More Bad Bugs

When performing fecal egg counts, we only count eggs from the two most important GI parasite species: Strongyles and Ascarids. However, it is not uncommon to find other parasite invaders. Some of these are shown below.

Small Strongyles

Single Stongyle Egg

Small strongyles have been reported in horses throughout the world with more than 50 species recognized. Small strongyles are considered the most prevalent and pathogenic parasites of horses today. Our heavy reliance on the repeated use of anthelmintics throughout the life of a horse is no longer an effective management technique. An understanding of the biology of cyathostomins, risk factors for infection and appropriate strategic use of effective anthelmintics is essential for the future management of this parasite group.


Anthelmintic treatment strategies originally designed to control large strongyles in horses were extremely successful in minimizing Strongylus vulgaris, S. endentatus, and S. equinus infections and reducing their prevalence in properly managed herds. Unfortunately, the popular yet outdated strategy of frequent, rotational deworming has inadvertently resulted in the selection of drug-resistant parasites,particularly small strongyles species–collectively known as cyathostomins. As a result, most of the parasite eggs seen on fecal exams come from one of many cyathostome species.

Strongyle in mucosa.

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.




Parascaris equorum (Ascarids)

Strongyle and Ascarid eggs

In addition to small strongyles, Parascaris equorum (commonly known as the equine roundworm, or ascarid) also constitutes a major threat to equine health. Unlike small strongyles, ascarids are most commonly in foals and younger horses. Adult horses usually have low numbers due to the development of immunity during the course of infection.


However, it is important to note that the reduced efficacy of anthelmintics and development of multiple drug-resistance to equine ascarids has been reported in many countries including the United States.

Anoplocephala magna, Anoplocephala perfoliata and Paranoplocephala mamillana (Tapeworms)

The three types of tapeworms that can infect horses are Anoplocephala perfoliata, Anoplocephala magna and Paranoplocephala mamillana, with A. perfoliata being by far the most common.

Anoplocephala magna adults

With tapeworms, the eggs develop in a lower segment of the worm’s body, which separates and pass out with the fecal material. However, this is not an ongoing process, making egg detection very inaccurate. Additionally, excreted tapeworm eggs do not float well using traditional fecal floatation methods. With that said, it is not unheard of to find a tapeworm egg while performing you fecal exam. However, due to the variables in egg recovery and the lack drug-resistance, we DO NOT include tapeworm eggs in our egg counts.


The most common of the three equine parasites, Anoplocephala perfoliata, is also considered the most problematic. A. perfoliata tends to conjugate at the ileocecal junction (the common opening between the ileum, or small intestine, the colon and the cecum). In extremely high numbers A. perfoliata may lead to obstruction of ileocaecal valve and mild colic, however, it is difficult to ascertain the amount of parasites in the animal since only about 50% of infected horses will have eggs in feces.

Praziquantel is known to be highly effective against tapeworms. Several pharmaceutical companies have developed combination products that offer a complete antiparasitic spectrum of activity. The current AAEP recommendation is that horses should be dewormed for tapeworms annually. Although praziquantel is safe, don’t overuse it as parasites likely will build resistance to it. Frequent use of dewormers puts tremendous pressure on the parasites to adapt to survive this continuous onslaught, so they select for resistance.

Strongyloides westeri  (Threadworm)

Important note: this is not the same as a large or small strongyle!

Larvated threadworm eggs with a strongyle egg

Strongyloides westeri is found in the small intestine of foals. Adult horses rarely harbor active infection; however, mares often maintain dormant larval stages within their tissues that are activated during the birthing process. The active parasite larvae move into the mammary tissue and subsequently, are transmitted to foals in the milk. Adult worms can also be acquired by skin penetration via infective larvae.

The eggs of S. westeri can be identified by their thin shelled larvated L1 stage that measure 30 x 50 um (approximately 1/3rd the size of a strongyle egg).

Eimeria leuckarti

Coccidia oocyst

Eimeria leuckarti is the only coccidia found in horses in North America. This protozoan invades the small intestine and colon. Experimentally, it has been shown to cause diarrhea, weight loss and death in foals, however, it is generally regarded as a “so what” finding in the average horse. The oocyst are heavy and usually require sugar centrifugal flotation to recover but salt floatation has been known to uncover a few stragglers.

Eimeria Leuckarti can be identified by its large, brown, thick walled, pryiform oocyst 71    – 88 X 49 – 63 um


Oxyuris equi (Pinworm)

Pinworm egg

Oxyuris equi is a common parasite found in the equine large intestine.  Females move toward the anus to lay their eggs, “cementing” them to the perianal region with a sticky, irritating substance. The irritating substance coating the eggs causes the horse to scratch excessively (i.e. rub against fence posts, stall wall, door, and feed bunk etc.) resulting in broken tail hairs and bare patches around the tail and hindquarters. Although many beautiful tails have been destroyed as a result of this parasite, they are easily treated with anthelmnintics and cause little damage to the overall health of the animal.

The most effective method of identification is the use of transparent adhesive tape to collect eggs for microscopic examination. However, from time to time eggs will be passed in the feces and found via floatation. Eggs are asymmetrical (flattened along one side), have a plug in one end and contain a single larvae 85-90 um


Individual horses vary greatly in their susceptibility to internal parasites; some limit infection almost entirely even without deworming drugs, while others carry very high parasite loads even with regular deworming.

Thus, we can also reduce anthelmintic use by only deworming the horses that really need it.