Sustainable use of anthelmintics in grazing animals

It is suggested that the major factor in avoiding the development of anthelmintic resistance is the percentage of worms that do not encounter the anthelmintics (worms in refugia). This in turn is determined by the numbers of larvae on pasture, the percentage of animals treated and whether any stages in the host can avoid the action of anthelmintic. To maintain anthelmintic efficacy the percentage of worms in refugia must be sufficiently large. In cattle, this should involve treating only first-year animals and using a different pasture each year for calves. For sheep, only animals that have to be treated should be dosed with anthelmintic and clean grazing strategies that involve the use of anthelmintics should be avoided. For horses, reliance should be placed on the removal of faeces from pasture and only treating when the animals' condition requires it. Without a change in anthelmintic use there is the likelihood of increasing numbers of cases for which no anthelmintic is effective and animal welfare may be compromised.     

Potential competitive interactions between feral horses and other grazing animals

The potential impacts on vegetation and watersheds in the western U.S. by feral horses are discussed. Feral horses select open habitats with relatively large amounts of grass. Horse dietsare largely grasses but shrubs are used in some areas. Utilization levels are proportional to animal numbers but equivalent numbers of horses may consume more than cattle because of differing digestive physiology. Predicted vegetation changes under heavy feral horses use may result in range dominated by unpalatable shrubs. Watershed conditions would probably not change except underheavy use by horses when soil compaction, soil disturbance and fouling of water holes could result.     

Effect of monensin, estradiol controlled release implants and supplement on performance in grazing steers

Five trials in five locations in the United States involving 512 steers were conducted to evaluate the effect of monensin [200 mg/d in .9 kg of supplement (Ru mensin )] and estradiol-controlled release implants ( Compudose ) administered alone and in combination on average daily gain (ADG) in steers on pasture. The effect of energy supplementation on rate of gain was also evaluated in these same trials. The initial weight of steers averaged 250 kg and the average duration of the five trials was 124 d. Estradiol-controlled release implants increased ADG by 15.6% (.095 kg/d; P less than .0001) and monensin increased ADG by 8.1% (.054 kg/d; P less than .05). The combination of estradiol-controlled release implant and monensin increased ADG by 27.4% (.168 kg/d). Treatment responses were additive relative to ADG response, with no interaction observed between the treatments. Nine-tenths kilograms of an energy supplement/d increased ADG by 12.4% (.073 kg/d; P less than .01).     

Potential environmental consequences of administration of anthelmintics to sheep

Anthelmintics, veterinary medicines for the control of endoparasites, enter into the environment largely through faeces of the treated animals. Sheep dung is a patchily distributed, ephemeral resource, with a functionally important decomposer community. The nature of this community and the pharmacokinetics of anthelmintics in sheep mean that the ecotoxic impacts of these drugs in sheep dung may differ markedly from those in cattle dung, where most research has been focussed. The period of maximum residue excretion is generally more transient in sheep than cattle dung, but low-level excretion may continue for longer, giving the potential for extended sub-lethal effects. Here, the environmental impacts of sheep anthelmintics, as well as alternative endoparasite control methods are reviewed. Impacts are discussed in terms of the potential for residues to enter into the environment, the toxicity and the impact on ecosystem functioning at an appropriate scale. Future research priorities are also discussed; these include the need for studies of the functional contributions of dung-colonising species, as well as the development of higher-tier ecotoxicological methods bridging the gap between laboratory and field experiments. Large-scale and long-term studies, including the development of appropriate models, are necessary to allow the consequences of anthelmintic administration to be assessed, particularly within the remit of sustainable animal production.     

Immunity and epidemiology of helminthiasis in grazing animals

Abstract

Extract

The recognition that animals develop immunity to a wide range of protozoan and helminth parasites first began about 80 yearsago with the observations of Smith and Kilbourne (1893 Smith, T. and Kilbourne, F. L. 1893. Cited by Soulsby (1957) [Google Scholar]) that cattle recovered from Texas fever (Babesia bigemina) were immune to subsequent infection. This was followed by reports of acquired immunity in various helminth infections including trichinosis (Ducas, 1921 Ducas, R. 1921. Cited by Tromba, F. G. (1972). [Google Scholar]), strongyloidosis (Sanground, 1928 Sanground, J. H. 1928. Some studies on susceptibility, resistance and acquired immunity to infection with Strongyloides stercoralis (Nematoda) in dogs and cats. Am. J. Hyg., 8: 507–538.  [Google Scholar]), haemonchosis (Stoll, 1928 Stoll, N. K. 1928. The occurrence of ‘self cure’ and protection in typical nematode parasitism. J. Parasit., 15: 147–148.  [Google Scholar]; 1929 Stoll, N. K. 1929. Studies with the strongyloid nematode Haemonchus contortus. I. Acquired resistance of hosts under natural reinfection conditions out-of-doors. Am. J. Hyg., 10: 384–418.  [Google Scholar]) and nippostrongylosis (Africa, 1931 Africa, C. M. 1931. Studies on the host relations of Nipposlrongylus muris, with special reference to age resistance and acquired immunity. J. Parasit., 18: 1–13.  [Google Scholar]).     

Factors affecting pharmacokinetics of benzimidazole anthelmintics in food-producing animals: the consequences and potential risks

Benzimidazoles are frequently and widely used veterinary anthelmintics. Unfortunately, an administration of these anthelmintics does not always result in the expected therapeutic success. Many host-related factors modify pharmacokinetic behavior and efficacy of a chosen anthelmintic. Pharmacokinetics of anthelmintics varies among animals of different species, sex and age. Also diseases, medication, feed and environmental conditions can significantly affect behavior of anthelmintics and resultant drug efficacy in animals. The presented review gathers information, gained in last 20 years, on factors which bring about the variability in performance of benzimidazole anthelmintics in food-producing animals. It is focused particularly on differences in absorption and metabolism of these anthelmintics as these stages of the pharmacokinetic process seem to be the most important for the overall anthelmintic efficacy. The consequences of abnormalities and alterations in pharmacokinetics of benzimidazole anthelmintics are summarized and discussed.     

Benefits and problems with cloning animals

Animal cloning is becoming a useful technique for producing transgenic farm animals and is likely to be used to produce clones from valuable adults. Other applications will also undoubtedly be discovered in the near future, such as for preserving endangered breeds and species. Although cloning promises great advantages for commerce and research alike, its outcome is not always certain due to high pregnancy losses and high morbidity and mortality during the neonatal period. Research into the mechanisms involved in the reprogramming of the nucleus is being conducted throughout the world in an attempt to better understand the molecular and cellular mechanisms involved in correcting these problems. Although the cause of these anomalies remains mostly unknown, similar phenotypes have been observed in calves derived through in vitro fertilization, suggesting that culture conditions are involved in these phenomena. In the meantime, veterinarians and theriogenologists have an important role to play in improving the efficiency of cloning by finding treatments to assure normal gestation to term and to develop preventative and curative care for cloned neonates.     

Biological control of helminths in grazing animals using nematophagous fungi

For the last 50 years the control of gastro-intestinal nematodes (GIN) in grazing animals has almost entirely been alleviated by the use of anthelmintics. Due to development of resistance against the drugs, especially in the GIN of sheep and goats it has become necessary to develop new, innovative strategies such as the use of nematode destroying fungi. Despite experiments to employ various species against plant and animal parasitic nematodes were already attempted in the 1930's, it was not until the 1990's when selection by simulating passage through the gastro-intestinal tract of cattle led to isolation of the fungus Duddingtonia flagrans that a major breakthrough was achieved. This fungus, producing sticky three-dimensional network and now isolated world-wide, is special due to its capacity to prolifically produce high numbers of thick-walled resting spores, chlamydospores. These spores survive passage through the gastro-intestinal tract of grazing livestock and are capable of growing and subsequently trap nematodes, including larval stages of parasitic nematodes. The great potential of this fungus as a biological control agent has been demonstrated through numerous trials with cattle, sheep, horses, and pigs. But these trials have also pointed towards some potential limitations in the activity spectrum of the fungus (Dictyocaulus, Nematodirus) beside the whole group of parasites spreading through infective stages protected inside resistant eggs (e.g. Ascaris, Trichuris). So far, in the few reported studies conducted, no negative environmental impact has been found, but it is important that further studies are conducted on this important issue. Although the potential use of D. flagrans chlamydospores has been verified through numerous trials it is necessary to develop practical delivery systems such as slow release devices, feed-blocks or similar to be able to implement this tool in future integrated control strategies. Such control strategies could include the use of biological control, grazing management, smart use of existing drugs, parasite resistant animal breeds, bioactive forages, and possibly vaccines.     

Researching the plant-animal interface: the investigation of ingestive behavior in grazing animals

Profitable livestock production from forages largely depends on efficiency of converting forages into products. Efficient grazing management systems require an understanding of the roles of system components. However, experimentation should be conducted with regard to the system as a whole rather than on the systems components in isolation. This may necessitate development of computer models. The short-term intake of forage by grazing animals is controlled both by the structure of the forage and by effects of the ingested forage on gut fill as moderated by the hunger-satiety complex. Intake can be defined as the product of bite size, rate of biting and grazing time. Measurement of these variables is facilitated by the use of esophageally fistulated animals and automatic recording devices. Bite size has the greatest influence on intake, with rate of biting and grazing time being compensatory variables. Sward structure influences bite size to varying degrees. In temperate grass swards, leaf surface height appears to be the dominant influence on bite size. But in tropical grass swards, leaf density and leaf:stem ratio have a greater influence on bite size than does leaf surface height. Alternative techniques to conventional grazing trials are described. Diversity of environments and forages in the U.S. requires further research into the development of grazing systems. In the future, small-scale trials and computer simulation techniques likely will be used to a greater extent.     

放牧家畜养分消化率的测定

放牧家畜养分消化率反映牧草供给量和营养品质、家畜的健康状况以及生产性能等,是草原管理的关键指标之一。放牧家畜养分消化率的测定方法主要有直接测定和间接估测两种,直接法包括全收粪法、指示剂法、近红外光谱法（NIRS）等,间接法主要有体内或体外发酵法、牧草品质和气候等预测法。全收粪法是较精确的测定方法,然而耗时、费力,对家畜放牧行为有较大影响,难以体现牧场饲草供给的空间异质性;指示剂法根据不溶物的回收比例估测养分消化率,对家畜放牧行为干扰较小,指示剂不易收集,也不适于野性较高的放牧动物;NIRS法效率高、劳动强度低、成本小、不影响家畜,能够大尺度地估测放牧家畜的养分消化率,需要大量的实测数据完善预测模型;气候估测法快捷、省时,精确性较差,适于大时空尺度。放牧家畜的养分消化率与家畜、草地、放牧方法、温度、降水等生物因子,环境因子和社会因子密切相关,与遥感、无人机（UAV）和人工智能等结合,可以准确、快速地测定放牧家畜养分消化率,为草地生态修复与健康管理提供支撑。     

A theoretical analysis of foraging efficiency of grazing animals

A mathematical model of the change in quantity of forage during grazing is constructed, assuming (1) that forage requirement by grazing animals is constant, and (2) that intake is equal to requirement when forage is in free supply but (3) that below a certain supply level intake becomes restricted and is proportional to supply. Grazing efficiency is then defined in terms of utilization of the forage present at the start of the period of restricted intake. The need, or otherwise, for a high grazing efficiency can be determined, given the acceptable level of utilization of forage. A particular part of the period of restricted intake is identified; the herd has to remain until well past the end of this part period, to achieve a high grazing efficiency. Because the animals are under stress during this time, it must be made as short as possible. Some ways of doing so are discussed. Subdivision of the grazing area into smaller camps can be an effective way of increasing grazing efficiency, but this method is not the only one which is practicable and it could even decrease efficiency. A formula for calculating the required number of camps is given. It can be used to give the stocking density required.     

Factors affecting pharmacokinetics of benzimidazole anthelmintics in food-producing animals: The consequences and potential risks

Benzimidazoles are frequently and widely used veterinary anthelmintics. Unfortunately, an administration of these anthelmintics does not always result in the expected therapeutic success. Many host-related factors modify pharmacokinetic behavior and efficacy of a chosen anthelmintic. Pharmacokinetics of anthelmintics varies among animals of different species, sex and age. Also diseases, medication, feed and environmental conditions can significantly affect behavior of anthelmintics and resultant drug efficacy in animals. The presented review gathers information, gained in last 20 years, on factors which bring about the variability in performance of benzimidazole anthelmintics in food-producing animals. It is focused particularly on differences in absorption and metabolism of these anthelmintics as these stages of the pharmacokinetic process seem to be the most important for the overall anthelmintic efficacy. The consequences of abnormalities and alterations in pharmacokinetics of benzimidazole anthelmintics are summarized and discussed.     

Prevention of selenium deficiency in grazing animals by annual topdressing of pasture with sodium selenate

The effectiveness, safety, economics and methodology of selenate prill topdressing in preventing selenium deficiency in grazing stock under various farming conditions on all groups of deficient soil are reviewed. Topdressing at the approved rate of 10 g/ha Se is effective for 12 months, and has a toxicity safety margin of about 20 times. This compares favourably with selenium dosing. However, materials for dosing usually cost less than one-sixth of those for topdressing. Considerations of convenience, correction of soil deficiency, and production profitability make topdressing an alternative for some farmers; particularly with severe deficiency, and a high stocking rate. Topdressing costs are least where one-sixth or less of the farm can be treated and grazed continuously for 2 months or more; as by ewes at mating. At the very small approved rate no adverse effects are foreseen on human or animal health, export sales, or the environment.     

The efficacy of triclabendazole and other anthelmintics against Fasciola hepatica in controlled studies in cattle

In eight controlled tests 274 cattle were used to assess the efficacies of triclabendazole, albendazole, clorsulon, nitroxynil, oxyclozanide and rafoxanide against Fasciola hepatica. Against one-, two- and four-week-old early immature fluke the mean efficacies of triclabendazole given orally at 12 mg/kg were 88.1, 95.3 and 90.7 per cent, respectively. Clorsulon, nitroxynil and rafoxanide administered at recommended dose rates showed negligible activity against these stages of the parasite. Against six- and eight-week-old infections the mean efficacies of triclabendazole at 12 mg/kg were 87.5 per cent and 95.7 per cent, respectively. Against F hepatica aged six weeks, albendazole and oxyclozanide showed no activity and clorsulon, nitroxynil and rafoxanide had only slight to moderate activity. The efficacies of triclabendazole, clorsulon, nitroxynil and rafoxanide against 10- or 12-week-old parasites were 100, 99.0, 99.1 and 90.1 per cent, respectively. Albendazole and oxyclozanide showed poor efficacy against 12-week-old infections.