Home -> Bison -> WD Time for Avermectins

Withdrawal Time Information for Ivermectin (Ivomec),
Doramection (Dectomax)
or Eprinomectin (Eprinex)

Murray  Woodbury DVM, MSc.

Specialized Livestock Research and Development Program
Department of Large Animal Clinical Sciences
Western College of Veterinary Medicine
University of Saskatchewn
Saskatoon, Saskatchewan    S7N 5B4

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The bison industry needs to support research and/or otherwise help fund a grant to study issues such as the withdrawal times for fenbendazole (Safe Guard) and other anthelmintics in bison. Injecting bison with these drugs and then analyzing tissues at increasing time intervals will provide information on how long animals must be held before slaughter. The first phase of such a study would involve the perfection and validation of the detection methods for these drugs in bison tissue. The second phase would use slaughtered animal (tissue) testing to determine the disappearance time for these drugs. However, until such time as this work can be completed and published, producers should understand that fenbendazole and most ivermectin-like drugs are not labelled (government approved) for use in bison. This means that ANY amount drug of found in a carcass, detected by methods for use in cattle, is a violation of current drug residue regulations and will result in the  condemnation of the affected carcass.

I once forwarded a request from the bison industry to provide recommendations to producers about the appropriate withdrawal times for ivermectin-like drugs to gFARAD. gFARAD stands for global Food Animal Residue Avoidance Database. It is a computer-based decision support system designed to provide livestock producers, extension specialists, and veterinarians with practical information on how to avoid drug, pesticide and environmental contaminant residue problems. Using this database, gFARAD personnel can sometimes estimate and recommend withdrawal or withholding periods where none currently exist. Below is a shortened version of the answer from Canadian gFARAD to the question about cattle doses of ivermectin (Ivomec), doramectin (Dectomax) or eprinomectin (Eprinex) given to bison in Canada.

 "As you can see, there are no approved minimum residue levels for ivermectin, doramectin or eprinomectin and no approved withdrawal times for endectocide residues in bison tissue in Canada. This means that if bison tissue is analysed by regulatory authorities in Canada, ANY detectable residues will be considered violative. According to a document dated October 2002, the limits of detection of the CFIA Saskatoon Laboratory, for these three endectocides in liver and muscle of cattle, horses, swine and sheep is 2 ppb.“ This is the target that we will need to aim for when establishing a withdrawal interval for the use of these drugs in bison in Canada in order to reduce the risk of violative residues being detected in bison tissue. A search of our database, of the veterinary literature and of regulatory documents provided us with some extremely limited information on the pharmacokinetics and residue depletion for these drugs in bison. Only one paper was found to report limited depletion information for ivermectin use in bison (Marley et al., J Wildlife Dis, 31 (1) 1995, 62-65). Since there was only one time point measured for residues, this study does not allow us to estimate a tissue half-life for bison. Upon inquiring with Merial, we were informed that the company did not conduct residue depletion studies in bison as this was not required to obtain their minor species claim for Ivomec in the United States."

 The reply from gFARAD goes on to present evidence that drug studies in deer rather than cattle might be best to estimate withdrawal times for bison. A half-life is the amount of time required for an animal to metabolize and reduce the drug concentration by one-half. The reply then continues:

"Tissue elimination half-lives are consistently and slightly longer in deer than in cattle. When using the cattle dose of ivermectin PO in deer, the measured level of residues in deer liver was 180 ppb at 7 days post-treatment. It would take a further 7 half-lives for this amount to be below the limit of detection of the CFIA (2 ppb). The recommended withdrawal interval for ivermectin pour-on given at the cattle dose in bison is therefore 7 days + ( 7 x 4.91 days) = 42 days. By following the same method of calculation, we found the following recommendations withdrawal intervals in bison for the endectocides for which deer data was available:

Drug-dose (formulation)

Initial residue level in deer (time of sample)

Number of half-lives to add to time of sample

Withdrawal interval in bison (days)

Ivermectin Injection 0.2 mg/kg

71 ppb (10 days)

6 X 5.8 days = 34.8

10 + 35 days = 45

Ivermectin Pour On 0.5 mg/kg

180 ppb (7 days)

7 x 4.91 days = 34.37

7 + 35 days = 42

Doramectin injection
0.2 mg/kg

65.5 ppb (10 days)

6 X 5.87 days = 35.22

10 + 35 days = 45

Therefore, the Canadian gFARAD recommends a withdrawal interval as per the above table which should be sufficient so that detectable residues are not found at slaughter. Because the Canadian gFARAD withdrawal recommendation is not an official withdrawal time and is based on data that has not been reviewed nor approved by the Veterinary Drugs Directorate of Health Canada, responsibility for residue violations rests with the prescribing or attending veterinarian.