Incidental Discovery of Conjunctival Thelazia callipaeda during Vitrectomy for Diabetic Tractional Retinal Detachment: A Case Report

Abstract

Introduction

Thelazia callipaeda is a parasitic nematode that infects the eyes of humans and other mammals. This case highlights its incidental discovery during ocular surgery and underscores the importance of preoperative vigilance.

Case Presentation

The parasite was identified intraoperatively in a 74-year-old female with a long-standing history of diabetes who was undergoing surgery for diabetic tractional retinal detachment in her right eye. She reported poor hygiene habits but no direct pet contact. The parasite, found within the conjunctival sac, was excised and confirmed by pathological examination. Postoperative care included antibiotic eye drops. At the 3-month follow-up, her best corrected visual acuity had improved to 6/120 with no recurrence.

Conclusion

This case emphasizes the critical role of comprehensive preoperative examination for the timely detection of ocular parasites to prevent complications. It also highlights the need for clinician awareness of Thelazia infection, particularly in high-risk individuals, and the importance of appropriate postoperative follow-up to optimize outcomes and minimize recurrence risk.

Introduction

Thelazia callipaeda, commonly known as the oriental eye worm, is a parasitic nematode that primarily inhabits the eyes of humans and other mammals [1, 2]. The disease is typically transmitted through the ocular secretions of animals such as dogs, cats, and rabbits, with Phortica variegata, the eye gnat, serving as the primary vector [2]. This fly feeds on the ocular secretions of humans and animals, especially dogs [2, 3]. When the fly ingests the first-stage larvae of the parasite present in the ocular discharge, the larvae develop into the infective third-stage larvae within the fly’s body. Upon subsequent feeding on the tear fluids of a host, the infective larvae are released from the fly’s mouthparts, entering the eye of a new host and causing infection [1–3].

Although human infections with T. callipaeda are relatively rare, reports of the disease have increased in recent years, likely due to global climate change, environmental shifts, and increased human-animal interactions [1, 2]. The incidence is particularly high in rural areas of Asia, with children, especially young ones, being considered high-risk groups [3–5]. The parasite typically resides in the conjunctival sac of the human eye, with the outer canthus of the conjunctiva being the most common site. However, it may also be found in the anterior chamber, lacrimal duct, lacrimal gland, eyelid, and subconjunctival tissues [1–3]. In rare instances, the parasite has been found in the anterior chamber and, even more infrequently, within the vitreous body [2, 6].

The clinical presentation of T. callipaeda infection varies widely, ranging from asymptomatic cases to more severe symptoms such as foreign body sensation, epiphora (excessive tearing), conjunctival hyperemia, and corneal lesions [7, 8]. In many cases, the infection is discovered incidentally during routine ophthalmic examinations or surgical procedures. Asymptomatic infections, in particular, pose significant diagnostic challenges, leading to delays in detection and treatment. For patients presenting with foreign body sensation, tearing, corneal damage, or unexplained uveitis, it is essential to routinely inquire about animal contact history or fly bites and perform a thorough slit-lamp examination [1, 3].

This case report presents the case of a 74-year-old female patient with a history of diabetes who underwent surgery for diabetic-related ocular complications. During the procedure, an incidental T. callipaeda infection was discovered in the conjunctival sac. This report highlights the diagnostic challenges associated with asymptomatic Thelazia infections and emphasizes the importance of comprehensive preoperative examinations, as well as the effective measures taken during surgery to prevent complications arising from the infection.

Case Presentation

The patient is a 74-year-old female with a 20-year history of diabetes mellitus, who presented with a complaint of blurred vision in the right eye persisting for the past 6 months. She did not report any increase in ocular discharge, redness, eye pain, photophobia, or tearing. On examination, the best corrected visual acuity in the right eye was limited to hand motions, and the intraocular pressure measured 15 mm Hg. The eyelids, lacrimal drainage system, and puncta were within normal limits. The conjunctiva, sclera, cornea, anterior chamber, and pupil shape were all normal (Fig. 1a). The right eye contained an artificial intraocular lens (Fig. 1a). The vitreous was clouded, and fundus examination revealed old laser marks in the upper retina, white vascular lines in the lower retina, and a large proliferative membrane in the posterior pole (Fig. 1b). B-scan ultrasonography of the right eye reveals retinal detachment (Fig. 1c). However, the patient did not see any proliferative changes in the left eye (Fig. 1d).

Fig. 1.

Clinical examination results of the patient. a Anterior segment photograph showing a quiet anterior segment with an intraocular lens in place, and no signs of active infection. b Scanning laser ophthalmoscopy of the right eye demonstrating severe proliferative diabetic retinopathy with laser scars and an epiretinal membrane. c B-scan ultrasonography confirming the presence of tractional retinal detachment. d Scanning laser ophthalmoscopy of the left eye (for comparison), showing no significant proliferative changes.

The patient underwent pars plana vitrectomy with a four-channel, two-handed membrane peeling technique for severe retinal proliferation. During the operation, the surgeon observed two white, linear worms within the superior and inferior fornices of the conjunctiva (Fig. 2a, b). The worm was distinctly different from the surrounding ocular tissues. Upon its discovery, the surgeon promptly paused the procedure and implemented necessary measures to prevent the parasite from remaining in the surgical field or causing any further complications. The surgical site was thoroughly irrigated with povidone-iodine to disinfect and minimize the risk of secondary infection. The worm was carefully excised and sent for pathological examination to confirm its identity and ensure complete removal (Fig. 2c). The surgical area was maintained under strict aseptic conditions throughout the procedure, and postoperative antibiotic eye drops were administered to prevent potential infection. No specific anthelmintic treatment was prescribed. The contralateral eye was also examined carefully, and no worms were found in the conjunctival sac.

Fig. 2.

Intraoperative discovery and pathological confirmation of Thelazia callipaeda. a, b Intraoperative identification of two milky-white, elongated linear worms (red arrows) within the palpebral conjunctival sac. c Histopathological findings of the worm in cross-section. The section demonstrates characteristic helminthic features, including a well-defined body wall with cuticle, hypodermis, and muscle layers, as well as an internal intestinal tract. Scale bars represent 100 μm.

Postoperatively, a detailed history revealed that the patient lives alone and has no known contact with pets. However, she reported poor hygiene habits, including rummaging through trash and collecting waste materials. Expert consultation with the Department of Parasitology at Jiangsu University School of Medicine confirmed the parasite as T. callipaeda, based on its morphological features and histopathological findings.

At the 3-month follow-up, the patient’s right eye best corrected visual acuity had improved to 6/120. A thorough ocular examination, including slit-lamp assessment and ocular B-scan ultrasonography, revealed no abnormalities or recurrence in the conjunctival sac or intraocular structures.

Discussion

T. callipaeda is a parasitic nematode that primarily infects the ocular tissues of various mammals, including humans [1, 9]. In recent years, reports of the disease have been increasing worldwide [1]. Although human infections are relatively rare, Thelazia infections have gained more attention due to environmental changes, climate shifts, and increased human-animal contact [3, 5, 9]. The disease is primarily transmitted through the ocular secretions of animals, such as dogs, cats, and rabbits, with P. variegata, the eye gnat, serving as the main vector [1]. This fly feeds on ocular secretions, including those of infected animals, thereby transmitting the first-stage larvae of the parasite to new hosts [1, 9]. In this case, a 74-year-old female patient, who lived alone in a town with poor sanitation, was incidentally found to have T. callipaeda infection in the conjunctival sac during surgery for diabetic-related ocular complications.

In clinical practice, diagnosing Thelazia infection is often challenging, especially in asymptomatic or mildly symptomatic cases [7]. Asymptomatic infections are particularly difficult to detect because patients typically do not report foreign body sensations or discomfort [7]. While the parasite was incidentally discovered and addressed during surgery in this case, for patients presenting with foreign body sensation, tearing, corneal damage, or unexplained uveitis, routine inquiry about animal contact history or fly bites, coupled with thorough slit-lamp examination, is essential. Timely diagnosis is crucial in preventing disease progression and complications.

Preoperative management is critical to preventing T. callipaeda infection from affecting the surgery. In this case, the parasite was not detected during preoperative examination. This oversight may be attributed to several factors: the parasite’s inconspicuous nature due to its small size and hidden location within the fornices; the likely presence of diabetic corneal neuropathy in this long-standing diabetic patient, which reduced ocular surface sensitivity and muted symptomatic cues [10]; a clinical focus dominated by the sight-threatening retinal detachment; and the superior visualization afforded by the surgical microscope intraoperatively. Therefore, special attention should be paid to a thorough examination of the conjunctival sac before surgery, including flipping the upper and lower eyelids to ensure there is no parasite residue. Additionally, a detailed eye examination with a slit lamp, including the tear ducts and eyelid area, can effectively identify potential T. callipaeda infections and other parasitic infections [1, 7].

The current standard treatments for T. callipaeda primarily include mechanical removal of the parasites and the use of specific anthelmintic drugs for prevention and therapy [1, 7, 11, 12]. The main medications for treating Thelazia involve local ocular anthelmintic agents, such as ivermectin and levamisole. Research has demonstrated that ivermectin eye drops (10 mg/mL, with a dosage of 6 µg per dose), diluted in 10% propylene glycol, are effective in eliminating the parasites [11]. Levamisole eye drops (5 mg/mL concentration) have also shown promising results in experimental settings, effectively clearing the infected Thelazia worms [12]. The mechanism of levamisole involves selectively activating acetylcholine-gated ion channels in the nematodes, causing spastic paralysis and ultimately leading to their expulsion [12]. Although the use of eye drops as a treatment option is limited, and many countries currently do not have specifically formulated eye drop preparations for this purpose, this therapeutic approach provides a viable option for localized treatment of Thelazia infections.

Beyond mechanical removal and topical agents, systemic anthelmintic therapy may be considered in specific clinical scenarios to eliminate possible residual larvae or to treat cases where manual extraction is incomplete or not feasible. Oral ivermectin has been advocated as an effective treatment for ocular thelaziasis in both human and veterinary practice [13]. A single dose of 0.2 mg/kg has been used successfully in the treatment of Thelazia infections in animals [14], and this regimen aligns with the standard dose for many human nematode infections. Another anthelmintic, levamisole, administered orally at 5 mg/kg, has also shown efficacy against ocular Thelazia in combination with ivermectin [15]. The rationale for systemic therapy lies in its potential to target larvae in the lacrimal glands or other tissues not accessible to topical agents. In the present case, systemic treatment was not administered as the parasites were completely and directly removed under surgical visualization, and the infection appeared to be localized and unilateral. However, systemic ivermectin could be a valuable adjunctive option for complex cases, such as suspected multifocal infection, recurrent infestation, or when surgical removal carries high risk.

However, when the parasite is directly accessible, as in our case, meticulous mechanical removal remains the immediate and definitive intervention. This principle is critical because the nematode’s internal contents are highly antigenic and potentially septic. For instance, a recent study demonstrated that Thelazia callipaeda secretes bioactive proteins such as macrophage migration inhibitory factor, which can actively modulate human immune cell responses, underscoring the potent immunogenicity of its components [16]. Traumatic rupture during extraction should be strictly avoided for two primary reasons: First, the release of such parasitic proteins and waste products can incite a significant localized chemical inflammatory response [1, 16, 17]. Second, rupture increases the risk of secondary bacterial infection. Therefore, the established clinical guideline emphasizes careful removal followed by thorough disinfection of the conjunctival sac [7].

Guided by the above principle, the surgical procedure should be promptly adjusted upon intraoperative discovery of the parasite. The procedure should be immediately halted, and meticulous removal should be performed. The surgical site should be thoroughly irrigated with povidone-iodine under strict aseptic conditions to minimize the risk of secondary infections. In this case, the worm was carefully excised and sent for pathological examination, ensuring complete parasite removal and reducing the potential for complications. Additionally, postoperative antibiotic eye drops were administered to effectively prevent infection. While specific anthelmintic eye drop formulations for humans are not currently available, regular postoperative follow-up and prompt management of any potential recurrences are essential to prevent reinfection.

This case also highlights the diagnostic challenges of T. callipaeda infections, particularly in asymptomatic or mildly symptomatic patients. To avoid missed diagnoses, it is recommended that if parasites are discovered during surgery, they should be immediately removed, and appropriate follow-up checks should be carried out. The patient’s history should be fully assessed, particularly regarding animal contact and other possible infection sources. Strengthening control measures for T. callipaeda, especially in high-risk areas, can effectively reduce the occurrence and transmission of this infection [1].

This case has several limitations that should be acknowledged. First, as with any single case report, the findings are descriptive and cannot be generalized. Second, due to the incidental nature of the intraoperative discovery, a dedicated preoperative parasitic screening was not performed, and the identification of the parasite was based on histopathology without molecular confirmation. Third, the follow-up period of 3 months, while adequate for assessing surgical recovery and absence of immediate recurrence, is insufficient to evaluate long-term outcomes. Despite these limitations, we believe this case provides a valuable clinical lesson.

Conclusion

T. callipaeda infection is often overlooked precisely because patients may remain entirely asymptomatic, leading to underdiagnosis. Enhanced clinician awareness, a thorough history of exposure, and a meticulous slit-lamp examination are paramount to preventing missed diagnoses. Clinicians should also ensure appropriate postoperative follow-up to prevent recurrence and optimize patient outcomes. Furthermore, implementing robust public health measures, such as vector control and routine deworming of companion animals in endemic regions, is vital to curbing the transmission of this and other emerging zoonotic diseases. The CARE Checklist has been completed by the authors for this case report, attached as online supplementary material (for all online suppl. material, see https://doi.org/10.1159/000550668).

Statement of Ethics

This study received ethical approval from the Research and Ethics Committee of Zhenjiang First People’s Hospital (Approval No. KYW001-06) and was carried out in compliance with the principles outlined in the Declaration of Helsinki. Informed consent was obtained from all participants. Written informed consent was obtained from the patient for publication of the details of their medical case and any accompanying images.

Conflict of Interest Statement

The authors declare no competing interests.

Funding Sources

This work was supported by the Natural Science Foundation of Fujian Province (2023J011588) and Zhenjiang Science and Technology Bureau under the Science and Technology Plan Project (Social Development) (FZ2025108).

Author Contributions

X.C., Y.B., and Y.G. were responsible for the conception and design of the manuscript. X.C. wrote the manuscript. Y.G. reviewed the manuscript. All authors read and approved the final manuscript for submission. X.C. and Y.G. contributed equally to this work.

Funding Statement

This work was supported by the Natural Science Foundation of Fujian Province (2023J011588) and Zhenjiang Science and Technology Bureau under the Science and Technology Plan Project (Social Development) (FZ2025108).

Data Availability Statement

The datasets generated and analyzed during the current study are not publicly available due to privacy and ethical restrictions but are available from the corresponding author upon reasonable request. Data requests will require approval from the Institutional Ethics Committee prior to sharing.