EFFICACY EVALUATION OF THE ANTI-TICK VACCINE GAVAC® AGAINST TEXAS OUTBREAK STRAINS OF RHIPICEPHALUS MICROPLUS AND R. ANNULATUS
Location: Tick and Biting Fly Research
Project Number: 6205-32000-031-27
Start Date: Aug 02, 2010
End Date: Sep 30, 2012
a)Conduct controlled stall studies to assess the level of efficacy achieved with the anti-tick vaccine Gavac® against Texas outbreak strains of Rhipicephalus microplus and R. annulatus. It is hypothesized that vaccination with Gavac® will lead to similar results as described in recent publications involving ticks from northern Mexico and south Texas producing 60-85 and 99-100% control of R. microplus and R. annulatus, respectively.
b) Determine if Bm86 anti-body titers correlate with tick efficacy and calculated control of tick fecundity. We hypothesize that Bm86 antibody titers at the time of infestation will correlate with acute tick mortality and control of IF.
c) Compare sequences between gene encoding Bm86 in the R. microplus Texas outbreak strain and the recombinant Bm86 antigen in Gavac® to assess identity and correlation to vaccine efficacy. It is hypothesized that Bm86 sequence identity will correlate with previously reported Gavac® efficacy in Mexico.
Two strains of ticks, one R. annulatus and one R. microplus, collected in Texas and 20 heifers will be used. Half of the animals will be vaccinated with Gavac® on weeks 0, 4, and 6 according to the label recommendations. The remaining half will be injected with only a saline/adjuvant carrier. On week 8, each animal will be infested with 250 mg (>2,500) larvae. Five of the Gavac®-vaccinated and 5 of the carrier-injected animals will be infested with R. microplus. The remaining 10 animals will be infested with 250 mg of R. annulatus. The Gavac label recommends a 6-month booster. To determine the efficacy of the vaccine before the boosting, all cattle will be returned to the barn at 4.5 months after the last vaccination and each will be infested with 250 mg of larvae using the identical treatment groups as the initial experiment. After all ticks have been recovered from the host animals (>31 days after infestation), each animal will be inspected for ticks and dipped in coumaphos by APHIS, VS Cattle Tick Inspectors according to the CFTEP standard operating procedures, injected with 2ml of Gavac® (6-month booster), and held on pasture for 4.5 months when the cattle will be brought into the barn and the entire process repeated a third time.
Engorged females will be collected and counted. From each tick collection on each animal, a random sample of up to 10 engorged females (whenever possible) will be saved to obtain ovipositional data. The percent hatch will be determined by counting the number of hatched and un-hatched eggs. After all data have been collected, the index of fecundity (IF) of ticks recovered from each animal on each day of the evaluation period will be calculated. The data obtained from cattle infested with R. microplus and R. annulatus will be analyzed as separate datasets. All response measurements (percent control, number of ticks per calf, IF, female weight, egg mass weight, percent egg hatch) will be assessed for normality and assumptions of equal variance. It is anticipated that most of these data measures will be analyzed by Student's t-test (SAS). Data in the form of percentages (percent control and percent egg hatch) will most likely be non-normal responses in the Beta distribution and will either be transformed using an arcsin square root calculation prior to analysis by t-test (Davey et al. 2001), or the response will be generalized using the appropriate distribution within the GLIMMIX procedure in SAS.
Before each immunization, and monthly between boosting immunizations, blood samples will be collected from each calf. Serum will be collected after centrifugation and stored at -80°C thereafter. Serum antibody titers will be determined using an antigen-specific indirect enzyme-linked immunosorbent assay (ELISA). This serum antibody concentration measure will be analyzed for differences between vaccinated and control cattle over time using a repeated measures analysis of variance (ANOVA) test (P< 0.05) within the MIXED procedure in SAS. Correlation analyses will also be conducted to compare antibody titers at the time of tick infestation and vaccine efficacy calculated in individual cattle (Almazan et al. 2010).