left untreated (group 3; controls). Animal weights, nematode egg counts in fecal specimens, and plasma pepsinogen concentrations were monitored monthly. At the termination of the study, 4 steers from each treatment group were slaughtered and necropsied and worm counts were determined. A set of parasite-free tracer calves (3/treatment group) were grazed with each treatment group for 1 month, beginning on Nov 22, 1983; a second set of tracer calves (3/group) were grazed with each treatment group for 1 month, beginning Apr 3, 1984. At the end of their respective grazing periods, tracer calves were held for 3 weeks and then were slaughtered and necropsied and their worm counts were determined. Mean nematode egg counts in fecal specimens of group 1 (MSRB treated) were significantly lower (P less than 0.05) than that of the TBZ-treated or nontreated steers. Differences in worm counts were not found between treatment groups. Differences in worm counts of tracer calves were not found among the 3 groups for November 1983 nor for April 1984. Steers treated with the MSRB had a higher mean weight gain (P less than 0.06) than did the control or TBZ-treated steers.     

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.     

Autumn acquisition of nematodes by calves given a morantel sustained-release bolus the preceding spring

Reinfection with nematodes late in the grazing season was assessed in calves treated the preceding spring with a morantel sustained-release bolus (MSRB). During an initial 155-day grazing period, MSRB-treated calves (n = 15) grazed a pasture used the preceding year for identically treated calves (MSRB pasture). Control calves (n = 15) were not given anthelmintic treatment in the spring and grazed heavily contaminated herbage for the initial 155-day period (control pasture). At the end of the initial grazing period, 3 calves from each group were killed for parasite recovery and counting, with control calves found to harbor 9.2 times more nematodes, compared with the MSRB-treated calves. Nematode counts from tracers killed periodically during the initial grazing period were of similar proportions, reflecting the much greater nematode exposure experienced by the control calves, compared with the MSRB-treated calves. At the end of the initial grazing period, 10 calves (5/group) were placed on a common, contaminated pasture after all were treated twice with fenbendazole (10 mg/kg of body weight, 7 days apart) while on concrete. The second grazing period was for 29 days, followed by a 3-day confinement on concrete. Then, the calves were killed and necropsied. During the 29-day grazing period, the MSRB-treated calves maintained their weight advantage over the control calves, and significant differences in nematode egg counts were not found between the 2 groups of calves. At necropsy, the MSRB-treated calves harbored 27.9% fewer nematodes than did the controls, indicating that prior therapeutic and prophylactic anthelmintic activities of the MSRB did not predispose the animals to enhanced acquisition of nematodes after MSRB protection.     

Evaluation of the morantel sustained release bolus in cows and calves

The efficacy of the morantel sustained release bolus (MSRB) in controlling gastrointestinal parasitism in beef cattle was assessed during the 1982 spring-autumn grazing season. Forty-eight cows and their calves were allotted to three equal groups. One group (T-1) served as a nonmedicated control group. One MSRB was administered to each calf of the T-2 group, and to each cow and calf of the T-3 group at the beginning of the study. The efficacy of the bolus was assessed by comparison of weight gain performance and parasitological data (fecal worm egg counts, herbage larval counts, worm counts from tracer and principal trial calves, and plasma pepsinogen level determinations). Though not statistically significant, treated calves from Group T-2 had a numerical mean weight gain advantage of 2.6 kg, and those from Group T-3 of 4.7 kg, over control calves. Average daily gains (ADG) for the three groups of calves were 0.69, 0.72, and 0.73 kg, respectively. Untreated cows from Group T-2 and treated cows from Group T-3 outperformed the control cows by 12.3 and 7.5 kg, respectively. Fecal worm egg counts from both groups of treated calves were significantly (P less than 0.01) lower than counts from control calves during the entire 169-day trial; notably, egg counts were reduced by 99% 28 days after MSRB administration to both groups of calves. There were no significant differences in the number of eggs counted from the three groups of cows, probably because of the very low numbers of eggs encountered. Mean total worm burdens of principal calves (six per group) necropsied at trial termination indicated a 91% (P less than 0.01) reduction in Group T-2 and an 87% reduction (P less than 0.01) in Group T-3. Worm-free tracer calves were introduced onto pastures every 28 days to monitor availability of infective larvae. The mean number of worms recovered at necropsy from tracer calves that grazed with control cattle increased as the season progressed. However, the numbers of parasites recovered each month from mid-August through mid-October from tracers that grazed pastures with treated cattle were lower (P less than 0.05) than those levels displayed at trial initiation. In addition, the mean numbers of worms from treated group tracers were lower than from the controls for each necropsy period.(ABSTRACT TRUNCATED AT 400 WORDS)     

Response of young cattle to anthelmintic treatment during autumn in a Mediterranean-type climatic environment

A worm control programme in which heifers were treated with anthelmintic on three occasions during autumn, was tested in the Mediterranean-type climatic environment of south-west Western Australia. The experiment aimed to determine if the treatments would prevent the heifers contaminating their pastures with worm eggs during autumn, thereby improving their growth performances the following winter. An attempt was made to measure the availability of infective larvae of abomasal worms on the heifers' pastures during early winter by counting the worms in steers, previously of low worm status, that grazed with the heifers from late autumn until the start of mating in mid-winter.

The anthelmintic treatments reduced the contamination of pasture for most of autumn. The treated heifers that grazed these pastures grew faster, and by the start of mating two months after the last treatment were about 22 kg heavier, than untreated heifers grazing contaminated pasture. At the end of mating six weeks later the difference was 45 kg in favour of the treated heifers. At this time half the heifers grazing contaminated pasture were treated with anthelmintic. The following month these heifers grew faster than those left untreated, but by late November they had not attained the wieght of the heifers grazing uncontaminated pasture.

The heifers that grazed uncontaminated pasture produced more calves the following autumn than did those grazing contaminated pasture. The abomasal worm counts of the steers, with a mean of about 46 000 worms, failed to reveal any difference between treatments in the availability of larvae of abomasal worms on pasture. However, it was concluded that the treatments probably exerted their effect on growth rates by reducing the number of infective larvae ingested by heifers grazing the uncontaminated pasture during winter.     

Efficacy of a morantel sustained-release bolus for the control of gastrointestinal nematodes in Florida dairy heifers

The effectiveness of a morantel sustained-release bolus (MSRB) in controlling gastrointestinal nematode infections was determined in 70 Holstein heifer calves with no prior grazing experience. Calves were allotted to 2 treatment groups and were maintained in Florida during the grazing season on divided pastures of equal area. One MSRB was administered to each calf in 1 group at the time of turnout onto spring pasture, whereas calves in the other group remained as nonmedicated controls. By the end of the grazing season, treatment resulted in significantly (P less than 0.05) lower worm burdens and significantly improved weight gains. The MSRB-treated calves were able to reach breeding weight 2 to 4 weeks earlier than did nonmedicated calves.     

Efficacy of morantel sustained release bolus against gastrointestinal nematodes in first season grazing Holstein calves

The efficacy of the morantel sustained release bolus (MSRB) in reducing gastrointestinal parasitism in first season grazing calves was evaluated during the summer--autumn grazing seasons of 1982 and 1983 in western Oregon. Each of 38 calves (1982) and 40 calves (1983) were randomly assigned to either control or treatment groups which were given MSRB on the day of turnout onto pasture. Mean worm burdens from tracer calves grazed with treated animals in 1982 and 1983 showed overall reductions of 86.4% (P greater than 0.05) and 84.3% (P less than 0.01), respectively, compared to tracers grazed with controls. Ostertagia ostertagi, Cooperia oncophora and Nematodirus helvetianus were the primary nematodes collected at necropsy. Twelve full-season 1982 tracer animals (6 treated and 6 control) indicated an 88.1% (P less than 0.05) overall reduction in mean worm burdens. Mean fecal worm egg per gram (EPG) counts of treated animals reflected a reduction of 69% (P less than 0.05) in 1982 and 90% (P less than 0.05) in 1983. Autumn inhibition of O. ostertagi was observed. In the 1982 trial the control animals showed a slight mean weight gain advantage over the treated group from Day 84 until Day 160 (trial termination) when the mean difference was 7.9 kg. The final mean weight gain advantage of treated animals in 1983 was 13.5 kg (P less than 0.05). These trials demonstrated that the MSRB was an effective anthelmintic for reducing gastrointestinal parasitism in grazing calves and for decreasing pasture larval contamination.     

A comparison of the morantel sustained-release bolus with conventional drenching for the control of gastro-intestinal nematodes in grazing calves

A comparison was made of the parasitological and production responses to the prophylactic (summer) and combined prophylactic and therapeutic (summer/autumn) administrations of the morantel intraruminal, sustained-release bolus (MSRB) with responses to conventional drenching in grazing dairy calves over an 8-month period. The MSRB was designed to release morantel tartrate at 150 mg/day for 90 days. In two concurrent trials, groups of calves grazed separate replicated pastures. In Trial 1, the combined summer (December) and autumn (March) administration of the MSRB was compared with a programme of six drenches (oxfendazole at 4.53 mg/kg) at 28-day intervals from December to May. In Trial 2, the summer administration of the MSRB was compared with a programme of three autumn drenches (oxfendazole) at 28-day intervals from March to May. There were no significant between-treatment differences in mean liveweight gain in either trial. In both trials, the low numbers of eggs passed by the MSRB-treated animals during summer (maximum mean 62 eggs per gram of faeces) were sufficiently high to produce a modest autumn larval peak (maximum 1990 larvae/kg herbage). In Trial 2, the summer use of the MSRB resulted in an 83% reduction in peak larval availability compared with that on pastures grazed by untreated calves during summer. Only the 6-drench regimen (Trial 1) prevented the accumulation of appreciable worm burdens of early 4th stage Ostertagia spp.     

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.     

Effect of nematode parasite infection on the performance of stocker cattle at high stocking rates on coastal bermudagrass pastures.

Studies were conducted during the summer of 2 successive years (1971 and 1972) to determine the effects of gastrointestinal nematode parasitism on performance failure of stocker beef steers grazed at high stocking rates on Coastal bermudagrass pastures. During each of the 2 years, 3 groups of steers were rotated at 7- to 10-day intervals on sets of three 1.63-ha pastures from mid-May to late October. (Each group was maintained on a given set of pastures.) In the 1971 program, from mid-May to early August (phase 1) each group consisted of 48 steers for a stocking rate of 29.6 steers/ha. From early August to late October (phase 2) the number of steers per group was reduced to 36 (stocking rate of 22.2 steers/ha) to improve the plane of nutrition. In the 1972 program, 2 groups were stocked at 48 steers per group (29.6 steers/ha) and a 3rd group was temporarily stocked with 58 steers (10 additional steers) to provide 2 animals each month for necropsy evaluation of worm burdens. With exception of the monthly removal of 2 steers from the 3rd group, the stocking rate in 1972 remained intact for the entire grazing season. In 1971, the 3 groups (groups 1, 2, and 3) were treated with thiabendazole (2 doses, 16 days between doses) before grazing. Parasite burdens and levels of pasture contamination were low initially and body weight gains were good during the 1st 3 months of grazing (phase 1). In the 2nd half (phase 2) of the grazing program, 1 group of steers given free-choice supplemental feed (1% of body weight/steer each day) and treated with anthelmintic (thiabendazole; 2 doses, 18 days between doses, during August) and another group, given supplemental feed only, had significantly better weight gains than did the 3rd group, not given supplemental feed nor treated with anthelmintic. There was no weight gain advantage derived from the reduced stocking rate in phase 2. Tracer calves, which were allowed to graze on pastures, developed an increased level of infection after August. Fecal egg counts in the principal steers and recovery of infective larvae from pasture paralleled the infection level in tracer calves. Necropsy evaluation of steers from groups 1, 2, and 3 at the end of the 1971 program indicated a similarity in total worm counts, but in the group not given supplemental feed nor treated with anthelmintic there was more generalized inflammation and a greater number of lesions in the abomasum (Ostertagia) and the caudal portion of the intestine (Oesophagostomum). Steers in the 3rd group also were in poorer condition. Initial worm burdens in the 3 steer groups (groups 4, 5, and 6) were low at the start of the 1972 program. One group was given freechoice supplemental feed (0.5% of body weight/steer each day) and treated with anthelmintics (thiabendazole in the 1st dose, levamisole in the 2nd dose, 12 days between doses, in May and August) and another group was given supplemental feed throughout the summer; the 3rd group was treated with anthelmintics (in May and August)...     

Use of the sustained-release morantel bolus in stocker calves in southern United States

Two groups of 21 mixed-breed heifers were wintered on separate permanent pastures. Each heifer from one group was administered a sustained-release morantel bolus on October 7 (day 0), and the other group remained as untreated controls. Body weights were determined and fecal samples were taken at 28-day intervals. At the onset of the trail and at every 56 days, 6 heifers were removed from each group for slaughter to determine the developmental stages and the number of gastrointestinal nematodes. In addition, 3 tracer calves that were free of gastrointestinal nematodes were released on each pasture for 28 days at the beginning of the trail and after the last experimental-group calves had been removed. The 6 calves slaughtered on day 0 of the trail had a mean of 5,544 gastrointestinal nematodes. Tracer calves released on day 0 and removed on day 28 of the trial acquired 31,143 and 30,530 gastrointestinal nematodes from the pastures containing the treated and control heifers, respectively. Throughout the trial, the number of nematodes in the control calves increased at each sampling date (mean, 126,168 worms), whereas the mean number of worms in the treated heifers was 45,458. Tracer calves placed in the pastures after the 168-day trail acquired significantly more worms (9,632 vs 2,899; P less than 0.05) from grazing the pastures with control heifers than from grazing the pastures with treated heifers. Counts of eggs per gram of feces were significantly different (P less than 0.01) between the 2 groups from day 28 through day 112.(ABSTRACT TRUNCATED AT 250 WORDS)     

The Development, Clinical Signs and Economic Losses of Gastrointestinal Parasitism in Feeder Cattle on Irrigated and Non-irrigated Dikeland and Upland Pastures

Investigations were carried out over three grazing seasons with parasitized and treated (control) steers on irrigated and non-irrigated upland and dikeland pastures. The stocking rate in each paddock was adjusted by either adding or removing animals so as to maintain as uniform a sward and rate of grazing as possible. Animals were weighed on and off the pastures and fortnightly during the grazing seasons.

During the first grazing season clinically normal steers shedding low numbers of gastrointestinal worm eggs contaminated the parasite-free pastures sufficiently to give rise to large residual pasture infections and clinical parasitic gastroenteritis in grazing stock during the second grazing season. Worm burdens of 100,000 to 200,000 Ostertagia ostertagi and Cooperia oncophora were established in several steers showing marked clinical signs. In spite of treatments with high dosages of thiabendazole in attempts to keep worm burdens at a minimum, there was a slow but gradual buildup of pasture infections in the paddocks grazed by the control steers over the three year period.

During the second and third grazing seasons there were significant differences in the daily rate of gain between the parasitized and control animals on both upland and dikeland pastures. The parasitized groups of steers had daily rates of gain ranging from 0.29 to 0.80 pounds less than their comparable control groups.

Under Maritime conditions, irrigation did not have a consistent effect on weight gains and development of parasitism.

     

Efficacy of the morantel sustained-release bolus in grazing cattle in North America

The efficacy of using a bolus containing morantel in a sustained-release preparation for controlling naturally acquired gastrointestinal parasitic infections in weaned calves and yearling cattle was investigated during the 1982 grazing season at selected sites in the United States and Canada. According to a common trial design under various climatic and management conditions, 10 field trials were conducted with the bolus. At the time of spring turnout, a bolus was administered to each calf or yearling in the treated group. Then, treated and control cattle grazed separate but equal areas of divided pasture(s). The epidemiologic pattern of parasitic gastroenteritis in control animals and the effect of treatment on this pattern was determined in each trial. Safety and practicality of use of the bolus also were established. When compared with untreated cattle (control), those given the bolus deposited significantly (P less than 0.05) fewer worm eggs (89% reduction) during the first 90 days of the grazing season, as well as significantly fewer (P less than 0.05) worm eggs (84% reduction) during the entire grazing season. Consequently, during the second half of the grazing season, larval populations on treated pastures remained significantly (P less than 0.05) lower (66% reduction), compared with numbers of larvae found on control pastures. For pastures grazed by treated and control cattle at trial initiation, mean worm counts recovered from tracer calves were equal, indicating comparable pasture contamination at the beginning of the grazing season.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of biological control through the daily application of spores of Duddingtonia flagrans in lambs kept under an evasive grazing system in the Netherlands

In 2004, an experiment was carried out to evaluate the effect of biological control through feeding spores of Duddingtonia flagrans on parasitic gastroenteritis in lambs, kept under an evasive grazing system. In total 66 lambs were used. Forty naturally infected 3-month old ram lambs were weaned in mid June, and divided into four groups of 10 lambs. On 21 June, G1–G4 were moved to four separate virtually clean plots, they were moved after 4 and 8 weeks to similar plots, and housed after 12 weeks to be necropsied 16 days later. The other 26 lambs had been raised helminth-free, and were used as pairs of tracer lambs. All but one of these pairs, were grazed during the last 2 weeks on each plot. The remaining pair (TA) was grazed during the last 2 weeks on pasture (30 August to 13 September) on the plot that had been grazed by G3 between 19 July and 16 August, to study inhibited development in Haemonchus contortus. All lambs were fed 200 g of concentrates daily throughout the whole period, and those of G1 and G2 were also fed 500,000 spores of D. flagrans/kg bodyweight daily.

The faecal cultures demonstrated a high reduction in yield as a result of fungal application. However, no differences between groups were seen in weight gain, faecal egg counts, pasture larval counts, worm counts and tracer worm counts. H. contortus was the dominant species, and it is obvious that the moves at 4-week intervals prevented the development of severe haemonchosis. This is in particular demonstrated by the much higher worm counts in the two TA tracer lambs grazed. Nevertheless, increases to high faecal egg counts 3 weeks after the first and second moves, indicated acquisition of infection before these moves and at least subclinical haemonchosis. This was supported with the worm counts of lambs and tracer lambs. A higher proportion of inhibited early L4 than in other tracers and than in the permanent lambs were found in the pair of TA tracer lambs. This indicates that moves to new pastures in late summer and autumn delays the onset of inhibition.     

Availability of infective larvae of parasitic nematodes of sheep grazing on Kikuyu (Pennisetum clandestinum) pastures in the winter rainfall area

Thirteen groups of 4 South African mutton Merinos grazed for 4 weeks with the flock on Kikuyu pastures and were slaughtered for total and differential worm counts at necropsy. Subsequently 12 groups of 8 week tracers grazed on the pastures and were killed for worm counts post mortem. The following were present in most sheep: Teladorsagia (syn. Ostertagia) circumcincta, Trichostrongylus axei, Trichostrongylus colubriformis, Dictyocaulus filaria and Oesophagostomum venulosum. Haemonchus contortus, Nematodirus spathiger and Trichuris skrjabini were less frequently recovered. Optimal conditions for infestation of grazing sheep occurred from June (late autumn)--October (spring) when mean temperatures in any 4 week period were less than 20 degrees C and a total of greater than 40 mm of rain fell on 8 or more separate days. When the mean temperatures exceeded 20 degrees C pastures were safe, sheep acquiring less than 1,000 worms in 4 weeks.     

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.     

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).     

Changes in gastro-intestinal helminth species diversity in lambs under mixed grazing on irrigated pastures in the tropics (French West Indies)

The development of gastro-intestinal helminth diversity was monitored in lambs grazing alone or grazing with heifers in the ratio one heifer to four lambs. Five successive cohorts of lambs were studied from January 1994 to May 1996. Each cohort of lambs grazed irrigated pastures of Pangola grass for 4 months (from weaning to 6 months of age). A total of 50 lambs was necropsied and their worms counted and identified at the end of each grazing period. Four heifers were also necropsied on one occasion. Special attention was dedicated to the identification of the most pathogenic worm, i.e. Haemonchus spp. Malate dehydrogenase polymorphism in H. contortus was studied in order to evaluate changes between cohorts and between grazing managements. The species diversity was estimated by Shannon diversity indices (main species or all species). It was higher in the mixed grazing group than in the lambs that grazed alone. Diversity increased in successive cohorts. This was due in part to the acquisition of Cooperia spp. of cattle origin. The increase in diversity in the mixed grazing lambs corresponded to the lower faecal egg excretion and better weight gains recorded previously in that group. There seemed to be no cross-transmission of H. similis found in heifers and H. contortus harboured by lambs. The latter species was not morphologically or genetically different in the lambs grazed alone or with heifers, indicating that the presence of cattle did not modify qualitatively the transmission of H. contortus.     

A new approach in the prevention of gastrointestinal parasitic infections in cattle

The efficacy of a morantel long-acting device in preventing parasitic gastroenteric infections throughout a whole year was evaluated in heifers and steer calves in south central Québec. Thirty-two calves, comprising nine Hereford steers, one Hereford heifer, fourteen Holstein crossbred steers and eight Holstein crossbred heifers, were allotted into two treatment groups and maintained throughout the grazing season on adjacent pastures resulting from equivalently dividing one original pasture. One morantel long-acting device was administered to each animal in one group at the time of turnout onto the pasture in the spring while the calves in the other group remained as nonmedicated controls. The parasitic infections incurred during the pasture season were primarily Ostertagia, Trichostrongylus, Cooperia and Nematodirus.

The following fall, after twelve calves were necropsied for worm counts, the twenty remaining ones were brought into the barn where they were kept throughout the winter with access to an outside yard. They received good quality hay and rolled barley (1 kg/head/day) up until the following May, at which time they were weighed and had fecal samples taken for egg counts. In contrast to the results observed among controls, the morantel long-acting device treatment group had an 87% reduction in fecal worm egg excretion and a 67.3 kg per calf increased weight gain after one year.

     

Sex-related susceptibility of bulls to gastrointestinal parasites.

The effects of sex and anabolic implants on fecal egg counts and pasture contamination were examined in 77 naturally infected yearling bulls, steers, and heifers of mixed beef breeds grazing the same contaminated pasture in southern Ohio in the spring of 1990. Fecal egg counts were compared in seven groups of 10-14 animals each, twice before anabolic implants and twice after implants. Comparisons were made between untreated bulls, steers, and heifers, between steers with and without implants, and between heifers with and without implants. Bulls had significantly (P < 0.01) higher fecal egg counts than steers, and counts from steers were not significantly different to those from heifers. There were no significant differences between counts from implanted and control steers or heifers. The results have important practical implications both for parasite control and for bovine parasite research. There is a need to pay special attention to young bulls when designing parasite control programs, and to distribute the sexes equally between groups when doing research trials with cattle of mixed sexes. Some previous studies on worm population dynamics and anthelmintic evaluation may need to be re-examined in the light of sex differences.