Prebiotics and probiotics are used for a similar purpose: to optimize the microbial
community of the gastrointestinal tract. However, they have different mechanisms of action.
A prebiotic is a substance that provides nutrients to specific and desirable
microbes in the digestive tract. Common prebiotics are various types of fermentable
compounds (usually carbohydrates). The goal of a prebiotic is to stimulate the growth of
beneficial organisms that already inhabit the gastrointestinal tract, particularly the
large intestine. A probiotic contains live organisms, such as Lactobacillus or other
microbes. The goal of the probiotic is to introduce beneficial organisms to the
gastrointestinal tract. To be effective, the probiotic organisms must be alive at the time
of ingestion and must be able to live in the gastrointestinal tract.
Answer by Dr. Cheramie, DVM, MS of the Merial Professional Services team
Yes, while it is unlikely a horse will become ill or suffer harmful effects from being dewormed too often, in the long term, all horses' health can be compromised by the development of parasite resistance to dewormers.
When deworming strategies were developed in the 1960's, the protocol was simple - treat every horse on an 8 week schedule with the newly available benzimidazole dewormer. A dramatic reduction in mortality from parasitic disease resulted. During the next 2 decades, as new dewormers became available, veterinarians recommended rotating between classes of products but, still treating every horse the same.
Parasites, however, responded to the chemical challenge by developing resistance. In the case of small strongyles, identified as the most prevalent parasite in adult horses today, there is evidence of their widespread resistance to 2 of the 3 major dewormer classes - benzimidazoles and pyrantels. Contributing to the development of small strongyle resistance is the common practice of rotating drugs, some of which are still effective against this parasite and some of which are not.
Experts say it's time to throw out these outdated practices. We now know only 20 to 30% of horses in a herd shed about 80% of the worm eggs. Thus, it doesn't make sense to treat every horse with the same 8 week frequency. Once you've determined how often each horse needs to be treated, it's important to make sure you're using products that are actually working against the target parasites on your farm. These practices are often called "strategic deworming" and are a better way to manage parasites and help avoid contributing to the development of resistance on your farm.
With the help of your veterinarian, the first step is to conduct a fecal egg count (FEC) on each horse, which will identify which parasites are present and which of the horses are high, medium or low egg shedders. Based on the results, your vet will recommend how often each horse needs to be treated. Your vet will also likely follow up with fecal egg count reduction tests (FECRT), used to then determine whether specific products are still effective against the parasite on your farm. Ultimately, you may find it appropriate to discontinue the use of some of the products that you were using.
Managing all parasites, including tapeworms, through a strategic deworming program may help save money in the long run as a broad spectrum product may be required less frequently for some horses.