Field evaluation of the oxfendazole pulse release bolus for the chemoprophylaxis of bovine parasitic gastroenteritis: a comparison with three other control strategies

Mathematical models to assess strategies for the control of gastrointestinal roundworms in cattle 2. Validation

A computer model of the population biology of Ostertagia ostertagi in young cattle and the effect of the parasite on animal growth has been constructed. It was validated against results from field trials where worm populations and rate of growth of cattle treated with anthelmintics were compared to similar groups of untreated animals. A close correlation between observed and predicted values for faecal egg and pasture larval counts was seen in the cattle which had been treated with an oxfendazole pulse release bolus at turnout. Timings and peak values were less accurately predicted in the untreated cattle. Although predictions for live weight gain during the grazing season indicated that the model may be overestimating the potential growth rate of cattle, it is considered that the model provides a suitable tool for comparing the effectiveness of different worming programmes under farm conditions. The computer simulation also allowed examination of the underlying influences and interactions between parasitological factors, such as numbers of immature and adult worms, and animal and pasture factors, such as sward height, grass consumption and feed conversion, in the parasite's influence on animal performance.

The determination at housing of exposure to gastrointestinal nematode infections in first-grazing season calves

Various parameter estimates were assessed at housing in calves that had been exposed to gastrointestinal nematodes during a first grazing season. The analysis involved 41 groups of first grazing season (FGS) calves on 15 different farms in Belgium and comprised groups that had received chemoprophylactic treatment and untreated controls. Serum pepsinogen levels gave the clearest division between chemoprophylactic-treated calf groups (all were <2.6 U tyr), and untreated calf groups in which sub-clinical (range: 2.0-4.1 U tyr) and clinical infections (range 3.7-6.3 U tyr) occurred. There was also a tight relationship between individual pepsinogen values and adult Ostertagia burdens obtained at slaughter. In chemoprophylactic-treated groups there was a significant negative relationship between mean serum pepsinogen levels at housing and the proportion of the grazing season covered by different chemoprophylactic systems. Although only limited data on crude adult Ostertagia antigen ELISA were available, a good relationship between optical densities and estimated exposure was also found. The parasitological parameters, faecal egg counts and pasture Ostertagia larval counts at housing, and weight gain per day, gave less clear divisions among the three categories (chemoprophylaxis, sub-clinical and clinical). Distinguishing how much exposure a calf group has experienced during a first grazing season could help in designing more appropriate control measures for the FGS calves in the next year, assuring good protection and at the same time allowing sufficient exposure for the development of acquired immunity to Ostertagia, and for this serum pepsinogen is recommended.

Gastrointestinal nematode infections of first-grazing season calves in Western Europe: general patterns and the effect of chemoprophylaxis

Research on the prevention of gastrointestinal nematode infections of cattle has mainly concentrated on comparing a specific chemoprophylactic treatment system to an untreated control group on a particular farm. Here, the results from analysis of 85 studies involving over 2000 first grazing season (FGS) calves put onto pasture for at least 4 months from late spring/early summer over a 26-year period in 13 countries in Western Europe are presented. Both control and chemoprophylactic treated FGS calf groups were considered. All chemoprophylactic systems (slow- and pulse-release boli, strategic treatments) were given early in the grazing season. Two general infection levels emerged--'sub-clinical' (32 studies) and 'clinical' (53 studies). The 'sub-clinical' infections were characterised by no clinical symptoms of parasitic gastroenteritis (PGE) being observed in the control groups. Mean faecal egg counts in the 'clinical' control groups were significantly higher than those for 'sub-clinical' control groups for almost the entire season with overall peaks of 275 and 100 EPG respectively. Maximum pasture larval counts were also significantly higher in the 'clinical' control groups with 44% of 'clinical' pastures > 10,000 L3 kg(-1) dry herbage by the end of the FGS, compared to only 15% of 'sub-clinical' pastures. There was a significant positive relationship between log transformed worm burdens from tracers put onto pastures for 2 weeks and the corresponding pasture larval count. No evidence of density dependence in tracer worm burden was observed. Weight gains in the 'clinical' control groups (375 g/day) were significantly lower than those of the 'sub-clinical' control groups (530 g/day). No symptoms of PGE were observed in any of the chemoprophylactic treated groups, but in those studies with an outbreak of PGE in the control group, the treated groups had significantly higher faecal egg and pasture larval counts than treated groups in 'sub-clinical' studies. The overall weight gain in chemoprophylactic treated calves in 'clinical' studies (600 g/day) was significantly lower than the chemoprophylactic treated calves in 'sub-clinical' studies (690 g/day), and was not significantly different from the weight gain of control calves in 'sub-clinical' studies. These results indicate that on heavily infected pastures, chemoprophylaxis will prevent PGE, but calves will still suffer production losses.

Gastrointestinal nematode infections of first-season grazing calves in Belgium: general patterns and the effect of chemoprophylaxis

Comparative analyses of the patterns of gastrointestinal nematode infections of first-grazing season cattle in Belgium are presented. The analysis involves 17 studies covering a 10 year period on 13 different farms in Flanders, Belgium. In all studies the calves were divided into an untreated control group, and one or two groups treated with chemoprophylactic systems. Two general infection levels emerged-'sub-clinical' (14 studies) and 'clinical' (three studies). The 'sub-clinical' infections were characterised by no clinical signs of parasitic gastroenteritis in the untreated control groups. Mean faecal egg counts remained low (less than 200), maximum pepsinogen levels only reached about 3500 mU tyrosine, and very small reductions in overall daily weight gain were observed compared with calves given chemoprophylaxis (less than 40 g day-1). Based on these results, on these 'sub-clinical' farms, chemoprophylaxis may not have been needed. In contrast, multiple salvage treatments of the control calf groups were required in the 'clinical' infections. Even with these salvage treatments mean faecal egg counts were high (more than 300), maximum pepsinogen levels were over 5500 mU tyrosine and there was a very large reduction in overall daily weight gain (more than 300 g day-1). However, it was not possible to predict either at turnout, or during the first month afterwards whether an infection on a particular farm would develop into a 'clinical' infestation. With the present data this prediction was possible from 8 weeks (Day 56) onwards, based on faecal egg counts and pasture larval contamination. It was also possible to predict using serum pepsinogen levels on Day 84. Therefore, one possible strategy for the effective control of gastrointestinal nematode infections of calves in temperate regions would be to evaluate faecal egg counts 2 months after turnout, and then only start treatment (i.e. metaphylaxis) if required.

Prophylaxis of bovine trichostrongylidosis and dictyocaulosis in the alpine region: comparison of an early and late administration of the oxfendazole pulse release bolus to first year grazing calves

The effect of the oxfendazole pulse release bolus (OPRB) administered at turnout, in May, or in mid-July on the development of infections with gastrointestinal nematodes and lungworms in first year grazing calves was investigated with three groups of nine animals, all grazing the same pastures in the Swiss midland region. In the calves of Group A (OPRB on 21 May) less than 20 eggs per gram of faeces (e.p.g.) and less than three Dictyocaulus larvae per 10 g (1.p.10 g) of faeces were shed for a period of 126 and 140 days respectively. Towards the end of the grazing period calves of this group excreted significantly higher levels of lungworm larvae (P < 0.05) compared with the other groups. Prior to OPRB administration on 14 July, the calves of Group B developed subclinical infections with trichostrongyles and lungworms which were similar to the untreated control Group C. By 4 weeks after bolus-administration the excretion of trichostrongylid eggs and lungworm larvae declined to levels below 20 e.p.g. and six l.p.10 g respectively. Serum pepsinogen values of the calves of Groups A and B were significantly lower (P < 0.05) compared with the control animals on days 84 and 98 after turnout and reflected mainly subclinical infections. With the exception of one Group A calf developing clinical parasitic gastroenteritis (p.g.e.) at the end of the season, clinical disease was not observed in the treated calves, while seven out of nine control animals grazing on the same pastures exhibited clinical p.g.e. with mean serum pepsinogen values exceeding 4400 mU tyrosine. Calves of Groups A and B gained significantly more weight (P < 0.01) compared with Group C (+36 kg and +41 kg respectively); differences between Groups A and B were not significant. The results indicate that in mixed grazing systems of treated and untreated calves, which are often found in the alpine region, administration of the OPRB in May or July provided good results with respect to performance of the calves. However, late administration of the OPRB has the additional advantage of coverage of the period of higher infection risk with trichostrongyles and lungworms in the late season until stabling, and should therefore be recommended.

Efficacy of a spring strategic fenbendazole treatment program to reduce numbers of Ostertagia ostertagi inhibited larvae in beef stocker cattle

Efficacy of a spring strategic treatment program with fenbendazole (FBZ) to reduce the accumulation of Ostertagia ostertagi inhibited early fourth-stage larvae (EL4) was investigated in two groups of crossbred beef heifers which were 7-9 months of age and ranged in weight from 155 to 223 kg. The cattle were allocated to groups and treated on 27 April (Day 0). Group 1 calves served as nontreated controls. Group 2 calves were treated with FBZ 10% drench suspension at 5 mg kg-1 on Day 0 and with 6-day courses of FBZ free-choice mineral (to provide 5 mg kg-1 per animal) on Days 28 and 56. Each group grazed on a separate 4.9 ha pasture for 105 days to 10 August. On 10 August the cattle were taken off pasture and each original group was re-allocated to subgroups of three cattle which were treated orally with FBZ (5 mg kg-1), oxfenbendazole (OXF, 4.5 mg kg-1) or left nontreated (CONT). The cattle were necropsied on Days 120 and 121. Mean actual and cumulative fecal egg counts indicated near total suppression of egg output in the strategically treated group (high of 5.2 eggs g-1 feces (EPG) on Day 28). Egg counts of the nontreated group remained above 100 or 200 EPG to Day 63 and then decreased to less than 100. Ostertagia was the predominant genus, followed by Cooperia on most sampling dates. The largest O. ostertagi worm burdens were recovered from the CONT-CONT subgroup; numbers of EL4 ranged from 18,922 to 51,137. Reduction in numbers of EL4 in original controls, treated with FBZ or OXF in August, were low, being 60.2% and 74.3%, respectively. The numbers of O. ostertagi recovered from subgroups originally treated strategically with FBZ were generally lower than in original controls. The largest reduction in O. ostertagi numbers was in the FBZ-CONT subgroup, which was not treated in August. Percent reduction values for O. ostertagi adults, developing L4 (DL4) and EL4 were 84.6%, 96.7%, and 99.0%, respectively. Percent reduction values for adults, DL4 and EL4 in the FBZ-FBZ and FBZ-OXF subgroups were 90.7%, 61.3%, 85.6% and 92.4%, 45.5%, and 73.0%, respectively. Variability in numbers of EL4 acquired during grazing by original controls and strategically treated cattle and variability in efficacy of August treatments was evident as observed in high outlier numbers of worms in all subgroups except the FBZ-CONT subgroup. Liveweights and gains were nearly identical in the two groups on 10 August.

Effect of chemoprophylaxis with avermectins on the immune response to gastrointestinal nematodes in first-season grazing calves

The influence of chemoprophylaxis on the immune response to gastrointestinal nematodes was evaluated in three groups of ten first-season grazing calves. Group C functioned as untreated controls, whereas Group L received two treatments, one at turn-out and one 8 weeks later, with doramectin (200 micrograms kg-1) and Group H three treatments, 3, 8, and 13 weeks post turn-out, with ivermectin (200 micrograms kg-1). At housing, five calves of each group were randomly selected for necropsied for worm counts. The remaining calves were challenged with 50,000 Ostertagi and 100,000 Cooperia oncophora and necropsied 4 weeks later. During the grazing season, Group C suffered from an outbreak of parasitic gastroenteritis which necessitated two anthelmintic treatments. In both treated groups no clinical symptoms were observed and growth rates were normal. Pepsinogen levels and antibody concentrations indicated a moderate and low gastrointestinal nematode infection in Group L and Group H, respectively. The Ostertagia and Cooperia worm populations present in the calves necropsied at housing were reduced in both treated groups in comparison with Group C. In the challenged calves, egg output was significantly higher in Group H than in Group L and Group C. Ostertagia and Cooperia worm burdens after challenge were high in Group H, moderate to low in Group L and low in Group C. This study indicates that highly suppressive prophylactic programmes, such as the ivermectin 3, 8 and 13 weeks schedule, can impair immune development against gastrointestinal nematodes during the first grazing season.

Control of gastrointestinal nematodes in first-season grazing calves by two strategic treatments with doramectin

Twenty first-season Holstein crossbred calves were used in a study conducted to investigate the efficacy of two strategic treatments with doramectin (s.c. 200 micrograms kg-1 bodyweight). They were allocated in two equal groups: Group D was treated with doramectin at turn-out and 8 weeks later and Group C served as control group. Tracers were used at turn-out, in August and before housing. At monthly intervals all principal animals were weighed, and their faeces were sampled for faecal egg counts and cultured for larval identification. Serum samples were collected for determination of pepsinogen. Animals were slaughtered for worm counts 3 weeks after housing. Clinical signs of parasitic gastroenteritis (PGE) in Group C necessitated a salvage treatment with pyrantel citrate. No signs of PGE were seen in Group D. The faecal egg output remained very low (less than 50 eggs g-1) in Group D, whereas in Group C a mean geometric maximum of 352 eggs g-1 was observed. Pepsinogen levels were below 2000 mU tyrosine in Group D, compared with maximum levels of 4190 mU tyrosine in Group C. The worm counts of the tracers at housing showed an overall reduction of 94% in the total number of gastrointestinal nematodes present on the D pasture, compared with the C pasture. Worm counts of the principal animals showed an 87.8% reduction in the total worm burden. The present results show clearly that doramectin given at turn-out and at Week 8 controls PGE in calves during the first grazing season.

Field evaluation of the efficacy of the fenbendazole slow-release bolus in the control of gastrointestinal nematodes of first-season grazing cattle

The ability of fenbendazole slow release bolus (Panacur SR Bolus, Hoechst) to control gastrointestinal parasitism in calves during their first grazing season at pasture was evaluated in two field trials. The infection level on both investigated farms was low and the control animals did not develop parasitic gastroenteritis. However, it was possible to demonstrate significant differences in the parasitological and biochemical parameters between the control and treated groups during the grazing season. Faecal egg counts and blood pepsinogen levels in the control cattle at both trials sites were significantly higher than those of the bolus-treated cattle.

Efficacy of doramectin in the prevention of gastrointestinal nematode infections in grazing cattle

Two studies were performed to investigate the efficacy of doramectin in the prevention of infection with Ostertagia ostertagi and Cooperia oncophora in grazing calves. In each study, 24 parasite-naive calves were randomly allotted to two equal groups and treated with either doramectin at 200 micrograms kg-1 or saline prior to mid-season turnout (Day 0) onto contaminated pasture. Faecal egg counts were carried out twice weekly from 15 to 64 days after turnout and the cumulative faecal egg count was calculated for each group of calves. In the doramectin-treated animals, eggs first appeared in the faeces 19 days and 22 days later than in controls for Studies 1 and 2, respectively. Mean cumulative faecal egg counts over the 64 days were reduced in the doramectin-treated groups by 71% and 87% for Studies 1 and 2, respectively (P < 0.01). The potential utility of injectable doramectin in the seasonal control of gastrointestinal nematode infestations in relation to these findings is discussed.

Persistent efficacy of doramectin against experimental nematode infections in calves

Three studies were conducted involving cattle exposed to experimental nematode infections. These studies were designed to investigate the prophylactic activity of a single subcutaneous treatment of doramectin at 200 micrograms kg-1 body weight against infections of Ostertagia ostertagi, Cooperia oncophora and Dictyocaulus viviparus. For each study, parasite-naive calves were randomly allocated to either a treated or a matched control group. One group received doramectin and the other received doramectin and the other received either no treatment or an injection of saline at 1 ml per 50 kg body weight by the subcutaneous route. Thereafter, all calves received a daily oral challenge of infective larvae of the particular parasite species on test in each study. Challenge of each pair of treatment/control groups continued for periods of 14, 21 or 28 days. An interval of 14-21 days was then allowed to permit the parasites which had established to mature, after which all animals were slaughtered and their worm burdens determined using standard techniques. Geometric mean worm burdens were calculated from the log worm counts and used to estimate percentage efficacy. Accumulated burdens of C. oncophora in doramectin-treated cattle resulting from a daily challenge infection for 14 or 21 days were reduced by 99.2% and 90.7% respectively, in comparison with those of non-treated control animals. For D. viviparus, burdens were reduced by 100% and 99.9% after a 21 or 28 day challenge, respectively. The corresponding figures for O. ostertagi were 99.9% after a 21 day challenge and 93.7% after a 28 day challenge.

Pathophysiology of infection with Ostertagia ostertagi in cattle

Infection with the abomasal nematode, Ostertagia ostertagi, is an important cause of impaired productivity in young cattle in temperate parts of the world. Such losses have been associated with marked changes in feed intake, gastrointestinal function, protein, energy and mineral metabolism, and in body composition. The reduction in feed intake is an important factor in the pathogenesis of infection and may account for a large part of the difference in weight gain between ad libitum fed control and infected calves. Despite the obvious importance of inappetance, only recently has an association been made between reduced intake, altered gut motility and elevated levels of certain gastrointestinal hormones, such as gastrin. It has been suggested that the elevated gastrin levels accompanying abomasal parasitism may impair reticulo-ruminal motility and slow down abomasal emptying, leading to a stasis of ingesta and a reduction in feed intake. The rise in blood gastrin levels may also be partly responsible for the marked hyperplasia of the fundic mucosa seen in abomasal infections. Pronounced changes in protein metabolism have also been associated with Ostertagia infection. Radioisotopic studies have demonstrated increased losses of albumin into the gastrointestinal tract which are accompanied by an increase in the rate of synthesis in the liver. Dietary protein breakdown in the abomasum is also likely to be impaired, although there is evidence of a compensatory increase in protein digestion in the lower gut of parasitised calves. Increased losses of albumin are not always accompanied by increases in faecal nitrogen, suggesting that albumin is broken down and recycled as ammonia. Radioisotopic studies in animals with intestinal nematode infections have demonstrated a marked reduction in muscle protein synthesis and an increase in protein synthesis in gastrointestinal tissue. Such changes in the balance of protein synthesis are likely to be brought about by alterations in the balance of certain metabolic hormones. Marked changes in energy metabolism also accompany Ostertagia infection. Parasitised calves exhibit a marked increase in non-esterified fatty acid levels, resulting from the mobilisation of adipose tissue, and a reduction in digestive efficiency of energy, probably associated with the increase in cycling of protein through the gastrointestinal tract and the compensatory increases in protein synthesis. Mineral metabolism may also be affected although relatively little work has been conducted in cattle. Changes in body composition reflect a reduction in deposition of muscle protein and fat, and an increase in bone content and water retention.

Bioavailability and bioequivalence of veterinary drug dosage forms, with particular reference to horses: an overview

The route of administration and formulation of the dosage form affect the bioavailability (rate and extent of absorption) of a drug and may thereby influence the intensity and duration of the pharmacological effect. Location of injection site may affect the plasma concentration profile of drugs administered as aqueous suspensions or sustained release parenteral preparations (procaine penicillin G). When absorption influences the rate of elimination ('flip-flop' phenomenon), the apparent half-life of the drug will be increased (cefazolin sodium, i.m.; meclofenamic acid, p.o.). Absorption generally approximates a first-order process and either the absorption half-life or the mean absorption time (statistical moment term) will provide an estimate of the rate of absorption. The method of corresponding areas is the usual technique employed in estimating the extent of absorption (systemic availability). Inherent in this technique is the assumption that clearance of the drug remains unchanged. In horses, the time of feeding relative to oral dosing has been shown to affect systemic availability (rifampin, trimethoprim) and pattern of absorption (phenylbutazone). Oral paste formulations (trimethoprim-sulphadiazine, ivermectin) are convenient to administer, allow precision in dosage compared with powders or granules added to feed, and could provide sustained release. Assessment of bioequivalence is based on relative bioavailability, using a reference dosage form, together with a measure of the uncertainty (variance) of the estimate. Bioequivalence relies on the concept that preparations of a drug which provide essentially equivalent plasma concentration profiles should produce the same therapeutic effect.

Interaction of Ostertagia and Nematodirus species in sheep and the potential of serum fructosamine determination in monitoring gastrointestinal parasitism

Serum fructosamine concentration falls with an increase in plasma protein turnover. Since this increase is a feature of gastrointestinal parasite infection at all sites, a falling serum fructosamine level may be of general application in the interpretation of parasite-related events. The circulating fructosamine level was investigated in the lamb as an indicator of protein-losing gastroenteropathy associated with two parasites at different sites, Ostertagia circumcincta (abomasum) and Nematodirus spathiger (small intestine). Infection with 10,000 N spathiger infective larvae daily for seven weeks produced only slight clinical signs, and only a small change in serum fructosamine levels. 2500 O circumcincta L3 daily gave no clinical signs in most animals, but serum fructosamine was more strongly affected in this group. Concurrent infection with both organisms caused some degree of diarrhoea in all lambs, and a sustained fall in serum fructosamine, more accentuated than that observed in either of the single infection groups, supporting the value of serum fructosamine determination in monitoring gastrointestinal parasitism.

Field evaluation of an experimental albendazole pulse release bolus in the control of parasitic gastroenteritis in first-season grazing calves

A field experiment is described which evaluates the use of an experimental albendazole pulse release bolus (E-bolus) on gastrointestinal parasitism. The bolus was administered at turnout to 9 first-season grazing calves and 9 animals were kept as controls on separate pasture. Dry summer months accounted for low infection levels in the calves; no cases of parasitic gastroenteritis were observed. The E-bolus introduced at the start of the grazing season provided moderate control of gastrointestinal parasitism. The beneficial effect of the treatment was demonstrated by lower strongyle egg counts and significant differences in ELISA extinctions for Ostertagia, Cooperia and Dictyocaulus. However pepsinogen values were elevated in the bolus-treated group, probably because of the long interval (31 days) between the pulsed treatments. Although not significant there was a trend for the bolus-treated group to gain more weight than the controls.

Controlled release devices for the delivery of anthelmintics in cattle

Control of cattle helminths is not so much a problem of drug choice, but of drug delivery and livestock management. Several effective anthelmintics are available for domestic livestock, but their efficiency in limiting infection and disease attributed to important parasites such as Ostertagia and Haemonchus has been mainly due to good management practice and strategically timed treatment based on detailed epizootiological studies of parasite transmission. In most situations, treated animals remain fully susceptible to reinfection if continually grazed on contaminated pastures, and this is the rationale behind techniques for continuous or multiple treatment with anthelmintics. In response to these treatment requirements, the animal health industry has developed controlled release devices, or boluses, that can be implanted orally into the rumen (Fig. 1) to release anthelmintics over an extended period - either in a delayed or pulsed fashion. In this article, Gary Zimmerman and Eric Hoberg discuss same of the most promising of such devices.