Managing Parsites in Ontario Sheep Flocks

Canadian Organic Science Cluster Success Story

Gastrointestinal nematodes (GIN) are a leading cause of diarrhea and weight loss in grazing sheep worldwide.  Globally, one of the major sources of GIN pasture contamination in the spring is the Peri-Parturient Egg Rise (PPER), whereby pregnant and lactating ewes experience an increase in fecal egg output at this time.  In earlier work on organic farms, we confirmed that the PPER occurs in Ontario ewes that lamb once a year in the spring; however, in Ontario, 20% of the sheep producers practice accelerated lambing whereby ewes lamb every 8 months, all year round.   This study was conducted to determine whether ewes lambing out of season still experience a PPER, leading to increased GIN pasture contamination at times of the year other than in the spring.  Six farms that practice accelerated lambing in Ontario were enrolled in the study and, on each farm, fecal samples were collected five times from 20 not pregnant/early gestation and 20 pregnant ewes over the gestation period, for three consecutive lambing seasons (spring, winter and fall).  The fecal samples were tested individually to determine the Fecal Worm Egg Count (FWEC).

Our results indicated that during the spring and winter lambing seasons, the overall FWEC was lower than in the fall lambing season, i.e. FWECs were highest in ewes that lamb in the fall.  Also, for ewes lambing in the spring and winter, the FWEC increased steadily over the lambing season, and peaked between early to late lactation.  In contrast, for ewes that lambed in the fall, FWECs peaked at late gestation, and then declined.  This decrease in fecal egg shedding in the fall coincided with when we usually observe a drop in environmental temperatures, which results in the parasites’ arrested development within sheep (a strategy for surviving the winter). 

In summary, the PPER was observed in all three lambing seasons, though the magnitude and duration of parasite egg shedding varied with the season. Collectively, these findings show that both environmental and animal factors play an important role in determining the level of fecal egg shedding of ewes. As such, they need to be considered when implementing parasite control strategies on Ontario sheep farms that practice accelerated lambing.


The Organic Science Cluster is part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada’s Growing Forward Policy Framework and is supported by contributions from industry partners.


Posted March 2012