Disseminated Halicephalobus gingivalis infection in a horse

Abstract

A 19-y-old American Saddlebred gelding was evaluated for epiphora of the right eye and generalized cachexia. Initial examination revealed anterior uveitis without ulceration, for which treatment was initiated. Despite the initial response to treatment, the signs progressively worsened to blindness. Histologic examination of the enucleated eye revealed granulomatous panuveitis and optic neuritis with intralesional nematode larvae identified as Halicephalobus gingivalis. Over time and despite anthelmintic treatment, blindness developed in the left eye along with neurologic signs, and the horse was euthanized. Disseminated halicephalobosis was diagnosed on postmortem examination, involving the heart, spleen, kidneys, oral cavity, tongue, left eye, lungs, CNS, adrenal glands, liver, and lymph nodes. Splenic involvement has not been reported previously, to our knowledge. Halicephalobosis is a sporadic parasitic disease that affects equids mostly and for which prognosis is poor despite aggressive systemic anthelmintic treatment. Parasitic granulomatous disease should be included as a differential diagnosis in equids with ocular or neurologic disease.

A 19-y-old, ~450 kg American Saddlebred gelding was presented in March 2020 with epiphora of the right eye of 2-d duration and generalized weight loss. The gelding was housed in a boarding facility with no history of recent travel. Retired from 3-d eventing, the horse was used for light general riding and was not receiving any medications. Being an intermittently high shedder of strongyles (>500 eggs/g [epg]), fecal egg counts were performed every 3–4 mo to guide deworming. Deworming was last done with moxidectin–praziquantel 4 wk before the initial evaluation.

Physical examination revealed a body condition score of 4 of 9 (Henneke scale) with moderate symmetrical wasting of the epaxial and gluteal musculature. In addition, there was mild anterior uveitis of the right eye with moderate blepharospasm, mild epiphora, and mild miosis. The cornea was clear, without any corneal ulceration or aqueous flare. The remainder of the physical examination, including evaluation of the left eye, revealed no significant abnormalities.

CBC and serum chemistry revealed leukopenia (1 × 10⁹/L; RI: 1.5–5.5 × 10⁹/L) and mildly decreased serum creatinine (71 µmol/L; RI: 80–130 µmol/L). Given the history of polyuria, endogenous adrenocorticotropic hormone (26 pmol/L; RI: 4–129 pmol/L), basal insulin (49 pmol/L; RI: 0–292 pmol/L), and blood glucose (5 mmol/L; RI: 4–7 mmol/L) were measured to screen for pituitary pars intermedia dysfunction and were within RIs. An agar gel immunodiffusion test for antibody to equine infectious anemia virus was negative. A modified Stroll fecal egg count revealed 0 epg.

Initial treatment for anterior uveitis of the right eye consisted of 1% atropine ophthalmic ointment (Bausch & Lomb) and neomycin and polymyxin B sulfates and bacitracin zinc (Triple antibiotic ophthalmic ointment; Bausch & Lomb) applied every 8–12 h. Oral flunixin meglumine paste (Banamine; Merck) was administered at 500 mg (~1.1 mg/kg) q24h for 3 d. There was initial improvement of the uveitis of the right eye following treatment but re-evaluation after a week confirmed anterior uveitis of the right eye with no corneal ulceration. Secondary treatment with topical 1% atropine ophthalmic ointment (Bausch & Lomb) q12h and neomycin and polymyxin B sulfates and dexamethasone ophthalmic ointment (Sandox) was initiated. In addition, 500 mg of oral flunixin meglumine paste (1.1 mg/kg; Banamine; Merck) q12h the first day, tapered to 250 mg q12h, was advised. Re-evaluation 3 d post-initiation of the new treatment protocol showed worsening uveitis. At this point, flunixin meglumine was discontinued and systemic dexamethasone (DexaJect; Henry Schein) treatment was initiated, initially with 20 mg of dexamethasone administered IV followed by 14 mg PO the following day, with topical atropine and corticosteroid ophthalmic ointments applied q6h as best able by the owner and barn caretakers.

Systemic dexamethasone elicited a good initial response, with a decrease in the degree of blepharospasm and epiphora. Because of concerns for the involvement of Leptospira sp., treatment with oral doxycycline at 10 mg/kg orally q12h was initiated. Three hours after the first dose of doxycycline, the right eye experienced a marked flareup of inflammation, after which doxycycline was discontinued; however, the uveitis became completely refractory, and the owner elected for enucleation 3 d later (16 d after initial onset). Standing transpalpebral enucleation of the right eye was performed with no complications. Ceftiofur crystalline-free acid (Excede; Zoetis) was administered IM at 2.2 mg/kg the morning of surgery, and a second dose was given 4 d later. Flunixin meglumine paste was administered orally at 0.5 mg/kg q12h for 5 d. The enucleated right eye was submitted for histologic examination with a resulting diagnosis of severe granulomatous panuveitis and optic neuritis with intralesional nematode larvae consistent with Halicephalobus gingivalis. The nematodes were present at the margins of the excised optic nerve, confirming that the infestation had disseminated beyond the eye. Thus, the horse was treated with high-dose ivermectin (1.2 mg/kg; Eqvalan, Boehringer-Ingelheim) to target systemic nematodes.

Post-operative recovery was uneventful, and the animal was returned to work under saddle and was monitored closely for any signs of systemic illness. Eight weeks after the enucleation, the horse was presented on emergency for sudden onset of blindness of the left eye, which progressed over the next few days with worsening neurologic signs including ataxia, dysphagia, and right circling. Repeat CBC and serum chemistry revealed no clinically significant abnormalities. Abdominal ultrasound revealed multiple, circular, well-circumscribed masses within both kidneys. A free-catch urine sample was obtained, but no nematodes were found within the sample. Based on deteriorating clinical signs and lack of response to treatment, a poor prognosis was given. The owner elected euthanasia, and an autopsy was performed.

On gross examination, there were firm, elevated pale-tan masses (up to 2 cm maximum dimension) on the mucosa of the tongue and extending into the muscle on cut section. Both kidneys were enlarged, with irregular capsular surfaces. Variably sized, up to 6-cm, pale-tan, elevated masses expanded the cortical surface of the kidneys. On cut section, these masses effaced the renal parenchyma and expanded into the medulla (Fig. 1). The adrenal glands and the medial iliac lymph nodes were enlarged and expanded by similar masses (3–8 cm diameter) that effaced the normal architecture. Similar nodules were also observed on the right ventricle (Fig. 2), liver, and spleen. Sections of the kidneys, lymph nodes, lungs, liver, spleen, gastrointestinal tract, pancreas, heart, adrenal glands, thyroid glands, spinal cord, brain, eyes, and skeletal muscle were collected for histologic examination.

Figures 1–6.

Disseminated Halicephalobus gingivalis infection in a horse. Figure 1. Pale-tan, variably sized granulomas in a kidney, and replacing an adrenal gland. Figure 2. Pale-tan nodule in the right ventricular wall. Inset: higher magnification of the nodule. Figure 3. Granuloma in the subendocardial myocardium of the right ventricular free wall seen in Figure 2. H&E. Figures 4, 5. Granulomatous inflammation centered on adult and larval rhabditid nematodes in a kidney. Figure 4. Both adults and larval nematodes are found within a dilated renal tubule. H&E. Figure 5. The inflammatory infiltrate consists of macrophages, lymphocytes, plasma cells, and eosinophils surrounded and separated by bands of fibrous connective tissue. H&E. Figure 6. Adult and larval forms of H. gingivalis with a rhabditiform esophagus (outlined in the highlight to the right) consisting of corpus (c), isthmus (i), and bulb (b). Scale bars = 50 µm.

Variably sized granulomas expanded most of the organs examined, including the heart (Fig. 3). In sections of the kidneys, the granulomas centered on numerous tangential and cross-sections of well-preserved nematodes (Fig. 4). The inflammatory infiltrates consisted of numerous epithelioid macrophages, multinucleate giant cells, numerous plasma cells, eosinophils, and a few lymphocytes surrounded and separated by abundant fibrous connective tissue (Fig. 5). The surrounding parenchyma was atrophic, with loss of glomeruli and tubules, shrunken, fibrotic glomerular tufts with thickened glomerular and tubular basement membranes. The interstitium was multifocally expanded by bands of fibrous connective tissue (Fig. 5). Tubules in many foci were moderately dilated and contained eosinophilic proteinaceous casts with numerous rhabditiform nematode parasitic larva, adults, and embryonated ova. The adult nematodes had a thin eosinophilic cuticle; a cylindrical body (13–20 μm diameter) with tapered ends; a characteristic rhabditiform esophagus with a distinct corpus, isthmus, and bulb; and dorsoventrally reflected ovary (Fig. 6). Larval nematodes were smaller (6–10 μm diameter) and also had the characteristic rhabditiform esophagus. Ova were round-to-oval, 10 × 35 μm, and occasionally contained larval nematodes.

In sections of the eye, the corneal endothelium was hyperplastic, and there was neovascularization of the stroma. The fibers of the corneal substantia propria were separated by clear space and dilated lymphatics with multifocal mixed inflammation. The anterior chamber contained loose strands of fibrin, scant hemorrhage, scattered neutrophils, macrophages, lymphocytes, plasma cells, eosinophils, and myriad adult and larval nematodes. Diffusely, there was thinning of the retina and multifocal retinal detachment with loss of layers. Extensive portions of the ciliary body, choroid, optic nerve, and retina were effaced by granulomatous inflammation with admixed adult and larval nematodes. The optic neuritis expanded the meninges and extended into the perineural adipose tissue. There was multifocal degeneration of the optic nerve with spongiosis and presence of a few dilated myelin sheaths. Similar granulomatous inflammation with intralesional nematodes was also observed in sections of tongue, heart, liver, spleen, adrenal gland, and lymph nodes. Sections of the cerebrum, cerebellum, brainstem, and cervical spinal cord had multifocal axonal degeneration, spheroid formation, and gliosis. No histologic lesions were observed in the pancreas, thyroid glands, and lungs.

Definitive diagnosis of disseminated halicephalobosis was made based on gross and histologic findings. H. gingivalis (previously known as Micronema deletrix) is a ubiquitous, free-living, saprophytic nematode that has been associated sporadically with opportunistic, granulomatous infections in horses,^2,5,8,11,16 zebras, ⁹ and, less frequently, cows ⁵ and humans.^13,14 Information on the infection, pathogenesis, and life cycle of this nematode is limited. In our case, consistent with previous reports,^2,5,12,16 the route of infection could not be identified. Proposed routes of infection include larval penetration through lacerated skin and mucous membranes, urogenital infection, and transmammary or possible prenatal or perinatal infection.^9,10 After the establishment of infection, hematogenous and lymphatic dissemination have been suggested given that free nematodes have been reported in the vessel lumens. ⁷ Adult nematodes have also been reported in urine, semen, and aqueous and vitreous humors.^11,18 The organs affected and lesions described in our case are consistent with published cases. Previous reports in horses include lesions in the CNS, kidneys, oral cavity, eyes, lungs, adrenal glands, and liver.^2,9,11,12,16 We also noted lesions in the spleen, which, to our knowledge, have not been described previously with Halicephalobus infection in horses.

Clinical presentation and signs in affected animals vary considerably depending on the organs affected and the extent of spread. Given the rarity of the disease, nonspecific clinical and laboratory findings, and lack of screening or confirmatory tests, a definitive diagnosis of halicephalobosis is challenging in the absence of lesions accessible for biopsy. The apparent clinical signs in our case were progressive uveitis and neurologic signs. Neurologic and renal lesions are overrepresented in the reported cases.^{1,5,8,9,11,12,16} Confirmatory diagnosis is based on histologic demonstration of granulomatous inflammation with characteristic intralesional nematodes.

Halicephalobus organisms have distinctive tissue histomorphology compared to other nematode parasites infecting horses. Adult Halicephalobus nematodes have a cylindrical body with tapered anterior end, pointed tail, rhabditiform esophagus with distinct corpus, isthmus, and valved bulb, dorsiflexed ovary, and ventroflexed uterus. ¹ Other equine rhabditid nematodes include Strongyloides westeri, Pelodera strongyloides, and Cephalobus spp. P. strongyloides affects dogs mostly and sheep rarely, with lesions limited to the skin of the ventral abdomen. ¹⁹ Percutaneous invasion is possible with S. westeri larvae; however, adult nematodes and eggs are not found in the skin. ³ Cephalobus spp. have a blunt tail and subtle differences in the shape of the esophagus differentiating it from Halicephalobus. ⁶ Other metazoan parasites reported to affect the equine CNS include Parelaphostrongylus tenuis, Setaria digitata, Strongylus vulgaris, Strongylus equinus, Angiostrongylus cantonensis, and Draschia (Habronema) megastoma. ²¹ S. digitata has been described in the eye of horses, and Habronema can affect periocular areas.^17,22 However, the size and distinct morphologic features of H. gingivalis enable accurate distinction from the other nematodes.

The prognosis of H. gingivalis infection is generally poor, and treatment is usually nonproductive in animals with CNS or disseminated infection, most likely because of the inability of drugs to penetrate the granulomatous lesions and/or to cross the blood-brain barrier. There are only a few reports of successful treatment of this infection in equids; all were localized infections outside the CNS.^4,15,20 It is important to consider parasitic granulomatous disease as a differential diagnosis in equids with ocular or neurologic disease.