Rachael Fouhy

There is a huge variety of products available for fly and lice treatment in sheep, so how do you choose the one that is best suited to your farm and how do you ensure it works to the best of its ability?

Resistance to some of the chemicals used to control external parasites has been around for a while, yet we don’t talk about it as much as we talk about drench resistance. In my area a change in weather patterns in recent years has resulted in a few more fly challenges than we have previously been used to.

When things don’t go to plan it can be hard to determine if the situation you are seeing on your property is because of resistance or the product not working as effectively as anticipated. It’s important you discuss any issues with your vet or local merchant. Common reasons we see for products not working as well as anticipated include:

  • Not using the product as per label directions. Choosing the best product for your wool length is key, as is mixing it to the correct label recommendations. Traditionally zinc sulphate was added to dip wash to prevent dermatophilosis. If you plan to do this make sure you check with the product manufacturer otherwise this may affect the ability to get enough chemical on the sheep. It is also very important it’s mixed well to be effective. Likewise stick to the mixing calculations on the container – don’t be tempted to be miserable and make it go a bit further. Measure chemicals and water, don’t just guesstimate. We all know the negative outcomes that come with underdosing with drench, and underdosing with these chemicals has the same outcome.
  • Not getting enough product on animals or in the key places. A classic example of this is the popular use of automatic jetting races for the control of lice. Yes, they are very convenient, but it is important to ensure all the jets are open and the sheep are not running too fast. Use a race that has jets that get the brisket and low areas of the sheep. Remember the idea is to achieve skin saturation. An option to mitigate the risk of not enough chemical is running the sheep through twice.
  • Not using the best product for your situation. It is really important to understand the product you are buying and have realistic expectations for it. Some of the labels can be a little vague, eg aids in the control of, works for up to 12 weeks etc. The efficacy here will depend on many factors including product formulation and factors in your control, like wool length, presence of dags, and whether it rains post-application. Some fly products have a knockdown claim so are suitable for struck sheep, while others (IGRs) act slowly and are not suitable for struck sheep.
  • Wool length – this is especially important for lice treatments. Products often have different dose rates for different length of wool. Off shears being less than 24 hrs after shearing with a flat comb, short wool 1-42 days post shearing, long wool 42 days + post shearing. Cover comb/blades will leave more wool and potentially alter the efficacy of backline treatments post shearing.

There are no rules requiring companies to display any information regarding resistance or resistance management, although this is being reviewed by the ACVM. Be aware that there is resistance in New Zealand and Australia to two chemical families of fly treatments, which has resulted in shortened and unreliable protection. There is also resistance developing to some of the lice treatments. These chemical families are still being sold by some outlets. Aside from correct product and application, actions you can take include:

  • using a different chemical to treat active strike than is used for flystrike prevention,
  • if treating twice in a season use different chemicals,
  • treat for lice when numbers are the lowest, either summer or off the shears,
  • consider using combination products,
  • keep accurate records, and
  • do not mix treated and untreated sheep – this is especially important with lice control.


  • Rachael Fouhy is a production animal veterinarian at Tararua Veterinary Services.