RESUMO
An experiment was conducted with the objective of evaluating whether a 5000 L3 larvae Haemonchus placei primary infection, a less pathogenic parasite species for sheep, could attenuate highly pathogenic Haemonchus contortus infections in lambs. Thirty nine 6-month-old lambs were divided in three primary-infection groups: (HcPI) H. contortus-primary infected lambs, (HpPI) H. placei-primary infected lambs, and (CTRL) non-infected control lambs. Later, these same individuals, then aged 10-month old, were cross challenged with Haemonchus parasite species, creating four groups: HcPI challenged with H. placei (HcPI-HpCH), HpPI challenged with H. contortus (HpPI-HcCH), HcPI challenged with H. contortus (HcPI-HcCH), and CTRL. After a 60-day challenge period, all animals were necropsied for gastrointestinal worm counts. HcPI faecal egg count average was found to be twice the HpPI FEC group (p<0.0001). The HcPI also showed lower packed cell volume averages compared to the other groups (p<0.0001). Both H. contortus- and H. placei-primary infections displayed immune responses with similar IgG levels. For the challenge trial, the larval doses used were not enough to trigger clinic infection signs in all treated groups, compared to controls, and H. placei primary infection was not able to maintain anti-H. contortus IgG levels in a subsequent H. contortus infection.