AgMemo Central Agricultural Region

Worm and flystrike risk increases following warm wet conditions

DAFWA has reminded sheep producers to consider the impact of recent summer rainfall on livestock husbandry requirements.
DAFWA has reminded sheep producers to consider the impact of recent summer rainfall on livestock husbandry requirements.

Recent widespread unseasonal rainfall has increased the risks of flystrike and worms to sheep flocks across the agricultural region.

The Department of Agriculture and Food, Western Australia (DAFWA) has urged sheep producers to take action now to protect their livestock and to prevent these risks from becoming exacerbated in coming months.


In normal summer with hot and dry weather the vast majority of scour worm eggs in sheep faeces die quickly, and never reach the larval stage that can infect sheep.

Normally, ‘scour worms’ (i.e. other than Barber’s pole worm) are not a significant risk during summer provided weaned lambs and hoggets were given an effective drench (i.e. drench with greater than 98% efficacy) last December.

Adult ewes in good body condition can tolerate small worm burdens, so can be left undrenched until late March or early April (“autumn drenching”).

This scenario has now changed with the widespread heavy rainfall during February in many agricultural districts.

However, if hot, dry conditions have subsequently occurred on your property there is no change to this advice.

The longer green pasture and mild temperatures remains, the higher the rate of survival of worm larvae in faeces being deposited on the ground now.

Consequently, the winter worm population will start from a higher base in the coming early winter period.

A short period of germinating pasture does not immediately cause an increase in scour worms in sheep.

However, if ewes had not been drenched before summer then it is recommended to drench all your sheep now rather than wait until late March or early April.

One way to assess the situation particularly if ewes were given a summer drench is to get a worm egg count done.

Using an effective ‘autumn’ drench (i.e. drench with greater than 98% efficacy) is essential otherwise future winter worm problems will be costly.

Re-check your ewes four weeks before the commencement of lambing by getting a worm egg count done.

Ineffective worm control now in your ewes will lead to reduced growth rates in your lambs this coming winter. 

If Barber’s Pole worm is present in your flock and green pastures persist or sheep graze perennial pastures then an outbreak may occur within 5 weeks post rainfall.

However, it is difficult to predict whether Barber’s Pole worm will be a problem even in high risk coastal areas without getting a worm egg count. 

Making use of tools like worm egg counts (taken at the right time), getting timely advice from consultants with knowledge of worm control or seeking information from Worm Boss will be good investments this year.    


The risk of flystrike has also increased.

Flystrike is usually a spring problem, which producers are used to managing, however much of the state’s sheep flock is now at increased risk of strike.

The recent heavy rains, followed by a return to warm weather have created perfect conditions for flystrike.

Predicting a producer’s risk of flystrike will depend on environmental conditions as well as how susceptible their sheep are.

The most susceptible sheep are highly wrinkled Merinos and flocks which have long wool and are uncrutched/unshorn over high risk times.

Crutching is usually a spring activity so many flocks will be at increased risk at the moment.

We also expect to see an uptick in fleece conditions such as fleece rot and dermo due to sheep being wet for prolonged periods.

These conditions make sheep more susceptible to flystrike.

Flystrike is a significant welfare issue, sheep at risk should be checked daily and strategic treatments such as crutching and strategic chemical controls used to manage the risk.

The key signs to look for include sheep that are on their own, possibly not grazing; dark stains on the wool, especially on the shoulders, back or crutch; tail twitching, rubbing or chewing at the affected part; and an offensive odour when close to the sheep.

When treating individual affected sheep, first clip all the wool from the affected area and extend the clipped area five centimetres or so into surrounding unaffected wool.

Do not rely solely on applying an insecticide powder or liquid without first clipping away the wool.

If you plan to use chemical treatments make sure they are registered for flystrike use.

Follow label directions and observe withholding periods for wool and meat as well as the export slaughter interval in order to avoid unacceptable residues.

A flystrike chemical planner is also available, in both a paper version and as an app for IPad or IPhone.

This will help to ensure that correct treatments are applied and withholding periods are observed.

For more see the Flystrike Management Tools webpage.

Information on flystrike control in sheep and lambs can be found at DAFWA’s Managing Flystrike in sheep webpage.

For more information  on flystrike contact Anna Erickson, Veterinary Officer, Narrogin on +61 (0)8  9881 0211

For more information on worms contact Danny Roberts, Veterinary Officer, Albany +61 (0)8 9892 8535.