This article reprinted with permission from Horseadvice.com an Internet information resource for the equestrian and horse industry since 1994. On the WWW at http://www.horseadvice.com . Copyright 2004Introduction

It is near impossible to overestimate the potential damage that intestinal parasites do to horses. Prior to development of the modern dewormers of the late 70's you could safely say that horses’ life spans were fairly well limited by that damage, particularly large strongyle larvae migration. The cause of the death did not happen all of a sudden but by damage accumulated over time. Even with modern dewormers, if not used appropriately, we pay for parasites with increase vet bills, increased feed bills and horses that don't look their best. Fortunately most deworming regimens have removed the dangerous large strongyle threat but other less dangerous parasites may be nibbling at your bank account daily.

How Parasites Cause Disease
There are two ways parasites hurt our horses:
The migrating larvae physically damage the horse’s tissue.
The adults both physically damage and consume the tissues.

Though most horse owners realize that intestinal worms consume the horse’s tissues, few realize that serious damage is done by the migrating larvae. The migrating larvae can be responsible for
colic
diarhorea
pneumonia in foals
skin diseases
paralysis

Older programs were designed to periodically kill off the adults. The aim of a modern parasite control program is to prevent the horse from picking up the larvae, which do much of the damage. Of course by consuming tissues the adults are also responsible for disease including:
decrease feed efficiency
weight loss
colic
poor quality coat
diarhorea.

Even if your horse does not have signs of disease, improperly designed deworming programs may have you increasing the amount fed to your horse and may have even turned to expensive supplements just to feed the worms.

Life Cycle of Parasites

Parasites in the Horse
The horse becomes exposed to parasites in the environment while the parasites are in their larval stages. How they are exposed depends on the species but most commonly while grazing. The larvae then burrow into the intestinal wall and some even migrate throughout the horse's body. During these migrations the larvae are maturing and may even encyst for a variable period before returning to the bowel. Back in the bowel as mature adults, they bite into the intestinal wall, begin to ingest tissues, and produce eggs, which pass out the horse in the feaces of the horses.

An important difference among parasites is how long it takes ingested larvae to become egg-producing adults. This determines how frequently you need to deworm. For example, let's say it takes six weeks (about the shortest time for egg production of equine parasites) for newly consumed parasite larvae to start producing eggs that pass out with the manure. If you deworm today with a product that kills 100% (No product is 100% but some are close) of the parasites in the horse, he will tomorrow go out pick up new larvae and start contaminating the pasture with parasite eggs again in six weeks.

Parasites on the Pasture
Once on the pasture the eggs hatch and the larvae begin to develop into more advanced stages that eventually become infective. How long after being excreted they became infective depends on species, temperature, and humidity. This can be as little as a couple of weeks and as long as a year or more. No matter how effective your deworming product is, horses instantly begin picking up parasite larvae from the pasture after being dewormed with a paste or tube. But the parasite burden on your pasture is determined by your deworming program, how many horses are on your pasture, and environmental factors.

Worms are not sensitive to winter's cold and they will persist on the pasture through the roughest winters. Most parasites are sensitive to dry, hot weather at least during part of their time on the pasture. Some stages of the developing larvae are resistant to drying however and may persist for months once they have achieved this stage. One way parasites deal with hot, dry weather is by staying in the manure balls which remain moist on the inside for long periods. This is probably one reason that horses avoid grazing around fouled areas and presents a management dilemma: toharrow or not to harrow.

Managing Pastures with Parasites
Harrowing will help kill larvae, fertilize the pasture, and reclaim fouled areas for use by the horses. It also increases your horse’s exposure to larvae that he may have avoided by habit, A good deworming program prevents the passing of worms into the feaces so the pasture can be safely harrowed. Stools contaminated by parasite eggs should not be harrowed and then horses allowed to graze immediately. Enough time must be allowed for the larvae to die. Just a month or two of hot dry weather is sufficient to kill the majority of the parasite larvae. Wet cool weather may allow the parasites to persist for months or even a year or more. Chain harrow when it is hot and dry and avoid having horses graze straight away if possible.

If the horse is on a deworming cycle that kills the adults before more eggs are released onto the pasture, it gradually become less contaminated with parasite larvae as time goes on. Once you learn that some of the worst damage done by a parasite is done during the early stages of larval migration in the horse you realize how important preventing your horse from being exposed to the larvae on the pasture is. This means that regular deworming is important. Haphazard deworming practices are almost as bad as none at all.

Tapeworm Infection a Significant Risk Factor for Colic
In the past tapeworms in horses was considered a fairly innocuous problem but recently a clinical survey using a new serological technique to detect tape worm infections found a strong correlation between the presence of tapes and two types of colic: cecal impactions and spasmodic colic. This study underscores that special precautions that a deworming program needs to include a consideration of tapeworm infestation. Standard deworming regimens are not effective against tapes.
Currently a simple, safe, and effective treatment for tapes is Strongid at a dose twice that of the calculated label dose. At this dose rate it is considered greater than 90% effective. Daily Strongid granules are also considered effective but this work is preliminary and not yet confirmed. Once yearly deworming with a double dose of Strongid paste is recommended. In areas where the parasite has been related to clinical disease two or three times a year deworming maybe needed.