Worming-Schedule

There are three general approaches to worming:

  • Broad-Spectrum Wormer. The horse is given a medication (usually a paste in a tube, given orally) targeted at a broad range of different types of worm parasites. This is typically give once every three or four months (thus 3-4 times per year).
  • Rotational Schedule. The horse is given a number of different types of wormers, alternating broad-spectrum wormers with worming medication targeted at specific worm types. Although slightly more complex, this approach is more effective in dealing with parasitic worms which are resistant to the broad-spectrum wormer medication. Rotational schedules typically involve worming more often, typically four to six times per year.
  • Continuous Worming. In this approach, the horse's daily feed includes low dosages of wormer, so they are medicated on a continuous basis. 

A worming schedule specifies WHEN the horse is wormed and with WHAT TYPE of worming medication. Following is an example, using the rotational schedule approach.

When Wormer Type
February General Wormer
April Tapeworm Wormer
July General Wormer
November Encysted Redworm Wormer
December Bot Wormer

Frequency

The above example involves worming a horse five times a year. However, in some circumstances it is sufficient to worm three times (or even less) per year and in other circumstances one would worm up to six times (or more) per year. The required frequency will depend on a number of factors:

  • Local Worms. The types of worms varies from region to region. For example, if you do not have redworms in your region, you can skip the above November worming in the above schedule. Likewise, if you do not have bots, you can skip the December worming in the above schedule. Consequently, you should ask a local vet which types of horse worms are a problem in your area. See Horse Worms for a description of the different types and associated treatments.
  • Environment. If your horse is pastured with a large number of other horses, especially if the other horses are not wormed at the same time, your horse will likely be reinfected by worms quicker and more intensively that otherwise. How you keep your horse is an important factor in the extent to which it is infected with worms and consequently how often it needs to be wormed. See worming alternatives for details on factors which determine worm infection rates.

In general, it is recommended to worm 4-6 times per year. Less that four times a year may be insufficient and may also be difficult to cover all the worm types as each wormer is effective against only certain types (see below for discussion). Over 6 times a year is considered of little additional benefit in terms of controlling the worm population, while having the drawbacks of additional expense and additional chemicals (which some consider to be counter to the horse's health). 

Types of Worming Medication

In addition to the different brands, there are different medication types (different chemicals). None of the chemicals are effective against all worms, so each wormer medication will remove only certain types of worms. For this reason, one needs to alternate the different types of worm medication (e.g. one time use a medication for tapeworms, the next time a medication which is effective against bots). 

Alternating brands (a common mistake) is not enough, since many brands use the same chemicals. Instead, one needs to choose wormers with different chemicals (that is, different medications). The yearly worming schedule needs to include different chemical wormers, so that all the worm types are addressed at some point during the year. 

A number of worm types can be treated by different chemicals. In such cases, one should try to alternate between the various effective 

Sequence

Some worm types have a specific time of year when they are most likely to infect horses. For example, bots infect horses during the summer, so it makes more sense to use a bot-effective wormer in the fall (just after the horse has been infected) rather than in the spring (before the horse becomes infected).

Special Considerations

If a horse has a heavy worm population, killing off all the worms at once can be dangerous to the horse. In such cases, consider using a gentle wormer medication or a moderate dosage to reduce the population and then a couple of weeks later worm the horse again. Circumstances which indicate that a horse may have a high worm population include:

  • If the horse has not been wormed regularly, its worm population is likely high.
  • If the horse has been ill, its immune system may be depressed and consequently its worm population may be high.
  • If the horse has been pastured with other horses which have a high worm count, it has likely suffered from high cross-infection rates (see Factors Affecting Worm Population).
  • If the horse exhibits symptoms of heavy worm infestation (e.g. high egg/larvae count in manure or underweight despite getting a good portion of food).

References

For further information on horse worms and their treatment, click on Guide to Horse Worms, Worming and Wormers.