Saturday, May. 25, 2024

Fighting The War Against Parasite Resistance

Once upon a time, a rotational program in which every horse was dewormed with a different product every six to eight weeks protected our animals from parasites quite well. But a dewormer administered to a horse that doesn’t need it is much like a person taking an antibiotic when he doesn’t have a bacterial infection; the chance of building resistance increases as you increase the frequency of chemical exposure to a bacteria or parasite.



Once upon a time, a rotational program in which every horse was dewormed with a different product every six to eight weeks protected our animals from parasites quite well. But a dewormer administered to a horse that doesn’t need it is much like a person taking an antibiotic when he doesn’t have a bacterial infection; the chance of building resistance increases as you increase the frequency of chemical exposure to a bacteria or parasite.

Many horse owners are reluctant to stop their rotational routine because they’ve never had a problem. So why should they change now? Just look at the incidence of MRSA and how the overuse of antibiotics has contributed to its emergence. Similarly, each year more and more farms across the country are diagnosed with “superworms” that are resistant to many products.

Considering that 20 percent of the equine population sheds 80 percent of the parasite eggs in any given population, it only makes sense that each horse should have his parasite burden status determined and addressed on an individual basis.

Fecal egg counts are simple tests performed on manure samples to determine a horse’s parasite burden. In mature horses strongyle eggs are the main focus. In horses less than 2 years old ascarid eggs are also counted.

It’s impossible to distinguish small versus large strongyles from a fecal count alone because their eggs look identical. It’s possible to culture the feces and differentiate the two, but this is not often done because both small and large strongyles pose a threat to the horse and high numbers need to be treated.

As always, consult with your veterinarian to help you design a program that is appropriate for your horse.

Staving Off Strongyles

In North America, five internal parasites pose the most significant health threats. In horses 2 years and older the major threats are large and small strongyles, tapeworms and bots.

Strongyles (also called bloodworms) pose the most danger and can result in poor coat, weight loss, colic and even death. They have well-developed mouth parts, including teeth, to enable them to latch on to the intestinal wall. Female strongyles lay large numbers of eggs almost constantly so a routine fecal egg count should detect them.

Due to the highly prolific nature of female strongyles, manure management is critical to decrease the number of larvae to which your horses are exposed. Strongyle eggs are passed in the manure and hatch to larvae when the temperature is between 45 and 85 degrees. Horses are infected by consuming infective stongyle larvae from grazing contaminated pastures or drinking contaminated water (such as a stream contaminated by manure).

Any horse grazing where manure is present is potentially at risk, but there are steps you can take to minimize their exposure. Decreasing the parasite burden within the horse will decrease its “pasture contamination potential” because fewer worms harbored translates into fewer eggs passed into the environment.

Removing manure from pastures on a regular basis is one of the most effective management treatments available. The frequency with which this needs to be done varies with temperature. The eggs passed in manure do not infect horses; rather the eggs must hatch and then progress through three larval stages before they are consumed by horses in order for them to establish an infection.

The rate at which strongyles may progress from eggs to infective (L3 stage) larvae varies with the temperature and can be in as few as three days with temperatures in the 80s to as many as a few weeks in cooler temperatures. Eggs can hatch in temperatures that exceed 85, but the resulting larvae die before they can progress to the infective stage. Freezing is fatal to strongyle eggs, which is helpful in the North; lack of sustained freezing temperatures in the South necessitates even better pasture management.

Depending on the number of horses and pasture size, removing manure may not be practical or even possible. Dragging manure is a common practice and is acceptable if the temperatures have exceeded 85 or have been below freezing for a week or so. However, dragging pastures when the temperature has ranged between 45 and 85 is not helpful as you are actually spreading the eggs and increasing the potential exposure of horses grazing the pasture.


Once the strongyle egg develops to infective larvae, a membrane surrounds them and makes them resistant to freezing, though they are still susceptible to heat. So horses in areas of the country that maintain warm summer temperatures are at far greater risk of parasitism in the winter than during the summer.

Strongyles can be divided into two major groups: large and small. There are several species of large strongyles, and they migrate through various tissues (lining of arteries that supply oxygen and nutrient-rich blood to the gut, liver, pancreas or peritoneum) for six to 11 months before returning to the gut to mature to egg laying adults.

Small strongyles do not migrate. Instead they invade the lining of the large intestine where they are protected from the immune system by a tough capsule. These are commonly referred to as encysted strongyles, and they are a challenge to treat because the protective capsule makes most dewormers ineffective. The encysted larvae can remain for many years and eventually emerge and mature to egg-laying adults within a few weeks when the adult population in the gut lumen dies off or is purged by a dewormer.

As a result, the time at which fecal samples are collected becomes critical. A dewormer may have successfully killed off the adults in the lumen, but before the post-treatment manure sample was collected encysted larvae had time to emerge and mature to egg-laying adults.

Taking Care Of Tapeworms

Several species of tapeworms can infect horses, and they can be difficult to detect using routine fecal egg counts. A blood test can detect antibodies to tapeworms, but that only demonstrates that a horse has been exposed and doesn’t necessarily mean he’s infected.

The most common type of tapeworm accumulates at the ileocecal valve (where the distal end of the small intestine connects with the cecum) and has been implicated as a cause for both ileal impactions and ileocecal intussusceptions. In an ileocecal intussusception, the ileum telescopes into the cecum and blocks off the flow of ingesta; surgical correction is the only viable treatment.

Horses are infected by ingesting forage mites infected with larval tapeworms while grazing. Segments of tapeworms called proglottids contain eggs. The proglottids detach from the tapeworm and are passed in feces, where they re-infect the pasture with eggs.

Currently it is recommended to treat horses with a product containing praziquantel one to two times per year.

Other Predators

Adult female bot flies look like honeybees. They fly around during the summer and deposit their tiny, yellow, sticky eggs on the hairs of the forelegs and under the jaw.

Eggs should be removed during daily grooming using a bot egg knife, block or similar tool. It takes five days for larvae to develop within the eggs, so removal twice per week should control them.

The larvae spend about a month in the oral cavity before moving on in the GI tract. Third stage larvae remain in the stomach and proximal duodenum for up to one year before passing out in the manure in late spring. Once in the manure the third stage larvae mature into adult flies in one to three months. Bot flies are most active during the summer and die off after a hard frost.

Ascarids are generally limited to youngsters less than 2 years of age because mature horses develop immunity. Ascarid eggs are very thick and remain viable for a long time—eight or more years—in pasture. Horses are infected by ingesting infective eggs while grazing. Migrating larvae pass through liver and lung tissue before maturing to adults 21⁄2 to three months after ingestion.


Pinworms are unique in that horses can get them without grazing. The eggs are sticky and can be picked up from common stalls and other living spaces. The female protrudes through the anus to lay eggs, which causes intense itching, and horses rub their tails, sometimes to a severe degree. Eggs are detected by placing a piece of clear tape on or around the horse’s anus then placing it on a slide to view under a microscope.

Intestinal parasites are a constant threat to grazing horses. But with proper management and strategic use of dewormers it is possible to keep your horse healthy without administering unnecessary chemicals. As the emergence of resistant parasites continues to increase, the need for more careful use of our currently available anthelmintics becomes ever more critical.

A Sample Program For Horses In Southern Regions

The following sample parasite control program is for horses older than 2 years of age. Consult with your veterinarian to establish a program that is tailored for your individual needs in regards to your horse’s age, lifestyle, health status and environment.

  • Begin by making a chart that includes the name of all horses that reside on the farm.Perform a fecal egg count prior to spring deworming. If any horse had an FEC greater than 200, recheck him two weeks after deworming.
  • Perform a fecal egg count prior to spring deworming. If any horse had an FEC greater than 200, recheck him two weeks after deworming.
  •  Administer Ivermectin or Moxidectin with praziquantel in February/March to all horses regardless of FEC. This will address parasites picked up over the cool months, which is when parasite transmission peaks in the South.
  • Perform FEC in September prior to administering dewormer; if any horse has an FEC over 200, recheck him two weeks after deworming.
  • Administer Ivermectin or Moxidectin with praziquantel in September (after first hard frost when most bot flies die off) to all horses regardless of FEC to address parasites picked up over the spring/summer grazing period including stomach bots, stomach worms and strongyles.
  • Consult with your veterinarian to determine which products to use if your horse has an elevated FEC at the recheck exam. Some will use a five-day double-dose treatment of fenbendazole followed by Ivermectin on the sixth day. This can be an effective treatment to address intestinal parasites resistant to traditional single-paste dewormers but needs to be administered under veterinary direction because it can be dangerous in horses that are debilitated or possess a high parasite burden.
  • The only alteration needed to make for horses that live in the North would be to shift the spring treatment to April and the fall treatment to October.

Guidelines For Getting A Fecal Egg Count

A fecal sample must be fresh and kept at a controlled temperature to avoid false negative results.

The fecal egg count only detects eggs. Thus a horse that passes a large number of eggs in his manure may not have an accurate count if the sample is not handled properly because the test can’t detect larvae. If you are collecting a sample during warm weather consider keeping it in the refrigerator because eggs can hatch in as few as 12 to 24 hours.

Fecal egg count reduction tests can determine how well a product works on a particular population. A manure sample is collected at the same time as treatment, and a second sample is collected two weeks later. It is important not to collect the sample too soon after administering a dewormer because you can end up with false positive results by seeing the eggs that are being released by the dying worms. If you wait too long to collect the post-treatment sample you can also have a false positive because enough time may have passed for the encysted larvae to emerge, mature and lay eggs.

Costs for fecal egg count tests range widely. Contact your state agriculture department because many states offer low-cost testing. Fecal egg count testing is quite cost effective in the grand scheme. Consider the following:

  1. Paste dewormer every two months at $12/tube = $72 per year
  2. Paste dewormer every six months at $12/tube = $24/year
  3. Fecal egg count every three to six months at $15/test = $30-$60/year

                                         Total= $54-$84/year

You do not necessarily expect to have 0 eggs per gram. Most experts suggest that you deworm only if the horse has 200 or more eggs per gram.

Kristyn Close is a fourth-year veterinary student at North Carolina State University College of Veterinary Medicine. She has competed in hunters and equitation and plans to pursue an equine internship and ultimately a large animal surgery residency program.

If you enjoyed this article and would like to read more like it, consider subscribing. “Fighting The War Against Parasite Resistance” ran in the April 9 issue. Check out the table of contents to see what great stories are in the magazine this week.




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