Performance of Barbervax® Vaccine, With Focus on Lactating Ewes and Weaner Merino Sheep

Abstract

<p>This thesis describes a series of studies designed to enhance and investigate the protective efficacy of Barbervax^® vaccine among the more susceptible group of sheep, particularly in weaner, pregnant and lactating ewes of the Australian Merino. The objective of the first study (Chapter 3) was to determine the protective efficacy of Barbervax^® vaccination when administered prior to lambing or at lamb marking in combination with short acting anthelmintic, to provide protection for lactating merino ewes. Single bearing Merino ewes (400) previously primed with Barbervax^® were randomly allocated into two treatment groups at pre-lambing and four treatment combinations at lamb marking. Ewes received Barbervax^® booster vaccination with anthelmintic (n=200) or anthelmintic alone (n=200) at pre-lambing. These two groups were further allocated into four treatment combinations at lamb marking as 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 (AV) 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 combination with anthelmintic, significantly reduced worm egg counts (WEC) assessed at seven 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. Bodyweight and packed cell volume did not differ between the treatment groups throughout the trial.</p> <p>The second study (Chapter 4) aimed to investigate of the protective efficacy of Barbervax^® at different levels of <i>H. contortus</i> larval challenge under field condition. Weaner Merino lambs (220) were randomly allocated to challenge treatment groups and given four Barbervax^® vaccinations or not vaccinated (control group). Animals within each treatment group were subjected to one of five levels of <i>H. contortus</i> 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 measurements were taken as in Chapter 3. Barbervax^® significantly (P < 0.0001) lowered faecal WEC in vaccinated weaners compared to unvaccinated controls. 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 range 60-75%). However, this protection did not reduce <i>H. contortus</i> infection to levels that would not require anthelmintic treatment at higher challenge levels.</p> <p>Lastly, this thesis has investigated the protective efficacy of Barbervax^® when coadministered with naturally exposed antigen from <i>H. contortus</i> surface larval antigen with the aim to enhance the protection period of Barbervax^® against <i>H. contortus</i> infection. Merino weaner ewes (132) were randomly allocated into six vaccination treatment groups (n=22). Sheep were vaccinated four times with either Barbervax^® (Bvax) or <i>H. contortus</i> L3 surface larval antigen (HcsL3) or combined vaccination (Bvax + HcsL3), Bvax + AlOH, HcsL3 + Saponin or unvaccinated control. Aluminium hydroxide (AlOH) and saponin adjuvants were used in HcsL3 and Bvax vaccines, respectively. Barbervax^® (Bvax), <i>H. contortus</i> L3 surface larval antigen (HcsL3) or combined vaccination (Bvax + HcsL3), Bvax + AlOH, HcsL3 + Saponin and unvaccinated control. Aluminium hydroxide (AlOH) and saponin were adjuvants used in HcsL3 antigen and Barbervax^® vaccine, respectively. The first three vaccinations were priming doses given at four week intervals and the fourth booster vaccination was provided nine weeks after the third priming vaccination. All animals were treated with Zolvix^TM monepantel (25mg/ml, Elanco) at the third vaccination and commencing two weeks later, trickle infected with 300 <i>H. contortus</i> infective larvae (L3) twice weekly later for five weeks. Worm egg count (WEC), packed cell volume (PCV), antibody titre and body weight were measured throughout the study as in Chapters 3 and 4. The administration of Bvax, and HscL3, alone or in combination, induced an antibody response against HcsL3 but only the Bvax and combined treatment elicited an antibody response to the Bvax antigen. Both vaccines targeting HcsL3 was confirmed by immunofluorescence staining of exsheathed <i>H. contortus</i> L3 larvae incubated in sera from the different treatments. However, only the booster vaccination in the Bvax treatment reduced WEC to levels below untreated controls, and none of the vaccine treatments had a significant effect on PCV and body weight.</p> <p>The results of this thesis demonstrated that Barbervax^® was ineffective for controlling <i>H. contortus</i> infection in ewes when booster vaccinations occurred during early-mid lactation and lambs failed to produce protective responses under high challenge. High levels of challenge require integration with measures that reduce pasture contamination increase host resistance and best use anthelmintics. Enhancing the level and duration of Barbervax^® protection by co-administration with <i>H. contortus</i> surface larval antigen was unsuccessful but warrants further investigation. Overall, the findings of this study will provide valuable information to aid in the development of effective Barbervax^® vaccination protocols for use in reproducing ewes and weaner Merino sheep.</p>