Parasite control practices on pasture-based dairy farms in the Republic of Ireland.

Dictyocaulus viviparus, Ostertagia ostertagi (nematode parasites), and Fasciola hepatica (trematode parasite) result in productivity losses on dairy farms and impact on animal health through clinical and sub-clinical disease. Parasite control in livestock systems is largely based on the use of chemoprophylactic agents (anthelmintics), grazing management, or a combination of both. The objective of this study was to document current parasite control measures employed by Irish dairy farmers in a predominantly pasture-based livestock system. A questionnaire survey of 312 geographically representative farmers was completed in 2009 with a follow up survey completed in 2011. Statistical analysis highlighted significant differences in chemoprophylactic usage between 2009 and 2011. In particular, an increase in the use of albendazole for both trematode (19% in 2009 to 36% in 2011) and nematode (30% in 2009 to 58% in 2011) control was observed. This was most likely due to flukicide restrictions introduced in the Republic of Ireland in 2010 for dairy animals. Logistic regression highlighted regional differences in chemoprophylactic use. Farmers in southern parts of Ireland, an area with good quality soil, less rainfall, and a higher density of dairy farms than other regions, were approximately half as likely to dose for F. hepatica and were more likely (OR>2.0) to use albendazole for both nematode and fluke control. Approximately 30% of respondents who used a chemoprophylactic treatment for nematodes, used a product which was 'unsuitable for purpose' (e.g. ivermectin for the treatment of F. hepatica), highlighting the need for increased awareness, continuing research, and regionally targeted education tools regarding optimal parasite control.

Efficacy of eprinomectin pour-on treatment in sheep naturally infected with Dictyocaulus filaria and Cystocaulus ocreatus.

The efficacy of eprinomectin on Dictyocaulus filaria and Cystocaulus ocreatus in naturally infected sheep was evaluated in the present study. In total, 30 infected sheep were randomly divided into two groups: treated (n = 15) and untreated (n = 15). A single pour-on dose of eprinomectin (0.5 mg/kg) was administered to the treated group. No medication was used in the untreated group. Faecal larval counts were performed on pre-treatment (day 0) and post-treatment (days 7, 14, 21 and 42) days. Eprinomectin was found to be 100% effective against D. filaria on day 7 post-treatment when compared with the untreated group and it maintained this effect on days 14, 21 and 42. However, the decrease in faecal larval count of C. ocreatus was found to be 86, 86 and 91%, on days 14, 21 and 42, respectively.

Efficacy of eprinomectin pour-on against Dictyocaulus arnfieldi infection in donkeys (Equus asinus).

A trial to assess the efficacy of eprinomectin (EPM) against the lungworm Dictyocaulus arnfieldi was carried out on 15, naturally-infected donkeys. Ten animals were treated with a 'pour-on' EPM preparation (at a dose of 0.5 mg/kg bodyweight), and five animals acted as controls. Faecal larval counts were carried out two days before treatment, on day of treatment and 7, 14, 21 and 28 days post-treatment with the anthelmintic. EPM was 100% effective in eliminating faecal larvae from day 7, until the end of study at day 28. No adverse drug-reactions or side-effects were observed in any of the treated donkeys.

Suppressive treatment of abamectin against Dictyocaulus viviparus and the occurrence of resistance in first-grazing-season calves.

Dictyocaulus viviparus cause severe lung infections and are endemic in some areas of temperate and tropical countries. Treatment is based on strategic nematode control programs using potent compounds with no reports of drug failure. Macrocyclic lactones (MLs) are available at different concentrations and combinations and have being used heavily by producers. The objective of this work was to determine the efficacy of the MLs ivermectin, moxidectin, doramectin, and abamectin, and the combination ivermectin plus abamectin in naturally infected calves (n=70). Initial infection was determined by necropsy of tracer animals. Faecal larval counts determined that none of the compounds used was able to eliminate D. viviparus for up to 28 days after treatment. The "Area Nova" strain was isolated for future work.

Targeted selective treatment of lungworm infection in an organic dairy herd in Sweden.

The effect of targeted selective anthelmintic treatment on the seroprevalence of the lungworm Dictyocaulus viviparus in cattle was investigated. The study was commenced on an organic dairy enterprise in Sweden in November 1998 after the observation of an outbreak dictyocaulosis in the herd, and then continued for almost 3 years. The first year sampling was conducted on a monthly basis and then biannually with the exception of between August and November 2000 when sampling was performed monthly following a second outbreak of dictyocaulosis. Throughout the study, blood samples were examined for specific IgG(1) levels from all animals in the herd that had been grazing for more than 3 months. At the first sampling occasion, 13% out of the 90 blood samples were seropositive. One month later, after targeted selective treatment with eprinomectin (Eprinex), Merial), the whole herd was seronegative. Seroprevalence then gradually increased and 1 year later it returned to levels similar to those observed at the start of the study. At turnout in April 2000, seroprevalence was 1.3% but it then rapidly increased to 28% and 30% in August and September, respectively. This increase was mainly due to an increase in FSG animals of which many were coughing. Consequently, all seropositive animals were injected with ivermectin (Ivomec), Merial) at 0.05 mg/kg body weight in late August 2000. Although all animals recovered, seroprevalence was only reduced to 12% 1 month later. The differences in seroprevalence after both of these anthelmintic treatments were probably attributed to the timing. The first deworming with eprinomectin was conducted in November when the infection already was transient, whereas ivermectin in connection with the second outbreak was injected in a more acute phase of the infection cycle. Infection levels in 2001 were low with seroprevalences of 2.3% and 5.6% in May and September, respectively. These results show that dictyocaulosis in Sweden can be effectively controlled by the use of macrocyclic lactones. However, the infection was not eradicated from the herd despite close monitoring of the seroprevalence and targeted selective treatment of every seropositive animal on two occasions.

Effects of condensed tannins and crude sesquiterpene lactones extracted from chicory on the motility of larvae of deer lungworm and gastrointestinal nematodes.

The objective of this study was to determine the effects of condensed tannins (CT) and an extract containing crude sesquiterpene lactones (CSL) from chicory (Cichorium intybus) on the motility of the first-(L1) and third-stage (L3) larvae of deer lungworm Dictyocaulus viviparus and the L3 larvae of gastrointestinal nematodes in vitro, using the larval migration inhibition (LMI) assay. The CT and CSL had a profound effect on the motility of the larvae displayed by their ability to inhibit larval passage through nylon mesh sieves. Incubation of lungworm L1 larvae in rumen fluid (collected from deer fed pasture) containing 100, 400 and 1000 microg CT/ml, inhibited 12, 28 and 41% of the larvae from passing through the sieves, respectively, while the incubation of L3 larvae with rumen fluid (pH 6.6) containing the same concentrations inhibited 26, 37 and 67% of L3 larvae from passing through the sieves, respectively. Gastrointestinal larvae seem more susceptible to CT than lungworm larvae especially at higher concentrations. CT inhibited 27, 56 and 73% of gastrointestinal larvae from passing through the sieves when used at a concentration of 100, 400 and 1000 microg/ml, respectively. CT were more effective (P<0.001) at reducing the motility of lungworm L1 and L3 larvae when added to the rumen fluid than when added to the abomasal fluid (pH 3.0). Addition of 2 microg polyethylene glycol/microg CT eliminated the inhibitory effect of CT against L1 and L3 larvae especially during incubation in rumen fluid, confirming the effect as due to CT. The CSL extract also showed similar inhibitory activity against L1 and L3 lungworm and L3 gastrointestinal larvae in both fluids, indicating that this extract was not affected by the pH of the fluid, and was more effective against L3 than L1 lungworm larvae. Condensed tannins appeared to be more effective than CSL at inactivating L1 and L3 lungworm and L3 gastrointestinal larvae in rumen fluid, but CSL were particularly effective against L3 lungworm larvae in abomasal fluid. Activity of these secondary compounds explains the reduced parasite problem of young deer grazing chicory.

The effect of treatment with eprinomectin on lungworms at early patency on the development of immunity in young cattle.

An experiment was carried out to study the effect of topical application of eprinomectin at early patency on the build up of infection and development of protection against Dictyocaulus viviparus in young cattle. Three groups of six calves were used and parasitological and blood variables were monitored at weekly intervals throughout the trial. At the start of the experiment calves in groups A and B were experimentally inoculated with 100 D. viviparus infective third-stage larvae (L3) for five consecutive days, whereas calves in group C served as uninfected controls. The calves in group A were each treated with eprinomectin (0.5mg/kg bodyweight) in a pour-on formulation at early patency at day 24 post the first inoculation, whereas the calves in groups B and C were left untreated. Seven weeks following anthelmintic treatment all groups were challenged with 1500 L3. Another 4 weeks later the animals were sacrificed and established worms in the lungs were counted. Moderate transient signs of lungworm disease occurred both in groups A and B. However, group B calves were found to be about 8 times more resistant than those in group A, whereas the naive infection controls in Group C was found to be about 35 times more susceptible to infection. Also the ELISA values showed that the course of infection was different between experimental groups. The eosinophil counts prior to and at the time of slaughter indicate that immunity was involved in the protection and the response was correlated with previous exposure and worm load. Weight gains differed significantly, but only between groups A and C and between groups B and C that on an average were approximately 13kg heavier at the termination of the experiment. It was concluded that eprinomectin was effective against established adult lungworms. However, the untreated calves (group B) developed a more marked resistance to lungworms compared to those that were subjected to anthelmintic treatment at early patency (group A). On the other hand, the cumulative number of excreted larvae was on an average 43 times higher in group B as compared to group A. Consequently, infected calves that remain out on pasture should be treated. This will restrain transmission of the parasite despite the fact that immunity is deteriorated.

Efficacy of moxidectin 1% injectable and 0.2% oral drench against natural infection by Dictyocaulus filaria in sheep.

Two separate trials (I and II) with 34 and 32 Churra ewes, respectively, and distributed into two groups, have been carried out to evaluate the efficacy of two different formulations of moxidectin at a dose rate of 0.2mg/kg body weight (b.w.) against natural infection by Dictyocaulus filaria in sheep. Trial I was designed to evaluate a 1% moxidectin injectable formulation, whereas in trial II a 0.2% moxidectin oral drench formulation was used. The efficacy was measured on the basis of the reduction of the faecal larval counts and of adult worm recoveries at slaughter. In each trial, a group of animals was treated on day 0 with moxidectin 1% injectable or moxidectin 0.2% oral drench and the other group acted as untreated control. When the faecal larval counts was compared within the treated groups, the efficacy was over 95% until day +13, and 100% at the remainder of the sampling dates after the application of injectable moxidectin, whereas in trial II, the larvae per gram (lpg) of faeces increased until the first sampling time post treatment (p.t.), day +6, and zero counts were recorded for all animals by the following days. On the basis of adult worm recoveries at necropsy, the efficacy of the treatment was 100% in both trials, however, adult worms were detected at slaughter for all control sheep. These results indicate that moxidectin 1% injectable and moxidectin 0.2% oral drench, administered at 0.2mg/kg b.w., were 100% effective against D. filaria infection in sheep. No adverse reactions to the treatments were observed in the animals.

Effect of condensed tannins extracted from four forages on the viability of the larvae of deer lungworms and gastrointestinal nematodes.

The inhibitory activity of condensed tannins extracted from four forage legume plants were evaluated by using a larval migration inhibition assay. The first (L1) and third (L3) stages of deer lungworm (Dictyocaulus viviparus), and the third stage (L3) of deer gastrointestinal nematodes were incubated with tannins extracted from Lotus pedunculatus, Lotus corniculatus, sulla (Hedysarum coronarium) and sainfoin (Onobrychus viciifolia). The tannins extracted from all the forages had inhibitory activity as measured by their ability to paralyse the larvae and inhibit them from passing through sieves. At the highest concentration used (1200 microg/ml) the tannins extracted from sainfoin had the highest activity against ensheathed L1 lungworm larvae (58 per cent), followed by L. pedunculatus (45 per cent), sulla (42 per cent) and L. comiculatus (35 per cent) when the larvae were incubated at 37 degrees C. The same trend, but with lower activities, was observed when the larvae were incubated at 22 degrees C. Anthelmintic activity against L3 lungworm larvae was evaluated by measuring the death rate of ensheathed L3 larvae after incubation with condensed tannins for two, 24 and 48 hours at room temperature (22 degrees C). The death rate was significantly higher (P<0.001) after 48 hours incubation than after two hours or 24 hours, and significantly higher (P<0.001) after 24 hours than after two hours incubation. Condensed tannins from sainfoin had the highest inhibitory activity followed by L. pedunculatus, sulla and L. comiculatus. The tannins from sainfoin also had the highest activity against L3 larvae of gastrointestinal nematodes, followed by L. pedunculatus, sulla and L. comiculatus. Exsheathed larvae of gastrointestinal nematodes were significantly more susceptible to the action of the tannins than ensheathed larvae.

Effect of forage legumes containing condensed tannins on lungworm (Dictyocaulus sp.) and gastrointestinal parasitism in young red deer (Cervus elaphus).

To investigate the effect of feeding forage legumes containing condensed tannins (CT) on internal parasitism, red deer calves were fed either lucerne (Medicago sativa; 0.1 per cent CT), birdsfoot trefoil (Lotus corniculatus; 1.9 per cent CT) or sulla (Hedysarum coronarium; 3.5 per cent CT) and trickle-infected with deer-origin gastrointestinal nematode and lungworm (Dictyocaulus sp.) larvae for 5 weeks, then slaughtered at 7 weeks. There was a significant negative linear relationship between dietary CT concentration and abomasal nematode burdens. No significant differences in faecal egg counts, lungworm burdens or voluntary feed intake were found. Deer fed sulla had higher liveweight gain, carcass weight and carcass dressing-out percentage, higher serum total protein and albumin concentration and lower serum gastrin concentration and faecal lungworm larval count, compared with lucerne-fed deer. Inclusion of sulla in diets for young red deer may reduce the impact of internal parasites and/or reduce the dependence on anthelmintic treatment.

Induction of protective immunity to Dictyocaulus viviparus in calves while under treatment with endectocides.

Three groups of five parasite-naive calves were used. The treatments were: (a) Group 1 calves were weighed on Day 0 and injected with doramectin at 200 microg/kg. From Day 1 to 19 they were dosed orally with 2000 infective larvae of Dictyocaulus viviparus. On Day 28 they were again injected with doramectin, and infected with D. viviparus larvae from Days 33 to 41. They were then left untreated until Day 81 when they were infected with 20 infective larvae of D. viviparus per kg body weight. They were killed on Day 110 and lungworms were counted; (b) Group 2 calves were immunised with oral lungworm vaccine on Days 0 and 28, and infected and slaughtered as Group 1 on Days 81 and 110, respectively; (c) Group 3 calves acted as infection controls. Blood samples were taken at Days 0, 21, 49, 77 and 110 for antibody tests to D. viviparus. At autopsy there were no significant differences between the number of lungworms from Groups 1 and 2 (Means 17.4 and 31.3, respectively); Group 1 had significantly less value than Group 3 (Mean 228) (p < 0.05). Increased antibody titres to the larval sheath of the infective larvae were observed from Groups 1 and 2, showing that the larvae in Group 1 had penetrated the intestine before being killed by the circulating anthelmintic. This experiment shows that if calves are exposed to infective larvae while under systemic endectocide cover, an immune reaction is stimulated.

The effect of simulated rainfall on the efficacy of doramectin pour-on against nematode parasites of cattle.

Two studies were conducted with doramectin topically administered at 500 microg/kg body weight to assess retention of therapeutic efficacy against nematode infections of cattle before, and after, simulated rainfall. In the first study, 50 heifers, with patent nematode infections, were allocated to one of five treatment groups. An untreated control group and one doramectin-treated group were not exposed to simulated rainfall. Simulated rainfall was applied at a rate of 25.4 mm of water in 35 min to three of the five groups: one group immediately before treatment, the second group 90 min after treatment, and the third group 24 h after treatment. Fecal samples were collected for determining egg counts 14 days after treatment. Percentage efficacy ranged from 97.3% to 100% in all treated calves, regardless of exposure to simulated rainfall. The second study involved 40 mixed-sex cattle that were allocated to one of four treatment groups (one saline control and three doramectin-treated groups). All cattle were confirmed to be free of nematode infections prior to oral gavage with infective larvae of Dictyocaulus viviparus, Cooperia oncophora, and Ostertagia ostertagi. Twenty-six days after infection, three groups were treated with doramectin pour-on and exposed to 20 mm of simulated rainfall over 40 min: one group 60 min before treatment, the second 20 min after treatment, and the third 40 min after treatment. Approximately two weeks after treatment, all cattle were necropsied for worm counts. In all treated groups, the percentage efficacy against O. ostertagi and D. viviparus was >99% to 100%. Percentage efficacy against Cooperia ranged from 97% to 98%. Results indicated that doramectin pour-on remains efficacious against nematodes of cattle when administered immediately before or after rainfall.

Persistent efficacy of doramectin injectable against artificially induced infections with Cooperia punctata and Dictyocaulus viviparus in cattle.

Three studies were conducted to evaluate the persistent efficacy of doramectin injectable solution against experimental challenges with infective larvae of Cooperia punctata and Dictyocaulus viviparus. In each study, four groups of ten randomly-assigned calves, negative for trichostrongyle-type eggs on fecal examination, were treated subcutaneously in the midline of the neck with saline (1 ml/50 kg) on Day 0 or doramectin (200 microg/kg = 1 ml/50 kg) on Day 0, 7, or 14. Two additional calves from the same pool of animals were randomly assigned as larval-viability monitors and received no treatment. On Days 14-28, approximately 1000 and 50 infective larvae of Cooperia spp. and D. viviparus, respectively, were administered daily by gavage to each animal in Groups T1-T4. On Day 28, the two larval-viability monitor calves were inoculated in a similar manner with a single dose of approximately 30000 and 2000 larvae of Cooperia spp. and D. viviparus, respectively. Equal numbers of calves from each treatment group were killed on Days 42-45, as well as the two viability monitor animals to enumerate worm numbers. A 2% or 5% aliquot of small intestinal contents and washings were examined for worm quantification and identification, while 100% of the lung recoveries were quantified and identified. For each study and across the three studies, geometric mean worm recoveries for each treatment group were calculated from the natural log transformed data (worm count + 1) and were used to estimate percentage reduction. In the three studies, doramectin injectable solution was 97.5% efficacious against lungworms for up to 28 days and was 99.8% efficacious in reducing infection resulting from challenge with infective larvae of C. punctata for at least 28 days post-treatment.

Persistent efficacy of doramectin pour-on against artificially induced infections of nematodes in cattle.

Two studies were conducted to determine the persistent efficacy of doramectin pour-on against an artificial, trickle challenge of mixed nematodes in calves. In each study, 42, 4-8 months old calves were randomly assigned into four groups of 10 animals each (T1-T4), plus two larval-viability monitor animals. All animals were treated with fenbendazole (10 mg kg(-1)) 14 days prior to the start of the study to clear any existing infection. Doramectin pour-on at 500 microg kg(-1) was used on each animal in Groups T2, T3, and T4 with intervals of 1 week (Day 0, 7, and 14, respectively). Calves in Group T1 were treated with saline solution on Day 0 and at the same volumetric rate (1 ml 10 kg(-1)) as the doramectin treated animals. All treatments were applied in a single passage along the midline of the back, from the withers to the tailhead. Subsequently, trickle inoculations with infective larvae were administered to all calves for 22 consecutive days (Days 14-35). Doramectin pour-on provided > or = 91.9% efficacy against challenge with Dictyocaulus viviparus, Haemonchus spp., and Ostertagia ostertagi for up to 35 days post-treatment and against challenge with Cooperia oncophora, Cooperia punctata, and Oesophagostomum radiatum for up to 28 days post-treatment.

Comparison of the persistent efficacy of the injectable and pour-on formulations of doramectin against artificially-induced infection with Dictyocaulus viviparus in cattle.

The persistent efficacy of the injectable and topical formulations of doramectin was compared against experimental challenges with infective larvae of Dictyocaulus viviparus in two separate studies. Four groups of 10 randomly-assigned calves, negative for lungworm larvae by the Baermann technique, were used in each study. Calves were treated subcutaneously in the midline of the neck or poured down the midline of the back with saline (1 ml/50 kg. injection: 1 ml/10 kg. pour-on) on Day 0 or doramectin (200 microg/kg = 1 ml/50 kg. injection: 500 microg/kg = 1 ml/10 kg. pour-on) on Day 0, 7, or 14. Two additional calves from the same pool of animals were randomly assigned as larval-viability monitors and received no treatment. Calves were inoculated daily with a gavage of approximately 100 larvae of D. viviparus from days 35 to 49 for the injectable study and days 28 to 42 for the pour-on study. The two larval viability monitor calves received approximately 3000 infective larvae in the same manner on Day 49 or 42 for the injectable and pour-on studies, respectively. Equal numbers of calves from each treatment group as well as the larval viability monitor calves were necropsied on days 14 and 15 after the last lungworm inoculation to enumerate the worm burden. The worms recovered were quantified and identified. For each study, geometric mean worm recoveries for each treatment group were back transformed from the natural log-transformed data (worm count +1) and were used to estimate percentage reduction. Doramectin injectable solution was 100.0% efficacious against lungworms for up to 49 days and the pour-on formulation was 100.0%, 93.1% and 81.5% effective in reducing lungworm infection resulting from challenge infection for up to 28, 35, and 42 days post-treatment, respectively.

Field evaluation of a topical doramectin formulation for the chemoprophylaxis of parasitic bronchitis in calves.

A study was conducted to evaluate the efficacy of two topical treatments with doramectin on the season-long control of lungworm and gastrointestinal infections in first grazing season (FGS) calves. At the start of the study, 20 FGS calves were randomly allocated into two treatment groups of 10 animals each. Calves in the D-group were treated with doramectin pour-on on days 0 and 56, at a dosage of 500 microg kg(-1) BW: calves in the C-group were designated as controls. A permanent pasture was divided in two blocks and these were randomly allocated to the treatment groups. Throughout the study, tracers (n = 32) were grazed on each paddock at 3-week intervals. Clinical signs of parasitic bronchitis (PB) were observed in the C-group in July and this necessitated two salvage treatments with levamisole. From day 28, post-turnout lungworm larvae were recovered from faeces of the C-calves until housing. No signs of PB were observed in the D-group during the entire grazing season. Shedding of lungworm larvae in the principals of the D-group did not occur until 112 days post-turnout. From the data obtained from the tracer calves. it appeared that larvae had overwintered on both pastures. On the C-pasture, the number of lungworms recovered from the tracer calves gradually increased to a peak in September, whereas on the D-pasture, the increase was observed only at the end of the pasture season. Both strongyle faecal egg counts and pepsinogen levels were relatively low in both groups throughout the present study. At the end of the grazing period (day 161). the principals were housed and treated with oxfendazole. During the housing period, all principal animals (D- and C-groups) and a third group of four helminth free animals (N-group) received a challenge infection with Dictyocaulus viviparus. It appeared that the different exposure to the parasite during the grazing season resulted in different establishment rates, i.e.. group C < group D < group N. The present results show that overwintering of lungworm larvae occurs in Belgium and that in such conditions, doramectin pour-on given at turnout and at 8 weeks controls PB in calves during the first grazing season.

Interpreting comparative experiments on parasite burdens in animals using randomization and canonical variates.

Comparative experiments on parasite burdens present difficulties for modelling and interpretation: data tend to have highly skewed distributions, some standard methods of testing for effects have low power, and there is a need to make allowance for dependencies among the various measurements. Randomization testing and canonical variates analysis are advocated as a means of minimising these problems.

Persistence of the efficacy of pour-on and injectable moxidectin against Ostertagia ostertagi and Dictyocaulus viviparus in experimentally infected cattle.

The persistence of the efficacy of moxidectin 0.5 per cent pour-on and moxidectin 1 per cent injectable against Ostertagia ostertagi and Dictyocaulus viviparus in calves was studied in two experimental trials. In the first trial two groups of seven calves were treated with either the pour-on or the injectable formulation, while a third group remained untreated. All the animals were infected daily from Monday to Friday with infective stages of O ostertagi and D viviparus between the day of treatment (day 0) and day 33, and were necropsied for worm counts three days later. The experimental design of the second trial was similar to that of the first but the period of infection was from 28 to 45 days after the treatment, and the necropsy was five days after the last infection. In both trials both moxidectin formulations had very high efficacies (99.6 per cent) against adult and developing stages of O ostertagi and D viviparus. The higher efficacy of the moxidectin pour-on preparation against early fourth stage larvae in both trials suggested that its effect was more persistent. It was calculated that the efficacy of moxidectin against O ostertagi persisted for at least five weeks for the injectable formulation and six weeks for the pour-on. The efficacy of moxidectin against D viviparus lasted for at least six weeks for both formulations.

Use of an ivermectin bolus against gastrointestinal nematode and lungworm infections in first-year grazing calves.

Two groups of first-year grazing cattle each were either left untreated as controls (group 1) or treated with an ivermectin bolus at turnout (group 2). Whereas group 1 suffered from dictyocaulosis, the bolus treated calves remained healthy. Shedding of lungworm larvae was completed prevented and gastrointestinal nematode egg output effectively reduced. During the 154 day grazing season the ivermectin bolus provided a season-long protection against parasitic gastroenteritis and bronchitis.