First Frost

FIRST FROST – do you need to wait for it to deworm?

The concept of waiting for the first frost before deworming has not come from any parasitology manual, but rather from livestock owners as a marker for season change. The frost itself does not influence worm burdens in the horse, or the ones on the pasture. Frosts in Australia are not harsh enough to reduce strongyle larvae in the environment; this takes extended time (3+ days) at below 0^oC, which does not happen outside alpine areas.

The key time for deworming horses in autumn is once all the adult bot flies have finished. Usually by first frost, winter has well and truly arrived, and the yearly lifecycle of the bot flies is at the point where all bot flies are currently inside horses as larvae in the stomach.

However, remember, that strongyles are the most important worm to target at any time of the year due their high prevalence and risk of disease if mismanaged. Additionally, frosts are not a significant factor for strongyles in Australia. For high-shedding horses, young horses, or those that are immunocompromised it is important to have them checked via FEC four times a year, and this may mean you need to treat them earlier in autumn for their health. If you have to treat your horses before the first frost for their health or for paddock management, bot flies can always be treated for again before the end of winter.

So, while a frost, or a cold night, may be taken as a sign of the seasons changing, it is not the definitive guide to treat. It is more important that you understand/know the worm burdens and EPG values of your horses, and the presence of worms/bot flies on your property to make an informed decision about when to treat. For example, if your adult horses, in good condition, are consistently low egg shedders (consistently low at least 2x a year on FEC for 2+ years), you can wait as long as you need to treat for bots. If you are unsure about your horses’ EPG values, get them checked via FEC: FECs are cheap, easy and can give you a lot of piece of mind, and allow much more mindful timing of treatments.0

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