Maintenance of acquired immunity to Trichostrongylus colubriformis in Merino sheep that are losing weight

SUMMARY The effect of loss of weight on maintenance of acquired immunity to Trichostrongylus colubriformis in Merino sheep was assessed in 2 experiments. Adult Merino sheep with previously acquired immunity to T colubriformis were housed in individual pens. Sheep were infected 3 times weekly with 2000 T colubriformis larvae. A diet based on chopped oaten and lucerne hay (experiment 1) or a pelleted diet based on lucerne hay and grain products (experiment 2), was offered in amounts to sustain a moderate gain in body weight, or was reduced step-wise to cause a loss in body weight. Worm egg counts were determined weekly. The reduction in feed on offer resulted in a mean weight loss of 4.0 kg over 9 weeks (experiment 1) or 4.8 kg over 7 weeks (experiment 2). Based on worm egg counts, there was no indication of a change in immunity to T colubriformis in either of the experiments. These results suggest that acquired immunity to T colubriformis in adult sheep that are exposed to infection is maintained during periods of moderate loss of weight associated with inadequate nutrition.     

Dietary impacts on the resistance of Merino lambs to Trichostrongylus colubriformis

AIM: To evaluate and compare the effects of a variety of diets on the resistance of young, lightweight Merino lambs to repeated low-dose infections of Trichostrongylus colubriformis.

METHODS: Ninety-six 12-week-old lambs were fed balanced or unbalanced diets, and given primary (at 19 weeks of age) or both primary (at 12 weeks) and secondary (at 19 weeks) infections of T. colubriformis. Suboptimal diets were low in total intake, rumen undegradable protein, total protein, readily available carbohydrate or minerals, or supplemented with cod liver oil. Liveweight was monitored fortnightly, protection was assessed from worm counts at the end of the study (25 weeks) and faecal nematode egg counts (FEC) conducted weekly, and mast-cell response assessed at the conclusion of the study from concentration of mast-cell protease in the jejunum. Concentrations of glucose in plasma were measured at 21 weeks.

RESULTS: Diets unbalanced for total protein, readily available carbohydrate, Mo or cod liver oil each increased the worm counts after 6 weeks of secondary infection (at 25 weeks of age) and FEC during secondary infection. Low intake of a highnutrient diet restricted liveweight gain but did not affect the development of resistance, as indicated by worm counts and FEC. Defi ciency of available carbohydrate resulted in reduced liveweight gain and reduced concentration of glucose in plasma. There was a reduction in jejunal mast-cell protease concentration in sheep fed unbalanced rations.

CONCLUSIONS: The parasitology results and reduction in the concentration of jejunal mast-cell protease observed in sheep fed unbalanced rations suggest that the nutrient defi ciencies and the fi sh-oil supplement may have acted via inhibition or retardation of the host's acquired immune response. Diets unbalanced for a range of components adversely affected the acquired protective response to T. colubriformis to a similar extent and appeared to involve a similar mechanism as each other. The quality of the diet was more important than quantity for the development of resistance, and soluble carbohydrate was an essential component.     

Enhancing immunity to nematode parasites in single-bearing Merino ewes through nutrition and genetic selection

The effectiveness of protein supplementation and genetic selection to enhance the resistance of periparturient Merino ewes to infection from gastrointestinal parasites was tested in a replicated grazing experiment. One hundred and twenty ewes from lines selected for increased resistance (R) to Haemonchus contortus or at random (C) were subjected to one of the three supplement groups that provided 0 or 250 g per day cottonseed meal for 5 weeks prior to, or for 6 weeks after the start of lambing. Faecal egg counts (FEC) of R ewes were consistently lower than those of C ewes but both groups exhibited a periparturient rise in FEC. Supplementation during the pre-partum period reduced FEC and increased ewe body weight gain. The benefits of pre-partum supplementation in reducing FEC continued to be apparent up to 10 weeks after supplementation ceased. There was a strong suggestion that the benefits to parasite resistance from protein supplementation were greatest in C ewes. Wool growth rates (15%) and birth weights (5%) were greater for C ewes but differences between the lines for lamb body weight had disappeared by day 97. The greatest benefit to resistance from protein supplementation was observed when ewes were experiencing a loss of maternal body weight. Conversely, no benefits to resistance were observed when ewes had moderate (78-107 g per day) rates of maternal weight gain. These results suggest that increased resistance as a result of protein supplementation is dependent on the prevailing supply and demand for scarce nutrients such as metabolisable protein (MP). Both genetic selection and protein nutrition are effective strategies to enhance host resistance to nematode infection during the periparturient period.     

Effects of molybdenum intake on primary infection and subsequent challenge by the nematode parasite Trichostrongylus colubriformis in weaned Merino lambs.

In a study designed to test the effect of molybdenum (Mo) on resistance to trichostrongylosis, the Mo content of the diet during primary infection of 8-month-old Merino lambs with Trichostrongylus colubriformis was found to affect the number of worm eggs found in the faeces during challenge and the total number of worms in the jejunum after six weeks of challenge infection. The optimal intake of Mo in this experiment was in the range of 4-8 mg sheep(-1)day(-1), approximately equivalent to feeding a diet containing 6-10 mg Mo kg(-1)dry matter (DM) and to 0.15-0.30 mg Mo kg(-1)liveweight. Lambs fed Mo at this rate showed a 90 per cent reduction in faecal egg count and total worm count six weeks after challenge compared with lambs fed quantities of Mo outside this range. The mechanism by which Mo exerted these effects was not defined, although the interactions of molybdenum, worm establishment and faecal egg count suggested that this trace element may be acting via an effect on the host's acquired immune response. This hypothesis is supported by the observed enhancement of immune responses (intestinal antibody and granulocyte numbers and in vitro worm-specific proliferation of lymphocytes) associated with Mo intake of 0.15-0.3 mg kg(-1)Lwt.     

Subclinical infection with the nematode Trichostrongylus colubriformis increases gastrointestinal tract leucine metabolism and reduces availability of leucine for other tissues

Gastrointestinal (GI) tract leucine metabolism was measured in 6- to 9-mo-old lambs subjected to trickle infection with Trichostrongylus colubriformis larvae and in separate animals that were not infected. Animals prepared with a jejunal catheter and with indwelling catheters into the aorta and the portal- (PDV) and mesenteric- (MDV) drained viscera were infused simultaneously with [1-13C] and [5,5,5-2H3] leucine to determine GI tract sequestration of leucine from arterial and luminal amino acid pools by tracer and tracee arteriovenous concentration differences. Leucine oxidative losses and net fluxes were also determined across the GI tract. Infection had no detectable effect on whole-body leucine flux, but it increased total GI tract leucine sequestration by 24% (P<.05) and GI tract oxidative losses of leucine by 22 to 41% (P<.01). Net PDV fluxes of leucine were decreased by 20 to 32% during the infection. The infection did not alter either the proportion of precursor leucine used by GI tract metabolism that was derived from the arterial leucine pool (.84 to .88) or the proportional sequestration of digesta-derived leucine during "first pass" absorptive metabolism (.12 to .18). These findings help to elucidate the metabolic basis for the reduced growth rates and nitrogen retention observed when animals are subjected to subclinical nematode infection.     

The influence of Ostertagia circumcincta and Trichostrongylus colubriformis infections on the pharmacokinetics of febantel in lambs

Plasma concentrations of febantel and its major metabolites fenbendazole, oxfendazole and oxfendazole sulphone were determined after oral administration of 7.5 mg/kg febantel in lambs before and 28 days after infection with 50 000 L₃ larvae of Ostertagia circumcincta or Trichostrongylus colubriformis. The febantel concentrations were always very low and only in a few samples higher than the detection limit. The mean decrease in AUC for the three metabolites for the infected sheep in comparison to the parasite naive sheep was 13.9%± 4.1% (mean± SEM) and 23.7%± 5.3% in the 0. circumcincta infected and the T. colubriformis infected lambs respectively. This reduction was only significant for the T. colubriformis infected group.
In order to determine a more complete pharmacokinetic profile, febantel was injected intravenously at a dose of 2.5 mg/kg in a further study.     

The establishment and development of Trichostrongylus colubriformis in goats

Twelve goats were inoculated with 20,000 infective larvae of Trichostrongylus colubriformis and two were killed on each of Days 4, 7, 11, 14, 18 and 21 after inoculation (DAI). The number of worms that established, and the site of development were recorded. Most of the worms established within the first 3 m of the small intestine. There was little relocation or loss of nematodes after establishment. The worms started to migrate from the mucosa to the lumen 11 days after infection. At 4 DAI, 63% of the worms were still at the late L3 stage; the remainder of the worm population had completed the third moult to the L4 stage. The population at 11 DAI comprised largely young adults. When 21 DAI was reached, about 57% of the female worms had eggs in their uteri.     

The intensity of resistance by mature Merino ewes against Haemonchus contortus and Tichostrongylus colubriformis in single-species and combined-species infection

Three-year-old, non-lactating and non-pregnant Merino ewes, raised on pasture under a program of strategic treatment with anthelmintic and found to be extremely resistant to "trickle" infection with Haemonchus contortus, were given single-dose infections with either H. contortus or Trichostrongylus colubriformis or both species together. The purpose was to ascertain the intensity of protective immunity against the 2 parasites in sheep with immunity acquired from a presumably slight exposure to infection. To provide a criterion, some infected ewes were immunosuppressed with corticosteroid, dexamethasone. Untreated ewes were extremely resistant to challenge infection with either 15,000 or 150,000 H. contortus or 15,000 T. colubriformis. Surprisingly, when mixed infection was given, egg counts for H. contortus were significantly elevated compared with infection by that species alone. Antibody to antigens from infective larval and adult H. contortus was measured in serum by enzyme-linked-immunosorbent assay (ELISA) during the course of infection. Serum titres against larval antigens were significantly depressed when infections with either H. contortus or T. colubriformis were permitted by immunosuppression with dexamethasone, whereas those against adult antigen were depressed when infection with T. colubriformis was permitted.     

Failure of Duddingtonia flagrans to reduce gastrointestinal nematode infections in dairy ewes

A field study was conducted on three Swiss farms to investigate the efficacy of Duddingtonia flagrans against naturally acquired infections of gastrointestinal nematodes in adult dairy sheep. On each farm the ewes were divided into two equal groups. One group received Duddingtonia during a period of 4 months at a daily dose rate of 10(6) chlamydospores per kilogram body weight, the second group acted as controls. At an overall moderate infection level in all farms D. flagrans did not have a significant effect on the observed parasitological parameters with the exception of a significantly reduced herbage infectivity in one farm. In contrast, the results from faecal cultures indicated a mean suppression of larval development during the fungus-feeding period between 82, 89 and 93% on the three farms, respectively. The discrepancy observed between the fungus efficacy in coprocultures and on pasture, which was also observed in several other studies deserves further research.     

Influence of adjuvants on immunity in rabbits vaccinated with infective larval somatic proteins of Trichostrongylus colubriformis

Adult New Zealand rabbits were vaccinated at 2-week intervals with three doses of 100 micrograms of infective larval somatic proteins (L3SP) administered subcutaneously with Freund's complete adjuvant (FCA) or beryllium hydroxide and then challenged orally with 10,000 L3. Groups of rabbits immunized orally with three doses of 5000 or 2000 L3 served as vaccination controls. Intestinal worm burdens on Day 21 after challenge revealed that beryllium hydroxide effectively potentiates the protective immunogenicity of L3SP. The level of protection obtained using the beryllium adjuvant (94.8%) was nearly as high as that in rabbits immunized with three doses of 5000 L3 (99.8%). Rabbits vaccinated using FCA showed very poor immunity (29.5%). Local and systemic antibody levels detected by radioimmunoprecipitation tests using 125I-L3SP showed very little correlation with the degree of protection. The beryllium hydroxide-treated group demonstrated significantly higher bile IgA antibody levels than other experimental rabbits. FCA-treated rabbits developed a much higher serum precipitating antibody response, detectable using gel double diffusion tests, than the beryllium group. Also, mucosal IgA antibody levels detected on Day 21 after challenge were significantly higher in the FCA group than in other groups.     

Regulation of the resistance to nematode parasites of single- and twin-bearing Merino ewes through nutrition and genetic selection

Periparturient Merino ewes obtained from lines of sheep that had been selected either for increased resistance to Haemonchus contortus (R) or at random (C) were supplemented, while grazing at pasture, with either nil or 250 g/day cottonseed meal (CSM) for the 6 weeks prior to or the 6 weeks after the start of parturition. Ewes from both supplement groups had lower (mean 66% reduction) faecal egg counts (FECs) during the postpartum period and this coincided with a period of maternal body weight loss. Factors which increased the rate of maternal body weight loss, such as pregnancy and lactation status, also increased FEC. Evidence is presented that the magnitude of the periparturient rise (PPR) in FEC in grazing ewes will be greatest during periods of maternal weight loss and at these times supplementation to increase metabolisable protein (MP) supply will be most effective in increasing resistance to nematode parasites. The resistance of R ewes to nematode parasites was greater than that of C ewes throughout the experiment and was sufficiently low such that anthelmintic treatment in a commercial environment may not have been required. Irrespective of actual FEC, ewes from all treatment combinations exhibited a PPR in FEC. Reduced FEC of R ewes resulted in reduced apparent pasture larval contamination after 18 weeks of continuous grazing but supplementation was ineffective in this regard. It is suggested that integrated parasite management (IPM) programs for periparturient ewes should make use of both protein supplementation and genetic selection to increase worm resistance and reduce dependency on anthelmintics for worm control.     

Resistance of Santa Ines and crossbred ewes to naturally acquired gastrointestinal nematode infections

This trial was carried out in Piracicaba, São Paulo State, Brazil, to comparatively evaluate the degree of resistance to naturally acquired gastrointestinal nematode infections in sheep of the following genetic groups: purebred Santa Ines (SI), SI crossbred with Dorper (DO × SI), Ile de France (IF × SI), Suffolk (SU × SI), and Texel (TE × SI). Fifteen ewes from each group were raised indoors until 12 months of age. At this age, they were moved to pasture that was naturally contaminated by nematode infective larvae and were evaluated from December to May, 2007. Rainfall ranged from 267 mm in January to 37 mm in April. Maximum and minimum mean temperatures ranged from 32.5 °C to 19.0 °C in March and from 25.9 °C to 12.8 °C in May. There was an increase in the mean number of eggs per gram of feces (EPG) after animals were placed on pasture with significant difference between the SI (80 EPG) and IF × SI (347 EPG) groups in January; and the DO × SI (386 EPG) and TE × SI (258 EPG) groups in May. The highest mean fecal egg count (FEC), 2073 EPG, was recorded for the TE × SI group in February. All groups showed a progressive reduction in body weight throughout the experiment of 12.0% (TE × SI) to 15.9% (SU × SI). In general, the animals with the highest FEC presented the lowest packed cell volumes (PCV); the highest correlation coefficient between FEC × PCV occurred in the SU × SI sheep in January (r = −0.70; P < 0.01). Similarly, there was an inverse relationship between FEC and blood eosinophil values, with the highest correlation coefficient in the TE × SI sheep in February (r = −0.64; P < 0.05). Immunoglobulin G (IgG) levels against Haemonchus contortus antigens increased in all groups as a result of the exposure to parasites and remained relatively constant until the end of the study, with the exceptions of SU × SI and TE × SI, which showed a rise in IgG levels during the last sampling that coincided with a reduction in mean FEC. In conclusion, crossbreeding Santa Ines sheep with any of the breeds evaluated can result in a production increase and the maintenance of a satisfactory degree of infection resistance, especially against H. contortus and Trichostrongylus colubriformis, the major nematodes detected in this flock.     

Measurement in vitro of a larval migration inhibitory factor in gastrointestinal mucus of sheep made resistant to the roundworm Trichostrongylus colubriformis

Lambs were challenged by dosing with 2500 T. colubriformis larvae per day for 34 weeks, rested for 4 weeks and then re-challenged with the infective larvae for a further 10 weeks. A technique for the measurement of inhibition of larval migration from agar gels was used to investigate the antiparasitic activity of mucus, obtained from the small intestine and abomasum of the lambs, at the end of the re-challenge period. Measurements were also made on ileal digesta samples collected at different times during the development of the initial resistance, the rest period and the re-challenge period, and on faeces collected during the re-challenge infection. The mucus from the small intestine and abomasum paralysed and inhibited larval migration from agar gels significantly more (P less than 0.01 and P less than 0.05, respectively) than corresponding mucus from parasite-free control animals. This inhibitory activity was also detected (P less than 0.01) in the ileal digesta of infected animals from Week 6 of primary dosing. The magnitude of the inhibition in the ileal digesta increased with time during the infection period and was again detected during re-infection (P less than 0.01). It was not detectable in resistant sheep towards the end of the rest period. Slight inhibitory activity was detected in faeces after 2 weeks of re-infection. These observations suggest that substances secreted into the lumen of the small intestine of infected animals are responsible for resistance against ingested larvae.     

Valine partitioning and kinetics between the gastrointestinal tract and hind limbs in lambs with an adult Trichostrongylus colubriformis burden

Intestinal parasitic infection increases the demand for AA because of increased protein synthesis in the intestine and increased luminal losses of AA, and these increased demands may be supported by increased mobilization of AA from the skeletal muscles. Two experiments were conducted to determine the effects of parasitic infection on valine kinetics within the gastrointestinal tract and hind limbs of lambs fed fresh forages. On d 1, lambs were given 6,000 stage-3 Trichostrongylus colubriformis larvae per day for 6 d (n = 6) or kept as parasite-free controls (n = 6) and fed fresh lucerne (Medicago sativa; Exp. 1) or fresh sulla (Hedysarum coronarium; Exp. 2). On d 48, valine kinetics within the mesenteric- (MDV) and portal-drained viscera (PDV) and hind limbs were obtained by carrying out concurrent infusions of para-amminohippuric acid into the mesenteric vein and indocyanin green into the abdominal aorta (for blood flow), and [3,4-(3)H]valine into the jugular vein and [1-(13)C]valine into the abomasum for 8 h (for kinetics). During the infusions, blood was collected from the mesenteric and portal veins and from the mesenteric artery and vena cava, and plasma was harvested. After the 8-h infusion, lambs were euthanized, ileal digesta were collected, and tissues were sampled from the intestine and muscle (biceps femoris). Tissues, digesta, and plasma were analyzed for valine concentration, specific radioactivity, and isotopic enrichment. In both experiments, intestinal worm burdens on d 48 were greater in parasitized lambs (P = 0.0001 and 0.003). In Exp. 1, parasitic infection increased (P = 0.03) the total valine irreversible loss rate (ILR) in the MDV and PDV. In Exp. 2, luminal ILR of valine in the MDV was reduced (P = 0.01); however, ILR of valine in the PDV was unaffected. Despite these changes within the MDV and PDV, parasitic infection did not affect the ILR of valine within the hind limbs, and valine transport rates were largely unchanged. We suggest that the increased mobilization of AA from the hind limbs that might have occurred in the early phase of inflammation was no longer required when the parasitic infection was established. The MDV and PDV data may indicate that the non-MDV parts of the PDV play an important role in this adaptation, which warrants further study.     

Cytokine and antibody subclass responses in the intestinal lymph of sheep during repeated experimental infections with the nematode parasite Trichostrongylus colubriformis

The expression of interleukin (IL)-4, IL-5, IL-10, IL-13, TNF-alpha and IFN-gamma genes, and parasite-specific IgM, IgG1, IgG2, IgA and total IgE levels, were monitored daily in intestinal lymph of sheep infected repeatedly with the nematode parasite Trichostrongylus colubriformis. Host genotype had a significant influence on IL-13 gene activity, with resistant-line (R) sheep consistently expressing higher levels of mRNA than susceptible-line (S) sheep. Mean gene expression of IL-13, IL-4 and IFN-gamma did not differ significantly between the first and second nematode challenge. Field-primed R and S as well as field-primed R and na?ve S sheep had lower mean gene expression of IL-5 and IL-10, respectively, during the second when compared to primary challenge. Genes for IL-13 and IL-5 were transiently and strongly up-regulated after nematode infection, particularly in animals with previous exposure to nematodes. Genes for TNF-alpha and IFN-gamma were also transiently up-regulated, but to a lesser extent and more typically after primary challenge. Na?ve sheep of both genotypes produced relatively little antibody response after primary challenge. A second nematode challenge resulted in large increases in the lymphatic levels of all antibody sub-classes which were significant for adult antigen-specific IgA and larval antigen-specific IgG1. In na?ve S line sheep, the larval-specific IgA and IgG2 response appeared delayed when compared to the R line animals. Field-primed R and S line sheep had relatively high lymphatic IgG1 levels prior to experimental infection and these did not change significantly afterwards. These results demonstrate that during nematode infections, the intestinal micro-environment of sheep is transiently skewed towards Th2 cytokine dominance, although IFN-gamma gene expression continues. This response is accompanied by increases of nematode-specific IgG1, IgA, IgG2 and IgM, as well as of total IgE in lymph plasma.     

Evolution of nematode community in grazing sheep selected for resistance and susceptibility to Teladorsagia circumcincta and Trichostrongylus colubriformis: a 4-year experiment

Field trials were undertaken to compare nematode population dynamics in two experimental flocks of rams selected for high and for low faecal egg counts (FECs) following two doses with 20000 infective larvae, 12000 Teladorsagia circumcincta (Tcirc) and 8000 Trichostrongylus colubriformis (Tcol) separated by a treatment. The selection was done by dosing 200 ram lambs (INRA 401 breed) and FECs proceeded with egg identification on days 28 and 35 post-each-infection. The 30 lambs with the lowest FECs constituted the group of permanent resistant rams (R), the 30 with the highest values the group of susceptible ones (S). Each group grazed separated pastures from April to November, this during 4 consecutive years. The contamination (number of eggs deposited) was estimated every 3 weeks on each pasture. The larval populations was measured by successive groups of tracer lambs. The infection of the permanent rams was done by slaughtering eight R and S rams on mid and end of each grazing season. Each autumn, new R and S ram lambs were selected to complete the R and S groups before turning out in spring. On third year, a third species (Haemonchus contortus, Hcon) was added to compare its behaviour with the two others.The regulation of parasite populations were studied by comparing what happens in R rams and pasture to S ones. The selected R ram lambs had a FEC of Tcirc 50% lower than S ones. This induced a similar lowered pasture contamination, a 25% decrease of infective larval population in years 2 and 3, and the worm burdens in R rams were the half of that in S rams after 2 years. The FEC of Tcol was 75% lower at the selection and induced similar difference in pasture contamination, but 65 and 96% reduction in L3 population, respectively, after 1 and 2 years. The worm burden was reduced by 99%. Adding Hcon species during the third year, in year 4, the L3 population was half in R pasture compared with S one, and R rams had 60% less worms of this species than S ones.This selection without any drench prevent outbreaks observed in young tracer lambs on S pasture. The comparison of selected resistant rams to selected susceptible ones and not to unselected controls gave more rapid information about the tendency of the regulation of the communities of nematode parasites.     

The effect of Johne's disease on production traits in Romney, Merino and Merino x Romney-cross ewes

AIM: To quantify the effects of clinical Johne's disease on the performance of Romney, Merino and Merino x Romney-cross ewes. METHODS: The performance of ewes was compared using eight birth cohorts (1971-1978). Merino and Merino-cross genotypes included New Zealand Merino and Australian Superfine Merino sources. Intensive monitoring of Johne's disease was undertaken over the production years 1975-1982. Positive diagnostic evidence of Johne's disease was established post mortem from lesions of granulomatous enteritis associated with high numbers of acid-fast bacilli. Over years, data on a total of 2,341 Romney ewes and 1,292 Merino and Merino x Romney-cross ewes were recorded, consisting of annual records of liveweight (LWT), greasy fleece weight (FWT), number of lambs born per ewe per year (NLB), and lifetime productivity of ewes. RESULTS: A total of 82 (3.5%) Romney ewes and 62 (4.8%) Merino and Merino x Romney-cross ewes were diagnosed with clinical Johne's disease over the 8-year monitoring period, equivalent to 0.9% and 1.2% annual cases for these breeds, respectively, of the ewes present at mating. The percentage of clinical cases (p<0.04) and the age at death from Johne's disease (p<0.02) were lower for Romneys than for Superfine Merinos. The mean age of death from Johne's disease was 3.41 (standard error (SE) 0.06) years, lower than the mean disposal age from the flock of 5.03 (SE 0.02) years for clinically normal ewes (p<0.001). In their final year of production, ewes with clinical Johne's disease had lower LWT by 5.3 kg (10.5% of the mean; p<0.001), lower annual FWT by 0.54 (SE 0.10) kg (14.2%; p<0.001), fewer NLB by 0.15 (SE 0.07) lambs (13%; p<0.05), and lower litter weaning weights by 3.6 (SE 1.3) kg (15%; p<0.01) compared with clinically normal ewes. The size of the production losses associated with Johne's disease depended on the age to which ewes survived. Considering all production years of ewes (up to 8 years), the total weight of lambs weaned by ewes with clinical Johne's disease was 30.9 (SE 3.4) kg lower (46%; p<0.001) than the total from clinically normal ewes. CONCLUSIONS: Clinical Johne's disease led to significant losses in LWT, FWT, NLB, and in the lifetime production of ewes, amounting overall to a 46% reduction in productivity (p<0.001). CLINICAL RELEVANCE: Productivity losses from clinical cases of Johne's disease would be of considerable economic importance in flocks with a high incidence of the disease. The lack of good diagnostic tests for Johne's disease in the live animal, and the lack of active surveillance programmes, has made it difficult to establish the true prevalence of Johne's disease in sheep flocks in New Zealand, and its economic consequences.     

The effect of diet fed to lambs on subsequent development of Trichostrongylus colubriformis larvae in vitro and on pasture

Contrasting herbage diets were fed to lambs to evaluate their effect on subsequent development of Trichostrongylus colubriformis larvae in faeces and on pasture. The diets had either no condensed tannin (CT), lucerne (Medicago sativa cv. Otaio), white clover (Trifolium repens cv. Tahora), or had moderate to high concentrations of CT, sulla (Hedysarum coronarium cv. Grassland Aokau), Lotus corniculatus (cv. Grasslands Goldie), L. pedunculatus (cv. Grassland Maku), Dorycnium pentophyllum, and Dorycnium rectum. Trials were carried out in summer (warm) and in autumn (cool and moist). In summer, egg viability was evaluated in vitro with egg hatch and larval development assays. In both seasons faeces were placed on pasture to compare recovery of eggs and larvae from faeces and larvae from herbage on the high and low fertility farmlets on the AgResearch Ballantrae Hill Country Research Station. D. rectum and D. pentophyllum diets decreased (P<0.01) egg hatching and larval development in laboratory assays relative to other diets. In summer, the number of larvae recovered from faeces placed on pasture was far greater (P<0.001) if the lambs had been fed lucerne than any other diet, whereas recovery was always lowest from faeces of sheep fed D. rectum and D. pentophyllum. Although dietary differences were lower in autumn than in summer, larval recoveries were lower (P<0.05) from faeces of lambs fed D. rectum and L. corniculatus than from white clover, lucerne and sulla diets. This study indicates that the diet of the host can have a significant impact on egg hatching and the subsequent development of T. colubriformis larvae in the laboratory and in the field. In particular, D. rectum consistently reduced T. colubriformis development. Effects measured in vitro generally under-estimated effects measured under field conditions.