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Study of the Population Dynamics of Gastrointestinal Nematodes in Organic Sheep Farms in Canada with an Attempt to Introduce Sustainable Parasite Control Measures

Over the past 3 years, sheep producers from central Canada participated in a study to help define how gastrointestinal parasites behave in sheep flocks in this part of the world, so that veterinarians and farmers can better control losses. Gastrointestinal nematode parasites, also known as GIN, infect lambs and adult sheep at pasture and can cause severe diarrhea, debilitation, and even death. Producers have used de-wormers (anthelmintics) for decades in the battle to prevent losses - but with the increasing problem of resistance of the worms to these drugs, as well as the desire to farm without having to rely on the use of chemicals - it is important to develop sustainable methods of control. To do this, we must first understand how the parasites behave under our climate and management practices, how the sheep respond to the infection, and which parasites are most important.

Thirty-two producers from Ontario and Quebec, representing certified organic, non-certified organic and conventional sheep farms, participated in the study from spring of 2006 to the fall of 2008. Fecal samples and pasture samples were collected monthly during the grazing season and also during the winter. Sheep and lambs were assessed as to body condition, evidence of diarrhea as well as anaemia. The numbers of parasite eggs were determined in the feces, as well as which parasites were most prevalent. Grass samples from pasture were assessed for presence of the free-living larval stages of the parasites. The history of pasture grazing was also determined at each visit. Producers were requested not to de-worm the sheep unless there was evidence of disease or if the fecal egg counts were dangerously elevated.

The results showed that there was tremendous variation from farm-to-farm in the level of parasites found. There were also significant differences between provinces and between years. However, some strong general trends were still evident. In the spring, adult ewes had the highest fecal egg counts (April-May). This is probably due to the commonly observed egg rise seen in late pregnant and early lactation ewes due to a relaxation in immunity. On pasture, the levels of free-living larvae were low in the spring but increased through the summer - initially because of the contamination from the feces of infected ewes, and then later from infected lambs. Dry weather delayed this rise, while wet weather hastened its development. The infection levels in lambs tended to peak in late July and August, then declined in the fall (October-November). Other pertinent findings included the following: The most commonly identified parasites (Teladorsagia [Ostertagia], Trichostrongylus and Haemonchus) can all survive the winter on pasture. Secondly, one of the management practices most strongly associated with high parasite loads was spreading manure on pasture. Lastly, organic sheep farms tended to have lower parasite counts than conventional farms.

Some recommendations that can be made from this study are that ewes should be de-wormed before lambing, and before being put to pasture in the spring, in order to prevent the egg rise around lambing. Young lambs may require preventive deworming 4-8 weeks after placement on pasture. In addition, they should ideally not be placed on spring pastures that were grazed by heavily infected sheep the previous grazing season. Instead, pastures should be used that were grazed by either adult sheep that tend not to have heavy loads due to acquired immunity, or an alternate species such as cattle (not goats). Finally, fecal egg counts in lambs should be regularly monitored, starting 4 weeks after being put to pasture - but no later than early to mid-July.

There are many other issues to take into account when designing a sustainable control program for gastrointestinal parasites. More information can be obtained from a handbook on the Control of Gastrointestinal Parasites of Sheep written for Canadian producers and veterinarians. A free copy can be downloaded at http://www.uoguelph.ca/~pmenzies/index.html

For farms that wish to farm organically:

Producers that wish to reduce routine use of anthelmintics in ewes in the spring, can choose to lamb indoors to reduce pasture contamination from the periparturient egg rise, and then either graze ewes and lambs on pasture that did not have sheep or goats the previous year, or hold the ewes and lambs off pasture until weaning and then graze the lambs on clean pasture. This must be accompanied by surveillance of lambs using fecal egg counts and should begin about 4 weeks after lambs are put to pasture and done at intervals not greater than every 4 weeks.

The Organic Production Systems-General Principles and Management Standards (CAN/CGSB 32.310-2006 Amended October 2008) state that "medical treatment for sick or injured livestock shall not be withheld to preserve their organic status. All appropriate medications shall be used to restore livestock to health when methods acceptable to organic production fail."

As per Section 6.7.9, when preventive measures fail to control parasites, anthelmintics can be used provided that:

• fecal samples or animal examination indicate that an individual or group of animals are infected with parasites;
• a producer has received written instructions from a veterinarian indicating the product to be used, the individual or group of animals to be treated, the dose and route of administration;
• withdrawal times are twice the label requirement or 14 days, whichever is longer;
• slaughter animals under one year of age can be treated only once and slaughter animals over a year of age can receive a maximum of two treatments;
• dairy animals needing more than two treatments per year (antibiotics and/or anthelmintics) will lose their organic status;
• a producer will create a written action plan (including timing), describing how they will adjust/change their parasite control plan to avoid similar emergencies.

Producers should always check with their organic certification body for proper interpretation of the regulations and standards prior to treating animals with products to make sure that they allowed to be used in their production system.