Parasites 

Most of the parasite eggs seen on a fecal exam come from one of many small strongyle species–collectively known as cyathostomins. Cyathostomins are ubiquitous parasites of grazing horses, and are currently considered one the most significant pathogenic internal parasites in horses. Drug-resistance among small strongyles has been intensely documented throughout the last decade. In addition to small strongyles, Parascaris Equorum, commonly known as roundworm, or ascarids, also constitute a major threat to equine health.

Unlike small strongyles, ascarids are found 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.

“The traditional one-size-fits-all treatment approach, dictated by the calendar alone, is not only inadequate but actually a recipe for disaster. Instead, the best method of parasite control requires a two-step process, implemented on a farm-by-farm and horse-by-horse basis. First, evidence must be collected to determine which dewormers still work in a particular herd and which have been rendered ineffective due to resistance. Then, the specific horses in need of deworming must be identified and a schedule for the administration of a targeted anthelmintics devised.” ~ Craig Reinemeyer, DVM, PhD.

The modified McMaster’s fecal egg count is a quantitative method used for examining strongyle and ascarid populations where large numbers of eggs are encountered. Eggs from other parasites, including tapeworms, are less informative, and typically are just noted as being present or absent.

Here are our primary targets:

Ascarids:

Unlike stronglyes, thick-shelled Ascarid eggs are resistant to freezing and drying and can therefore survive for long periods in the environment. The outer layer is sticky and ensures that eggs will be present almost everywhere in a horse’s environment.

Identification of equine ascarid eggs:

• Round

• Clear to dark brown

• Thick shelled

• Single celled egg

• Measures approximately 90-100 μm
  

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

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.

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)

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.

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!

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

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)

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

Non-Floating Parasites

Although not as prevalent as strongyles, roundworms or tapeworms, there are other equine parasites that are important to recognize and combat. The lesser known intruders listed on this page cannot be identified via fecal flotation but may equally impact your horse’s health.

Dictyocaulus arnfieldi (Equine lungworm)

Donkeys are the common host of D.arnfieldi and are considered to be the primary source of pasture contamination for horses. The adult female worm produces eggs that already contain first-stage larvae, which hatch before they’re passed out in the donkey’s manure. The larvae become infective in pasture within about five days. When the eggs are ingested they migrate by way of the lymphatic system to arrive in the lungs in another five days. In a patent infection, egg laying begins about 28 days after initial infection in the lungs, and the larvae travel up the trachea via coughing. Once in the throat, they’re swallowed and make their exit via the intestinal tract. It is rare for lungworms to successfully reproduce in horses, however, larval lungworms living in the lumen of the bronchial tree will cause serious lung damage in horses. Interestingly, donkeys harboring the lungworms usually do so without any outward sign of disease. They serve as “quiet contaminators”.

Diagnosis is based on clinical signs, epidemiology, and presence of first-stage larvae in rectal collected feces. First-stage larvae or larvated eggs can be recovered by performing a Baermann on fresh feces. Bronchial lavage can also be used to reveal D.arnfieldi infections in horses without a patent infection.

L1 lungworm Larvae

Clinical signs of D. arnfieldi infections include: chronic bronchitis, chronic cough and eosinophilia.  Since a horse with clinical signs is unlikely to have a patent infection a good approach is to check the donkey and treat the horse.  At this time there are no reports documenting drug-resistance in D. arnfieldi and it can be treated with ivermectin and moxidectin. Advising clients to avoid donkey-horse cohabitation is a good way to prevent this disease in the horse.

Gasterophilus spp. (Bots)

Female bot flies are often observed on warm, sunny days hovering near horses in an attempt to attach their sticky “glue-like” eggs to various areas of the horse. Bot flies are common virtually everywhere horses are kept, with two major species found in the United States: Gasterophilus nasalis and Gasterophilus intestinalis.

G. nasalis lays its eggs on the hairs under the jaw. The eggs hatch spontaneously five to six days after being deposited, and the larvae crawl downward to the chin until they pass between the lips and enter the mouth. G. intestinalis lay their eggs on the hairs of the forelegs and shoulders of the horse where the larvae are stimulated to emerge by the horse licking or biting the attached eggs.

Although the migration patterns of G. nasalis have not been researched, it is most likely similar to G. intestinalis in which the larvae spend about a month in the oral cavity where they burrow into the mucosa of the tongue or pockets in the spaces between the upper molar teeth. Molting occurs into second-stage larvae and the parasites migrate to the stomach. G. intestinalis larvae attach in clusters to the non-glandular part of the stomach whereas G. nasalis larvae are usually found in a small dilatation in the first few inches of the duodenum.

Bot larvae use their mouth hooks to attach to the wall of the stomach or duodenum for up to 12 months. When late spring arrives the larvae release their grip on the mucosa and pass out with the manure to pupate (mature into adult flies) in the soil. Adult bot flies emerge from the pupae in three to nine weeks depending on the temperature and the cycle repeats itself.

There is very little evidence associating Gasterophilus infections with clinical illness. Most horses can support substantial populations of these parasites without apparent disease. This is not to say that bots don’t have an impact on the horse’s health; their presence can cause disease too subtle for current detection methods. Particularly heavy Gasterophilus infections have been associated with abscesses and gastric ulceration. Some researchers have implicated them in cases of stomach rupture and peritonitis, but others argue that the presence of bots might be coincidental, not causative.

Fall is the best time to treat for bots (about 30 days after the first hard frost). Additionally, those tiny yellow bot eggs glued onto your horse’s legs, chest, shoulders, and maybe under his chin can be removed with a sharp blade or bot block to prevent initial ingestion.

Habronema and Draschia “Stomach Worms”

Habronema muscae, H. microstoma, and Draschia megastoma inhabit the glandular portion of the equine stomach, with a special predilection for the margo plicatus (a sharp demarcation that separates the glandular and non-glandular stomach). H. microstoma and D. megastoma deposit larvae, but H. muscae lay eggs containing larvae. The larvae are ingested by house and stable fly maggots which are also developing in manure. The larval worms develop inside the maggot, becoming infective third-stage larvae at about the time that the adult fly emerges from its pupa. Larvae are deposited on the muzzle, eyes, sheath, or open wound as the flies feed. Larvae that are licked and swallowed by the horse will eventually make their way to the stomach where they will mature.

With the exception of Draschia megastoma which was once associated with the formation of fibrous nodules (but have become relatively rare due to the advent of modern dewormers),  Habronema species are of little consequence in the stomach. However, their larvae are responsible for a skin condition known as cutaneous habronemiasis, a.k.a. summer sores. These granulomas develop when the larvae are deposited in minor wounds and moist areas of skin, such as the conjunctiva of the eye. Cutaneous habronemiasis is characterized by very rapid production of granulation tissue that refuses to heal during the fly season, the granulomas are also extremely itchy, and secondary trauma often occurs as the horse attempts everything possible to find relief.

Larvae can be difficult to find, a Baermann technique should be used to isolate the parasite. Fly control is a great way to prevent transmission.