Fall becomes a reminder of internal parasite control needs. We've fought hordes of flies and mosquitoes over the late spring and summer. As fall weather lessens these flying populations, attention can again come to focus on the internal parasites we generally don't see.

The list of internal parasites has remained constant over several decades. They include large strongyles, small strongyles, roundworms, bots, pinworms and tapeworms. Large strongyles develop and take blood meals in the large intestine. There are only 3 major species of this parasite, and with the development of modern deworming compounds in the last 15 to 20 years, large strongyles have been nearly eliminated in some situations.

Small strongyles have largely replaced the large strongyles in importance as an internal parasite of horses. There are many more species (40), they are much more prolific at egg laying, and their ability to go dormant in an encysted form in the intestinal wall make them easily overshadow large strongyles. In addition, the damage done to blood vessels and digestive tract tissue by their migrating larval stages keep small strongyles on the top of the list used to plan deworming strategies.

Roundworms still hold greatest significance for young horses, usually birth to 6 months. Adult horses develop immunity to roundworms that aids them in holding this parasite at bay.

Bots and pinworms are both regarded more as nuisance parasites than ones of great health significance.

Bots develop in the stomach after the horse ingests bot fly eggs licked off the hairs of the horse's front legs. That makes fly control and manual removal of bot fly eggs from leg hair very important as preventive measures for this parasite.

Pinworms are seldom a problem in horses older than 1 to 2 years of age. The eggs for this parasite are laid around the horses' tailhead and anus. The eggs and the glue that holds them in place are irritating and cause affected horses to rub and create hair loss.

Tapeworms have garnered increased attention with the development of a new deworming compound specific for tapeworms (Praziquantel). They have been known to be present in horses for a long time. But their significance has been debated. Severe infestations can play a role in intestinal spasms, colics and impactions, but they are not known to create these adverse effects alone.

All of these parasites are pretty easy to control with the large number of quality dewormers and the many forms available for use. The general product classifications are broken into three: pyrantels (Strongid, Continuex, Rotation 2, etc.); macrocyclic lactones (Ivermectin and Moxidectin); benzimidazoles (Safeguard, Panacur, Benzelmin, etc.); combined macrocyclic lactones (Ivermectin or Moxidectin in combination with Praziquantel for tapeworms).

Selection of the proper deworming product and program requires understanding the environment your horse is in, age of the horses involved, and overall risk of parasite exposure and re-exposure. Your local MFA Agri Services dealer can help with making those decisions.

Dr. Tony Martin is manager of animal health for MFA Incorporated.