For wormers to effectively treat internal parasite burdens, every horse must be dosed with the appropriate amount of wormer according to their bodyweight. If horses are underdosed with wormer, the internal parasites are exposed to a sub-lethal amount of the drug. This may leave a population of worms that are able to tolerate the wormer.
It is important to protect the wormers that remain effective as best we can. One of the most important ways of doing this is to ensure that animals are dosed to the correct bodyweight, preferable erring on the side of a slight overdose.
Prior to worming, very few horses are accurately weighed on a weighbridge or horse scales. More often than not, the dose of wormer a horse receives is based on a visual estimation of its weight. Unfortunately, visual interpretation of a horse’s weight is very unreliable. Studies have shown that horse owners and handlers tend to under, rather than overestimate their horse’s bodyweight.
Most people underestimate weight by approximately 20%. This means that many horses may be unintentionally under-dosed with wormer simply because a more accurate weight estimation method has not been used. All Virbac wormers such as EQUIMAX®, STRATEGY-T®, EQUIMAX® ELEVATION, EQUIMAX® LV are now available in doses large enough to treat all horses weighing up to 700 kg.
There are two very serious implication of under-dosing:
It is always better to slightly overdose than underdose.
Most wormers on the market in Australia only treat a horse up to 600kgs some as little as 575kg or even 450kg, however many horses can weigh more than the amount a single syringe of wormer is designed to treat.
A survey of Australian horses was conducted with regards to their size and weight to get an accurate picture of the “average” Australian horse. This survey sampled many horses from a different variety of breeds and uses and produced some interesting findings. One thing that was very clear from the survey was that horses in Australia are generally heavier than it was thought.
It is interesting to note that this survey was conducted during drought conditions, and therefore the majority of horses were of average body condition. There was only one horse in the survey larger than 17 hands, so large animals were not actively sought. This survey indicated that a significant proportion of horses will be underdosed if only one syringe of wormer is administered.
Virbac has taken into account this evolution in bodyweight by modifying its syringes to accommodate the treatment of horses that weigh up to 700kgs in a single dose.
Horse scales or a weighbridge are the most accurate way to weigh your horse. Unfortunately, most horse owners do not have access to a set of scales suitable to weigh their horses on.
The best alternative to scales is a weight tape or using the weight estimation formula. This is done by: