THE EFFECT OF DEXAMETHASONE ON A SINGLE AND A SUPERIMPOSED INFECTION WITH HAEMONCHUS-CONTORTUS IN SHEEP

Experiments of the same design were run with non-immune and immune sheep to highlight protective immunity against H. contortus. Sheep were given a single infection with H. contortus or were given a second infection 7 days later. In half the sheep dexamethasone treatment was given to coincide with either the initial or the second infection. In non-immune sheep there was no indication that the first infection with the parasite influenced the magnitude of the second infection. Immunosuppressive treatment showed that major protective responses reduced the size of both the first and second challenge infections in sheep made immune by previous exposure to infection. However, responses to the second challenge could be abolished despite their effective operation against the first challenge. Parasites of longer standing in the host than 7 days appear to be resistant to the major protective response. Arrested larvae were seen in the experiment with non-immune sheep but not in the one with immune sheep, a difference ascribed to the different times of year in which experiments were run and not to differences in the physiological status of sheep. Both corticosteroid treatment and superimposed infection triggered developmental arrest in arrest-prone larvae.