Investigation of the combined efficacy of two Haemonchus contortus vaccines in weaner Merino sheep.

This study was conducted to investigate whether co-administration of Barbervax® (Bvax) with Haemonchus contortus surface larval antigen (HcsL3) would increase the protective efficacy and duration of protection against H. contortus infection in weaner Merino sheep. A total of 132 10-month-old weaned Merino ewe lambs were randomly allocated into six treatment groups (n = 22). Sheep were vaccinated four times with either Barbervax® (Bvax), H. contortus L3 surface larval antigen (HcsL3), combined vaccination (Bvax + HcsL3), Bvax + AlOH, HcsL3 + Saponin or remained as unvaccinated controls. Aluminium hydroxide (AlOH) and saponin adjuvants were included in HcsL3 and Bvax vaccines respectively. The first three vaccinations were given at 4 week intervals and the fourth vaccination provided as booster, 9 weeks later. All animals were treated with Zolvix™ (monepantel 25 mg/mL, Elanco) at the third vaccination and commencing two weeks later, artificially trickle infected with H. contortus L3. Worm egg count (WEC), packed cell volume (PCV), antibody titre and bodyweight were measured throughout the study as was specific antibody directed against each antigen using ELISA. The administration of Bvax and HcsL3, alone or in combination, induced an antibody response against HcsL3 but only the Bvax and the combined treatment elicited an antibody response to the Bvax antigen. The targeting of HcsL3 by each vaccine was confirmed by immunofluorescence staining of H. contortus L3. However, only the booster vaccination in the Bvax treatments reduced WEC to levels below untreated controls. The HcsL3 vaccine did not reduce WEC in this experiment and co-administration with Bvax did not improve the efficacy and duration of protection against H. contortus infection.

Protective efficacy of Barbervax® in Merino weaner sheep trickle infected with five doses of Haemonchus contortus infective larvae.

Barbervax® protects sheep against H. contortus infection; however, the level of protection afforded by the vaccine at different levels of larval challenge under field conditions has not been reported. Therefore, this study was designed to investigate the protective efficacy of Barbervax® at increasing doses of trickle infection with H. contortus third stage larvae (L3). Merino lambs (220) were randomly allocated to challenge treatment groups and received Barbervax® vaccinations at approximately 8, 11, 15 and 22 weeks of age or not vaccinated (control group). Animals within each treatment group were subjected to one of five levels of H. contortus L3 trickle infection (0, 300, 600, 1200 and 2400 L3/week in two split doses). Trickle infections started two weeks after the third vaccination (week 0). Worm egg count (WEC), packed cell volume (PCV), antibody titre and bodyweight were measured at week 2 (start of trickle infection), week 5 and then every two weeks for 11 weeks. Vaccinated weaners had a significantly (P < 0.0001) lower faecal WEC than unvaccinated control animals. Vaccination induced significant specific antibody responses that were not influenced by level of trickle infection. Vaccination significantly reduced the number of weaners requiring salvage anthelmintic treatment (9.4% vaccinated weaners versus 33.6% unvaccinated). Vaccine protective index based on WEC was similar at all challenge levels (overall mean> 60%) and at the higher challenge levels did not reduce H. contortus infection to levels that would not require anthelmintic treatment. Therefore, it is suggested that under conditions of high larval challenge the use of Barbervax® should be integrated with other control methods.

Evaluation of Barbervax® vaccination for lambing Merino ewes.

The Barbervax® vaccine, directed against Haemonchus contortus, has been commercially available in Australia since 2014. However, to date the optimal timing for booster vaccination of lambing ewes has not been identified. Therefore the aim of this experiment was to compare the efficacy of Barbervax® when administered to ewes one week prior to the commencement of lambing versus at lamb marking 7 weeks later. A total of 400 single bearing Merino ewes, run on a commercial sheep property in the New England region of New South Wales, previously primed with Barbervax® were enrolled in the project. The experiment incorporated a cross-over design with two treatments imposed pre-lambing; Barbervax® booster vaccination with anthelmintic (n = 200) or anthelmintic alone (n = 200). At lamb marking, there were four treatment groups (n = 100) and anthelmintic was administered to all treatment groups. The four treatment combinations were Barbervax® booster vaccination at pre-lambing and lamb marking (V-V), booster vaccination only at pre-lambing (V-A), booster vaccination only at lamb marking (A-V) or anthelmintic only at both at pre-lambing and lamb marking (A-A). Pre-lambing Barbervax® booster vaccination of ewes induced an antibody response and, when used in conjunction with anthelmintic, significantly reduced worm egg counts (WEC) assessed at 7 weeks (lamb marking) and 13 weeks later, relative to treatment with anthelmintic alone. Booster vaccination plus anthelmintic treatment at lamb marking was not effective at reducing WEC compared to anthelmintic treatment alone. Body weight and packed cell volume did not differ between the treatment groups throughout the trial. In conclusion Barbervax® booster vaccination at pre-lambing provides additional protection for reproducing ewes given an effective short acting anthelmintic at this time.

A simplified Barbervax® vaccination regimen in lambs to evoke immunological protection to Haemonchus contortus.

This study assessed the feasibility of altering the pre-weaning vaccination schedule of the commercially available Barbervax® vaccine directed against Haemonchus contortus, to avoid the 2nd priming vaccination which typically falls between lamb marking and weaning. Merino lambs (n = 175) born to maiden ewes, located in the Northern Tablelands, New South Wales, were randomly allocated to treatment groups (n = 35) and subjected to five different vaccination regimes. One group remained as unvaccinated controls and another had the full set of three priming doses. The other three groups were vaccinated only at marking and weaning receiving a double dose of vaccine at marking and/or weaning. The hypothesis tested was that reducing the interval between lamb marking and weaning to 6 weeks, and providing a double dose of vaccine at marking, weaning or both would remove the need for the second priming vaccination between lamb marking and weaning. This priming vaccination in the vaccination protocol necessitates an additional mustering of ewes with young lambs at foot and is a time consuming and costly exercise which increases the risk of mis-mothering. Blood and faecal samples were collected at frequent intervals for worm egg count (WEC), larval differentiation and H. contortus vaccine specific ELISA antibody analysis. Overall, the results supported the hypothesis, and it was found that antibody titres and WEC reductions equivalent to the registered vaccine regimen were achieved by the alternative regimens. This finding requires further investigation under a wider range of conditions. Deviation from the registered vaccination protocol would constitute off-label usage, and at this time and until further evaluations are done these deviations are not recommended.

Immune-mediated responses account for the majority of production loss for grazing meat-breed lambs during Trichostrongylus colubriformis infection.

The hypothesis tested in this experiment was that Trichostrongylus colubriformis infection would reduce growth rates of grazing meat-breed lambs; however production loss would be reduced by suppression of the host immune response. The experiment had a 3×2 factorial design using 6-7 month old meat-breed lambs which remained uninfected or infected (IFY) with 2000 or 4000 T. colubriformis L3/week for 12 weeks and were immunosuppressed (SUPY) using methylprednisolone acetate once weekly or remained non-immunosuppressed (SUPN). Immunosuppression increased worm egg counts (WEC) of infected lambs (SUPY 2421 eggs per gram (epg), SUPN 1154 epg on day 84, p<0.05) and T. colubriformis burdens (p<0.05-0.10) and reduced circulating eosinophils (p<0.05 on days 11, 42, 56 and 84) and intestinal total antibody titres (p<0.02). There was a significant (p<0.05) interaction between the main effects of infection and immunosuppression with infection having a larger negative effect on the liveweight of non-immunosuppressed lambs. The immunological response of the host to T. colubriformis infection accounted for 75% of the overall cost of infection (3.1kg) with the majority of this cost occurring during the first 35 days of infection. In contrast, most of the cost associated with the direct effect of infection occurred after day 35. These results confirm in grazing meat-breed lambs that the host's immunological response to T. colubriformis infection is the major component of production loss.

Removal of tapeworm (Moniezia spp.) did not increase growth rates of meat-breed lambs in the Northern Tablelands of NSW.

This experiment tested the hypothesis that growth rates of meat-breed lambs would not be affected by infection with tapeworm (Monieza spp.). Two experiments, conducted in successive years (2012 and 2013) on a commercial sheep farm on the Northern Tablelands of NSW, assessed growth rates of meat-breed lambs, between 4 and 6 months of age, following the removal of the cestode, Monieza spp. (or commonly referred to as tapeworm). In 2012 and 2013, 93 and 85 lambs respectively were randomly allocated to two treatment groups. One group (Prazi) was treated with praziquantel, levamisole and abamectin to remove tapeworm and gastrointestinal nematode infection (GIN) while the second group (Control) was treated with levamisole and abamectin to remove only GIN. Tapeworm prevalence and egg counts of Control lambs ranged from 25 to 77% and 7 to 730 eggs per gram (epg) respectively and were significantly (p<0.005) reduced in Prazi lambs, following treatment, at all time-points in both years. Pre-treatment GIN worm egg counts ranged between 1684 and 3368 epg with Haemonchus contortus the dominant species. Post-treatment GIN worm egg counts were similar between Prazi and Control groups, expect on one occasion (Day 65, 2013) when GIN worm egg counts were expectantly higher (p<0.005) in Control lambs. No significant difference in growth rates were observed between treatment groups in either year with overall group mean daily bodyweight gains being 95 and 81 g/day (p=0.053) in 2012 and 132 and 134 g/day (p=0.784) in 2013 for the Prazi and Control groups respectively. This experiment confirmed that removal of tapeworm burdens did not increase growth rates in meat-breed lambs on a commercial sheep farm in the Northern Tablelands of NSW.

Persistent challenge with Trichostrongylus colubriformis and Haemonchus contortus larvae does not affect growth of meat-breed lambs suppressively treated with anthelmintics when grazing.

This experiment tested the hypothesis that persistent challenge with anthelmintic susceptible Trichostrongylus colubriformis and Haemonchus contortus larvae would not affect growth of grazing, meat-breed lambs when suppressively treated with anthelmintics. The experiment was a 2×2 factorial design using 6-7 months old White Suffolk X Border Leicester/Merino (meat-breed) lambs which were either infected with 2000 T. colubriformis and 300 H. contortus L3/week (IF) or remained uninfected (UIF) for 9 weeks and were either treated (TX) with a combination of short and long-acting anthelmintics or remained untreated (UTX). Lambs grazed as one flock and were rotated between paddocks to avoid autoinfection from pasture. Lambs were humanely euthanised on day 63 and the abomasum and small intestine collected to determine total worm burdens and tissue antibody response specific to T. colubriformis. As expected, worm egg count (WEC) and worm burden were significantly higher in IF UTX lambs (p<0.001). WEC was dominated by H. contortus and peaked at 2,325 epg on day 63 but remained at zero for the other treatment groups for the duration of the experiment. Tissue antibody responses were evident in IF lambs (titres; 9982 vs 2767, p=0.012) but treatment had no effect (titres; 5912 vs 5349, p=0.829). Lambs grew an average of 2.6 kg during the experiment with no difference between IF TX and UIF TX groups (p=0.432). Elevated tissue antibody responses were not associated with differences in growth. Results from this experiment support the hypothesis that persistent larval challenge with anthelmintic susceptible H. contortus and T. colubriformis will not affect growth of grazing, meat-breed lambs when suppressively treated with effective anthelmintics. Therefore the use of sheep suppressively treated with effective anthelmintics appears to be a valid substitute for gastrointestinal nematode-free lambs in field experiments.

Nutrient partitioning of Merino sheep divergently selected for genetic difference in resistance to Haemonchus contortus.

This research was designed to determine if divergent genetic selection for resistance to Haemonchus contortus had produced correlated changes in the metabolism of amino-nitrogen in the absence or presence of H. contortus infection. Partitioning of amino acid-nitrogen between tissues was determined in 42 Merino weaner wethers from the CSIRO Haemonchus selection flock, increased resistance to Haemonchus (IRH), decreased resistance to Haemonchus (DRH) and random bred control (C) selection lines. Weaner wethers were fed a restricted diet (9.8 MJ ME/kgDM, 86 gMP/kg DM) calculated to allow a gain of 125 g/d bodyweight throughout the experimental period and were either worm-free or trickle infected with H. contortus. At 8 weeks post-infection animals were injected with (15)N-labelled duckweed directly into the abomasums. Animals were euthanased at either 6 or 24h after the injection to collect tissue samples for calculation of percentage recovery of (15)N in tissue and to determine abomasal worm counts. Worm egg count and worm counts at week 8 of infection were lower in animals from the IRH line. IRH animals had a lower N digestibility, increased oxidation of amino acids and lower N balance but whole-body protein flux was unaffected. Amino acid metabolism, as assessed from (15)N uptake and excretion in response to H. contortus infection, differed between IRH and DRH animals. In IRH animals a greater recovery of (15)N in the thymus and abomasal smooth muscle indicated greater partitioning of amino acids towards the immune response. In DRH animals an increased recovery of (15)N in the spleen, in response to infection, may be a possible adaptation. It appears that divergent selection for worm egg count has not been associated with symmetrical changes in amino acid metabolism, but rather the partitioning of amino acid resources reflects each selection line's independent response to infection.

Rumen function and digestion of Merino sheep divergently selected for genetic difference in resistance to Haemonchus contortus.

The aim of this experiment was to investigate the effect of Haemonchus contortus infection on rumen function and digestion of Merino sheep from lines divergently selected for genetic resistance to H. contortus. Rumen function and whole-tract digestibility were determined in 29 Merino weaner wethers from the CSIRO Haemonchus selection flock, increased resistance to Haemonchus (IRH), decreased resistance to Haemonchus (DRH) and random bred control (C) selection lines. Wethers were fed a restricted low quality roughage diet (5.9 MJ ME/kg DM, 29 g MP/kg DM) to maintain weight and were either uninfected (NIL) or infected with H. contortus (INF). Measurements were taken at week 7 of infection, while animals were housed in metabolic crates. Animals were then euthanased to determine abomasal worm counts. IRH had significantly lower worm egg count than DRH and C lines however, adult worm and larval counts, though lower in IRH animals, did not differ significantly. DM and OM digestibility and in sacco degradability of IRH and DRH selection lines were greater than C animals. Rumen function of animals from the IRH line was altered in response to H. contortus infection with an increase in fluid outflow and turnover rate and a decrease in propionic acid concentration. These changes may be a component of a greater host resistance and begin to explain the failure of IRH animals to translate lower worm egg count into greater bodyweight gain.

Voluntary feed intake and diet selection of Merino sheep divergently selected for genetic difference in resistance to Haemonchus contortus.

This research was designed to determine if divergent selection for resistance to Haemonchus contortus had produced correlated changes in voluntary feed intake and diet selection. Voluntary feed intake, diet selection and production were determined in 54 Merino weaner rams from the CSIRO Haemonchus selection flock, increased resistance to Haemonchus (IRH), decreased resistance to Haemonchus (DRH) and random bred control (C) selection lines. Weaner rams were fed ad libitum either a high (9.2 MJ ME/kg DM, 90 g MP/kg DM) or moderate (6.3 MJ ME/kg DM, 30 g MP/kg DM) quality diet and given the choice between the two diets, when uninfected (NIL) or infected with H. contortus (INF). Symmetrical response to divergent selection for worm egg count (WEC) was not matched by a symmetrical change in feed intake and there was no difference in diet selection between selection lines. Feed intake, growth and wool production of DRH animals remained the same as that of IRH, yet DRH animals had five times greater WEC than IRH. This study begins to explain the mechanisms that allow resistant animals to effectively prevent establishment and/or development of H. contortus, by maintaining a greater immune response to infection through higher circulating eosinophils, plasma globulin and IgG(1) antibody titres. Susceptible animals have displayed resilience by improving feed conversion efficiency and increasing protein synthesis.