The effects of extended-release eprinomectin on heifer performance and fescue toxicity and an evaluation of parasite resistance

Anthelmintics are commonly used by livestock producers. Parasite resistance has been reported in ruminants in multiple macrocyclic lactones, after their use over time. As new versions of anthelmintics are released to the market, evaluations for resistance need to be conducted. In addition, the use of anthelmintics has also been reported to be beneficial at reducing negative impacts of fescue toxicity. Previous work has shown improvements in growth performance of growing animals when treated with either ivermectin or extended-release eprinomectin. No work has determined the mechanism for this improvement though. The objective of this thesis was to determine if parasite resistance to extended-release eprinomectin was present on a cow-calf operation after three years of use and to explore CYP450 activity and liver function as potential mechanism for extended-release eprinomectin to mitigate fescue toxicity in fall-born beef heifers grazing endophyte-infected tall fescue. Only one extended-release anthelmintic is currently on the market for ruminants and only limited data exist relative to parasite resistance. Therefore, the objective of this experiment was to determine if parasite resistance to extended-release eprinomectin (LongRange) was present in an operation with 3 yr of prior eprinomectin use and impacts on heifer growth performance. Fall-born Angus × Simmental heifers (224 ± 22 d of age) were randomly assigned 1 of 3 treatments: extended-release eprinomectin (ERE), extended-release eprinomectin and oxfendazole (COMBO), or saline control (CON). Control cattle had greater FEC from d 55 to 83 compared to ERE and COMBO heifers, although FEC was not different any other time point. Fecal egg count reduction (FERT) for ERE and COMBO were above the 90% threshold (91% and 98% reduction, respectively). Packed cell volume was similar between treatments throughout the experiment. Treated cattle had greater BW and BCS beginning on d 83 and 55, respectively. Parasite resistance to extended-release eprinomectin was not present based on FERT, and treated heifers had greater growth performance compared to CON heifers. The mechanism causing increased growth performance of growing cattle treated with extended-release eprinomectin has not been determined. Therefore, the objective of this experiment was to evaluate CYP450 and liver enzymes as potential mechanisms of extended-release eprinomectin mitigating fescue toxicosis in beef heifers grazing endophyte-infected tall fescue. Fall-born Angus × Simmental heifers (42 heifers; 170.2 ± 20.0 kg initial body weight (BW); 222 ± 21 d of age) were stratified by d -2 fecal egg count and BW and were assigned to one of six groups (7 heifers per group) using a stratified randomized design. Groups were then assigned to one of two treatments: extended-release eprinomectin (ERE; n = 3), or saline control (CON; n= 3). Body weight and BCS was improved in ERE heifers beginning on d 112 and 28 respectively. Respiration rates tended to be improved and HCS was improved in ERE heifers on d 55 and 83. Serum prolactin and total urinary ergot alkaloids were similar between treatments. In addition, ERE cattle tended to have greater total CYP3A4 activity. Liver enzymes including alkaline phosphatase and gamma-glutamyl transferase were greater in ERE heifers and total protein tended to be greater than control. Extended-release eprinomectin improved heifer growth performance, HCS, and tended to improve RR. In addition, extended-release eprinomectin tended to increase total CYP450 activity, and also had greater levels of several liver enzymes in heifer grazing endophyte-infected tall fescue.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste