Managing parasites and resistance in cattle

This project measured the concentrations of ivermectin (a macrocyclic lactone) reaching target cattle tissues (abomasum and small intestine) and the target parasites (Ostertagia and Cooperia) when given to cattle orally, by injection or pour-on. 


While it is known that different routes of drench administration can result in variable efficacy against some parasite species it is not understood why this occurs or why different organs appear different. Despite evidence that pour-on administration is the least effective and most likely to select for resistant parasites, these products are the most common for the cattle market in New Zealand today. Solid evidence explaining why one route of administration is better or worse than others is required to understand if a change on-farm is required. 

Current data indicates that in cattle, oral drenches are more effective than injections or pour-ons and this is probably because orals deliver a higher concentration of active ingredients to the target parasites in the gut.  

Despite this information, most cattle farmers continue to use pour-ons because of their ease of application. This almost certainly results in failure to properly control Cooperia (with an associated production loss) and the likely promotion of drench resistance in Ostertagia.

Key programme results

Route of administration is likely to influence the exposure of ivermectin for different parasite species:

  • for most parasite species (e.g. Cooperia species), oral administration of ivermectin will deliver the highest exposure of parasites to the drug
  • for Ostertagia ostertagi, administration of ivermectin by injection is likely to be the preferred option for achieving maximum drug exposure.

Ivermectin concentrations were highest in the small intestine following oral administration.

Benefit to farmers/industry

This knowledge provides farmers and veterinarians with scientific evidence on the type of administration of drench that is most effective in cattle and that the most effective administration may differ depending on the parasite targeted. Changes to the most effective way of administering drench will help reduce the likelihood of drench resistance and improve productivity.


Scientific publication


  • Two B+LNZ field days.

Timeline and investment

This was a 20-month project. B+LNZ investment was $95,000 over the duration of the project.


This work was led by AgResearch and co-funded by B+LNZ and AgResearch.

Lead scientists: Dave Leathwick and Tania Waghorn.
B+LNZ point of contact: Suzanne Keeling.