Assessment of the Fasciola hepatica saposin-like protein 2 (rFhSAP2) vaccine candidate in sheep: Immunogenicity without protective efficacy

Fasciolosis caused by Fasciola hepatica is an economically important parasitic disease of livestock and a major zoonotic disease in humans. Vaccination is a promising strategy to reduce the impact of fasciolosis and dependence on fasciolicides. Saposin-like protein 2 (FhSAP2) has previously shown high immunogenicity and reductions in fluke burden in experimental animal models; however, its efficacy has not been evaluated in sheep. Twenty-one female sheep were randomly assigned to three groups (n = 7): vaccinated, adjuvant control, and infection control. The vaccinated group received 400 μg of rtFhSAP2 formulated with Montanide ISA 201 VG, the adjuvant control received Montanide ISA 201 VG alone, and the infection control received saline. Animals received three subcutaneous immunizations at four-week intervals and were subsequently challenged orally with 200 F. hepatica metacercariae. Vaccine efficacy was evaluated based on fluke burden, fecal egg counts, and egg viability. Antigen-specific IgG, IgG1, and IgG2 antibody responses were assessed by ELISA. Vaccination with rtFhSAP2 resulted in a 16.4% reduction in fluke burden compared with the infection control group (p > 0.05). Reductions in fecal egg counts ranged from 8.9% to 34.5% (p > 0.05). Fecal egg viability remained high, ranging from 88.9% to 91.3% among groups (p > 0.05). Gallbladder-derived egg viability was lower in vaccinated sheep (56.9%) than in the infection control (75.9%) and adjuvant control (73.6%) groups (p > 0.05). Vaccination induced a strong and sustained humoral response. IgG and IgG1 levels were significantly higher in vaccinated sheep than in both control groups from weeks 4–24 and 4–32, respectively. IgG2 levels were also significantly higher from weeks 4–24, although the response was lower than that of IgG1. Overall, rtFhSAP2 formulated with Montanide ISA 201 VG was highly immunogenic in sheep, but the humoral response did not translate into significant protection against F. hepatica.