Osteopathic examination of the horse’s spine is tremendously helpful in finding the presence of L4 large strongyle larvae at the mesenteric artery area. The horse with a larval problem will have restrictions in one side of the back all the way from Th12 to L4. This restriction can be found on either side. Horses with EPM, which is often found with the presence of L4 larvae; may be overly flexible on the opposite side of their spine.

We have found that the presence of the L4 larvae in the area of the anterior mesenteric artery of the horse weakens the immune system and allows other diseases, such as EPM to enter and infect. Texas A and M University has done antibody titers on a large population of horses here in Texas, and have found that 85% of horses tested had antibodies to EPM. Many of these horses never presented with clinical signs. I believe that a horse with a healthy immune system can attack and kill the invading protozoa without the horse showing clinical signs. A horse that has a weakened immune system is more likely to develop clinical signs of the disease. Young horses just going into training, horses on the road competing at a different location every weekend, and horses with any type of other stress going on in their bodies, such as ulcers; are also more prone to develop a full blown case of clinical EPM.

Other signs of L4 parasites present in the horse include a mane that breaks easily, dry, unhealthy hair coat, grumpy disposition, decreased ability to perform, diarrhea, and colic. Common performance issues include uneasiness in the roping box, failure to make tight barrel turns, leaving the barrels with less force, not getting to the bottom of their stops, unwillingness to jump, and less desire to lift their back in dressage.

An extreme presence of L4 larvae can cause a thromboembolic colic. This type of colic is caused when the strongylus vulgaris parasite (bloodworm) gets into the arteries near the intestines of the horse. Here they can cause the blood to clot and the clots can block off blood supply to some of the arteries that feed the intestines. This inflammation and infarction (death of the intestinal tissue) cause this type of colic episode. Some horses may not be obviously uncomfortable, others may show a low grade colic with a mild fever. Others can have a severe colic.

The lifecycle of the Strongylus vulgaris can take anywhere from six to eleven months. These larval forms are not killed by freezing conditions, however some of them may die in very hot, dry, climates. They eventually become L4 larvae and migrate from the large intestine to the anterior mesenteric artery, where they cause inflammation. I often find the presence of these L4 larvae causes the walls of the anterior mesenteric artery and the dorsal aorta to thicken. Most of these horses have a negative fecal egg count when tested. The adult worms have been killed, so there are no eggs, but the L4 larvae are persistent in the tissues.

I treat these horses carefully, depending on their state of health. If the horse is strong and healthy, I will proceed with the following:

I suggest using colloidal silver during the deworming process to protect the horse’s intestines from trauma and inflammation caused by the dewormer and the dying parasites. I use homemade colloidal silver, or Silver Biotics brand commercial colloidal silver. I follow the human dose recommended on the bottle for the commercial product. I dilute about two dropperfulls of the homemade colloidal silver in a little distilled water and pour it over the horse’s feed morning and night. I start the colloidal silver about 2 days prior to the first deworming. If the horse is prone to or has ulcers, I also like to use a probiotic such as Advanced Biological Concepts Pro-Bi or Kam Animal Products KLPP. I will start the probiotic a couple of days before the deworming and continue it and the colloidal silver through the entire deworming process.

If the horse is weak, I may put the horse on an organ cleanse before starting the deworming protocol. This will prepare the organs for the toxins released by the dying parasites. I use a product specially formulated for my clinic, called Stage 1.

If the horse has not been dewormed in over 4 months, or de-worming history is unknown, I will start with a dose of ivermectin before the rest of the protocol. The ivermectin will do a slow gentle kill off of any adult worms in the intestines, so that there is not a sudden kill of lots of worms, which could cause an impaction colic.

Wait at least 3 weeks after giving the ivermectin and then begin the larvicidal protocol.

Larvicidal Protocol:
First, you want to give Panacur Powerpak in a sufficient amount for your horse’s weight. You can purchase additional single doses of Panacur if you are treating a horse over 1250lbs. The Panacur Powerpak administers a double dose of dewormer every day for 5 days in a row. Be sure that you are giving your horse a double dose that is sufficient for his weight. You can safely overdose the fenbendazole by a hundred pounds or so. Better to give a little over than give a little under the horse’s weight. If your horse has an allergy to fenbendazole, you can use an oxibendazole product at a double dose instead. I have found the oxibendazole to be a little more severe on the horse’s gut, and prefer the Panacur Powerpak. I also prefer the name brand over the generic product. We get better results using the name brand.

Wait ten days for the horse to recover from the dewormer; but not long enough for the L4 larvae to recover…..and give the first dose of moxidectin to the horse’s weight (NOT a double dose). The larvae will still be affected by the Panacur Powerpak and unable to migrate away from the dewormer in the gut and blood vessels.

Wait another ten days after the moxidectin and give a Quest Plus, (moxidectin plus praziquantel) again at a dose equivalent to your horse’s weight.

You want to give your horse a sufficient amount of Quest to kill the parasites, so it must be at least enough for an animal of his weight. If you don’t give enough, you will just teach the parasites to be resistant to the dewormer. If you give too much Quest, you can make your horse sick.

Let’s talk Prevention:

First, a complete management program.

  1. Minimize the number of horses per acre to prevent overcrowding.
  2. Pick up and dispose of manure a minimum of twice a week
  3. Compost the manure in a pile instead of spreading it on the pasture
  4. Mow and harrow pastures periodically to expose the parasite larvae to heat and drying. Remember, freezing will not kill the larvae.
  5. Rotate pastures with other animals if possible, to break up the cycle of equine parasites
  6. Use a feeder to keep hay and grain from sitting directly on the ground.
  7. Use a charcoal block or flea comb to remove bot eggs from the horse’s legs before they can lick and ingest them.
  8. Set up a deworming program that fits your area and type of horse keeping.

The American Association of Equine Practitioners is aware of the problem of the worms developing resistance to the dewormers we currently have access to. One thing you can do to prevent further development of resistance to the dewormers we are currently using is to make sure to give your horse a dose that is adequate for his weight. If you dose your horse for less than his weight, you will make all the worms sick, and kill NONE of them. They will develop resistance to the dewormer. Many states are successfully incorporating a system of only deworming the horses that are shedders of parasite eggs. It seems that only a small portion of the horses seem to be carrying most of the parasites. We call these horses “shedders”. If we only deworm the “shedders”, theoretically we will decrease the chance of resistance developing. If your horse does show a high egg count after a certain type of dewormer, you can try using a different ingredient (don’t just buy a different name brand of dewormer, make sure you are using a different ingredient).

Here are some ways to battle the resistant parasites:

Regular osteopathic exams can help the horse to have a strong, healthy immune system; which helps him to be a low shedder. If a horse sees an osteopathic practitioner twice a year for examination, you can find and treat L4 larvae before they can do a lot of damage.

Monitor the ERP, Egg reappearance period. Depending on the dewormer you use, this is the amount of time after deworming before you begin to see eggs in the feces. Here are some examples:
Moxidectin: 10 to 12 weeks
Ivermectin: 6 to 8 weeks
Fenbendazole: 4 to 5 weeks

Perform a FECRT, Fecal egg count reduction test. Test the horse’s feces before you de worm and again 2 weeks after to determine if the product was effective or not. To be effective, there should be a 90% reduction in the egg count.

20% of the pastured horses will usually shed 80% of the parasite eggs found. Separate out the low shedders from the high shedders and deworm accordingly. We must de worm smart in order to prevent further parasite resistance. Be sure and give your horse sufficient dewormer for his weight. The tubes of dewormer are usually made for a horse that is between 1000 and 1250 pounds. If your horse weighs more than that, you must purchase an extra tube of the dewormer to achieve an adequate dose for your horse. When you deworm with less than the optimal dose, you are teaching all the worms how to be resistant to the dewormer, and you are killing NONE of them.

Deworm the low shedders less often than you deworm the high shedders.
Example: For Low Shedders:
Deworm 2 to 3 times a year, depending on your location. In Texas, where we never get hot and dry enough to kill the larvae; I like to deworm a minimum of 4 times per year. Use Moxidectin in the spring and fall to kill as many larvae as possible. Consider giving a Panacur Powerpak first if you have strongyle larvae in your area.

For Moderate Shedders:
Deworm 3 to 4 times per year, depending on your location. When I was working in southern Arizona, where it is hot and dry, we only dewormed our horses twice a year, and saw very little problems with parasites. In Texas, where the humidity protects the larvae year round, you may want to increase the amount of deworming.

For High Shedders:
Deworm these horses every other month. 5 to 6 times per year. Use moxidectin a minimum of 3 to 4 times in your rotation, and consider preceding it with Panacur Powerpak once or twice a year. You can also do a fecal egg count 4 weeks beyond the kill period for the last dewormer given. Test 3 grams of fresh fecal material, keeping the samples cool and in air tight containers until you can have them tested. A sugar flotation test is preferred. More eggs are found in the fresh sample, and if the clinic sends the fecal out for testing, egg counts are less reliable.

I have not had great success using the fecal test to determine which horses to deworm and which ones to not deworm here in Texas. Perhaps this method will work if the horses are not exposed to any pasture. Most of my clients have horses that graze on some type of pasture, and the ones who do not have parasites in their feces can still develop L4 larvae at their anterior mesenteric artery when they are not dewormed for an extended period of time. We have also found that if we check the feces in the clinic we find many more parasite eggs than the lab does if we send the fecals out. It seems that the eggs are very fragile and seem to disintegrate before they reach the lab, but if the feces are checked while it is still very fresh, more eggs are visualized.

In this part of central Texas, if your horses are on any pasture at all, it is my recommendation to deworm them every 2-3 months with alternating dewormers.

Here is an example deworming schedule for my area, near Houston Texas:

Jan/Feb Double dose of pyrantel pamoate
Mar/April oxibendazole or fenbendazole
May/June moxidectin with praziquantel
July/August ivermectin
Sept/October moxidectin with praziquantel
Nov/Dec oxibendazole or fenbendazole

Consider using double doses of the oxibendazole or fenbendazole in high shedders

Parasite load appears to be more linked with the horse’s strength of immunity than anything else. The best way to protect your horse from worms is to keep his immune system very strong. See other articles on this website for how to build a healthy immune system in your horse.

The horses that we see in the clinic with EPM (Equine Protozoal Myelitis) these days are much less neurologic than the presentation many years back. I believe that a strong immune system will help protect your horse from EPM. Research says that 85% of the horses in Texas test as being exposed to EPM, yet only a small percentage exhibit any clinical signs. The horses appear to be developing resistance to the protozoa. We often find that EPM will come in after the horse has parasites at the anterior mesenteric artery, which weakens the immune system. We first treat the horse to kill the L4 larvae at the anterior mesenteric artery before we address the EPM. We will often give a product called Karbo Pellets and Epic liquid to improve the immune system of the horse that has the EPM. There is a natural product called Sefacon that works very well as a protozoal killer. The Sefacon cannot be given within 5 days of any type of dewormer. We carry these products here at the clinic. See our online store or call for prices. (979)243-4969.