Worm egg count results monitor parasite egg shedding in horses. They can be used to categorise animals into low, medium and high egg shedders, and to guide treatment thresholds appropriately.
This allows us to treat the horses most likely to be contributing to parasite contamination on the pasture, while avoiding unnecessary anthelmintic use. Leaving some parasites unexposed to treatment, known as refugia, is an important strategy for slowing the development of resistance.
The CANTER Guidelines for Sustainable Equine Parasite Control state: "Treatment cut-offs of 200–500 eggs per gram (EPG) can lead to reductions in overall faecal egg output of 95% (Kaplan and Nielsen, 2010). Therefore, choosing a threshold in this range is recommended when using the commonly applied McMaster FEC method."
This allows us to treat the horses most likely to be contributing to parasite contamination on the pasture, while avoiding unnecessary anthelmintic use. Leaving some parasites unexposed to treatment, known as refugia, is an important strategy for slowing the development of resistance.
The CANTER Guidelines for Sustainable Equine Parasite Control state: "Treatment cut-offs of 200–500 eggs per gram (EPG) can lead to reductions in overall faecal egg output of 95% (Kaplan and Nielsen, 2010). Therefore, choosing a threshold in this range is recommended when using the commonly applied McMaster FEC method."
Research defines the thresholds of low, medium and high shedders slightly differently, but the principle is consistent. All horses should be tested, egg shedders identified, and treatment targeted to those most likely to contribute to pasture contamination.
For adult horses, CANTER define: 0–200 epg of strongyles as low shedders (estimated 70–90% of the herd), 200–500 epg as moderate shedders (5–20%), and >500 epg as high shedders (0–10%), typically determined using the McMaster method.
At Westgate Labs we undertake all worm egg count tests using a Modified McMaster method.
Based on this guidance, we recommend treatment for moderate and high egg shedding horses with worm egg count results over 200 epg for small redworm.
This threshold sits within the evidence based range outlined in current guidelines and has been widely used in targeted parasite control programmes.
In recent years some prescribers have begun advising treatment only for high egg shedding horses over 500 epg. This reflects an increasing focus on reducing anthelmintic use and preserving drug efficacy, an aim that we strongly support. Minimising unnecessary treatments is essential if we are to slow the development of parasite resistance. However, parasite control programmes must balance the goal of reducing drug use with the need to maintain low levels of pasture contamination and protect individual horse health. At present we feel the evidence base supporting a universal move to the higher threshold remains limited, particularly across the wide range of management systems found in practice.
For that reason our current recommendation remains a treatment threshold of 200 epg. This approach has been used successfully for over 27 years with many thousands of horses tested through Westgate Labs, providing a precautionary and well established benchmark within targeted parasite control programmes. Westgate Labs does not sell wormers. Our recommendations are based solely on parasite monitoring data, published research, and our long standing experience working with horse owners and veterinary professionals.
Equine parasite control is an evolving field and we continue to follow emerging research closely. As new evidence becomes available, we remain open to adapting our guidance where it is supported by robust data. For now, we believe a treatment threshold of 200 epg represents a balanced and pragmatic approach, helping to protect horse health while supporting the responsible use of anthelmintics as part of a sustainable parasite control strategy.
References CANTER Guidelines (Second Edition). 2026. https://canterforhorses.org.uk/guidelines/
For adult horses, CANTER define: 0–200 epg of strongyles as low shedders (estimated 70–90% of the herd), 200–500 epg as moderate shedders (5–20%), and >500 epg as high shedders (0–10%), typically determined using the McMaster method.
At Westgate Labs we undertake all worm egg count tests using a Modified McMaster method.
Based on this guidance, we recommend treatment for moderate and high egg shedding horses with worm egg count results over 200 epg for small redworm.
This threshold sits within the evidence based range outlined in current guidelines and has been widely used in targeted parasite control programmes.
In recent years some prescribers have begun advising treatment only for high egg shedding horses over 500 epg. This reflects an increasing focus on reducing anthelmintic use and preserving drug efficacy, an aim that we strongly support. Minimising unnecessary treatments is essential if we are to slow the development of parasite resistance. However, parasite control programmes must balance the goal of reducing drug use with the need to maintain low levels of pasture contamination and protect individual horse health. At present we feel the evidence base supporting a universal move to the higher threshold remains limited, particularly across the wide range of management systems found in practice.
For that reason our current recommendation remains a treatment threshold of 200 epg. This approach has been used successfully for over 27 years with many thousands of horses tested through Westgate Labs, providing a precautionary and well established benchmark within targeted parasite control programmes. Westgate Labs does not sell wormers. Our recommendations are based solely on parasite monitoring data, published research, and our long standing experience working with horse owners and veterinary professionals.
Equine parasite control is an evolving field and we continue to follow emerging research closely. As new evidence becomes available, we remain open to adapting our guidance where it is supported by robust data. For now, we believe a treatment threshold of 200 epg represents a balanced and pragmatic approach, helping to protect horse health while supporting the responsible use of anthelmintics as part of a sustainable parasite control strategy.
References CANTER Guidelines (Second Edition). 2026. https://canterforhorses.org.uk/guidelines/
|
Strongyles
|
Ascarids
|
|||||
|
Low
|
Moderate
|
High
|
Low
|
Moderate
|
High
|
|
|
Foals: EPG
|
0-500
|
500-1000
|
>1000
|
0-500
|
500-1500
|
>1500
|
|
Proportion of animals
|
70-90%
|
5-20%
|
0-10%
|
70-90%
|
5-20%
|
0-10%
|
|
Yearlings: EPG
|
0-500
|
500-1500
|
>1500
|
0-200
|
200-500
|
>500
|
|
Proportion of animals
|
20-50%
|
20-50%
|
20-50%
|
90-100%
|
0-5%
|
0-5%
|
|
Adults: EPG
|
0-200
|
200-500
|
>500
|
0
|
0-100
|
>100
|
|
Proportion of animals
|
70-90%
|
5-20%
|
0-10%
|
95-100%
|
0-1%
|
0-1%
|
The table refers to parasite faecal egg counts as typically determined using the McMaster method. Low, moderate, and high faecal egg count levels (in eggs per gram, EPG) for different age groups for strongyles and ascarids, respectively, showing the proportions (as percentages) to expect in each age category.