Habronema muscae in Horses: Stomach Worms and Cutaneous Habronemiasis (Summer Sores)
Etiology and Taxonomy
Habronema muscae is a spiruroid nematode belonging to the family Habronematidae. Adult worms inhabit the equine stomach, predominantly the glandular portion near the margo plicatus. Two related species, Habronema microstoma and Draschia megastoma (formerly Habronema megastoma), also occur in horses but are less common. Habronema muscae is the principal species responsible for gastric and extra-intestinal disease, including cutaneous habronemiasis (summer sores). The parasite requires an obligate intermediate host, the house fly Musca domestica and related species, for larval development.
Lifecycle and Transmission
The lifecycle of Habronema muscae is indirect. Adult females in the equine stomach produce larvated eggs that are shed into the lumen and passed in the feces. First-stage larvae (L1) hatch from eggs in the environment and are ingested by larval or adult Musca domestica flies. Within the fly, larvae develop through two molts to become infective third-stage larvae (L3). L3 larvae migrate to the fly's proboscis and are deposited onto the horse's lips, nostrils, or preexisting wounds when the fly feeds. Horses become infected by swallowing L3 larvae during grooming or feeding, or by larval deposition directly onto wounds.
The prepatent period from ingestion to adult egg production is approximately 56 to 64 days. Most adult worms survive for several months before being expelled.
graph TD
A[Adult worms in horse stomach], > B[Larvated eggs in feces]
B, > C[L1 larvae in environment]
C, > D[Larvae ingested by Musca domestica]
D, > E[L2 and L3 development in fly]
E, > F[L3 larvae migrate to fly proboscis]
F, > G[Fly deposits L3 on horse lips/nostrils/wounds]
G, > H[L3 swallowed or enter wound]
H, > I[L3 mature to adults in stomach or cause cutaneous lesions]
I, > A
Epidemiology
Habronema muscae has a worldwide distribution, with higher prevalence in tropical and subtropical regions where fly populations are abundant. Prevalence in horses ranges from 10% to 80% depending on management conditions. Free-ranging horses and those kept outdoors with poor sanitation are at greater risk. The disease is seasonal, occurring during warm months when flies are active. Cutaneous habronemiasis (summer sores) typically presents in spring and summer, hence the name "summer sores." Ocular habronemiasis is less common but also follows a seasonal pattern.
Clinical Forms
Three clinical forms of habronemiasis are recognized: gastric, cutaneous, and ocular.
Gastric Habronemiasis
Most gastric infections are subclinical. Heavy worm burdens may cause mild gastritis, ulceration, and even nodular hyperplasia at the margo plicatus. Clinical signs include intermittent colic, poor appetite, weight loss, and diarrhea. Draschia megastoma more commonly produces large granulomatous nodules, but Habronema muscae can also induce an eosinophilic inflammatory response.
Cutaneous Habronemiasis (Summer Sores)
Cutaneous habronemiasis is the most clinically significant form. It occurs when L3 larvae are deposited onto wounds, abrasions, or moist skin (commonly the medial canthus of the eye, prepuce, or distal limbs). Larvae invade tissue and induce a severe eosinophilic and pyogranulomatous reaction. Lesions are characterized by exuberant granulation tissue, necrotic debris, caseous material, and serous exudate. Typical features include:
- Raised, ulcerated, nonhealing wounds with a cobblestone appearance.
- Pruritus leading to self-trauma.
- Fibrotic nodules containing yellow, gritty "sulfur granules" (necrotic eosinophils and debris).
- Lesions bleed easily and are often secondarily infected with bacteria.
Lesions persist through the fly season and regress in winter, only to recur the following spring. Chronic cases may lead to extensive fibrosis and disfigurement.
Ocular Habronemiasis
Ocular involvement affects the conjunctiva and lacrimal caruncle. Clinical signs include epiphora, conjunctivitis, chemosis, and nodular lesions at the medial canthus. The eye is often kept closed. Corneal ulceration may occur secondary to mechanical irritation. Diagnosis is based on finding characteristic granulomas.
Pathology
Gastric lesions consist of small erosions, ulcerations, and mucosal thickening. Microscopically, an eosinophilic infiltrate with plasma cells and lymphocytes is present. In cutaneous habronemiasis, histopathology reveals a central necrotic core surrounded by palisading epithelioid macrophages and an outer zone of eosinophils and neutrophils. This granulomatous reaction is characteristic. The "sulfur granules" are aggregates of necrotic eosinophils.
Diagnosis
Diagnosis is based on clinical signs and confirmed by detecting larvae or adults.
Gastric habronemiasis: Fecal examination using flotation or sedimentation may detect larvated eggs, but egg shedding is intermittent. Fecal examination has low sensitivity. Gastroscopy with visualization of adult worms attached to the gastric mucosa is more reliable. Worms are small (1–2.5 cm), white, and coiled.
Cutaneous habronemiasis: Diagnosis is primarily clinical based on the appearance and seasonality of lesions. Deep scraping or biopsy of the granulation tissue may reveal L3 larvae (approximately 500–600 μm long) in histologic sections. Biopsy is the gold standard. The presence of eosinophilic granulomas with necrotic centers is highly suggestive. Culture for bacteria is recommended to rule out secondary infection.
Ocular habronemiasis: Examination of conjunctival scrapings or biopsy may show larvae. Cytology often reveals numerous eosinophils.
Molecular diagnostics: PCR assays targeting ribosomal DNA (e.g., ITS-2 region) have been developed for Habronema species and can differentiate them from other nematodes. PCR offers higher sensitivity than microscopy and can be applied to fecal samples, gastric biopsies, or tissue specimens. Commercial PCR panels for equine parasites may include Habronema.
Serology: No validated commercial serologic tests are available for Habronema infection. Research using ELISA for detection of antibodies against excretory-secretory antigens has shown promise but is not yet clinically applied.
Differential Diagnosis
Cutaneous habronemiasis must be differentiated from:
- Bacterial granulomas (e.g., Staphylococcus aureus, Rhodococcus equi).
- Fungal granulomas (e.g., Histoplasma, Cryptococcus).
- Pythiosis (caused by Pythium insidiosum).
- Equine sarcoid or squamous cell carcinoma.
- Foreign body granulomas.
Ocular forms should be distinguished from conjunctival foreign bodies, bacterial conjunctivitis, and Onchocerca ocular lesions. Gastric habronemiasis may resemble other causes of colic or gastritis.
Treatment
Treatment targets both adult worms and larval stages. Ivermectin (200 μg/kg PO or paste) is highly effective against Habronema adults and L3 larvae. Moxidectin (400 μg/kg PO) also has efficacy. A single dose usually resolves gastric infection, but cutaneous lesions may require repeat treatment (every 2–4 weeks during fly season) to kill newly deposited larvae. Many clinicians use a combination: an avermectin for systemic effect and a topical steroid (e.g., betamethasone valerate) to reduce inflammation. Surgical debridement of exuberant granulation tissue is often necessary for large or chronic summer sores. Cryotherapy or laser ablation may be used for focal lesions. Topical insecticides applied to the wound can prevent reinfestation. Systemic glucocorticoids (prednisolone 1 mg/kg PO q24h for 5–7 days) can reduce eosinophilic inflammation but should be used cautiously. Ocular lesions may be treated with topical ivermectin (1% solution) and corticosteroid ophthalmic drops.
Antibiotics are indicated if secondary bacterial infection is present. Wound management includes cleaning with povidone-iodine solution, debridement, and bandaging. Fly control is essential to prevent reinfection.
Control and Prevention
Control of Habronema muscae centers on breaking the lifecycle by reducing fly populations and protecting wounds.
- Fly management: Use of insecticidal sprays, baits, traps, and environmental sanitation (removing manure and decaying organic matter) reduces Musca domestica breeding sites. Biological control agents such as parasitic wasps (Spalangia spp.) can also be deployed.
- Wound protection: Cover all wounds, including castration sites and superficial abrasions, with fly-repellent dressings. Use insect-proof bandages and frequent monitoring.
- Regular deworming: Strategic anthelmintic treatment with macrocyclic lactones (ivermectin or moxidectin) at intervals of 6–8 weeks during the fly season can reduce larval release into the environment. Year-round rotational deworming programs should incorporate fecal egg count reduction testing to monitor efficacy.
- Pasture management: Frequent removal of manure decreases fly breeding substrate. Composting manure adequately kills eggs and larvae.
- Quarantine: New horses should be dewormed with ivermectin upon arrival and kept under fly-proof conditions for two weeks.
Public Health Considerations
Habronema muscae is not a zoonotic pathogen. However, Musca domestica can mechanically transmit other pathogens (e.g., enteric bacteria) to humans. Good hygiene in stables and around horses reduces this risk.
References
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- Reinecke RK, Rowland JC, Steyl JC. The diagnosis of Habronema muscae infection in horses. Journal of the South African Veterinary Association. 1990;61(4):179-181.
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