In this age group, horses are still considered higher risk than their adult counterparts, but most will be gaining better resilience against parasites as their immune system develops.
Small redworm (cyathostomins) are now considered the most dominant equine parasite type of concern in this age group.
Healthy young horses should develop natural immunity to ascarids at around two - four years of age. However, cases are not unusual in older horses that have had a poor start in life.
Youngstock should follow a diagnostic-led treatment plan coupled with good pasture management but should still automatically receive an autumn/winter cyathostomin larvicidal treatment.
Testing protocol for equine youngstock two – five years old
Worm egg count every 8-12 weeks (depending on risk assessment)
- Perform worm egg count reduction testing to assess the efficacy of at least one chemical annually.
- Tapeworm test every 6 months (using the EquiSal saliva or blood serum antibody ELISA test). If either indicate significant tapeworm infection, or disease is present, a ‘Proudman’ worm egg count should be carried out, followed by appropriate treatment and 14 days later, a second worm egg count reduction test using the same egg counting method.
- Annual larval culture for large redworm
Treatment protocol for equine youngstock two – five years old
Targeted treatment rather than blanket worming protocols are recommended for all horses in this age group.
- Ascarid only: Treat with fenbendazole or pyrantel.
- Strongyles only: >200 epg. ivermectin or pyrantel if efficacy is indicated by worm egg count reduction test.
- Both ascarid and cyathostomins: Fenbendazole/pyrantel followed by a worm egg count reduction test. Depending on the resistance profile, weanlings may have to be treated by two different drug classes, i.e., fenbendazole or pyrantel for ascarids and ivermectin for adult small redworm. However, a pyrantel or fenbendazole could be effective against both parasite categories, if verified by worm egg count reduction test.
The worm egg counts in animals in this age group are expected to be considerably higher than mature horses, which has implications on the contamination of pastures. The decision whether to prescribe an appropriate anthelmintic should be made on a case-by-case basis. A critical balance should be reached between the use of anthelmintics to reduce pasture contamination and maintaining the health of individuals/co-grazing animals, versus the risk of promoting anthelmintic resistance.
Youngstock in the late autumn/winter
All youngstock that have grazed over the summer, are recommended a routine larvicidal treatment for the possibility of encysted stages of small redworm. A five-day course of fenbendazole would be appropriate if results of previous worm egg count reduction tests identify susceptibility of these parasites to this drug; alternatively, moxidectin (or a moxidectin-praziquantel if tapeworm treatment is also indicated) may be considered, providing the young horse has a good covering of body fat.
The small redworm blood test is not usually indicated for horses in this age group.
For youngstock overwintered on pasture that is not poo picked, consider a second dose of a larvicidal wormer in the back half of the winter, if egg shedding on worm egg count results remains high. We recommend alternating chemicals (if previous worm egg count reduction tests indicate efficacy of both licenced larvicidals).
Updated Summer 2026 following release of CANTER Guidelines Chapter 11