The resistance of sheep to Fasciola hepatica: studies on the pathophysiology of challenge infections.

Determinations of circulating eosinophils, plasma proteins and glutamate dehydrogenase, voluntary drymatter intake, and plasma loss in the faeces were carried out on three groups of sheep infected with Fasciola hepatica. One group was previously uninfected, one had been treated to eliminate a 15-weeklong preliminary infection, and one had previously been exposed to five infections each terminated after one week. Compared with the controls, both challenged groups showed earlier and greater peaks of eosinophils and the group previously exposed to multiple short-term infection showed a more gradual and persistent rise in glutamate dehydrogenase. The trends of plasma globulin suggested that an anamnestic response occurred only in the group which previously harboured adult flukes. The latter group also showed, at 12-14 weeks after challenge, the lowest intake of dry matter and the greatest loss of plasma in the faeces. At post mortem examination, similar numbers of flukes were recovered from challenged and control sheep. It was concluded that there was no evidence from the present experiment to show that preliminary infection of sheep with F hepatica conferred any significant resistance to the physiological effects of a challenge infection.