Some intestinal nematodes occur commonly but are of limited clinical importance in horses and therefore should not dictate parasite control programs. These include bots (Gasterophilus spp), stomach worms (Habronema spp and Trichostrongylus axei), pinworms (Oxyuris equi), and threadworms (Strongyloides westeri).
Although all of these parasites can cause GI pathology in horses, they are very rarely associated with GI disease. However, O equi and Habronema spp are capable of causing clinical signs in other body systems, such as skin or mucosal membranes, which may require anthelmintic treatment.
Gasterophilus spp in Horses
Horse bots are the parasitic larvae of botflies (Gasterophilus spp). In general, bots are considered benign, even though they may cause some pathology. Bots can be identified during oral inspection or gastroscopy.
Bots are extremely common in horses, and several species can infect horses. The two most common species are Gasterophilus intestinalis and Gasterophilus nasalis. Other species are known to infect working, feral, and wild equids but are rarely, if ever, encountered in sport and leisure horses. By far the most common and abundant species to infect horses is G intestinalis.
The adult botfly glues eggs onto hairs on the horse's body. G intestinalis eggs tend to be found on the distal legs and around the neck and withers; G nasalis eggs are typically found in the submaxillary region. Bots are eventually ingested through grooming and can cause inflammatory reactions during migration within the oral cavity and by attachment to the stomach wall.
With G intestinalis, hatching is stimulated by licking or grooming; G nasalis eggs hatch spontaneously. With both parasites, larvae find their way to the horse's mouth and enter the oral cavity. They first migrate within the tissues of the tongue for up to 21 days; after that, they can be found in periodontal pockets surrounding molars and premolars.
After approximately 4 weeks in the oral cavity, bots relocate to the stomach as second instars, where G intestinalis attaches in the nonglandular portion surrounding the margo plicatus, and G nasalis attaches in the pylorus. Larvae develop into third instars (see third-instar image) before they are passed out in feces.
Courtesy of Dr. Martin K. Nielsen.
Altogether, Gasterophilus larvae can spend up to 8–10 months within the horse. Outside of the horse's body, they pupate in the soil for 3–5 weeks before adult botflies emerge.
Clinical Findings
The gastric stages of Gasterophilus can be numerous but rarely produce clinical signs; however, erosions and ulcerations do occur. Gastric perforation and esophageal impaction have been reported on rare occasions. The most pronounced clinical signs are associated with the oral stages of infection, including salivating, head shaking, and problems with chewing.
Clinical laboratory findings are typically unremarkable.
Diagnosis
Oral inspection to observe second instars
Gastroscopy to visualize second and third instars attached to gastric mucosa
Oral inspection of horses with Gasterophilus spp infection can show larvae within the diastemata between molars and premolars. Lesions at the base of the tongue can also be observed. Gastric stages are easily visualized with gastroscopy. In addition, bots can sometimes be observed in feces. However, fecal flotation and fecal egg counts are not useful to diagnose this parasite.
Observation of botfly eggs glued onto a horse's hair can indicate infection. However, such observation documents only general exposure of the herd, because horses are likely to acquire infection from each other while grooming.
Treatment
Ivermectin
Moxidectin
Ivermectin (0.2 mg/kg, PO, once) has high efficacy against oral and gastric stages of Gasterophilus spp. Moxidectin (0.4 mg/kg, PO, once) has documented efficacy against gastric stages; compared with ivermectin, however, its efficacy is more variable and generally lower.
Prevention
Gasterophilus spp are not considered primary parasitic pathogens; however, treatment with ivermectin during autumn should help decrease larval burdens into the winter. It is not necessary to wait until the first frost to achieve a marked decrease in bot numbers. Concerned horse owners could be instructed to manually remove botfly eggs from hair as they occur.
Zoonotic Risk
Gasterophilus spp pose no zoonotic risk.
Habronema spp in Horses
Equine stomach worms—Habronema muscae, Habronema microstoma, and Draschia megastoma—infect the gastric mucosal lining and cause catarrhal gastritis. Draschia has become rare in domestic horses. In their gastric stage, Habronema parasites rarely lead to GI signs; however, they do also cause the cutaneous condition often referred to as "summer sores." Macrocyclic lactones are still believed to be effective against the gastric stages of Habronema.
Habronema and Draschia spp are vector-borne parasites of horses that use muscid flies as intermediate hosts. Adult parasites establish themselves in a horse's stomach after the horse ingests either larvae deposited by flies around its mouth or dead flies carrying larvae. Within the stomach, parasites become adults in approximately 8 weeks.
Habronema adults are found in close contact with gastric mucosa; however, they do not produce clinical signs. Draschia spp have become very rare in domestic horses.
Clinical Findings
The gastric stage of Habronema spp infection has been described as a catarrhal gastritis; however, clinical signs have not been reported during the gastric stage. Similarly, a pulmonary form of habronemiasis has been described, but the clinical implications are unclear.
Draschia spp have been described to cause large fibrous masses near the margo plicatus. However, the clinical importance of these masses is unclear, and they are rarely encountered.
Diagnosis
Fecal flotation or egg counts
Observation of adult worms by gastroscopy
Habronema eggs can be difficult to see microscopically, and they appear to be vulnerable to osmotic pressure from the flotation medium. Therefore, to be reliable in the diagnosis of gastric infections caused by Habronema spp, fecal flotations and fecal egg counts must use low-specific-gravity flotation media (zinc sulfate). At 6–25 mm long, adult worms can be identified on gastric mucosal membranes by gastroscopy.
Treatment
Macrocyclic lactones
Gastric stages of Habronema spp are expected to be susceptible to macrocyclic lactones (ivermectin [200 mcg/kg, PO, once] or moxidectin [400 mcg/kg, PO, once]).
Prevention
Control of muscid flies could, in theory, decrease infection pressure by Habronema spp; however, this presumed effect has not been tested experimentally.
Zoonotic Risk
Habronema spp pose no zoonotic risk.
Oxyuris equi in Horses
Equine pinworms are commonly found worldwide. There are two species: Oxyuris equi and Probstmayria vivipara. However, P vivipara has not been found to be clinically relevant and is not discussed further.
The infective stage of O equi is the embryonated egg. Upon ingestion by a horse, L3 larvae are released and invade mucosal walls, primarily in the ventral colon and, to a lesser extent, the cecum. They enter the intestinal lumen as L4 larvae and move to the dorsal colon, which is the predilection site for the adult stage. Gravid females migrate through the descending colon and rectum to deposit their eggs in packets onto perianal skin (8,000–60,000 eggs in one deposit) and then die in the environment.
Infection by O equi can occur in horses of any age and takes approximately 5 months to complete. Adult females can sometimes be observed in the horse's feces and, because of their stout appearance (length 5–10 cm), can be mistaken for ascarids. However, their tapered posterior ends can confirm identification.
Clinical Findings
Although O equi larval stages cause inflammatory reactions in the mucosal walls of the cecum and ventral colon, associated clinical signs have not been described. Egg masses can produce local perianal irritation and pruritus that can progress to tail rubbing. Yellow or brown egg masses can sometimes be observed in the perianal area. Clinical laboratory findings are typically unremarkable.
Diagnosis
Microscopy to identify O equi eggs collected from perianal skin
Observation of adult females in feces
In cases of O equi infection, direct microscopy of egg mass material from the perianal area reveals numerous eggs, which are morphologically distinguishable from other equine helminth egg types (see O equi egg image).
Courtesy of Dr. Martin K. Nielsen.
A sample of O equi eggs can be collected by pressing the sticky side of a piece of cellophane tape onto the perianal skin and then applying the tape to a glass slide for microscopy. Alternatively, a lubricated tongue depressor can be used to gently scrape the skin surface for subsequent microscopy.
Fecal flotation and egg counts are not useful in diagnosing O equi infection, because the eggs are generally not released into feces. Female worms can sometimes be observed protruding from the anus as they deposit eggs or can be found in feces or attached to rectal sleeves after rectal examination.
Because tail rubbing can be due to multiple factors, differential diagnoses for this behavior should be considered.
Treatment
Benzimidazoles
Supportive care
Ivermectin and moxidectin should not be expected to work against O equi, because there are reports of resistance to this drug class. Benzimidazoles (fenbendazole [5 mg/kg, PO, once] or oxibendazole [10 mg/kg, PO, once]) are effective anthelmintics. Pyrantel efficacy against O equi varies.
Attempts to administer anthelmintics via rectal lavage are not useful, given that O equi adult females are only transiently found in the rectum and their predilection site is the dorsal colon.
Supportive care for O equi infection consists of thorough washing of the perianal area and treatment of skin lesions, as appropriate.
Prevention
Little is known about measures that effectively prevent O equi infection. However, stable and stall hygiene could be considered, because eggs can be deposited onto various objects as a result of tail rubbing.
Zoonotic Risk
O equi poses no zoonotic risk.
Strongyloides westeri in Horses
Strongyloides westeri is an intestinal threadworm (see S westeri adult image) that commonly infects young foals. It has been associated with small intestinal enteritis, resulting in diarrhea. However, this parasite's pathogenicity remains unclear. Ivermectin and oxibendazole are reported to be effective against intestinal stages in foals.
Courtesy of Dr. Martin K. Nielsen.
Strongyloides westeri infection commonly occurs in foals worldwide. Few large-scale prevalence studies exist; however, Strongyloides eggs are typically found in the feces of 5–50% of managed foals < 2 months old.
Lactogenic transmission is believed to be the most important route of S westeri infection in young foals. In their tissues, mares harbor larvae in a hypobiotic arrested stage that become reactivated at foaling and migrate to the mammary gland.
S westeri larvae can be isolated in milk 5 days post partum, but the highest concentration occurs 10–14 days post partum. Larvae have been identified in milk until approximately 45 days post partum. Positive egg counts have been reported in foals as young as 5 days old.
S westeri infection can be transmitted via two other routes—oral ingestion and percutaneous invasion; however, the importance of these routes as means of infection is unknown. Parasites obtained via either of these routes are believed to migrate through the lungs, whereas lactogenically acquired parasites do not migrate through extraintestinal tissues in foals.
Two additional features distinguish S westeri from other intestinal helminths of horses: it is a facultative parasite (able to complete and maintain its life cycle without entering a host), and only females are parasitic.
Clinical Findings
S westeri was once speculated to cause foal heat diarrhea, because this condition is often observed approximately 10 days post partum, when some foals can have positive S westeri fecal egg counts. However, bacterial and viral intestinal pathogens are more likely to be implicated in causing diarrhea at this age.
S westeri has been associated with enteritis and diarrhea in horses; however, no distinct disease has been described. Epidemiological studies suggest that disease can occur if Strongyloides egg counts exceed 2,000 eggs/g, and experimental inoculations with the parasite have led to enteritis.
The percutaneous route of S westeri infection can lead to dermatitis on distal extremities, and a hyperactive, “frenzied” behavior has been described in yearlings exposed to an infective environment.
Diagnosis
Fecal egg counts
Coproculture
Like strongyle eggs, Strongyloides eggs are ovoid; however, they are shorter, have thinner walls, and often contain larvae (see S westeri egg image). Because of their thin walls, the eggs tend to collapse in flotation media with specific gravities > 1.25, so a specific gravity of approximately 1.20 is recommended.
Courtesy of Dr. Martin K. Nielsen.
Diagnosis of disease associated with S westeri infection in horses is based on the number of Strongyloides eggs identified by fecal flotation or egg counts and on the presence of third-stage larvae, as identified in coproculture.
Treatment
Anthelmintics
S westeri infection in horses is generally treated with an anthelmintic, either oxibendazole (15 mg/kg, PO, once) or ivermectin (0.2 mg/kg, PO, once). There are no reports of anthelmintic resistance in this parasite.
Prevention
To prevent lactogenic transmission of S westeri in horses, farm managers and horse owners commonly deworm mares around the time of foaling. However, the effectiveness of this procedure is unknown. No data are available on the efficacy of available anthelmintics against migrating tissue stages within the mare, and there is no information on the most effective dose, timing, and duration of such treatments.
Zoonotic Risk
S westeri poses no zoonotic risk.
Trichostrongylus axei in Horses
Trichostrongylus axei, the stomach hair worm, is a trichostrongylid parasite that infects a variety of hosts, including sheep, goats, cattle, and horses. Infections in humans have also been described.
The environmental life cycle of T axei is similar to that of cyathostomins, with the third-stage larva being the infective stage. Once ingested, larvae penetrate the gastric mucosa, develop into adult worms, and emerge into the gastric lumen. Egg production begins 3–4 weeks after infection.
T axei is seldom encountered in domestic horses and is not considered a primary parasitic pathogen.
Clinical Findings
Infection by T axei can result in catarrhal gastritis with small lesions that can coalesce to form larger erosions and ulcers. Clinical signs have not been well described; however, the parasite has been implicated as a cause of weight loss in horses.
Diagnosis
Coproculture
The eggs of T axei are morphologically indistinguishable from strongylid eggs of horses and ruminants. Diagnosis of T axei infection can be based on the presence of third-stage larvae, as identified in coproculture.
Treatment
Macrocyclic lactones or benzimidazoles
T axei is expected to be susceptible to treatment with macrocyclic lactones (ivermectin [200 mcg/kg, PO, once, or moxidectin [400 mcg/kg, PO, once]) or benzimidazoles (fenbendazole [5–7.5 mg/kg, PO, once] or oxibendazole [10 mg/kg, PO, once]). Benzimidazole resistance has been reported in T axei that infect sheep, and similar resistance could develop in T axei that infect horses.
Prevention
Infections by T axei rarely occur in horses without ruminant contact. Preventive measures have not been specifically recommended for this parasite.
Zoonotic Risk
T axei infections in humans have been described. Infection occurs via the ingestion of infective larvae present on contaminated lettuce, cabbage, or other produce.
Key Points
Gasterophilus spp infection is very common in horses worldwide; however, horses are usually unaffected.
Clinical implications of the gastric stages of Habronema spp infection are unknown.
The equine pinworm, Oxyuris equi, is common around the world and widely resistant to macrocyclic lactones.
Strongyloides westeri infection commonly occurs in young foals; however, only extremely rarely has it been reported to cause disease.
Trichostrongylus axei infection is rare in domestic horses, and it is not known to cause clinical disease.
For More Information
American Association of Equine Practitioners (AAEP): Internal Parasite Control Guidelines.
Reinemeyer CR, Nielsen MK. Review of the biology and control of Oxyuris equi. Equine Vet Educ. 2014;26(11):584-591.
Also see pet owner content regarding GI parasites of horses.
