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Objective: To evaluate efficacy of topically applied eprinomectin against inhibited early fourth-stage larvae (IL4) of Ostertagia spp in calves., Animals: 4 groups (n = 6 [replicates]) for dose titration; 2 groups (n = 8 calves [replicates]) for dose confirmation., Procedure: 2 dose titration studies-0, 125, 250, and 500 micrograms of eprinomectin/kg of body weight-Louisiana and Georgia- and 2 dose confirmation studies of selected therapeutic dosage (500 micrograms/kg) in Scotland and France. Monitor calves were used to determine inhibition percentage of Ostertagia IL4. Test calves were ranked by weight in replicates of 4 (titration trials) or 2 (confirmation trials) animals each, and within replicates, were randomly allocated to treatment groups. Drug treatments were done on day 0, and animals were euthanatized by replicate, with holding time between treatment and euthanasia varying among trials from 14 to 27 days., Results: Observations indicated high efficacy (> 99%) of 500 micrograms of eprinomectin/kg in removal of Ostertagia IL4. Ostertagia and Cooperia were only genera common across sites, with efficacy of aforementioned dosage against adult and larval stages of both genera consistently high (> 99%). Results of 1 or both titration studies (500 micrograms/kg) indicated > 99 to 100% efficacy against adult Haemonchus placei, Trichostrongylus axei, T colubriformis, Bunostomum phlebotomum, Dictyocaulus viviparus, and Oesophagostomum radiatum. Lower efficacy values were observed at minimal (125 micrograms/kg) dosage. In France, 500 micrograms/kg was 85% effective against Trichostrongylus spp adults; however, numbers of control calves infected with Trichostrongylus spp and degree of infection were low. Adverse reactions were not evident., Conclusion: Eprinomectin given topically (500 micrograms) was highly effective against Ostertagia IL4 and other common nematodes of cattle.

Eight controlled trials were conducted, using 96 cattle of European breeds, to evaluate the efficacy of abamectin against natural and artificially acquired infections of adult and fourth-stage larvae of all the economically important gastrointestinal nematodes and lungworms in Germany and the United Kingdom. Half the animals were treated on one occasion with abamectin at a dose of 200 micrograms/kg bodyweight given subcutaneously while the other half remained untreated. Worms were counted 14 or 21 days after treatment or 28 days after the last infection. The treatment was highly effective (> 99 to 100 per cent) (P < 0.05) at removing immature (L4) stages and adult worms of the following species: Ostertagia ostertagi (inhibited larvae included), Trichostrongylus axei, Haemonchus contortus, Bunostomum phlebotomum, Cooperia species Oesophagostomum radiatum and Dictyocaulus viviparus. Naturally acquired adult C surnabada and Trichuris discolor infections were also significantly (P < 0.05) reduced. For Nematodirus helvetianus the efficacy varied from 89.8 to > 99 per cent (P > 0.1 to < 0.01). Abamectin gave full protection against the gastrointestinal nematodes O ostertagi, H contortus, Cooperia species and O radiatum for at least seven days and against the lungworm D viviparus for at least 14 days after treatment.

Sequential in vivo measurements of total body water, exchangeable sodium and exchangeable potassium, made by radioisotopic dilution techniques, were used to determine changes in body composition in calves exposed to natural infections with gastrointestinal nematodes during their first grazing season. Two groups of calves were studied, one of which received a sustained release anthelmintic bolus at turn out. Over the grazing season the bolus-treated calves showed a significantly improved pattern of bodyweight gain, compared with the untreated control calves. There were also significant increases in both total body water and exchangeable potassium, as a percentage of the bodyweight, in the bolus calves compared with the controls. These findings indicated that body protein as a percentage of bodyweight was increased in the bolus animals. The bolus-treated calves also had significantly lower serum pepsinogen concentrations, faecal egg counts and worm burdens.

An experiment was carried out simultaneously in Glasgow and in Wageningen to investigate possible differences between the local strains of Ostertagia ostertagi and Cooperia oncophora. In each location calves of the local Friesian breed were infected with 100,000 larvae of either the Glasgow or Wageningen strain of O ostertagi or C oncophora. At both locations the calves received the same diet. The Glasgow strain of O ostertagi was more pathogenic than the Wageningen strain and a larger proportion of the worm burden was found in the abomasal mucosa. The number of ova per female was greater in the Wageningen strain. For C oncophora the Wageningen strain gave rise to higher worm burdens and longer worms. Differences were also present between locations. The British Friesians had higher worm burdens of C oncophora and the worms of this species were longer in this host. Compared with the Dutch Friesians the British calves had a higher proportion of O ostertagi in the mucosa. This experiment showed how difficult it is to compare data from the literature because of differences in parasite and host strains and laboratory techniques.

An experiment was conducted in calves to investigate the effect of sustained release and pulse release anthelmintic intraruminal boli on the development of pathophysiological changes following daily infection with Ostertagia ostertagi and Cooperia oncophora for six weeks. After infection various pathophysiological changes were detected including increases in serum pepsinogen concentration, enteric plasma protein losses and in the catabolic rate of albumin. Such changes developed rapidly in the unprotected calves following patency after 17 days and persisted until the termination of the study. There were indications that the sustained anthelmintic release device was more efficacious than the pulse anthelmintic release device in reducing the worm burdens and early pathophysiological changes associated with infection. It was found at necropsy that the release of anthelmintic by the oxfendazole pulse release bolus had been delayed in several calves.

Calves which had received daily concurrent infections of 2000 Ostertagia ostertagi and 10,000 Cooperia oncophora infective larvae over a 42-day period displayed a range of clinical signs typical of acute parasitic gastroenteritis including inappetence, weight loss, hypoalbuminaemia and diarrhoea. There were consistent and significant depressions in both digestive efficiency and nitrogen retention. Radioisotopic studies revealed marked disturbances in protein metabolism which were associated with high losses of plasma proteins into the gastrointestinal tract. On post mortem examination larval establishment of O ostertagi was seen to be high. The adverse changes were prevented by the prior administration of a morantel bolus.

The epidemiological features of three different isolates of bovine Ostertagia spp under similar initial levels of larval challenge were compared in the field. Two of the isolates, consisting mainly of Ostertagia ostertagi, and a low proportion of Skrjabinagia lyrata conformed in epidemiological behaviour with those investigated by previous workers, though the worm burdens which established did not give rise to the expected clinical signs. The third isolate behaved in a different way, yielding very high faecal egg counts which were followed by high pasture larval counts, heavy worm burdens and severe clinical disease. This isolate, while consisting mainly of O ostertagi and a few S lyrata, also contained a proportion of O leptospicularis, and it is suggested that this species may influence the dynamics of the host-parasite relationship in bovine ostertagiasis.

After experimental infection of three groups of calves with three different defined isolates of Ostertagia spp sensu lato a marked variation in the infectivity and degree of pathological change was noted. The most highly infective and pathogenic isolate contained approximately 30 per cent O leptospicularis in addition to O ostertagi (70 per cent) and a negligible number of Skrjabinagia lyrata. The other two isolates consisted solely of O ostertagi (approximately 90 per cent) and S lyrata (approximately 10 per cent) and showed comparable infectivity and pathogenicity to that commonly recorded. The greater pathogenicity of the isolate containing O leptospicularis was attributed to the higher infectivity and the greater damage caused to the gastric mucosa by individual parasites when compared with O ostertagi.

Observations of growth, nitrogen (N) balance and digestibility were made during the first winter housing period upon three groups of calves which had previously been naturally infected with Ostertagia ostertagi and which were left either untreated (A) or treated with fenbendazole on three occasions (B) or every two weeks during the entire grazing season (C). The diet given contained sufficient metabolisable energy but only about 60 per cent of the recommended crude protein to sustain a daily growth rate of 0.5 kg. Growth rates were only 50 per cent of that expected on a basis of metabolisable energy input with untreated calves A being poorest. Water intake and output was greatest in group A. There were no significant differences in digestibility between groups although group A had the lowest apparent crude protein digestibility. N balance was always lowest in group A and highest in group C but group B demonstrated a marked increase in N retention with time after housing. Differences in N retention were mainly accounted for by increased urinary N excretion. It is concluded that, under conditions of suboptimal protein intake, nematode infection in the growing calf can markedly affect production even after efficient anthelmintic treatment.

A 16-day infection of Dictyocaulus viviparus in two groups of calves was treated with levamisole and fenbendazole respectively. Five days afterwards the calves were reinfected with 4000 larvae and necropsied 21 days later. Although the lungworm burdens of the two groups of calves were reduced by about 70 per cent compared to a control group the clinical signs of dyspnoea, tachypnoea and coughing were indistinguishable from a primary infection. This was due to pulmonary emphysema, oedema and an acute epithelialising pneumonia apparently associated with the death and disintegration of lungworms in situ, the result of an incompletely developed immune response. The results are compared with those obtained with the lungworm vaccine. It was concluded that the outcome of any system of "control" which depends on drug therapy and reinfection is unpredictable and that vaccination offers the only effective method of prophylaxis.

Ivermectin, one of the new avermectin group of anthelmintics, was more than 99 per cent effective in removing all stages of Ostertagia ostertagi including inhibited larvae, and adult Trichostrongylus axei when administered to cattle orally at 100 microgram/kg body-weight or subcutaneously at 100 and 200 microgram/kg body-weight. High efficacy (> 98.6 per cent) was also obtained against adults and inhibited larvae of Cooperia oncophora at 100 microgram/kg orally or 200 microgram/kg by subcutaneous injection. The latter treatment caused an 82.3 per cent reduction of adult Nematodirus helvetianus.

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

Calves which had received daily infections of Cooperia oncophora over a six week period showed inappetence, weight loss, impaired nitrogen retention and a loss of plasma proteins into the gut. During this period the infections occurred in the duodenum, jejunum and ileum and many larval stages were present in the mucosa of the small intestine. At necropsy, on weeks 3, 6 and 10 after the first infection, stunting and thickening of the villi were noted with excessive mucus production. Prior administration of a morantel slow release bolus prevented these changes. By week 12 most of the worms had been expelled yet a loss of plasma proteins to the gut was still occurring.

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

Friesian calves given a low level infection of the abomasal parasite Ostertagia ostertagi over a six week period displayed a mild diarrhoea with high faecal egg counts and elevated plasma pepsinogen values. At necropsy on day 23 abomasal lesions characteristic of ostertagiasis were widespread. At 42 and 84 days oedema and congestion were also prominent. Total worm burdens on days 23 and 42 were similar but a marked decrease had occurred by day 84. Feed digestibility and nitrogen economy were not markedly affected but radioisotopic measurements demonstrated an increase in albumin disappearance and catabolic rates, and plasma faecal clearance during the course of the infection. Prior administration of a morantel sustained release bolus to a group of similarly infected calves reduced the total worm burdens to less than 50 per cent of those recorded in the infected calves on days 23 and 42 and this fell to 3 per cent on day 84. Abomasal damage and the adverse pathophysiological changes associated with infection were prevented in this group.

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

The sequential development of Type I and Type II ostertagiasis over a 2-year period in the same naturally infected cattle is described for the first time. Particular reference is made to biochemical and serological changes. Positive relationships were demonstrated between the clinical signs of both Type I and Type II disease, and marked increases in the levels of plasma pepsinogen, plasma gastrin and antibody titres to adult Ostertagia antigen. At necropsy, there were significant relationships between the combined total of adult and developing 5th stage larvae of Ostertagia spp. and the levels of both plasma pepsinogen and gastrin. By the end of the second grazing season the cattle had acquired an immunity to infection with Ostertagia spp. and had very low burdens of this parasite at necropsy. However some of these cattle maintained elevated plasma pepsinogen levels when under natural challenge by Ostertagia spp. larvae and the aetiology of these changes and the problems of diagnosis using this parameter are discussed. Similar trends of infection were observed for Cooperia oncophora, although resistance to the parasite developed more rapidly.

The interaction of the morantel sustained release bolus with the development of immunity in calves vaccinated with two doses of gamma irradiated (40 Kr) Dictyocaulus viviparus larvae was investigated under laboratory conditions. A total of 37 helminth-naive calves were used. Eight calves were used in the first part of the study to test the efficacy of a larval vaccine prepared by using gamma rays delivered from a cobalt source. In the second part of the study, four groups of four groups of four calves each were vaccinated and of these, all the animals in two groups each received a bolus. The remaining three groups (two groups of four and one group of five calves each) remained nonvaccinated with each animal in one group receiving a bolus. All the calves were challenged with approximately 2000 lungworm larvae four months postvaccination. In order to simulate possible field conditions, two of the vaccinated groups and two of the nonvaccinated groups were given a trickle infection of 800 lungworm larvae over a four-week period, three months prior to challenge. Based on a comparison of clinical signs, pathology and lungworm burdens at necropsy, the vaccination of the calves conferred a significant degree of protection (P less than 0.001) to a subsequent challenge compared with controls. The introduction of a morantel sustained release bolus and/or a trickle infection had no effect on the high degree of protection engendered by the vaccination. Nonvaccinated calves given a trickle infection, with or without a bolus, were also highly immune to challenge.

In studies on the control of parasitic gastroenteritis in calves and sheep, involving an annual rotation of pastures grazed by these host species, it was shown that young cattle could play an important role in the epidemiology of Nematodirus battus, a species usually regarded as a parasite of lambs. Thus, young cattle readily acquired heavy burdens of N battus in spring and the contamination of pastures with eggs from these infections resulted in significant populations of larvae on the herbage, which were infective to both calves and lambs grazed on these pastures in the following year. Although the majority of the N battus eggs hatched in the spring, some hatched in the autumn. The calves developed a strong immunity to N battus during the grazing season as demonstrated by the absence of worms at necropsy in the autumn, despite the presence of infective larvae on the pasture.

Two studies are described which demonstrate the persistent activity of ivermectin injected subcutaneously into cattle at 200 micrograms/kg in preventing the establishment of induced infections with the gastrointestinal parasites Ostertagia ostertagi and Cooperia oncophora and the lungworm Dictyocaulus viviparus. These results indicated a reduction in mean worm count compared with the control group for O ostertagi of more than 99, 45 and 94 per cent with a seven, 14 or 21 day interval between treatment with ivermectin and the administration of infective larvae, respectively, in trial 1 and more than 99, more than 99 and 99 per cent at seven, 10 or 14 days, respectively, in trial 2. Corresponding values against C oncophora were 99, 0 and 45 per cent at seven, 14 and 21 days in trial 1 and more than 99, 84 and 31 per cent at seven, 10 and 14 days in trial 2. Against D viviparus, reduction in counts were more than 99, 98 and more than 99 per cent at seven, 14 and 21 days, respectively, in trial 1 and 100, 100 and 100 per cent at seven, 10 and 14 days, respectively, in trial 2. The relevance of these results to the build-up of infective larvae on pasture and infection in cattle is discussed.

A mathematical model based on development and mortality rates, and incorporating data on the infectivity, fecundity and migratory behaviour of Ostertagia ostertagi, was used to predict the level of pasture contamination and the occurrence of clinical ostertagiasis in grazing calves during 1975 and 1976. A comparison of the predicted and observed events showed a good correlation.

Abomasa were collected from 183 dairy and 186 beef cows in south-west Scotland and examined for the presence of nematodes. One hundred and fifty-two of the dairy cows (83 per cent) and 154 of the beef cows (83 per cent) had Ostertagia species worm burdens of less than 1000; 4 per cent of both dairy and beef animals had more than 10,000 ostertagia present. The only other nematode detected was Trichostrongylus axei and this was present in low numbers in 23 cows (13 dairy and 10 beef). A marked seasonal fluctuation was observed in the proportion of adult and fourth larval stages of ostertagia present. Over 70 per cent of the burdens from October through to April were fourth larval stages. In contrast less than 20 per cent of the burdens from May to September were fourth stage larvae.

The results of a questionnaire circulated in 1984 concerning parasitic worm control on cattle and sheep farms in a veterinary practice in the west of Scotland are reported. Control by grazing management or anthelmintic treatment was used in 92 per cent of the cattle herds and in all the sheep flocks. The use of anthelmintic drugs was greatest on farms where grazing control was also practised. Benzimidazoles were the most frequently used anthelmintic drugs.

Two groups of yearling cattle which had been treated with ivermectin either three and eight, or three, eight and 13 weeks after turn out to trichostrongyle contaminated pasture in their first grazing season were exposed in the following season to natural challenge with helminth parasites. To assess their immunity to this challenge each group shared a pasture with parasite naive first season calves. No anthelmintic treatments were administered at any time during the year. Throughout the grazing period the yearlings showed normal respiratory rates, negative faecal lungworm larval counts, and, relative to the calves, low faecal trichostrongyle egg counts. All the first season calves developed patent lungworm infections and on one occasion the mean respiratory rates of each group of calves were significantly greater than those of the yearling cattle. At the end of the grazing period, from early May until late September or October 1986, the cattle were removed from pasture and together with parasite naive controls challenged with either 10 or 22 third stage larvae of Dictyocaulus viviparus/kg bodyweight and necropsied between 18 and 23 days later. Although the experimental challenge resulted in relatively heavy lungworm infection of the naive controls, none of the yearlings and only three of the 11 calves which had been at pasture were found to be infected. However, large numbers of arrested fourth stage larvae of Ostertagia ostertagi were present in all the naturally infected yearlings and calves.

Experimental infections of calves were carried out with either isolates of predominantly Ostertagia ostertagi, pure O. leptospicularis or a mixed isolate of equal numbers of both these species. The total worms established on day 21 for the mixed species from a total inoculum of 100 000 infective larvae, was 1.2 times greater than from 100 000 larvae of the O. ostertagi isolate and 3.3 times that of the pure O. leptospicularis isolate. The increased establishment in the mixed inoculum referred to both O. ostertagi and O. leptospicularis (days 17 and 21). These differences were both highly significant (P less than 0.01). The severity of the pathological changes was also greater in the mixed infections. It is suggested that these findings must be taken into account when control measures involving alternate grazing of sheep and cattle are being employed.

Four groups, each of six male Friesian calves, were set-stocked on separate 0.66 ha paddocks from May 7 until October 23 1986. Each of the animals in groups 1 and 4 was dosed with an oxfendazole pulse release bolus at turn out whereas the animals in groups 2 and 3 were left untreated. Parasite-free naive tracer calves were introduced into each paddock for a limited period 12 days after turn out and again at the end of the trial. No adverse reactions or clinical signs were observed in either of the groups of calves which received boluses. The development of clinical parasitic gastroenteritis in both the untreated groups necessitated the humane slaughter of two animals and emergency anthelmintic treatment of the remainder. The lower plasma pepsinogen concentrations, and lower faecal egg and larval counts and worm burdens post mortem, together with the absence of clinical signs of parasitic gastroenteritis and bronchitis in the treated calves, confirmed the high efficacy of the bolus treatment.

An unusual epidemiological pattern in bovine ostertagiasis is described in which clinical ostertagiasis occurred in calves grazed on a silage aftermath from late July. The calves were treated with the anthelmintic levamisole prior to grazing the aftermath and the latter had not been grazed since the previous autumn. Analysis of the herbage larval counts suggests that delayed emergence of overwintered Ostertagia ostertagi L3 from the soil was the most likely source of the infection.

In the West of Scotland the epidemiology of parasitic bronchitis in grazing calves was studied over a two year period with the aid of tracer calves and herbage examinations for Dictyocaulus viviparus larvae. The observations of both years emphasised the importance of overwintered lungworm larvae as a source of disease. In the first year it was shown that the ingestion and development of these overwintered larvae were, by themselves, directly responsible for severe morbidity, high faecal larval counts and deaths. In the second year it was shown that pasture ungrazed during the winter and spring and from which a hay crop was removed in mid-summer was still capable of producing clinical parasitic bronchitis in susceptible calves within three to four weeks of their introduction in later summer. In both years there was some evidence that the outbreaks appeared to be associated with the sudden availability of infective larvae on the herbage. The possibility that such larvae may have survived for many months in the soil is discussed. Despite the heavy challenge with lungworm larvae experienced by the grazing calves in the first year those vaccinated with lungworm vaccine survived, their clinical signs were mild and of short duration and their faecal larval output was greatly reduced.

The epidemiology of ostertagiasis in south west Scotland was studied in groups of cattle grazed through two successive grazing seasons separated by a period of winter housing. Towards the end of the first grazing season (September) the numbers of infective larvae (L3) on the pasture had increased to high levels (up to 24,000 L3 per kg) which resulted in high faecal egg counts, worm burdens, plasma pepsinogen levels and the occurrence of clinical ostertagiasis in the calves. By late spring (May) at the onset of the second grazing season, there was an almost complete mortality of the overwintered L3 on the pasture followed by the appearance of moderately high numbers of a new population of L3 in September (up to 9000 L3 per kg). The latter increase in the numbers of L3 was reflected by negligible faecal egg counts, low worm burdens and a moderate elevation of plasma pepsinogens in the second year animals. It therefore seems that although young cattle acquire a good immunity to Ostertagia ostertagi after one season at grass the small infections established in the early part of the second season are capable of contaminating the pasture to levels which could be dangerous for susceptible stock. An allergic reaction in the abomasal mucosa could be the basis of the elevated pepsinogens present in the second year animals.

In May 1985 four groups of 10 calves, aged between four and five months, were turned out on to separate, permanent pastures of equal area which had been seeded during the previous few days with larvae of Dictyocaulus viviparus. One group acted as a control, the second was vaccinated with lungworm vaccine before turnout and treated with thiabendazole three, eight and 13 weeks after turnout, while the third and fourth groups were given ivermectin three times (three, eight and 13 weeks after turnout) and twice (three and eight weeks after turnout), respectively. A severe outbreak of parasitic bronchitis resulted in the death of three control calves within five weeks of turnout and parasitic bronchitis and gastroenteritis affected the second group of calves after approximately four months at pasture. The calves given ivermectin excreted no lungworm larvae and remained free of clinical parasitism throughout the trial.