Four factors to consider when choosing a wormer for your horse:
The age of the horse:
Foals are at significant risk of disease due to roundworm infections and they should receive their first worming treatment at around 8 weeks of age. All horses less than 5 years old are more likely to have significant worm burdens than horses between 5 and 15 years of age as horses develop immunity to worms over time. Horses over 15 years of age become increasingly more susceptible as their immunity wanes.
The weight of horse/s to be treated:
The most common mistake horse owners make when treating their horses for worms is under or overestimating their horse's bodyweight. It isn't easy to access a set of scales that are capable of weighing your horse, however, with the aid of a tape measure and some mathematical calculations, it is easy to get a fairly accurate idea of your horse's weight. Measure your horse in centimetres around their "heart girth" and their length from the point of the shoulder to the point of their buttock. Multiply heart girth by heart girth by length and divide by 11877 to get your horse's weight in kilograms.
The time of year:
Unfortunately, in most parts of Australia, seasonal conditions are ideal for the development of parasites for most of the year, however, Spring and Autumn are always critical times to treat horses for worms. The different life cycles of worms means that they will be most active in the horse at different times of year e.g. bots and strongyles ( redworms) in the cooler months. It is critical to ensure that the active ingredient you use will kill the species of worms that are likely to be infecting your horse at each particular time of year.
The active ingredients in the product:
Horse wormers contain either a single active ingredient or a combination of ingredients from four different chemical classes.
Tetrahydropyrimidines or THPs (Pyrantel and Morantel): This class of chemicals treats a variety of worms including some tapeworms but does not treat bots. Horses under 2 years of age should be treated with a wormer containing Pyrantel or Morantel because they are reliably effective against roundworm.
Benzimidazoles or BZs (Oxfendazole, Oxibendazole and Fenbendazole): This group treats the majority of worms in horses except bots and tapeworms. There is resistance to this class of chemical in small strongyles when used on their own, however they are useful for the treatment of mectin resistant worms when used in combination with other classes of wormers.
Macrocyclic Lactones or Mectins (Abamectin, Ivermectin and Moxidectin) treat a wide range of parasites, however, they do not treat tapeworm and roundworm are commonly resistant to this class of chemical.
Isoquinolines (Praziquantel) This ingredient is used in combination with other classes of chemicals in horse wormers because it is very effective against tapeworms.
