Resistance to experimental infections with Haemonchus contortus in Romanov sheep

Abstract

Responses to immunization with aggregated human serum albumin (HSA) and to repeated experimental infections with H. contortus were studied in 51 female lambs of the Romanov breed, born from 8 sires and 36 dams. The 8 sires were of haemoglobin genotype Hb AB; the 51 lambs were distributed into 3 groups of 17 each, corresponding to the 3 genotypes HbAA, HbAB and HbBB. In addition, the experimental lambs were typed for antigens of the major histocompatibility system (OLA). The parasitological findings were the following: a repeatability of faecal egg counts between successive infections, a negative correlation between peak faecal egg counts and self-cure intensity, a positive correlation between faecal egg counts and degree of anaemia, an acquisition of immunity to the parasite by previous contact with the parasite and a reduction of this immunity by anthelmintic treatment. According to the genetic investigations, there were significant sire effects on variables reflecting the resistance. The faecal egg counts did not seem to be related to the haemoglobin system, but might be affected by 1 or several genes located in the OLA complex or close to the latter. The humoral response to HSA showed a negative correlation to parasite resistance.