Worm Management in Sheep


The use of anthelmintics should be incorporated into an integrated parasite management program with the goal of slowing the development of drench resistance and minimising production loss. Once the parasites develop resistance to an anthelmintic there is little or no reversion to susceptibility. So the aim is to delay the development and severity of drench resistance.

Important Worms

Uncontrolled outbreaks of worm infections can kill sheep, particularly young sheep. For Barber’s Pole worms, sheep can go from being perfectly healthy to severe anaemia and death within as little as two or three weeks. Black Scour worms and (Small) Brown Stomach worms, can also lead to similar proportions of the flock dying, but this typically takes many months, during which the sheep scour and lose weight, making it obvious to the farmer that there is a problem.

Three important roundworm parasites of sheep in Australia are the Barber’s Pole worm (Haemonchus contortus), the Black Scour worm (Trichostrongylus colubriformis or T. vitrinus) and the (Small) Brown Stomach worm (Teladorsagia or Ostertagia circumcincta). The Thin-necked Intestinal worm (Nematodirus spp.) is sometimes important, particularly where pastures are very short following a dry spell or overstocking. In each case, the adult and juvenile parasitic worms live in the digestive tract of the sheep (abomasum for Ostertagia and Haemonchus), small intestine for Trichostrongylus and Nematodirus) where they suck blood (Haemonchus) or feed on the lining of the gut. Liver flukes (Fasciola hepatica) are also important in some parts of the country in both sheep and cattle. The average life-span of an individual adult worm is around 3 months. Female worms lay large numbers of eggs, which pass out in the dung. Numbers of eggs vary according to species, and range from a few hundred eggs per worm per day for Ostertagia, to 10,000 eggs per female worm per day for Haemonchus.

Life Cycle and Environmental Conditions

Eggs in the dung-pats hatch under warm, wet conditions and grow into infective larvae, which can leave the dung-pats following rain and swim onto the pasture where they can be ingested by a grazing sheep. When an infective larva is eaten by a sheep, it begins its development through several parasitic stages that result in a mature adult male or female worm 3 weeks later. There are similar numbers of male and female worms in a worm population.

Hatching and development from egg to infective larva can take as little as 5 or 6 days under ideal conditions of warmth and moisture, or many weeks if temperatures are low or moisture is limiting. The various species have different temperature and moisture requirements, generally Haemonchus and Trichostrongylus require higher temperatures and moisture levels for hatching and development than do Telodorsagia and Nematodirus.
worm life cycleInfective larvae are the longest-lived stage in the life-cycle, as most of their lives are spent waiting to be picked up by a sheep. They do not feed, but can withstand extremely hot, dry conditions in dung pats for several months. Once they leave the dung-pats following rain, their survival time is dependent on prevailing temperatures. Temperatures of 20°C or more encountered in spring or summer cause most larvae to die within 2-3 months. Larvae developing in autumn or winter will face much lower temperatures, resulting in increased survival times of 6 months or more. A few larvae can survive on pasture for up to a year.

In general, the higher the temperature, the shorter the survival time on the ground, and the lower the temperature, the longer the survival time. Frosts or snow have no effect on larvae of these species. If anything they increase their survival and keep them fresh and healthy. The main effect of frosts and low winter temperatures is to prevent hatching and development of eggs to larvae, and to improve the survival of infective larvae that have already developed.

Drenching Tips

  • Aim to reduce the frequency of drenching
  • Wethers, dry ewes and older sheep develop natural immunity to worms and will require less frequent drenching than lambs and hoggets
  • Only drench when necessary based on worm egg count of mob
  • Conduct FECRTs (drench trials) every 2-3 years to determine rotation options, by finding out which drenches or combination of drenches remain effective on the property
  • Never underdose.  Drench to the heaviest in the mob. Split mobs where there is a wide variation in weights
  • Use effective broadspectrum drenches in annual rotation.  Conduct a drench test to find out which drenches are effective on the property
  • Never use a broadspectrum drench when a narrow spectrum is adequate
  • Monitor the effectiveness of worm control, particularly in weaner sheep
  • Prepare low risk pastures to drench young sheep onto
  • Wean early to give weaners the advantages of the best feed and lower worm areas
  • Ensure adequate protein and energy intake to keep weaners growing and for the development of immunity to worms

Conducting a Drench Test

The faecal egg count reduction test (FECRT) is a practical method of testing for resistance to broadspectrum drenches in sheep. It compares % egg count reduction for each worm to the efficiency of each drench against that worm. By using the FECRT producers are able to find out which drenches or combination of drenches remain effective on their property and plan a rotation program to help slow down the development of resistance. The FECRT is very flexible as it allows the farmer or veterinarian to select and test the efficacy of any individual drench or combination of drenches that they think worthwhile. Deciding on which drenches to test will depend on results from previous test and drench usage patterns on the property. The aim of the trial is to define drench options for the producer.

  • Sheep should be young homebred sheep usually undrenched weaners

  • Sheep should not have been drenched with a broadspectrum drench in the previous 8 weeks

  • 15 sheep are included in each group, although faecal samples are only taken from 10 sheep

  • One group of sheep remains undrenched throughout the trial, and is known at the control group. Trial sheep should be run on the same paddock from which they came for the whole trial (10-14 days).  They can run with the main mob, even if these have been treated.


    Step 1:  Draft off a sufficient number of even-sized weaners from the mob.

    Step 2: To allocate sheep to groups, run them into a race and sequentially mark heads with appropriate colour(s) on the forehead.  Proceed down the race until all sheep have been marked.

    Step 3: Run sheep through the race and draft into groups.  Return the control group to main mob.

    Step 4: Using scales weigh a few of the largest sheep.  Calculate the dose required for each drench based on the heaviest weaner in the mob.

    Step 5:  Prepare drenches and check gun delivery.  Set gun to deliver correct dose. Squirt 10 doses of the first drench to be administered into measuring cylinder, divide total volume by 10 to give output of gun.  Adjust gun and repeat procedure with second drench gun to ensure both deliver correct dose.  Preferably use two guns and backpacks. To minimise contamination wash gun and pack and check dose and gun delivery when changing drenches.

    Step 6: Return each group of 15 sheep to the race and administer the drench(es) for that group.  Take care that each sheep in the group is treated.

    Step 7: Repeat until all groups of sheep have been treated.

    Step 8: Return all weaners to the main mob.

    Step 9: 10-14 days later collect faecal samples. Muster mob early in the morning. Draft marked lambs into colour groups.  Minimise stress, it is difficult to collect adequate samples from stressed lambs.

    Step 10. Collect individual dung samples from 10 of the lambs in each group, a minimum of 10-15 pellets per sample.  Samples are collected from the rectum of each sheep using the rubber glove provided with the kit, and put into the appropriate containers. Keep samples cool and ensure delivery to the testing laboratory in a timely manner.