Comparative study of early-season prophylaxis using ivermectin with lungworm vaccination in the control of parasitic bronchitis and gastroenteritis in cattle

A stochastic model to investigate the effects of control strategies on calves exposed to Ostertagia ostertagi

Predicting the effectiveness of parasite control strategies requires accounting for the responses of individual hosts and the epidemiology of parasite supra- and infra-populations. The first objective was to develop a stochastic model that predicted the parasitological interactions within a group of first season grazing calves challenged by Ostertagia ostertagi, by considering phenotypic variation amongst the calves and variation in parasite infra-population. Model behaviour was assessed using variations in parasite supra-population and calf stocking rate. The model showed the initial pasture infection level to have little impact on parasitological output traits, such as worm burdens and FEC, or overall performance of calves, whereas increasing stocking rate had a disproportionately large effect on both parasitological and performance traits. Model predictions were compared with published data taken from experiments on common control strategies, such as reducing stocking rates, the ‘dose and move’ strategy and strategic treatment with anthelmintic at specific times. Model predictions showed in most cases reasonable agreement with observations, supporting model robustness. The stochastic model developed is flexible, with the potential to predict the consequences of other nematode control strategies, such as targeted selective treatments on groups of grazing calves.

The efficacy of two isolates of the nematode-trapping fungus Duddingtonia flagrans against Dictyocaulus viviparus larvae in faeces

A series of experiments was carried out to examine the effects of two different isolates of the nematode-trapping fungus Duddingtonia flagrans to reduce the number of free-living larvae of the bovine lungworm, Dictyocaulus viviparus. A laboratory dose-titration assay showed that isolates CI3 and Troll A of D. flagrans significantly reduced (P < 0.05 to P < 0.001) the number of infective D. viviparus larvae in cultures at dose-levels of 6250 and 12,500 chlamydospores/g of faeces. The larval reduction capacity was significantly higher for Troll A compared to CI3 when lungworm larvae were mixed in faecal cultures with eggs of Cooperia oncophora or Ostertagia ostertagi and treated with 6250 chlamydospores/g of faeces. Both fungal isolates showed a stronger effect on gastrointestinal larvae than on lungworm larvae. Two plot trials conducted in 1996 and 1997 involved deposition of artificial faecal pats containing free-living stages of D. viviparus and C. oncophora on grass plots. Herbage around the pats was collected at regular intervals and infective larvae recovered, counted and identified. These experiments showed that both D. flagrans isolates reduced the number of gastrointestinal as well as lungworm larvae in faecal pats. During both plot trials, the transmission of C. oncophora larvae, but not D. viviparus, from faecal pats to the surrounding herbage was clearly affected by climatic conditions. After collection of faecal pats from the grass plots one month after deposition, the wet and dry weight of pats as well as organic matter content were determined. No differences were found between the fungus-treated and non-treated control pats. This indicated that the rate of degradation of faeces was not affected by the addition of the fungus.

Effects of preventive anthelmintic treatment on acquired resistance to gastrointestinal nematodes in naturally infected cattle

The objective of this study was to investigate the influence of different types of chemoprophylaxis in first season grazing calves on their resistance against a natural reinfection with Ostertagia ostertagi and Cooperia oncophora in the second grazing season. Thirty helminth-naive crossbred calves were randomly divided in three groups of 10 animals. The animals of group B received an ivermectin sustained release bolus on day 0. The calves of group D were treated on days 0 and 56 with a subcutaneous injection of doramectin (0.2 mg kg(-1) BW). Group C was the untreated control group ('immune' controls). Although exposure to gastrointestinal nematodes in the first grazing season was only limited, the chemoprophylactic treatments in groups B and D resulted in three distinctly different infection levels (group C > group D > group B). At the start of the second grazing season, six helminth-naive steers (group N, 'susceptible' controls) were turned out together with the second season animals. After 3 weeks of grazing, the 'susceptible' controls were slaughtered, together with four animals from each other group. Parasitological and immunological parameters indicated that resistance to reinfection with Ostertagia was reduced in the chemoprophylactic treated animals, and was negatively related to the degree of suppression of host-parasite contact in the first grazing season (group C > group D > group B > group N). None of the groups had developed a complete resistance against Cooperia yet. A negative relationship was observed between reduction of first grazing season exposure, and weight gains early in the second grazing season. The remaining animals stayed on pasture until the beginning of November. At the end of the second grazing season, levels of acquired resistance against Ostertagia infection were similar in all groups, and all animals had become immune against Cooperia. No effect of first year chemoprophylaxis on total weight gains could be demonstrated. Because of discrepancy between pasture larval counts and tracer worm counts, it was not possible to draw firm conclusions on the effect of chemoprophylaxis on pasture infestation levels in the second year.

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.

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.

The effect of different infection levels on acquired resistance to gastrointestinal nematodes in artificially infected cattle

The objective of this study was to evaluate the effect of different infection levels of Ostertagia ostertagi and Cooperia oncophora in a simulated 'first grazing season' on the resistance of calves to an artificial challenge infection. The infection levels were determined by the infection schedules and the chemoprophylaxis used. Thirty six 7-11-month old Holstein-Friesian bull calves were randomly divided into four groups. The animals of group B received an ivermectin sustained release bolus (ISRB) on day 0. The calves of group D were treated on days 0 and 56 with a subcutaneous injection of doramectin (0.2 mg kg(-1) BW). Group C was the untreated control group. The calves of group N were used as helminth-naive controls, while the animals of groups B, C and D were trickle infected for 24 weeks. The infection schedules were designed to simulate the expected infection pattern for each treatment group under set-stocked conditions in temperate climate areas. After the last infection, all animals were treated with oxfendazole. One week later, all animals received a challenge infection of 50,000 O. ostertagi L3 and 100,000 C. oncophora L3, spread over 10 consecutive days. During the primary infection period the faecal egg output and the serum pepsinogen and antibody levels reflected the different levels of host-parasite contact between the groups (group C > group D > group B > group N). After the challenge infection, faecal egg counts, total Ostertagia burden, size of the adult worms and abomasal globule leucocyte counts all indicated a positive relationship between the level of Ostertagia infection during the primary infection period and the level of acquired resistance. A reduction of host-parasite contact during the primary infection period, as a consequence of the infection schedule and the chemoprophylaxis used, resulted in a diminished level of resistance to the artificial challenge infection with O. ostertagi. Faecal cultures and small intestine worm counts indicated that all previously infected groups had acquired a high degree of resistance to the Cooperia challenge infection.

Immunity development against Ostertagia ostertagi and other gastrointestinal nematodes in cattle

The immunological response to gastrointestinal nematodes is complex and it will take time and financial support to dissect the different components of protective immunity in ruminants. The present review briefly summarises different aspects and manifestations of protective immunity in cattle against gastrointestinal nematodes, especially Ostertagia ostertagi, and the factors that may interact with the development of this immunity.

Effect of chemoprophylaxis with an ivermectin sustained-release bolus on acquired resistance to gastrointestinal parasites in cattle

The influence of chemoprophylaxis with an ivermectin sustained-release bolus in the first grazing season on the resistance of cattle to gastrointestinal nematodes during the following grazing season was investigated. In 1993 and 1994 dairy replacement calves were either given one bolus at the start of their first grazing season or left untreated. The two groups were grazed separately on a pasture that was divided into two similar sized paddocks. Faecal egg counts, serum pepsinogen and antibody levels were measured to evaluate host-parasite contact. Pasture infection levels were estimated by pasture larval counts and worm counts in tracer calves. After winter housing the animals were monitored during their second grazing season on a pasture that was also divided into two similar sized paddocks. Acquired resistance to gastrointestinal nematodes was evaluated by faecal egg counts and weight gains. Again, pasture infection levels were determined and pepsinogen and antibody levels were measured. During the first grazing seasons gastrointestinal nematode infections were controlled very effectively by the bolus, as shown by the greater weight gains, the negligible faecal egg counts and the low serum pepsinogen and antibody levels in the treated calves. In contrast, all parameters showed extensive parasite-host contact in the untreated animals. The efficient prophylaxis in the treated groups resulted in low levels of larval contamination on the paddocks grazed by the treated animals, compared to moderate infection levels at the end of both first grazing seasons on the paddocks grazed by the untreated animals. During the second grazing seasons (1994 and 1995) the faecal egg output was low in all groups. Although in the previously treated animals faecal egg counts were consistently higher, the differences were minimal, resulting in comparable levels of larval contamination on both paddocks. Serum pepsinogen and antibody levels were not significantly different between the groups and indicated a similar level of larval uptake on both paddocks. No negative effect of the previous chemoprophylaxis on the clinical condition and the weight gain of the second season grazing animals was observed.

The effects of first-season strategic and tactical ivermectin treatments on trichostrongylosis in the first- and second-season grazing

A 2 year study was conducted to evaluate the effects of first-season strategic or tactical treatments with ivermectin on trichostrongylosis in heifer calves in the first and second-season grazing. Three groups of each eight Holstein-Friesian calves were turned out in early May onto a permanent pasture naturally contaminated with trichostrongyle larvae. Two of these groups were given ivermectin either as strategic treatments (Weeks 3, 8 and 13 after turnout) or as tactical treatments (Weeks 14, 18 and 22 after turnout); the third group served as untreated controls. The strategic ivermectin treatments prevented build-up of high herbage infectivity from mid-summer onwards as shown by low trichostrongyle egg outputs, serum pepsinogen levels and serum antibody responses. In spite of exposure to continuous high larval challenge in late season, the pathogenic effects of worm loads in calves receiving the tactical ivermectin treatment were significantly suppressed. The performance of the strategically treated calves tended to be higher than that of the tactically treated calves in the first-season grazing; yet, there was no statistical difference. During the following summer, all three groups were grazed in a single herd together with a new group of eight first-season calves. No anthelmintic treatments were given to any animals during the season. From late August until the end of the season all animals were given weekly experimental challenge infections. Following the challenge infections, the first-season calves developed clinical parasitic gastroenteritis, whereas the second-season heifers showed no symptoms. At post-mortem it was found that worm burdens mainly consisted of early fourth-stage larvae (L4) of Ostertagia ostertagi (> 97%). Fewer adult worms were recovered from the untreated animals than from the treated ones. However, serum anti-parasite IgG1 responses and post-mortem worm counts suggested that the untreated heifers harboured markedly fewer adult O. ostertagi than the previously treated ones, indicating a higher level of immunity against adult worms. However, this difference did not have any clinical impact in this experiment.

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.

A comparison of early and mid grazing season suppressive anthelmintic treatments for first year grazing calves and their effects on natural and experimental infection during the second year

A comparison was made of the efficacy and parasitological sequelae over 2 years, of continuous and intermittent periods of anthelmintic suppression applied both early and in the middle of the first grazing season of calves. Five groups of 15 calves grazing separate paddocks within the same field were allotted to one of the following treatment regimes during their first year at grass: Group 1, untreated controls; Group 2, treated with ivermectin injections at 3, 8 and 13 weeks after turnout; Group 3, treated with ivermectin injections at 10, 15 and 20 weeks after turnout; Group 4, treated with a morantel slow release intraruminal bolus at turnout; Group 5, treated with a morantel slow release bolus at 10 weeks after turnout. Five animals from each group were slaughtered at the end of both grazing seasons. Two months after the end of the second season the remaining five calves were challenged with an experimental infection of 250,000 third-stage larvae (L3) of both Ostertagia ostertagi and Cooperia oncophora. All treatment regimes protected the respective calves from parasitic gastroenteritis. Over the 2 year observation period Groups 2 and 4 showed significantly better weight gain than other groups, and at the end of the first season, they were found to harbour significantly fewer O. ostertagi in the early fourth stage of development. During Year 1, Groups 2 and 3 excreted much lower percentages of Ostertagia spp. eggs than other groups. In Year 2, Group 2 excreted a higher percentage of Ostertagia spp. eggs although the total egg output was approximately half that of Group 1 during the same period. The results showed that the effects of anthelmintic suppression on egg output of different nematode species was affected by the activity of the anthelmintic used.

Pathophysiological studies of sheep during the build-up of immunity against Ostertagia leptospicularis and after a challenge infection

Four groups of three lambs were used to investigate the pathophysiological changes during the development of a protective immunity against Ostertagia leptospicularis induced by a trickle infection. Three groups (A, B and C) were infected daily with 1000 third-stage larvae (L3) for a period of 26 weeks; group D remained uninfected until challenged. Egg excretion ceased after about 10 weeks in groups A, B and C and at the end of the trickle infection no parasites were found in group A sheep after slaughter. The sheep of groups B and C were highly refractory to two challenge infections, with 100,000 and 200,000 L3, given four and 17 weeks after the end of the trickle infection. Both challenge infections were followed by short term increases in the concentrations of gastrin and pepsinogen in the serum. Group C sheep were immunosuppressed with flumethasone during the second larval challenge and had higher pepsinogen concentrations but similar gastrin concentrations to the untreated sheep of group B.

A field study of the ivermectin sustained-release bolus in the seasonal control of gastrointestinal nematode parasitism in first season grazing calves

The effect of an ivermectin sustained-release bolus (I-SRB) on the epidemiology of nematode parasites and on calf productivity was evaluated in a field trial under Northwestern European conditions. Twenty parasite-naive female Friesian calves (principals) aged 5-9 months were used together with six male Friesian tracer calves. Principal calves were allocated by restricted randomization on day 0 body weight to either an untreated control group or a group given one I-SRB, designed to deliver 12 mg ivermectin per day for 135 days, orally on day 0. Each group was grazed on adjacent paddocks, naturally contaminated with parasitic nematode larvae, from 13 May 1991 (day 0) until housing on 30 September (day 140). Body weights of principal calves were recorded and individual blood and faecal samples taken at regular intervals throughout the trial. Pasture nematode contamination was monitored by larval counts on herbage and by worm counts of tracer calves grazed on each paddock from day 126 to day 140. Nematode contamination levels on the control paddock did not rise until the end of the grazing season, as a result of a mid-summer drought period. The period of exposure to a high larval challenge was too short to provoke body weight losses and clinical parasitic gastroenteritis in control calves. Use of the I-SRB resulted in zero faecal egg counts of trichostrongyles during the whole pasture season, thereby preventing a build-up of parasitic gastrointestinal nematodes on pasture.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of host immunity on the epidemiology of trichostrongyle infections in cattle

The acquisition of immunity to the important trichostrongyles of cattle is reviewed and the subsequent effect on the epidemiology of trichostrongylosis is discussed. Examples of factors which can delay or impair the development of immunity to these nematode infections are presented. These include the interactions with anthelmintic prophylactic programmes, nutrition, intercurrent infections, pregnancy and lactation.