Biotic potential and pathogenicity of a Haemonchus contortus susceptible laboratory isolate compared to a resistant field isolate in Brazil

Abstract

The high prevalence of anthelmintic resistance of Haemonchus contortus is threatening sheep farming. In this context, resistant populations’ replacement using susceptible isolates appears to be a strategy to overcome this situation. The aim of the present study was to evaluate the ability of a susceptible H. contortus laboratory isolate in comparison with a multiresistant field isolate to infect lambs and to study its behaviour in field conditions. Sixteen worm-free lambs were experimentally infected with 4000 infective larvae (L3) of a H. contortus anthelmintic-resistant field isolate (Group R, n = 8) or a laboratory isolate susceptible to anthelmintics (Group S, n = 8). Groups were placed in separate clean pastures, where they were kept for 266 days. Faecal egg counts (FEC) showed maximum values between 21- and 42-days post-infection (PI) and decreased over time, with no statistical differences between groups (P > 0.05). The pathogenicity of the isolates was similar, as lambs from both groups showed a decrease in packed cell volume and total plasma protein values in the beginning of the study, coinciding with the highest FECs. Production of L3 were similar for both groups, when evaluating faecal cultures kept in vitro or L3 recovered from pasture samples and from faecal pats collected from the paddocks, which suggests that both isolates have the same potential of development in vitro and in the environment. The degree of resistance/susceptibility to thiabendazole (TIA), levamisole (LEV), monepantel (MON) and ivermectin (IVE) was evaluated using the Larval Development Assay (RESISTA-Test©) 160 days PI. The resistant isolate showed a high resistance factor (RF) to TIA (141), LEV (822), MON (34) and IVE (196), while the RF of the susceptible isolate remained below three, which indicates high susceptibility to all drugs tested. Our study demonstrates that the susceptible isolate was able to infect lambs and did not show higher pathogenicity nor any differences on its biotic potential when compared to the resistant field isolate. These results indicate that the susceptible isolate has the potential to be used to replace a resistant population of H. contortus, which should be validated in long-term studies under commercial sheep farming conditions.