The overall goal for worm management in adult horses is to deworm them as little as possible. For the majority of adult horses, one or two treatments per year is all that is needed. However, the key to getting your horse onto a schedule of one deworming treatment a year, is to pick the best time of year to do it.
In Southern Australia, the optimal time to deworm your horses, and to target as many parasites as possible, is in mid to late autumn. This time of year ties in best with the lifecycles of bot flies and small strongyles. At this point, all the bot flies are at a stage of their lifecycle where they are inside the horse (see figure 1), and hence susceptible to dewormers. Currently, in late summer/early autumn, there are still bot flies flying around and as eggs on your horses’ coats. Deworming now can be considered a wasted treatment, as it will not kill these stages of the bot flies. If horses were treated now, a secondary treatment would be needed in winter to mop up any bot fly larvae that infect your horse in early autumn.
Figure 1: Lifecycle of bot flies in Southern Australia. This lifecycle takes 12 months, therefore bots only need to be treated for once a year
Late autumn is also a good time of year for an annual strongyle ‘clean-out’. Small strongyles are the most common worms horses will have, and while seasonal or 6-weekly treatments to remove strongyles is no longer advised, a single yearly treatment can help prevent high burdens developing. The choice of late autumn for this treatment also due to the lifecycle of small strongyles; some larvae may encyst in horses’ intestinal walls over spring/early summer. In late summer/early autumn, as the weather cools, these larvae will begin to emerge from their cysts. This is usually a slow process that happens in nearly all horses with little to no side effects. If horses are dewormed prior to all these encysted larvae emerging, the removal of the adult worms can trigger a mass emergence of larvae (rather than a slow emergence) which can lead to condition loss or even colic in extreme circumstances.
Waiting until all the larvae have safely and slowly emerged before deworming, and removing both adult worms and emerged larvae in one treatment may be the safest option.
When it comes to making the decision about when it is the right time to deworm, wait until the nights cool and the adult bots have disappeared. This may be around the ‘autumn-break’ (mid-April/Easter time) or once the first frosts have started. The “first frost” method simply means it is cold enough that the bots will be finished. Australia frosts are not cold enough to actually kill any worms in the ground – these need consistent days of below zero temperatures. Later in autumn is a safer option if your horses are in good condition heading into winter. However, do not be too late – bot fly pupae will be passed out in the manure in late winter. Depending on your local conditions, this single treatment can be done anywhere between mid-April through to the end of June.
For the northern parts of Australia that are considered sub-tropical or tropical, the seasonality of bot flies and strongyle lifecycles will not be as clear as down south. This means there is a less define window when it is best to perform an annual deworming. If there is a clear time of the year when adult bot flies are not present (middle to late winter), choose this time. Otherwise, aim for the time of year between the dry season and the wet season.
Any other worms horses may need treatment for include ascarids, tapeworms and pinworms. Ascarids are generally present only in young horses, which should be having FECs done 3 – 4 times a year, on which ascarid infections will be diagnosed if present. Any ascarid infection should be then treated regardless of the time of year. Tapeworms are relatively uncommon, and a single treatment once a year with a tapeworm specific paste (containing praziquantel) will be enough to keep tapeworms under control. However, drug resistance in tapeworms is developing and so the use of praziquantel dewormers should be limited only to horses that have a current tapeworm infection (WormCheck now offers a tapeworm specific FEC). Lastly, pinworms are relatively non-pathogenic, and so a single treatment once a year may also help keep them under control - although treatments for pinworms should be considered on a case by case basis.
Although all horses in should be treated in autumn, it is still recommended that all horses have a FEC done in autumn. FECs are a quick and cheap way to monitor your horses health, and also paddock management. Changes or spikes in EPG values may indicate illnesses (e.g. EMS/cushings) that need further investigation, or poor pasture management. Additionally, once a year, each property should test the efficacy of the dewormer used. This required before and after FECs, so it is best practice to do a FEC prior to deworming and another FEC 2 weeks after to make sure that drug resistance is not developing. In autumn, whatever dewormer that is used must contain a drug that is active against bot flies. For healthy adult horses, ivermectin or abamectin pastes are more than suitable. The addition of praziquantel is only needed if tapeworms are a problem. Moxidectin, while it is effective against bot flies, should only be used when required (in young/sick/high shedding horses) as drug resistance is slowly developing and so its use should be limited. “Rotational” dewormers (oxfendazole/fenbendazole) are not active against bot flies.