New School of Thought on Parasite Management

         Often as the Small Ruminant and Equine Extension Agent I get calls regarding horses and parasites. In recent years the industry has begun to see a shift in the way we approach and handle internal parasites of many species, particularly horses.  In the past, parasite management in adult horses was largely based on knowledge that is now considered to be outdated. The purpose of this article is to discuss the new approach to parasite management in adult horses. It is important to note that these are merely researched based suggestions and it is still important for one to work closely with their veterinarian in developing a farm specific parasite program.

         In order to fully understand the scope of the parasite issue, horse owners must understand the parasites at hand, dewomers, and resistance. It is important to understand why these changes in parasite managment have occurred. Recent research has demonstrated the following:

1.    Switch in type of parasite focus. Large strongyles have for the most part been eradicated and new parasites of focus are small strongyles and tape worms. A recent survey performed in the Southeast United States, focusing on small strongyles, found that 95% of herds demonstrated a resistance to panacur, 53% of herds were resistant to Anthelcide ® and 40% of herds were resistant to Strongid ®. Horse owners should note that small stronglyes affect all grazing horses so focus should not be for complete eradication but rather to treat only when needed. Small stronglye parasites are low pathogenic, meaning they only produce a disease when parasite loads are high.

2.    Increase in anthelmintic (dewormer) resistance across the board, which is the ability of worms to survive treatment; this problem has occurred due to old school of thought where one should rotate type of dewormer every two months and the lack of accurate dosing resulting from not having a true weight of the horse.

3.    Adult (over age of 3) horses vary greatly in regards to needing to be dewormed due to immune response.

Dewormers can be grouped into 3 drug classifications: Benzimidazoles, Pyrimidines, and Macrocyclic Lactones. Knowing the drug classification of a dewormer can help owners to stop rotation of dewormers.  The table below denotes these classifications and common horse dewormers that fall into their category.

Drug Class	Common Dewormers
Benzimidazoles	Fenbendazole (Panacur ® ) and Oxybendazole
Pyrimidines	Pyrantel (Strongid ® )
Macrocyclic Lactones	Ivermectin and Moxidectin (Quest ® )

         There are many factors to consider when developing a parasite management protocol for your farm. When working with your veterinarian these are a few things horse owners should take into consideration:

1.    Ivermectin is a larvicidal (will kill larvae) and a boticide. If used every 6 months large strongyles will be eliminated from your farm. It is equally important when introducing a new horse on farm one should deworm them immediately, stall them for a minimum of 4 days to prevent worm contamination.

2.    Horses develop an immunity to roundworms.

3.    Eggs can last for years in the environment so preventing contamination is essential.

4.    Perform fecal egg counts. Fecal egg counts will provide one with the type of parasites and load of parasites. Knowing what worms and how many are present one can determine how to treat or even if treatment is necessary.

5.    Fecal egg count reduction tests can provide horse owners with information necessary to determine if a dewormer is still working on farm.

EPG = Egg Per Gram

                EPG Pre-treatment – EPG 14d post-treatment  x 100 = FECRT      

                           EPG pre-treatment

6.    Products containing praziquantel (Equimax ®) are effective against tapeworms. Tapeworms are carried by the pasture mite which dies after first hard frost. So, deworming with a product containing praziquantel is recommended yearly.

7.    Pasture management is a vital aspect of parasite management. Overpopulating and overgrazing pastures will inevitably lead to parasite transmission. Picking up manure piles in pastures instead of dragging them are now considered better options to reduce contamination.