Equine Veterinary Newsletter
Dr. John H. Berends
Dr. Berends' Equine Veterinary News Letter includes
the latest headlines and information about equine health, immunizations,
and care techniques. You can subscribe to Dr. Berends' Equine Veterinary
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John H. Berends
2012 Winter Newsletter
This news letter will focus on a discussion of equine internal parasites and treatment for same. For many years, the horse owning population (on the advice of their veterinarian) has practiced rotational deworming programs centered upon frequency and deworming product rotation. Due to the development of parasite resistance, the equine veterinary community has recognized the need to restructure these programs. Some of the rational thinking that has led to the newer recommendations is discussed in this newsletter.
Horses all get internal parasites, but not all horses get sick from them. However, their presence in the horse population does pose a risk. The typical life cycle of the parasite has an external and internal phase (outside and inside of the horse’s body). A simple explanation of this cycle is: the parasite eggs are shed onto the pasture through the horse’s manure. In this state (egg) they are not infective to the horse. These eggs must develop to a third stage larvae before they can infect the horse via ingestion from pasture grass. This L3 stage has a very tough protective layer surrounding it and is not affected by “killing frost”. The egg development to L3 does not occur during cold winter temperatures, however. The L3 returns to the horse’s digestive tract (via ingestion) and matures to the adult stage inside your horse. Eggs are produced and excreted in the feces thereby returning to the pasture and completing the cycle.
Internal damage to the horse’s digestive tract (as well as the vascular system in the case of blood worms) can occur. Colic and diarrhea can result as symptoms caused by parasites. Horses 4 years old and younger are most severely affected. Also, horses that already are challenged by another disease can be further weakened/debilitated by these parasites. A parasite free status does not exist in the horse. Additionally, there has been no species eradication of any internal parasite in the horse. This means that from the time before the existence of the popular dewormers we know today until now, we’ve only been able to create a tougher, more resistant strain of equine parasite. Once drug-resistance develops there is no way back to the original strain. This means we have been in the process of creating our own monster. Since the introduction of the benzimadazoles in the 60’s, pyrantel in the 70’s, ivermectin in the the 80’s and moxidectin in the 90’s there have been no new equine anthelmintic drugs developed.
Making sure the animals are not shedding worm eggs during the first 3 months of the grazing season reduces the build-up of infectious larvae on the pasture later in the season. Successful deworming requires a seasonal program with multiple treatments given at strategic times to prevent the build-up of parasite contamination in the environment. Treatments need to be strategically timed in the season when transmission is most likely to occur. In Michigan, parasite transmission is limited during the winter months. Therefore, transmission occurs most successfully during the spring and again in the fall of the year.
Parasite control programs should be designed for farms on an individual basis using dewormers that are successful for the environmental parasite population found on those farms. This farm specific program requires the use of fecal egg counts and identification of which parasites are endemic on each farm. If you are interested in setting up an individualized deworming program for your horses, contact me to discuss this opportunity. Over the course of your horse’s life, it will make your deworming program the most effective for your horse’s health and save you money by using only the dewormers that your individual horses require.
