Parasite Control Options for Horses

A comprehensive parasite control program involves more than deworming your horse on a regular basis. The most important feature is the ability to reduce the number of parasites and eggs in the environment. Secondly, the program should be effective with the fewest number of treatments necessary. Finally, the program should be broad spectrum to control many different types of parasites.

Preventative medication is a very important component of parasite control. The bad news is that there is no single program that works for all situations. However, there are many different programs available, these include:

Targeted Dosing: This strategy involves testing the level of parasite burdens in individual animals. Standard fecal egg counts should be performed once monthly. Also tapeworm testing via fecal testing or serology (blood testing) should be done twice yearly. All animals that are positive over a certain cut off level should be treated. A yearly treatment for "bots" should also be included during the winter. This program is only appropriate for adult horses and should be considered on a farm with a dedicated manager where good grazing management is in place.

Strategic Dosing: This strategy involves treating all pastured animals at regular intervals with an appropriate product. The interval between dosing can be determined by the egg reappearance period (ERP)of the medication, which is shorter for young animals. The ERP is the period after medicating an animal with a dewormer until there are significant numbers of parasite eggs present again in the feces. The animals are only treated during the spring/summer season when the risk for increased egg loads is highest.

Interval Dosing: This strategy is the one most commonly used. It is similar to Strategic Dosing; however, animals are treated year round at regular intervals. As the duration of parasite kill varies from product to product and even between farms, the interval between doses should be determined by the ERP or by guidelines set by your veterinarian based upon products used. This program may be appropriate for farms where there are frequent new additions to the group, at more casually managed (hobby) farms and in young animals.

Daily Deworming: This strategy involves the addition of a parasite control medication to the horse's daily ration. This program is appropriate for most adult grazing horses; however, additional periodic deworming with other products is usually necessary. Twice yearly treatment with ivermectin (Eqvalan, Phoenectrin, Zimectrin), ivermectin/praziquantel (Equell) or moxidectin (Quest) has been recommended. This program can select somewhat for resistant organisms since the parasites are continuously exposed to a low level of the drug.

Click here for more information about deworming your horse.

Source: Jeremy D. Frederick, DVM, University of Minnesota

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.