Four winter worm counting myths busted
24 December 2018
how and when to test in winter
There's a common misconception that worm egg counts shouldn’t be used over winter as ‘worms aren’t active’ and ‘small redworm are all encysted’ in the gut wall - therefore not egg laying to be counted in a test. Our response? Well, not quite… There are still a number of instances when a worm egg count can provide very useful information between the months of December and February.
While it’s true that worm activity slows below 6°C it certainly doesn’t cease and even in 2017/18 winter which was considered a cold one by meteorological standards, there were still many days where average daily temperatures exceeded this.
Furthermore neither ascarids or large redworm go through an encysted stage and not all species of small redworm inhibit and so adults of these parasites remain active and egg laying all year round.
Worm counts can be used in winter to:
- Test new horses in quarantine; if a horse of unknown worming history is being introduced to a herd then we would advise a worm count at any time of year to detect and treat any worm burden present to prevent potential pasture contamination.
- To support a decision on whether a second winter dose for the possibility of encysted redworm is required;
For many horses a single proactive treatment of either moxidectin* or 5 day fenbendazole is sufficient to treat for the possibility of encysted redworm. However high density groups of horses (especially youngsters) turned out on poor or over-grazed pasture for even part of a day are at much higher risk from inhibited larval cyathastomins. Such populations should be treated with a suitable chemical at the start of the winter, preferably after a couple of hard frosts, and tested at the end of the winter with a worm egg count. Where redworm infection is present a second treatment for larval cyathastomins may be advised by your prescriber.
- To test for wormer resistance; the winter dose is an ideal time to add in a resistance test to your programme. Following a positive redworm or roundworm count that has required worming treatment or a winter worming dose for the possibility of encysted redworm, a second worm egg count can be conducted 10-14 days after treatment to test for drug efficacy. We want to see the second test come back as <50 e.p.g. no eggs seen, to indicate that treatment has been effective.
- To test for potential ascarid and large strongyle infection.
*We try to reserve moxidectin for the winter dose because of its efficacy in treating larval stages of redworm where possible.
To this end the lab still sees many high worm egg counts through the winter. If you have any particular concerns about a horse and think a parasite burden could be part of the problem then don’t hesitate to send a sample for analysis. It could still prove a very useful tool.