The emergence of parasite resistance is managed by limited anthelmintic use. This can be undertake using a coordinated response to parasite control
1. Pasture management
Pasture management is essential to reduce transmission of parasites and their resistance genes between individuals and includes
a. Safe removal of faeces from pasture on a minimum of a weekly basis during the peak grazing season
b. Rotation of grazing animals to use others (eg ruminants) to reduce the parasite burden on pasture
c. Resting pasture for prolonged periods
d. Segregation of horses that are known to have a persistently high shedding from remaining populations
e. Isolation of horses with an unknown parasite burden before turnout to pasture
2. Targeted selective treatments use of anthelmintic agents in adult horses
a. Avoiding fixed interval deworming
b. Only administering products to control parasites in adult animals with a proven parasite burden:
i. Developing a parasite control program with up to date veterinary advice
• Veterinary surgeons should review the current best evidence based on the BEVA/AAEP guidelines or other national guidelines where these exist locally
• SQPs should follow guidance based on industry guidelines working within their scope of practice.
a. Anthelmintic resistance is a veterinary clinical diagnosis and therefore the diagnosis and management of anthelmintic resistance must involve the attending veterinary surgeon
ii. Only administering anthelmintics to horses at pasture, or co-housed, during the grazing season.
iii. Using faecal worm egg counts to determine the parasite burden in individual horses and only dosing those animals with greatest burdens
iv. Targeted use of the small strongyle assay (SSA) to identify horses with a low parasite burden.
• At this time, some certainty can be achieved from using a low ELISA result as evidence of low parasite burden. However the contrary is not certain, and the use of targeted anthelmintics should be based on current or seasonal understanding of faecal worm egg counts.
• In animals where FWECs are not available from the grazing season, those animals should be assumed to be at a higher risk of disease, except where SSA data suggests there risk profile is low.
• SSA should not be used in place of regular worm egg counts taken through the grazing season in resident horses
c. Rotating classes of dewormer only when resistance is identified, not on a routine basis
3. Strategic use of appropriate anthelmintics in young stock
a. Selecting anthelmintic agent based on the current parasite burden
i. Ascarid parasites have a different pattern of resistance to strongyles but are commonly encountered up to 6 months of age
ii. Use of faecal egg counts to determine the prominent pathogen from 6 months of age 9393
iii. Without laboratory testing it is not possible to know which parasites are important in a particular foal and therefore which anthelmintics are most effective
• Macrolides (eg moxidectin and ivermectin) are generally not effective against ascarids due to the widespread development of resistance
• Benzamidazoles (eg fenbendazole) are generally not effective against small strongyles (cyathostomins) due to widespread development of resistance
b. Pasture management including
i. Collection of faeces and removal from fields
ii. Rotate pastures and avoid using the same fields from year to year with youngstock
iii. Managing stocking density to reduce transmission
