title: Survival and development of ivermectin-resistant or susceptible strains of Haemonchus contortus under field and laboratory conditions.
abstract: The free-living development of three strains of Haemonchus contortus was studied in two experiments. Day 21 faecal samples were collected from lambs infected with either a susceptible strain, a laboratory-selected ivermectin (IVM) resistant strain or a South African field strain showing multiple anthelmintic resistance, which included IVM. No eggs hatched in samples cultured at 4 or 10 degrees C. At 22 degrees C the laboratory-selected strain showed the highest rate of development while at 27 degrees C the susceptible strain produced the highest yield of third stage larvae (L3): at both temperatures the field strain showed the lowest percentage development to L3. The second experiment was a field study carried out in southern Brazil. Faeces containing either an IVM-susceptible or an IVM-resistant strain of H contortus were placed in two series of grass plots during each of three summer months. Soil subsequently yielded more larvae than did grass suggesting migration or mechanical transport into the soil. For plots contaminated during the first two months there was no significant difference in recovery rate between the two strains (P > 0.05). When contamination occurred during the third month, the IVM-resistant strain produced significantly higher recovery rates (P < 0.05) from both pasture and soil.
authors.affiliation: Department of Veterinary Parasitology, University of Glasgow.
authors.forename: F A
authors.initials: FA
authors.lastname: Echevarria
authors.forename: J
authors.initials: J
authors.lastname: Armour
authors.forename: M F
authors.initials: MF
authors.lastname: Borba
authors.forename: J L
authors.initials: JL
authors.lastname: Duncan
mesh_terms: D000818:Animals; D004351:Drug Resistance; D005243:Feces / Q000469:parasitology; D006188:Haemonchiasis / Q000188:drug therapy / Q000662:veterinary*; D006189:Haemonchus / Q000187:drug effects*; D007559:Ivermectin / Q000627:therapeutic use*; D008828:Microbiological Techniques / Q000662:veterinary; D010270:Parasite Egg Count; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy / Q000469:parasitology*; D012988:Soil Microbiology
chemical_list: D007559:Ivermectin