Ask the vet: deworming

Tired of knowing which dewormer to use? Or when to deworm your horse(s)? Learn all of the answers by posing your questions on the topic of deworming during the month of February to our AAEP expert, Dr. Chanda Moxon.

1
Question: Are there any particular dewormers that will cause horses to have problems with recurrent uveitis? I have a grey mare that has recurrent uveitis, and I was told that certain dewormers can trigger an attack. Is there any truth to that? 
Answer: 
Equine Recurrent Uveitis (ERU) is one of the most common eye disorders in horses. It is an immune mediated disease with several hypersensitivities, and no specific cause. The hypersensitivities include, leptospirosis, brucellosis, Streptococcus equi (strangles), Onchocerciasis, and hoof abscesses just to name a few. The most common hypersensitivity can be linked to Leptospirosis. With this being said, ERU can be very frustrating to link back to a particular cause.

Onchocerciasis is a parasite that is associated with connective tissue. They are not found in the intestinal tract of horses like most parasites. They produce microfilaria, which most often migrate to the dermis and/or to the eyes of horses. The clinical sign most often seen with Onchocerca is dermatitis. There may be areas of scaling, ulcerations, alopecia, and pruitis on the skin. 

Onchocerca can be diagnosed by having your veterinarian perform skin and/or cornea biopsies .The treatment includes Ivomec or moxidectin, which are found in many of the intestinal dewormers. If your horse has a history of ERU and has been diagnosed with onchocerciasis, during treatment, the dying of microfilaria can incite ERU.

This question had me researching in a number of places for an answer. Equine Recurrent Uveitis can have so many causes and treatment options. I recommend working closely with your veterinarian and perhaps even taking your horse to an equine ophthalmologist, especially if your horse is showing any signs of skin problems. There could also be other parasites that I am not aware of that could cause the flare ups after deworming.
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Question: My mare has been rubbing her tail very aggressively, off and on for a year, against posts, walls, etc. One veterinarian told me it was pin worms. I have tried to use paste dewormers, including a dose two weeks apart of Panacur. It seems better, but not completely 100%. Do you have any suggestions? 
Answer: 
I agree with your veterinarian that the “tail rubbing” is likely due to Oxyuris Equi or Pin worms. We should also consider other causes of pruitis in horses, such as an allergic response to the insect Culicoides and/or the fungi, dermatophytes (ring worm). And for male horses, sometimes a simple sheath cleaning will help when they are rubbing their tails, while in females cleaning the udders will help.

But, let's get back to assuming it is Pin worms, a tape test can be performed by your veterinarian to diagnose them. Since your horse seems to respond temporarily after deworming, I would expect we are dealing with either parasite resistance with your dewormers or an environmental contamination issue and your horse keeps re-infecting herself. Pin worms have become resistant to some of the deworming products available. I would recommend using a Pyrantel Pamoate paste every four weeks for at least two treatments. I would also decrease environmental contamination by wiping the rectal area with baby wipes and clean all water/feed buckets, as Pin worm eggs are very sticky and attach to objects in the stall allowing for your horse to be easily re-infected.
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Question: I just moved to Las Vegas, Nevada from Michigan and would like to know if I need to change my deworming schedule for my horses and deworm for different types of parasites since the weather is so much different in Michigan than here in Nevada.
Answer: 
This is a great question; however, many theories are changing in the way we think of managing equine parasites. The rule of thumb in histories past was to deworm as the seasons changed or every three months.

There are many considerations involved when dealing with intestinal parasites such as the age of the horse, the size of pasture, the number of horses grazing a pasture, and if pasture rotation is available. We have not had any new antiparasitic agents developed since the mid 1990’s, the current deworming products we have, we need to make them last. With this being said, there has been a great deal of parasite resistance developing with our current dewormers. The recommendations we are now making for “when to deworm” is to perform fecal egg counts before deworming. This is a very simple test that your veterinarian can run in the clinic and at that time your veterinarian can detect what parasites are found and give you a better understanding as to what deworming product will best meet your horse’s needs, as well as offer other environmental management recommendations. Often times, I have found that we are deworming when there is not a need to deworm at all. In Nevada, I would recommend taking a fresh fecal sample to your veterinarian every 3 months. I would expect with the dry environment, you will be deworming less often than you were in Michigan.

This new way of thinking is to act more as a preventative management and attempt to decrease your horse developing resistance, as well as keeping a chemical out of your horse’s body if there is no need to deworm.