As horse-carers it is your job to know and understand the lifecycles of the parasites that infect your horses. Management of worms is not just about the worms that live inside your horse. One could be so bold as to say that the worms inside your horse are the least important part of the whole equation. In the long term, as with all infectious diseases, it is far more important to control/manage the source of the infection, rather than the infection itself.
The most important part of (strongyle) worm management in horses is managing the free-living larvae that live on your horses’ pasture. These are the infective stage, and the amount of larvae present on a pasture will have the greatest impact on the amount of worms living inside your horse.
In spring, when the weather warms up, the development of larvae from an egg (deposited in the horses’ manure) into an infective larvae speeds up. In optimal conditions (~15-25oC) this occurs in as little as 5 days. The overall lifecycle takes about 5 - 6 weeks, with a period of time where the larvae have to mature into an adult worm inside the horse before they can begin laying eggs again.
This is where you need to start thinking about the timing of the lifecycle and what is going to happen to your pasture over the next 5 months (Aug – Dec). Eggs will be shed, develop, be ingested, mature, begin shedding eggs again. In as little as five weeks, one egg can turn into an adult female worm that will be shedding 1000’s of eggs per day. Over spring, that can be up to four sets of lifecycles; the larvae will survive on the pasture throughout all of spring, so throughout each lifecycle, the number of larvae will continue to build up.
Horses will have immunity in place to limit the establishment of new worms, however the more infective larvae they are exposed too, the more likely it is their worm burden will increase. Additionally, when horses are exposed to high pasture larval loads, they are more likely to develop encysted larvae burdens. These encysted larvae may not cause a problem until the start of autumn, or even mid-winter the following year; what you do now is not just to manage worms during spring, but also for months in advance.
In spring, your job is to limit/manage how many larvae are on the pasture. How can you do this?
Avoid manure build up. Sometimes it is difficult to stay on top of manure pick up so chose other ways such as not over stocking and resting paddocks if you cannot collect all manure.
Get all your horses FEC-ed, and then deworm your high shedding horses. If you go into spring by eliminating the majority of eggs being shed, you will be one step ahead. This does not mean deworming all of your horses. You can stop 95% of egg shedding by only deworming half your horses. Deworming those horses that are only shedding a few eggs will only cost you more in wormers and continue to drive drug resistance. High-shedding, young and immunocompromised horses may need to be FEC-ed early in spring and also again towards December.
Cross-graze, if possible. Rotate through other livestock (cattle, sheep etc) to clean off horse worm larvae. How you do this is up to you – it depends on paddock size, how fast the grass is growing, stocking rates.
See the WormCheck website for details on how to organise your FECs for this spring.
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