Deworming Program for Control of Parasites of Sheep in Ontario
by Delma Kennedy, OMAF Sheep Specialist

Using information on the lifecycle of internal parasites, susceptibility to parasites and pasture management it is possible to reduce the number of times that it is necessary to deworm sheep, and over time, to stop deworming mature ewes completely.

Dr. Lautenslager, a clinical parasitologist, developed a strategic deworming program for Ontario sheep producers, which was first presented in the late 1980s. The primary objective is to reduce the number of parasites on the pasture to a level that the sheep can manage without having to be dewormed as often. Several producers around the province have successfully modified this strategy for their individual farm conditions and production systems.

The most critical internal parasite of sheep in Ontario is the Haemonchus contortus, or Barber pole worm. As with most internal parasites, the lifecycle of this worm is about three weeks long with four larvael stages before becoming an adult worm. If conditions are good, eggs hatch on pasture and develop through three larvael stages, with only the third being infective. To complete the lifecycle sheep must eat the worm at the third larvael stage, and after ingestion the fourth larvae stage forms in the sheep maturing into an adult worm and producing eggs so the cycle continues.

Parasites are programmed to survive. Eggs require warmth and moisture for optimum development. Months where the temperature is above 10ÞC and there is greater than 50mm of rainfall, are favourable for development of parasites. Temperature affects the speed of development, in cooler conditions the lifecycle takes much longer to complete. At temperatures of 20 or 25ÞC, the lifecycle completes much faster. Hot, dry and windy conditions are the most effective at killing the egg and larvael stages on grass. In May and June most larvae that have survived over the winter will die in 3 to 4 weeks unless ingested by sheep. At the end of the summer, a large number of the larvae picked up off of the pastures burrow into the gut wall and become dormant. Most of the time, these larvae are dormant until spring when the animal returns to grass, but if the animal becomes stressed or immunity drops, as it can at lambing, larvae can hatch into adult worms.

Mature animals build up varying degrees of resistance or immunity to infection through exposure, but lambs have virtually no resistance to worms. Small numbers of parasites on a pasture can be dangerous for lambs in warm and wet weather conditions. By about 8 months of age some resistance has been developed as long as the animals have been exposed to parasites on pasture.

Basic Strategic Deworming Procedure:
To ensure that animals going to grass don’t deposit large numbers of eggs in the manure and reinfect the pasture it is important to reduce or eliminate over-wintering of parasites in sheep and deworm at the right time in the spring. Once on pasture, sheep eat the available larvae that have survived the winter, therefore animals must be dewormed when they have eaten as many larvae as possible but before these larvae develop into adult worms shedding large numbers of eggs. After deworming, animals must always be moved to safe grazing within 12 to 24 hours.

To start this spring:
YEAR 1

1) Deworm all ewes in the flock after lambing and before going to grass.

2) Deworm all animals 3 to 4 weeks after going to grass – this is critical. This allows sheep time to ingest any gastrointestinal nematodes that have survived the winter and allows the inhibited larvae in the sheep to develop to the adult stage. Then animals are dewormed before the pasture is re-contaminated.

3) During the last two weeks of July, take approximately 20 – 40 individually packaged fresh fecal samples (6 – 8 pellets each) from each group of sheep being pastured and have them analyzed by your veterinarian. If the group on pasture is made up of ewes and lambs, take fecal samples for each group. Your veterinarian can assist in determining the most accurate number of samples for the flock size and expected parasite load.

a) If fecal samples indicate high worm counts, deworm all ages in the group and move to a pasture that has not been grazed by sheep that year or with the longest rest period.

b) If fecal samples indicate low worm counts, deworming is not necessary in these groups.

4) At winter stabling, deworm all ages with a product that is effective against dormant parasites. Some products are only effective against adult parasites. Different deworming products are effective for different stages of the parasite’s lifecycle. Your veterinarian can advise you on the appropriate product to use in your farm situation.

YEAR 2
Follow steps above but skip step 1

YEAR 3
Follow steps above but skip step 1 and 4

YEAR 4
Follow steps above but skip steps 1,2 and 4

Fecal analysis (step 3) must be done every year, forever. If the July sample comes back high for your mature ewes in any year you must start the cycle over at year 1. It is important to note that this program is designed for the mature flock. Lambs and stock under 24 months of age will need to be more closely monitored for parasite build-up.

It is important with a strategic deworming program to ensure that when animals are dewormed, that the deworming works. Careful attention should be taken to ensure that animals receive the proper dosage. Weigh some of the biggest and smallest animals to ensure that you have an accurate idea of the weights of animals being treated. Check your equipment to ensure that the dose that is supposed to be delivered is actually delivered. Under dosing will create parasites that are resistant to the dewormer being used. If you are concerned that the parasites in your sheep are resistant to a particular dewormer, your veterinarian can assess resistance by analyzing a number of fecal samples before and after deworming. Deworm all new animals entering the farm prior to being exposed to the rest of the flock. Resistant parasites can be passed from farm to farm in the sheep. Changing the dewormer you use on a yearly basis will help to minimize the development of resistance. Your veterinarian can help you to develop a strategic deworming program for your operation that will avoid resistance.

Be aware of the symptoms of parasitism. Clinical signs are anaemia, which causes the membranes of the eye, mouth and vulva to be very pale, bottlejaw, which causes a swelling below the jaw and scant, hard manure. These animals are often found dead. This indicates a heavy burden of adult worms. A sudden change in weather favourable to parasite development can result in this situation, particularly in younger animals that haven’t built up immunity. If an animal is found dead of parasitism, all of the animals in the group should be dewormed immediately. Any delay could result in further deaths.

The two biggest barriers to success with strategic deworming is the availability of safe pastures and the ability to deworm all animals 3 to 4 weeks after going to pasture. A safe pasture is one that has been harvested for hay, has not been grazed by sheep for several months after winter or that contains seasonal forages. Deworming 3 to 4 weeks after going to pasture can be difficult if not impossible for producers who are lambing on grass, however in this situation it is most critical. Hormonal changes in late pregnancy and during lactation cause a depression of the ewe’s immune system. This can allow dormant larvae to hatch en masse and develop into adult worms. Also, female worms are less inhibited by the ewe’s immune system and therefore lay more eggs. If ewes and young lambs have to be moved long distances to handling facilities, split ewes into groups that are manageable before lambing starts. For those with portable handling facilities, consider taking the handling system to the sheep. Consult your veterinarian on treatment regimes suitable for pasture lambing.

The relationship between sheep, parasites and the environment is dynamic, which means that what works this year may not work exactly the same next year. Changes in temperature and moisture affect the ability of parasites to reproduce. As it is rare to have summer temperatures and moisture levels that are the same, in the same months, two years in a row, planning ahead and having potential safe pastures will ensure that you can adapt to the challenges caused by the weather. It may be necessary to modify the strategic deworming plan above to fit into your unique farm situation. Your veterinarian and pasture specialist can assist you in finding a way to make strategic deworming work on your farm year after year.