Authors

Discussion

Multiple studies have attempted to develop a vaccine to control Fasciola hepatica infection in cattle and sheep, evaluating a number of F. hepatica proteins. Despite being a logical vaccine target, surface tegument proteins have not yet been studied. Here we report results from 5 trials in cattle evaluating 9 recombinant tegument proteins (Tetraspanins (TSP) 2 and 3; Annexins (Anx) 2, 3 and 8; novel Tegument proteins  (Teg) 1, 5, 22 and 25, as well as the Transforming growth factor beta (TGF-β) like homologue (termed FhTLM) protein from F. hepatica, as vaccines, both alone or as double/triple/quadruple combinations. Tetraspanin 2 was also evaluated fused to the E. coli heat-labile entero-toxin B subunit LTB adjuvant (termed LTB-TSP2) using intranasal vaccination of cattle. Glutathione S-transferase (GST), a known efficacious vaccine protein, was also tested in combination with TSP2, Anx2 and Anx3/Anx8. The FhTLM protein was evaluated in combination with GST only. In total, 17 different vaccine groups were assessed.

Groups of female Angus/Angus cross cattle (n=6-7/group; age 6-18 months) were vaccinated with proteins formulated in Freund’s adjuvant as a means to screen a large number of proteins, in an attempt to identify an optimal vaccine combination. In one trial, a nanoparticle commercial adjuvant was tested. After experimental infection with metacercariae, we analysed total IgG and IgG1/IgG2 subtype antibody responses, faecal egg counts and adult fluke numbers recovered from livers at completion of the trial. We also assessed the fraction of animals which showed the lowest fluke numbers (ie. less than the lowest value in the Control group) as another indicator of protection since economic loss is associated with low fluke counts (<30 flukes). A high % of animals with low fluke counts will result in lower herd production losses which is the key parameter for a commercial vaccine.

There was considerable variation in vaccine efficacy between the various proteins tested, using single or combination vaccines. There were no reproducible negative associations between IgG1/IgG2 responses to individual vaccine proteins and low fluke counts. Some reductions in FEC values were observed but were variable between groups. Neither LTB-TSP2 or TSP2 alone, nor combinations of Teg1/5/22/25, induced significant protection. Significant reductions in mean fluke numbers/group (38-48%) were observed in 4 vaccine groups with various combinations of TSP3, Anx2, Anx3 or Anx8 as well as with FhTLM+ GST. However, there was variation between trials in vaccine efficacy with certain combination vaccines. Vaccine efficacy, based on % reductions in mean fluke counts, was closely associated (r ² = 0.915) with the fraction of animals in each group showing fluke counts less than the lowest value in control groups. The data suggest that, under our experimental conditions, 90-100% of animals show relatively low fluke counts with a vaccine efficacy of only 43-48%.

The results indicate that tegument proteins and FhTLM+ GST are potential candidates for a commercial fluke vaccine in cattle and the data will be discussed in the context of how best to assess fluke proteins as vaccines going forward.