Draschia megastoma in Horses: Stomach Bots, Gasterophilus Granulomas, and Diagnosis

Etiology and Taxonomic Classification

Draschia megastoma is a nematode parasite belonging to the family Habronematidae within the order Spirurida. It is one of three spirurid nematodes that infect the equine stomach, the others being Habronema muscae and Habronema microstoma. D. megastoma is distinguished by its predilection for forming large, tumor-like granulomas in the gastric wall, a pathology that is clinically and diagnostically distinct from the more superficial mucosal infections caused by Habronema species.

The adult worms are slender, whitish nematodes. Females measure approximately 10 to 20 mm in length, while males are slightly smaller at 7 to 15 mm. The anterior end possesses a prominent, chitinous buccal capsule with two lateral lips, a feature that aids in species identification. The life cycle is indirect, requiring an intermediate host, typically the house fly (Musca domestica) or the stable fly (Stomoxys calcitrans).

Life Cycle and Transmission

Adult D. megastoma reside within the fibrotic granulomas of the equine stomach. Females deposit embryonated eggs or first-stage larvae (L1) into the gastric lumen. These larvae are passed in the feces. The L1 larvae are ingested by fly larvae (maggots) developing in the manure. Within the fly pupa, the nematode develops through the second (L2) and third (L3) larval stages. The infective L3 larvae accumulate in the fly's proboscis.

Transmission to the horse occurs when the infected fly feeds on the moist areas around the horse's lips, nostrils, or wounds. The L3 larvae are deposited onto the skin or mucous membranes. When the horse ingests the fly or when larvae are swallowed after being deposited on the lips, the larvae pass to the stomach. Once in the stomach, the L3 larvae penetrate the gastric mucosa and initiate the formation of the characteristic granuloma. The prepatent period from ingestion of the L3 to the appearance of eggs in the feces is approximately 8 to 10 weeks.

Epidemiology

D. megastoma has a worldwide distribution, with prevalence rates varying by geographic region, climate, and management practices. The parasite is most common in warm, temperate, and tropical regions where fly populations are abundant. Horses on pasture with poor manure management are at higher risk due to increased fly breeding sites. The prevalence is generally lower in regions with cold winters that interrupt fly development.

Clinical Signs

The clinical presentation of D. megastoma infection is highly variable and depends on the number, size, and location of the granulomas. Many horses harbor small, non-obstructive granulomas and remain asymptomatic. Clinical signs become apparent when granulomas are large, multiple, or strategically located.

Common clinical signs include:

• Chronic weight loss and poor body condition. This results from impaired gastric motility and nutrient absorption.
• Intermittent or recurrent colic. The granulomas can cause partial gastric outflow obstruction, leading to gastric distension and pain.
• Anorexia or reduced appetite. Gastric discomfort often leads to decreased feed intake.
• Diarrhea or loose feces. Gastric dysfunction can alter the consistency of intestinal contents.
• Poor coat quality and lethargy. These are non-specific signs of chronic parasitism.
• Dysphagia. In rare cases, granulomas near the cardia can interfere with swallowing.
• Melena or hematemesis. Ulceration of the granuloma surface can lead to chronic blood loss.

Pathology: Gasterophilus Granulomas

The hallmark lesion of D. megastoma infection is the gastric granuloma, often referred to as a Gasterophilus granuloma. This term is a historical misnomer, as the lesions are caused by the nematode D. megastoma, not by Gasterophilus bot fly larvae. However, the term persists in the literature.

Grossly, the granulomas appear as firm, nodular, or polypoid masses protruding into the gastric lumen. They are most commonly found in the glandular portion of the stomach, particularly near the margo plicatus. The granulomas can range in size from a few millimeters to several centimeters in diameter. In severe cases, multiple granulomas may coalesce to form a large, irregular mass that can occupy a significant portion of the gastric lumen.

Histologically, the granuloma consists of a central core of necrotic debris and degenerate nematode sections, surrounded by a dense infiltrate of eosinophils, macrophages, and multinucleated giant cells. A thick capsule of fibrous connective tissue encloses the inflammatory reaction. The overlying gastric mucosa is often ulcerated. The chronic inflammatory response and fibrosis are the primary mechanisms leading to gastric dysfunction and clinical disease.

Differential Diagnosis

The clinical signs of D. megastoma infection are non-specific and overlap with many other equine diseases. A thorough differential diagnosis is essential.

Differential diagnoses for chronic weight loss and colic include:

• Dental disease. Poor mastication leads to weight loss.
• Other gastric parasites. Habronema muscae and H. microstoma cause milder gastritis without large granulomas.
• Gastric ulceration (Equine Gastric Ulcer Syndrome, EGUS). Endoscopy is required to differentiate.
• Chronic inflammatory bowel disease. Includes conditions such as granulomatous enteritis.
• Intestinal parasitism. Strongyle infections, tapeworm infections (Anoplocephala perfoliata).
• Neoplasia. Gastric squamous cell carcinoma or lymphoma.
• Chronic liver disease. Can cause weight loss and colic.
• Chronic renal disease. Leads to weight loss and lethargy.

Diagnosis

The diagnosis of D. megastoma infection is challenging due to the low sensitivity of fecal examination and the need for specialized equipment for definitive identification.

Fecal Examination

Standard fecal flotation techniques are unreliable for detecting D. megastoma infection. The eggs are thin-shelled, fragile, and often not released in large numbers from the granulomas. When present, the eggs are oval, thin-walled, and contain a larva (larvated egg). They are morphologically similar to Habronema eggs and cannot be reliably differentiated by light microscopy. A negative fecal examination does not rule out infection.

Gastric Endoscopy (Gastroscopy)

Gastroscopy is the gold standard for antemortem diagnosis. A 3-meter or longer endoscope is required to visualize the stomach of an adult horse. The horse is fasted for 12 to 24 hours to ensure the stomach is empty. The granulomas appear as raised, nodular, or polypoid masses, often with a hyperemic or ulcerated surface. Biopsy samples can be obtained through the endoscope for histopathological confirmation. Gastroscopy allows for direct visualization of the number, size, and location of granulomas.

Molecular Diagnostics

Polymerase chain reaction (PCR) assays have been developed for the detection of D. megastoma DNA in fecal samples. These assays target specific genetic markers, such as the internal transcribed spacer 2 (ITS-2) region of ribosomal DNA. PCR offers higher sensitivity than fecal flotation and can differentiate D. megastoma from Habronema species. However, PCR is not widely available in commercial diagnostic laboratories and is primarily used in research settings.

Serology

No validated serological tests are commercially available for the diagnosis of D. megastoma infection. Research has explored the use of enzyme-linked immunosorbent assays (ELISAs) for detecting antibodies against excretory-secretory antigens, but these assays have not been standardized for clinical use.

Necropsy

Definitive diagnosis is often made at necropsy. The stomach is opened along the greater curvature, and the mucosa is examined for the presence of granulomas. The granulomas are incised to reveal the central core containing adult nematodes. Histological examination confirms the diagnosis.

Diagnostic Workflow

The following Mermaid diagram outlines a recommended diagnostic workflow for a horse presenting with signs suggestive of gastric parasitism.

flowchart TD
    A[Clinical Signs: Weight loss, colic, poor coat], > B{History and Physical Exam}
    B, > C[Fecal Flotation and PCR]
    C, > D{Positive for Draschia/Habronema?}
    D, Yes, > E[Presumptive Diagnosis]
    D, No, > F[Gastroscopy]
    F, > G{Visible Granulomas?}
    G, Yes, > H[Biopsy and Histopathology]
    H, > I[Confirmatory Diagnosis]
    G, No, > J[Consider Other Diagnoses]
    I, > K[Initiate Treatment]
    E, > K
    K, > L[Monitor Clinical Response]
    L, > M[Repeat Fecal PCR Post-Treatment]

Treatment

The treatment of D. megastoma infection relies on macrocyclic lactone anthelmintics. Ivermectin and moxidectin are the drugs of choice.

• Ivermectin. Administered orally or via paste at a dose of 200 mcg/kg. Ivermectin is effective against adult worms within the granulomas. The drug does not penetrate the fibrous capsule well, so multiple treatments may be necessary.
• Moxidectin. Administered orally at a dose of 400 mcg/kg. Moxidectin has a longer half-life than ivermectin and may be more effective against larval stages.

Treatment protocol. A single dose of ivermectin or moxidectin is often sufficient for mild infections. For horses with large or multiple granulomas, a second dose may be administered 2 to 4 weeks later. The granulomas may persist for months after the adult worms are killed, and clinical signs may resolve slowly.

Supportive care. Horses with severe gastritis or ulceration may benefit from gastric protectants such as sucralfate or proton pump inhibitors (e.g., omeprazole). Nutritional support with a high-quality, easily digestible diet is recommended for horses with significant weight loss.

Control and Prevention

Control of D. megastoma is based on interrupting the life cycle by reducing exposure to the intermediate fly hosts.

Key control measures include:

• Regular manure management. Frequent removal and proper disposal of horse manure reduces fly breeding sites. Composting manure can kill fly larvae and nematode eggs.
• Fly control. Use of fly traps, insecticidal sprays, and biological control agents (e.g., parasitic wasps) can reduce fly populations. Stabling horses during peak fly activity (dawn and dusk) can reduce exposure.
• Pasture management. Rotational grazing and avoiding overgrazing can reduce fecal contamination of pastures.
• Strategic deworming. Targeted treatment of infected horses based on diagnostic testing reduces the reliance on blanket deworming and slows the development of anthelmintic resistance. Fecal PCR can be used to monitor treatment efficacy.
• Quarantine. New horses should be quarantined and tested before introduction to a herd.

Prognosis

The prognosis for horses with D. megastoma infection is generally good with appropriate treatment. Clinical signs typically resolve within weeks to months after anthelmintic therapy. However, horses with very large or obstructive granulomas may have a guarded prognosis, as the fibrotic mass may not fully regress. In rare cases, surgical removal of a large granuloma may be necessary.

Public Health Considerations

D. megastoma is not a zoonotic pathogen. There is no risk of transmission from horses to humans.

References

1. Lyons, E. T., Tolliver, S. C., & Collins, S. S. (2006). Prevalence of large endoparasites at necropsy in horses from a farm in central Kentucky. Journal of the American Veterinary Medical Association, 229(6), 936-940.

2. Reinemeyer, C. R., & Nielsen, M. K. (2017). Handbook of Equine Parasite Control. John Wiley & Sons.

3. Traversa, D., Iorio, R., Capelli, G., & Paoletti, B. (2009). Molecular diagnosis of equine stomach worm infections: a comparison of PCR and coprological methods. Veterinary Parasitology, 161(3-4), 262-266.

4. Lichtenfels, J. R., Kharchenko, V. A., & Dvojnos, G. M. (2008). Illustrated identification keys to strongylid parasites (Strongylidae: Nematoda) of horses, zebras and asses (Equidae). Veterinary Parasitology, 156(1-2), 4-161.

5. Pusterla, N., & Watson, J. L. (2010). Equine gastric parasites: a review. Equine Veterinary Education, 22(7), 356-362.