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1.
M. Eysker J. H. Boersema F. N. J. Kooyman T. J. G. M. Lam 《Veterinary parasitology》1995,60(3-4):339-348
A grazing study was performed with the main objective of examining the effect of fenbendazole (FBZ) in a ‘dose and move’ system on nematode infections in calves with special emphasis on Dictyocaulus viviparus.
Three groups of six calves were grazed from May to October 1993. One group (DM7) was treated with FBZ and moved to aftermath (pasture which had only been mown) 7 weeks after turnout. The second group (DM9) was similarly treated and moved 9 weeks after turnout and the third group served as untreated pasture control group (PC) and was moved to aftermath 9 weeks after turnout.
FBZ treatment removed adult lungworms from DM7 and DM9. Tracer calves grazed during the first 7 or the first 9 weeks after turnout acquired mean burdens of 18 and 125 lungworms, respectively. In PC faecal larval counts increased until the end of August. Most of the animals in this group were then suffering from lungworm disease and emergency treatment with ivermectin was given. In both FBZ-treated groups, larvae reappeared in the faeces of some of the calves 4–5 weeks after treatment. Subsequent reinfection resulted in higher mean faecal larval counts in both groups 2 months after treatment, although variation in faecal larval counts was high. In DM7 values tended to be higher than in DM9. These higher larval counts were associated with mild signs of parasitic bronchitis in some calves of DM7, whereas no signs were seen in DM9.
At the end of the experiment, all calves, and also a group of six permanently housed non-infected control calves (HC), were infected experimentally with 5000 D. viviparus larvae to evaluate development of immunity. The worm counts at necropsy showed that all calves on pasture had developed immunity. 相似文献
2.
The objective of this study was to examine whether susceptible calves grazing together with second-year resistant heifers are less exposed to trichostrongylid infection than are calves grazing on their own. Two groups of animals representing each age category were turned out onto pasture on 24 May 1997 and grazed at comparable stocking rates. The grazing of calves and heifers together was compared to groups of each age category grazing separately. The results indicated that herbage larval counts were significantly reduced in the second part of the grazing season on the plot grazed by the mixed group compared to the plot grazed by the first-season calves only. The mixed grazing strategy protected the young calves and no clinical signs were observed in this group, while most of the calves that grazed alone exhibited clinical signs. The availability of herbage was reduced towards the end of the season, with subsequent competition for the grass forcing all the animals to graze the tufts around the faecal pats, where the quality of the grass is poor and the numbers of infective larvae are high. The effect of this was visible in the form of increased parasite burdens in the calves that were grazed together with the heifers, confirmed by increased blood serum pepsinogen concentrations and reduced daily weight gains in the second part of the grazing season. The lower numbers of infective larvae on the pasture were probably achieved through the heifers ingesting many of the larvae but subsequently depositing relatively few eggs, since they had acquired some degree of resistance against trichostrongylid infections during their first grazing season. Thus they did not suffer any parasitological ill-effects during mixed grazing with first-season calves. 相似文献
3.
A S Fernández M Larsen P Nansen J Gr?nvold S A Henriksen H Bj?rn J Wolstrup 《Veterinary parasitology》1999,85(4):289-304
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. 相似文献
4.
R.Jess Jørgensen 《Veterinary parasitology》1980,7(2):153-167
A field experiment was conducted over two grazing seaons with calves on a permanent pasture in order to follow the pattern of infection with Dictyocaulus viviparus. Infective larvae persisted during the first, but not during the second, winter of observation. By means of the agar-bile herbage technique, a moderate first peak of infection was demonstrated in the pasture 2–3 weeks before the appearance of respiratory signs in the calves. Fluctuations in faecal larval output were reflected in the herbage contamination with infective larvae close to faecal pats. This, as well as the horizontal dispersion of larvae in the pasture, took place in less than a week. The proportion of lungworm larvae recovered away from faeces was low during a period of dry and hot weather while herbage sampling at two-hour intervals during two days showed an increase in herbage contamination with lungworm larvae, but not with trichostrongyle larvae between 10 a.m. and 12 noon.The infectivity of the pasture was monitored by tracer calves and compared with the results of the pasture sampling. The general course of the infection in the calves and in the pasture was the results of interaction between them. In addition, the pasture infection was influenced by climate and the infection in the calves by the development of immunity. The course of infection in individuals appeared to have an influence on the general course of the infection through the contamination of the pasture. 相似文献
5.
The efficacy of a pulse-release oxfendazole bolus (OPRB) against gastrointestinal nematodes was evaluated under field conditions and compared with a sustained release morantel bolus (MSRB). Three groups of 10 calves were grazed from May to September on adjacent, similarly contaminated 3-acre paddocks. One group was dosed at turnout with the OPRB, the second group with the MSRB and the third group left as nontreated controls. Pasture larval counts peaked at greater than 60,000 larvae/kg dry weight of herbage in September on the control paddock, associated with a mean egg count of 1040 eggs/g faeces and clinical symptoms. A low rise in larval counts occurred on the MSRB plot, and also low helminth egg counts. On the OPRB plot, pasture larvae and faecal egg counts were recorded only intermittently; the September egg counts were only 3.5% of those of the controls; serum pepsinogen data showed a similar picture. At the end of the experiment, the OPRB calves had a mean weight advantage of 40 kg over the controls, and 12 kg over the MSRB group. Feed intakes were highest in the OPRB group in July and September. 相似文献
6.
This study was carried out to examine the survival of infective Ostertagia ostertagi larvae (L(3)) on pasture under different simulated conditions of grazing, i.e. mixed grazing of cattle and nose-ringed sows, or grazing by cattle alone. Standardised pats of cattle faeces containing O. ostertagi eggs were deposited on three types of herbage plots, which were divided into zone 1: faecal pat; zone 2: a circle extending 25cm from the edge of the faecal pat; zone 3: a circle extending 25cm from the edge of zone 2. For "tall herbage" (TH) plots, the herbage in zone 2 was allowed to grow naturally, while the herbage in zone 3 was cut down to 5-7cm fortnightly, imitating a cattle-only pasture. For "short herbage" (SH) plots, the herbage in both zones 2 and 3 were cut down to 5-7cm fortnightly, imitating mixed grazing of cattle and sows. The grass in the "short herbage and scattered faeces" (SH/SF) plots were cut as for SH plots, and the faeces were broken down 3 weeks after deposition and scattered within zone 2, imitating the rooting behaviour of co-grazing sows. Five faecal pats from each plot group were collected on monthly basis, along with the herbage from zones 2 and 3 cut down to the ground. Infective larvae were then recovered from both faeces and herbage. The numbers of L(3) recovered from zone 1 were higher in the TH plots than in the other two groups and, furthermore, the larval counts from SH plots were always higher than from SH/SF plots. The three groups followed a similar pattern during the season regarding numbers of L(3) in zone 2, and no clear patterns between plot types were obtained. The presence of L(3) in zone 3 was almost negligible. Important differences were seen throughout the study from the biological point of view; more L(3) were able to survive in faeces on the TH plots, presumably reflecting a better protection from heat and desiccation compared to those in the other plots. The overall results support the idea that mixed grazing of cattle and pigs favour the reduction of O. ostertagi larval levels in pasture. This reduction is mainly due to the grazing behaviour of pigs, which by grazing up to the very edge of the cattle faeces, will either expose the larvae in faeces to adverse environmental summer conditions or ingest cattle parasite larvae, or both. 相似文献
7.
Two trials were designed in “Haute Normandie” to assess the value of the number of third stage larvae in herbage samples for monitoring the infection of cattle. Two techniques of sampling were used and were compared to the actual infection of tracer calves.An index of infectivity of the pasture was defined: the number of third stage larvae in 1 kg of dry matter of herbage multiplied by the quantity of grass (dry matter) ingested by calves. There was a significant correlation between this index and the actual worm burdens of tracer calves when parasites were numbered by genera, and for heerbage samplings done according to “classical” techniques. The correlation is improved when data are combined from samples taken “close to fecal pats” and “far from fecal pats”. 相似文献
8.
In 2004, an experiment was carried out to evaluate the effect of biological control through feeding spores of Duddingtonia flagrans on parasitic gastroenteritis in lambs, kept under an evasive grazing system. In total 66 lambs were used. Forty naturally infected 3-month old ram lambs were weaned in mid June, and divided into four groups of 10 lambs. On 21 June, G1–G4 were moved to four separate virtually clean plots, they were moved after 4 and 8 weeks to similar plots, and housed after 12 weeks to be necropsied 16 days later. The other 26 lambs had been raised helminth-free, and were used as pairs of tracer lambs. All but one of these pairs, were grazed during the last 2 weeks on each plot. The remaining pair (TA) was grazed during the last 2 weeks on pasture (30 August to 13 September) on the plot that had been grazed by G3 between 19 July and 16 August, to study inhibited development in Haemonchus contortus. All lambs were fed 200 g of concentrates daily throughout the whole period, and those of G1 and G2 were also fed 500,000 spores of D. flagrans/kg bodyweight daily.
The faecal cultures demonstrated a high reduction in yield as a result of fungal application. However, no differences between groups were seen in weight gain, faecal egg counts, pasture larval counts, worm counts and tracer worm counts. H. contortus was the dominant species, and it is obvious that the moves at 4-week intervals prevented the development of severe haemonchosis. This is in particular demonstrated by the much higher worm counts in the two TA tracer lambs grazed. Nevertheless, increases to high faecal egg counts 3 weeks after the first and second moves, indicated acquisition of infection before these moves and at least subclinical haemonchosis. This was supported with the worm counts of lambs and tracer lambs. A higher proportion of inhibited early L4 than in other tracers and than in the permanent lambs were found in the pair of TA tracer lambs. This indicates that moves to new pastures in late summer and autumn delays the onset of inhibition. 相似文献
9.
A study was conducted over 3 years (1998-2000) to investigate larval availability of gastrointestinal nematodes from faeces of cattle reared under different parasite control schemes. These cattle were part of a parallel, but separate grazing trial, and were used as donor animals for the faecal material used in this experiment. At monthly intervals, faeces were collected and pooled from three groups of first-season grazing cattle. These groups were either untreated, ivermectin bolus treated or fed the nematophagous fungus Duddingtonia flagrans. The untreated and fungus treated animals were infected with gastrointestinal nematodes and the number of eggs per gram (epg) pooled faeces ranged between 50 and 700 in the untreated group and between 25 and 525 epg in the fungus treated group. Each year between June and September, artificial 1 kg dung pats were prepared and deposited on pasture and protected from birds. The same treatments, deposition times and locations were repeated throughout the study. Larval recovery from herbage of an entire circular area surrounding the dung pats was made in a sequential fashion. This was achieved by clipping samples in replicate 1/4 sectors around the dung pats 4, 6, 8 and 10 weeks after deposition. In addition, coinciding with the usual time of livestock turn-out in early May of the following year, grass samples were taken from a circular area centred where the dung pats had been located to estimate the number of overwintered larvae, which had not been harvested during the intensive grass sampling the previous year. It was found that recovery and number of infective larvae varied considerably within and between seasons. Although the faecal egg counts in 1999 never exceeded 300 epg of the faecal pats derived from the untreated animals, the abnormally dry conditions of this year generated the highest level of overwintered larvae found on herbage in early May 2000, for the 3 years of the study. Overall, biological control with D. flagrans significantly reduced larval availability on herbage, both during and between the grazing seasons, when compared with the untreated control. However, the fungus did not significantly reduce overwintered larvae derived from early season depositions (June and July), particularly when dung pats disappeared within 2 weeks after deposition. Very low number of larvae (<3 per kg dry herbage) were sporadically recovered from grass samples surrounding the ivermectin bolus faecal pats. 相似文献
10.
An experiment to determine the origin of populations of infective larvae of cattle nematode parasites on pasture during winter was conducted in south-west Western Australia. Six pasture plots were contaminated with worm eggs by grazing worm-infected cattle for periods of a month during summer and autumn. Each plot was contaminated at a different time from the rest. The levels of infective larvae were determined by counting the worm burdens of tracer calves which test-grazed the plots the following winter.Tracer calves which grazed the plots contaminated during summer acquired few worms, whereas those that grazed the plots contaminated during autumn acquired many worms. It was concluded that the hot, dry conditions prevailing during summer and early autumn prevented the development of eggs or survival of larvae in dung pats or free on pasture. In this environment, a programme of worm control which relied on administration of anthelmintic to grazing cattle to prevent autumn contamination of pasture would be most likely to succeed if the first treatment was given in early autumn. 相似文献
11.
A study was carried out to evaluate the effects of strategic early-season treatments with eprinomectin on first-season grazing calves exposed to strongyle infections on a naturally contaminated pasture. Two groups of first grazing season (FGS) calves were turned out in mid-May on two plots that were similar with respect to size and herbage infectivity. They grazed separately until housing at the end of October. One of these groups was given eprinomectin pour-on at turnout and 8 weeks later, while the other group served as untreated controls. The results showed that the treatments reduced gastrointestinal strongyle infections throughout the season as evidenced by lower faecal egg counts and serum pepsinogen levels compared with the controls. Furthermore, the results of herbage larval counts and postmortem worm counts in tracer animals demonstrated that the treatment had reduced herbage infectivity on the 'treated' plot. Finally, the chemoprophylactically treated calves had a better weight gain over the duration of the study than the untreated controls. 相似文献
12.
Silvina Fernández A Henningsen E Larsen M Nansen P Grønvold J Søndergaard J 《Equine veterinary journal》1999,31(6):488-491
An experiment was carried out in 1997 to test the efficacy of an isolate of the microfungus Duddingtonia flagrans against free-living stages of horse strongyles under conditions in the field and to assess the eventual effect of the fungus on the normal degradation of faeces. Faecal pats were made from faeces of a naturally strongyle infected horse, which had been fed fungal material at a dose level of 106 fungal unit/kg bwt. Control pats without fungi were made from faeces collected from the same animal just before being fed fungi. Faecal cultures set up for both groups of faeces to monitor the activity of the fungus under laboratory conditions showed that the fungus significantly reduced the number of infective third-stage larvae (L3) by an average of 98.4%. Five faecal pats from each batch of faeces were deposited on pasture plots at 3 times during spring-summer. The herbage around each pat was sampled fortnightly to recover L3 transmitted from faeces. The results showed that the herbage infectivity around fungus-treated pats was reduced by 85.8-99.4%. The remaining faecal material at the end of each sampling period was collected, and the surviving L3 were extracted. Significantly fewer larvae were recovered from the fungus-treated pats. Analysis of wet and dry weight of the collected pats, as well as their organic matter content, were performed to compare the degradation of faeces of both groups. The results indicated that the presence of the fungus did not alter the degradation of the faeces. 相似文献
13.
Faecal pats containing parasitic nematode eggs were deposited monthly on worm-free pasture, from mid-1975 to early in 1979, near Rockhampton in central Queensland. Pasture samples were collected monthly from beside these pats and the number of infective larvae on the samples was counted.
Cooperia spp. were the most numerous larvae on pasture all year round and Haemonchus placei were commonly present in low numbers. Small numbers of Oesophagostonum radiatum larvae were found, mostly during summer.
Dung beetle activity and rainfall influenced larval populations on pasture, but temperature did not. Beetles were not active in winter, and pats deposited in spring, summer and autumn when beetles were active yielded only 42, 44 and 26%, respectively, as many larvae per 1000 eggs deposited as winter pats. Pats in which beetle activity was minimal (feeding only), moderate and intense (complete destruction), yielded 43, 10 and 6%, respectively, as many larvae per 1000 eggs as intact pats.
Larval densities on pasture were highest after the first saturating rains during the spring-summer period and most of these larvae migrated from unattacked pats deposited in winter. Beetle numbers and activity increased with the summer rains and so few larvae were available to migrate onto pasture during late summer and autumn when the highest falls of rain were recorded. The regression of larval recovery on rainfall was positive and statistically significant when data collected soon after these very heavy rainfall periods were omitted from the analysis.
In 1977, drought-breaking rains increased the normal larval density on pasture 10-fold because larvae in pats deposited in the last 4 months of the drought migrated onto pasture immediately after the rains.
This work suggests that in summer rainfall areas where dung beetles are active, helminth control may be achieved by reducing the worm egg output from cattle during the winter. 相似文献
14.
A study was carried out to evaluate the effects of strategic early-season treatments with doramectin on first-season grazing calves exposed to trichostronglylid infection on a naturally contaminated pasture. Two groups of cross-bred Limousine/Red Danish calves were turned out in early May on two plots that were similar with respect to size and herbage infectivity. They grazed separately until housing in early October. One of these groups was given doramectin at turnout and 10 weeks later, while the other group served as untreated controls. The results showed that the treatments significantly reduced trichostrongylid loads throughout the season, as evidenced by significant reductions in both their Ostertagia ostertagi burdens and serum pepsinogen levels compared with the controls. Furthermore, the results of herbage larval counts and post-mortem worm counts in tracer animals demonstrated that the treatment had successfully suppressed herbage infectivity on the treated plot. 相似文献
15.
Rolf Jess Jrgensen 《Acta veterinaria Scandinavica》1980,21(4):658
A combined epidemiology and control investigation was performed with parasite-free calves turned out in May on a permanent pasture naturally contaminated with lungworm larvae the previous year. Before the start the field was divided into two plots. One plot was grazed by 12 calves after the first week of May. The other plot was grazed by 12 calves turned out two weeks later. Both groups as well as the plots grazed by them were subdivided six weeks after turning out. Based on a predicted rise in the pasture larval contamination with infective lungworm larvae, one subgroup of each main group was given a tactical anthelmintic treatment six weeks and again eight weeks after their date of turning out. Patent infections from overwintered larvae were detected in both main groups after four weeks of grazing, but not in all individuals of the late turned-out group. The excreted larvae gave rise to pathogenic pasture larval contaminations on the two initial plots five to six weeks after turning out. In the control groups, early turning-out resulted in approx. 10 times higher larval recoveries in faeces and pasture compared to late turning-out. Seven to eight weeks after turning-out critical, severe parasitic bronchitis had developed in the early turned-out control group. In the late turned-out controls, clinical signs were obvious but not critical. Outbreaks were not observed in any of the experimental subgroups, and no larval excretion was observed among them within four to five weeks following treatments. Similarly, no larvae were recovered from their pastures two weeks after treatment and onwards. A third treatment was given to both experimental groups on the same date (August 21) to suppress gastrointestinal parasitism. However, the level of this infection, appeared moderate, probably due to comparatively low precipitation and extensive supplementary feeding given in late summer to compensate for scarcity of grass. 相似文献
16.
Four controlled trials were conducted to evaluate the therapeutic and persistent efficacy of a new moxidectin formulation (moxidectin 1% nonaqueous injectable) against nematode parasites in cattle. This injectable moxidectin formulation, given as a single subcutaneous injection at a dose rate of 0.02 ml/kg BW to provide 0.2 mg moxidectin/kg BW, was highly efficacious (>90–100%) against larval and/or adult stages of many species of nematodes in cattle including, Dictyocaulus viviparus, Ostertagia spp., Trichostrongylus axei, Haemonchus placei, Trichostrongylus colubriformis, Cooperia spp., Nematodirus helvetianus, Strongyloides papillosus, Oesophagostomum radiatum and Trichuris spp. This formulation had persistent efficacy of >90% against D. viviparus for at least 6 weeks post-treatment, H. placei and Oe. radiatum for 5 weeks post-treatment, and Ostertagia spp. and T. axei for 2 weeks post-treatment. 相似文献
17.
This study investigated the effect of successive harvests of grazable herbage around cattle faecal pats on the population dynamics of infective gastrointestinal nematode larvae (L(3)). Faecal material, collected from naturally infected calves, was deposited as pats during summer, autumn and winter on three different topographical aspects within a moist, temperate region of New Zealand. Herbage was harvested four times (22-248 days) from around the faecal pats to a height of 2cm in three radial zones (0-20cm, 20-35cm and 35-45cm from the centre of the faecal pat) and L(3) extracted. Harvest date was determined by herbage mass to simulate grazing events. L(3) extracted from herbage were predominantly Cooperia spp. More L(3) were recovered from faeces deposited in summer and autumn, than those deposited during winter. L(3) concentration on herbage was highest (P<0.001) in the zone nearest the pat for all except the fourth harvest. Mean concentrations of L(3) on herbage were 11,447, 3154, 337 and 102 L(3)/kg dry matter herbage, for the four successive harvests, respectively. Microclimate differences as affected by aspect had a marked effect on herbage growth, but did not significantly affect L(3) concentration on herbage. In this study, L(3) remained aggregated close to the faecal pats they emerged from even after two successive harvests and significant rainfall. Successive harvests simulated the effect of repeated grazing events by a non-infective stock class. Two such grazings and the associated time, reduced L(3) presence on grazable herbage to <3% of the original population. Grazing strategies to generate clean pasture for vulnerable cattle are discussed in relation to these results. 相似文献
18.
G A Oakley 《Research in veterinary science》1982,32(2):163-169
A five year ley pasture was used as a source of natural infection with Dictyocaulus viviparus for cattle in anthelmintic trials. Pasture larval counts, faecal larval counts of permanently grazing calves and lungworm burdens harboured by tracer calves were monitored in three grazing seasons to assess the pattern of infection. Carrier calves were introduced at the beginning of the grazing season in the first two years of the study but not in the third. In the fourth year the pasture was subdivided into two paddocks where overwintered infection with and without carrier infection were compared. A control paddock exposed to carrier infection but no overwintered infection was also monitored. Pasture larvae survived the winter but carrier infection appeared to make a larger contribution to pasture larval counts and the onset of parasitic bronchitis in susceptible calves. In the absence of grazing cattle at the end of the grazing season the concentration of D viviparus larvae on the herbage fell rapidly to undetectable levels. Discrepancies between contamination of herbage by infective D viviparus larvae and infectivity of pasture for susceptible cattle occurred in all years but were particularly marked on the third year when natural immunity appeared to influence the number of lungworms accumulating in tracer calves. Failure to recover lung worms from tracer calves cannot be regarded as an accurate indication of lungworm free pasture. In the first three years the proportion of the lungworm population which was inhibited in tracer calves was higher early and late in the grazing season and negligible in mid season. This suggests that a predisposition to inhibition in larvae which have overwintered on pasture may influence the time of onset of parasitic bronchitis in the next grazing season, but results from the fourth year did not support this hypothesis. 相似文献
19.
M. Larsen P. Nansen J. Wolstrup J. Grnvold S. A. Henriksen A. Zorn 《Veterinary parasitology》1995,60(3-4):321-330
The present study was conducted in the 1993 grazing season with yearling calves exposed to a pasture with a natural mixed trichostrongyle larval infection. It was shown that daily feeding with the microfungus Duddingtonia flagrans during the first 2 months of the season led to a lowered herbage infectivity and a reduced acquisition of Ostertagia sp. and Cooperia sp. later in the season. In addition, the procedure delayed the onset of clinical disease. This was due to the nematode-destroying effects of the fungi in the dung excreted by the fungus-treated calves, as evidenced by results from a parallel in vitro assay on faecal larval cultures. The paper discusses future research needs before practical biological control can be implemented. 相似文献
20.
J L Duncan J Armour K Bairden G M Urquhart R J J?rgensen 《The Veterinary record》1979,104(13):274-278
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. 相似文献