Deworming horses has always been sort of like going to the dentist: You know it’s necessary, but you’d just as soon skip it. Fortunately, nowadays, worming products are convenient to administer, have proven to be safe, even for pregnant mares and baby foals.

Most horse owners have, at one time or another, asked themselves or their veterinarians these questions (Answers provided by Thomas Bello, D.V.M., Ph.D.):

Q:Is it best to alternate deworming agents at each deworming treatment so my horse will be protected from major parasites?

A:There is controversy in the veterinary profession concerning rotational deworming. According to the old school of thought, a different class of dewormer should be used at each treatment. However, some veterinarians now feel it’s okay to use one basic dewormer for a period of time, and, if necessary, complement it with treatment for bots and tapeworms. Periodic fecal tests will indicate if your dewormer is working satisfactorily.

Q:Are large strongyles the major cause of worm-related colic?

A: Not anymore, because almost every broad spectrum dewormer on the market is effective against large strongyles.

The current major culprits of worm-related colic are small strongyles, which burrow into and often pierce the gut wall of the large intestine, and which have developed a resistance to benzimidazoles at normal everyday therapeutic dosage. Bots, at high levels, have also been known to cause colic, even culminating in a ruptured stomach.

Q:Do worms only inflict damage during certain stages in their life? If so, is it most effective to deworm during peak egg-laying season?

A:The goal of deworming is zero worms. But, just because no eggs are present doesn’t mean that no worms are present. In fact, during “quiet” times, adult worms are either preparing to lay eggs or are in the migrating larval stage –– during which time 90 percent of all damage occurs. It is best to consult with your veterinarian to develop a deworming schedule based on the life cycles of worms.

Q: If my horse is ill and taking a medication, is it best to discontinue feeding it a daily dewormer?

A:Each case is different. Your veterinarian is your partner; you should consult with him/her regarding discontinuing daily dewormers, or administering any dewormer, when your horse is ill.

Q:Is bimonthly worming better than daily deworming; daily deworming better than bimonthly deworming?

A:Both methods of deworming are effective, so it boils down to questions of personal preference and cost.

Daily feed dewormers provide protection, but you might need to add ivermectin or organophosphates to your horse’s deworming schedule to protect him from bots. If you use an organophosphate, pay careful attention to dosing instructions to avoid toxic side effects.

Q:Is the best way to determine the efficiency of a dewormer by assessing fecal egg counts?

A:Fecal egg counts as a measure of parasite control are certainly helpful. But, they only demonstrate the effectiveness of the testing. Fecal egg counts do not account for the number of migrating larvae that are still present and burrowing into the horse’s intestinal and gastric lining, often provoking severe – sometimes fatal – results.

Q:Is it true that freezing kills strongyle larvae and ascarid eggs, thereby decreasing the need to maintain pastures (pick up manure), rotate horses to different pastures, or clean stalls and pens, as often in cold weather?

A:Dry heat and freezing cold weather cause strongyle larvae and ascarid eggs to become immobile. However, when the weather changes, they reactivate. And, when your horse eats either larvae or eggs, they are resurrected in the digestive system.

This problem is compounded because strongyle larvae have a protective overcoat that enables them to survive in pastures, and ascarid larvae develop inside of the nearly indestructible ascarid egg. In addtion, ascarid eggs are hard and sticky, enabling them to adhere to anything (footwear, hoofs, grass, stall walls, feed buckets) in a stall or barn for long periods of time, often years. When your horse eatsthem, he ingests not only the eggs, but also the infective larvae inside.

Q:Is tube deworming the only way I can be sure my horse will be dewormed properly?

A:Tube, daily-feed, and paste dewormers are all absorbed equally. There are advantages and disadvantages to each method.

Some horses become unruly when a tube is passed through their noses into their stomachs. Some horses spit out some of the paste after you have turned your back. Some horses might not ingest all of their daily dose of dewormer in their feed. The method to use is up to you and your veterinarian.

Q:Is it true that if a horse is fat, slick, and shiny, he doesn’t have any worms?

A:No. A horse can have worms, and yet look perfectly healthy. –