Interactions between chemoprophylaxis and immunity to bovine parasitic gastroenteritis and bronchitis: pilot studies using an oxfendazole pulse release bolus

Immunomodulatory effect of various anti-parasitics: a review

This paper reviews the immunomodulatory effects (immunosuppression or immunoactivation) of various anthelmintics including levamisole, fenvalerate, dieldrin, carbofuran, aminocarb, thiabendazole, fenbendazole, oxfendazole and ivermectin. The induced modulation of immune function may occur via direct and/or indirect mechanisms. The immunomodulatory effects of these anti-parasitics have been studied in a variety of bacterial (e.g. brucellosis, salmonellosis, paratuberculosis, mastitis), viral (e.g. infectious bovine rhinotracheitis, Herpes, foot and mouth disease), parasitic (e.g. onchocerciasis, coccidiosis, ascariasis, schistosomiasis) and neoplastic diseases. Some antiparasitics have also been used to boost immunity in a number of human diseases including leprosy, Hodgkin's disease, rheumatoid arthritis, and in adjuvanted therapy of colorectal cancer. The ability to stimulate the immune response of animals offers a new means of disease intervention. Future research on immunomodulatory effects of anti-parasitics, for humans and domestic farm animals, will provide additional methods of treating immunosuppressed subjects. The immunopotentiating or immunosuppressing activity of anti-parasitics will dictate whether co-administration of vaccines and anthelmintics or administration of vaccines during the window of immunoactivation is justified or not.

Issues and challenges in developing ruminal drug delivery systems

Ruminants have a specialised digestive system that contains anaerobic bacteria and protozoa capable of digesting the cellulosic materials that are so common in plant materials. In addition, their distinct digestive system can change the metabolism and mode of action of some nutrients, medicines or other bioactive materials when delivered orally or may provide opportunities for alternative oral dosing strategies. In particular, there is interest in administering a relatively large depot of some drugs into the rumen, which then provides for a prolonged and sustained release of small quantities of these drugs over time. Any strategy to develop a new ruminal drug delivery system must take into account the characteristics of the digestive system of ruminants and its specific bioactive application. For example, in the case of products to control parasitic infections, the development of the host's immunity against the nematodes, which can be acquired during the pasture season, must be considered; likewise, where pharmacologically active materials are used to manipulate a particular metabolic or biochemical process, one must always be aware of interactions with other processes, which might eventuate. This article reviews the necessary concepts, the issues and the challenges to construct ruminal drug delivery systems.

Dictyocaulus viviparus: re-emerging or never been away?

For over 40 years a highly effective vaccine against the bovine lungworm has been commercially available. The use of it successfully reduced the number of outbreaks in calves. However, the past decade has seen a dramatic increase in lungworm outbreaks in adult cows in the UK. This might indicate that Dictyocaulus viviparus is re-emerging as a significant parasite in the dairy cattle industry. Much is still unknown, and here the most important aspects requiring urgent attention are put into perspective.

Resilience of second year grazing cattle to parasitic gastroenteritis following negligible to moderate exposure to gastrointestinal nematode infections in their first year

The influence of gastrointestinal nematode infections on performance of four groups of female Holstein Friesian calves was monitored until the end of the second grazing season (SGS). In the first year three groups were grazed and one group (G4) was permanently housed. General and grazing management during the first grazing season (FGS) was arranged such that G1 acquired moderate infections, G2 low infections and G3 very low infections with gastrointestinal nematodes. These infections were monitored through faecal egg counts, differentiation of faecal larval cultures, pasture larval counts, serum pepsinogen values, ELISA with a recombinant Cooperia oncophora protein, weight gain, tracer worm counts and sentinel worm counts. In 1998 all four groups were grazed together as one herd from 23 April to 26 October and infections were monitored with the same techniques with the exception of sentinel calves. In the FGS weight gain was higher in G4 than in the other groups and higher in G3 (28. 6kg) than in G1. Weight gain of G2 was intermediate to G1 and G3 but did not significantly differ from either group. In the SGS weight gain in G4 was far less than in any other group and the mean weight at the end of the experiment was 41.9, 38.6 and 50.9kg lower than G3, G1 and G2, respectively. Though no significant differences were observed between G1, G2 and G3 at the end of the experiment it was obvious that the weight gain advantage of G3 over G1 at the end of the FGS had disappeared. Parasitological and serological findings in the SGS indicated that G3 and G4 had build up less immunity during the FGS compared to G1 and G2. The conclusion of the experiment is that resilience to parasitic gastroenteritis in the SGS depends on the level of exposure to nematodes in the FGS. However, problems with poor weight gain only will be expected when exposure is very low in the FGS and high in the SGS.

The effect of first season chemoprophylaxis in calves on second season pasture contamination and acquired resistance and resilience to gastrointestinal nematodes

Two groups of second grazing season cattle, which had been treated with an ivermectin-sustained-release bolus (ISRB) in their first grazing season, were monitored during consecutive years (1995 and 1996) on the same second grazing season pasture. In the preceding year (1994), this pasture had been grazed by yearlings that had not received chemoprophylaxis in their first grazing season. The objectives of the study were, firstly, to investigate whether the chemoprophylactic-treated yearlings were less resistant to gastrointestinal nematodes upon subsequent exposure, and hence excreted more strongyle eggs compared to the control yearlings; secondly, whether an increased susceptibility of the previously treated animals resulted in a yearly increase of the pasture infestation on the second grazing season pasture; and finally, whether this affected the second year weight gain of the animals. In 1996, the yearlings that had been chemoprophylactic-treated in 1995 excreted higher numbers of nematode eggs, compared to the previously 'untreated' yearlings. In addition, the proportion of Cooperia larvae was markedly higher in the faecal cultures from the chemoprophylactic treated-animals, suggesting a negative effect of preventive treatment with an ISRB on the acquired resistance of the animals. However, there was no evidence that the slightly higher egg output in the previously treated yearlings had an effect on the larval contamination of the second grazing season pasture. A significant yearly decrease in the second season average daily weight gains was observed, but it could not be inferred from the results of the parasitological parameters that the differences in second year growth were caused by different levels of resilience between chemoprophylactic-treated and -untreated animals. As the study covered three consecutive second grazing seasons, an effect of differences between years (e.g. in weather conditions or grass growth) on the results cannot be excluded.

Integrated control of nematode infections in cattle: a reality? A need? A future?

Helminth infections are a major cause of production loss in cattle. Great progress has been achieved in the design of control strategies for these infections. Control is based mainly on the use of anthelmintics, and these have become more potent and easier to administer. However, the most effective control is possible only through the integration of different approaches. Moreover, an increasing number of disadvantages of chemotherapy/prophylaxis--biological, economical and environmental--have been suggested. In sheep, the high incidence of anthelmintic resistance has simply forced veterinarians/producers to adopt alternative control strategies; in cattle, no real need for deviation from the actual control programmes seems to exist. Therefore, the following questions are discussed: (1) Based on the distribution of cattle worldwide, what are the target parasites? (2) Can we continue to rely on control based mainly on the use of (highly effective) anthelmintics? (3) What are the prospects for non-chemical control? (4) Who will develop and implement integrated control systems? (5) In the case of parasite control in Western Europe, has it been efficient and can/need it be changed? (6) How can we integrate helminth control in the general design of herd disease control?

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.

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 development of protective immunity against gastrointestinal nematode and lungworm infections after use of an ivermectin bolus in first-year grazing calves

Interactions between treatment with an ivermectin bolus at turnout and immunity to bovine parasitic gastroenteritis and bronchitis were examined. Immunity related parameters, i.e. development of clinical disease, parasite development and stimulation of parasite specific antibodies were examined for two grazing seasons and compared with untreated second season cattle (immune control) and untreated parasite naive cattle (nonimmune control). With respect to gastrointestinal nematodes, clinical condition, body weight development, faecal egg counts and pepsinogen concentrations of the bolus treated animals were not significantly different from the respective values of untreated immune cattle, showing a considerable degree of resistance in both groups compared to the parasite naive cattle. With respect to lungworms, untreated immune cattle were protected against clinical disease, but two of eight animals shed larvae again. Bolus treated animals transiently showed mild clinical symptoms and six of seven animals shed low numbers of larvae again, whereas all parasite naive cattle shed high numbers of lungworm larvae and showed signs of disease during the whole grazing season. In spite of the effective treatment with an ivermectin bolus during the first year a considerable resistance to gastrointestinal nematode and lungworm infection was present in the second grazing season.

Control of gastrointestinal parasitism in calves with albendazole delivered via an intraruminal controlled-release device

The efficacy of albendazole in an intraruminal controlled-release device against gastrointestinal nematodes in calves was evaluated under field conditions. Calf productivity was monitored during the first and second grazing seasons. Two groups of parasite-naive Holstein Friesian heifer calves were grazed from May to October on adjacent, similarly contaminated paddocks. One group was given a Captec bolus at turnout, the group was left untreated. Pasture larval counts peaked on the paddock grazed by the untreated calves at 23,000 1 kg-1 dry matter in October. This precipitated clinical parasitic gastro-enteritis in the untreated calves with a mean peak faecal egg count of 335 epg, high blood pepsinogen and gastrin concentrations and a mean worm burden of 79,614 at 100 days post-turnout. Pasture larval counts on the paddock grazed by the treated calves remained less than 3000 1 kg-1 DM and the mean faecal egg count was zero until mid-August, peaking at 146 eggs per gram of faeces (epg) in October. The treated calves had body liveweight advantage of 19 kg at the end of September. During the second grazing season the first season untreated calves were refractory to infection. The first season treated calves showed signs of parasitic gastro-enteritis, although not to the extent exhibited by a group of first season parasite-naive tracer calves which were grazed on the same paddock.

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.

Effect of treatment with an ivermectin sustained-release bolus on productivity of stocker beef calves

Three groups of 30 crossbred beef steers, 8-10 months of age and ranging in weight from 158 to 320 kg, were used to compare effects of treatment with an ivermectin sustained-release bolus or two ivermectin injectable treatments on parasite control and productivity in relation to untreated controls during a 168 day winter-spring grazing period. Each group of 30 consisted of five cattle on each of six separate 1.6 ha pastures. Treatments on Day 0 (12 December) were: Group 1: untreated controls; Group 2: ivermectin injectable at 200 micrograms kg-1 bodyweight, s.c., on Day 0 and Day 56; Group 3: ivermectin sustained-release bolus to deliver ivermectin at 12 mg day-1 over approximately 135 days. All cattle were weighed at 28 day intervals and fecal samples were collected for egg per gram counts (EPG). Geometric mean EPG for Group 3 remained consistently less than 1.0 after Day 0 and were highest (2.4) on Day 168. All group EPG were significantly different (P < 0.01) by Day 56, and EPG of Group 2 had increased to 10.5 following initial treatment and to 42.8 on Day 112. With the exception of a low mean EPG of 6.8 for Group 1 on Day 112, EPG of the group were consistently highest (range 24.9-36.0) to the end of the experiment. Ostertagia ostertagi was predominant, along with smaller proportions of Haemonchus placei and Cooperia spp. Throughout the experiment Group 3 had highest liveweights and gains that were most often different from those of Group 1 at P < 0.01 or greater.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Oxfendazole treatment of non-parasitized lambs and its effect on the immune system

Ten parasite-free lambs were drenched with oxfendazole on days 0 and 28 and, one day after each drench, were injected with human erythrocytes and ovalbumin. Ten other antigen-injected lambs were not drenched (controls). Lymphocytes collected 3 days after each antigen injection and cultured in RPMI 1640 plus 5% fetal calf serum (FCS) and lymphocytes collected 3 days after the first and 3 and 7 days after the second antigen injection and cultured in 50% autologous serum had decreased blastogenic activity compared with control lymphocytes. After the second drench, decreased blastogenesis was seen with lymphocytes collected on days 3 and 7 and cultured in 5% FCS and concanavalin A (Con A) and on day 3 when cultured in 5% FCS and phytohaemagglutinin (PHA). Decreased blastogenesis was also seen with lymphocytes collected 7 and 29 days after the second injection of antigen and cultured in 50% autologous serum plus Con A and on days 3, 7 and 29 when cultured in 50% autologous serum and PHA. Significantly depressed antibody responses to both antigens were seen after the second drench. The serum complement level was depressed 3 days after the second injection of antigen. Serum nitric oxide levels were significantly depressed 3 and 21 days after the first and 7 and 21 days after the second injection of antigen. There were no differences in levels of growth-promoting hormones but the drenched lambs gained significantly more weight than the controls.

Gastrin and pepsinogen changes during an Ostertagia ostertagi challenge infection in calves

Daily changes in serum gastrin and pepsinogen concentration have been studied during two types of infection with Ostertagia ostertagi in calves. In a first experiment two calves were trickle infected (10 times 10,000 L3 Ostertagia) and two animals received a single infection of 100,000 L3 Ostertagia. Gastrin and pepsinogen changes are discussed in relation to adult wormburdens. The second experiment involved 8 calves and was designed to investigate pepsinogen and gastrin changes following a challenge infection in previously sensitized calves. The high dosed group was infected with 5,000 L3 O. ostertagi during 30 days, the low dosed group received 500 L3 O. ostertagi and group 3 served as uninfected control. At day 41 post infection all animals were treated with oxfendazole and on day 61 challenged with 100,000 L3 O. ostertagi. Only in the high dosed group a distinct pepsinogen and gastrin reaction was noticed. Both parameters dropped to almost preinfection levels after treatment. Two days post challenge a moderate rise (+/- 1,000 mU tyr) of the pepsinogen concentration was observed in the previously infected animals and gastrin showed a temporary slight increase in several animals 8 to 10 days post challenge. The effect of treatment and challenge infection is discussed in relation to gastrin and pepsinogen changes and immunity.

Chemoprophylaxis and immunity to parasitic bronchitis in cattle--a field experiment comparing topical ivermectin and an oxfendazole intraruminal device

Seeder calves infected with Dictyocaulus viviparus were used to contaminate a field divided into three similar paddocks. Twenty-four autumn-born calves were allocated to three matched groups; one group was given topical ivermectin treatments (0.5 mg/kg) at 3, 8 and 13 weeks after turnout (Day 0); each member of a second group was given an oxfendazole pulse-release intraruminal device (OPRB) at turnout; while a third group was kept as untreated controls to monitor the natural epidemiological pattern of events. Severe pasteurella pneumonia exacerbated by lungworm infection occurred in the controls after Day 24. Two died and repeated doses of antibiotic and anthelmintic therapy were necessary to save the remainder. Clinical signs were much milder in the ivermectin and OPRB groups and resolved with only a single dose of antibiotic. The OPRB group excreted some lungworm larvae at this time, but none was detected in the faeces of the ivermectin group during the grazing season. At housing, five calves from each group and four lungworm-naive calves were challenged with D. viviparus larvae. The infection became patent in all challenge-control calves, but no larvae were passed by any of the trial animals. Post-mortem worm-counts revealed percentage takes for the challenge controls, trial controls, ivermectin and OPRB groups of 16.7, 0.01, 0.9 and 0.2, respectively. All trial groups had therefore developed a substantial immunity.

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

Twenty-four first-season calves were randomly allocated to three similar groups (1-non-treated controls; 2 - vaccinated against lungworm; 3 - treated with ivermectin 21, 56 and 91 days after turnout) and set-stocked on separate pastures. Parasitic bronchitis became evident in the controls and signs of parasitic gastroenteritis were seen later in the vaccinates, but the ivermectin-treated calves remained healthy. After autumn housing, all were kept as a single group for the winter. The following summer, six of the ivermectin-treated animals and their matching vaccinates were grazed together, without further prophylaxis, along with six first-season calves. The latter displayed evidence of mild but debilitating pulmonary and gastrointestinal parasitic disease while both groups of yearlings remained healthy (with the exception of one diarrhoeic vaccinate). Thus, the immune status of the ivermectin-treated animals appeared to be comparable to that of the vaccinates. This conclusion was supported by parasitological observations and by artificial challenge at the end of the second grazing season.

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.

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.