ATTEMPTED IMMUNIZATION OF SHEEP AGAINST FASCIOLA-HEPATICA USING GAMMA-IRRADIATED METACERCARIAE

The potential of gamma-irradiated Fasciola hepatica metacercariae to v accinate sheep against fascioliasis was examined. The effect of the si ze of the inocula of irradiated metacercariae and the level of gamma-i rradiation on the recovery of non-irradiated fluke was assessed follow ing homologous challenge. Groups of Merino wethers were vaccinated wit h a single infection of either 500 or 2000 metacercariae, previously e xposed to either 30, 100 or 400 Gy of gamma-irradiation. No significan t reduction of fluke burdens were observed in any group, although a no nsignificant 20% reduction was observed in sheep vaccinated with 2000 metacercariae irradiated with 100 Gy. A second trial was conducted in which groups of sheep were vaccinated with 2 doses, given 4 weeks apar t, of 2000 metacercariae, previously irradiated at either 70, 100 or 1 50 Gy. In both trials parasite viability was severely affected by dose s of gamma-irradiation of 30 Gy or greater and no mature flukes were r ecovered from control sheep given metacercariae attenuated with 70 Gy or greater. A strong humoral immune response to somatic F. hepatica an tigens was observed in all sheep: Only sera from sheep receiving 70 Gy irradiated metacercariae recognised the 2 candidate liver fluke vacci ne molecules, F. hepatica glutathione S-transferase and cathepsin-l pr oteases. No reduction was observed in either the number of flukes or t he production of fluke eggs in any vaccinated group. Vaccination appea red to affect the development of the challenge fluke population, resul ting in reduced hepatic damage during migration, as measured by levels of serum glutamate dehydrogenase, and an increase in mean fluke weigh t.