By: Akanksha
Two ophthalmologists at Kangwon National University Hospital in Chuncheon-si, South Korea, recently removed 11 live Thelazia callipaeda parasitic roundworms from the eyes of a previously healthy 71-year-old crop farmer.
Some of these worms were breeding, while others were pregnant with larvae. The infection developed after tear-feeding flies introduced the zoonotic parasite into the conjunctival sacs of the patient while feeding on ocular secretions.
A slit-lamp examination of the patient revealed writhing nematodes within the eyes, so the doctors administered local anesthetic and removed the worms individually using fine forceps.
The case is detailed in an online report dated October 24, 2026, and in the October 29, 2026 edition of The New England Journal of Medicine.
The 71-year-old crop farmer initially approached Kangwon National University Hospital for cataract surgery. For around two months before his visit, he had been experiencing a persistent sensation of having something stuck in both of his eyes, along with blurred vision and redness.1
Midway through a pre-op examination, the ophthalmologists performed a slit-lamp examination, which revealed conjunctival hyperaemia, dilated blood vessels causing redness in the conjunctiva membrane, corneal epithelial erosion, and multiple white, translucent nematode worms in the upper and lower fornixes of the conjunctiva in both eyes.1
Bilateral involvement is relatively uncommon in reported human thelaziasis cases. A 2025 systematic review of 134 documented cases across 18 countries found that 90.3% of infections affected only one eye, making the involvement of both eyes in this patient a notable clinical feature.3
Instead of administering antiparasitic anthelmintics, the doctors removed the worms manually using local anesthetic. Eleven live worms ranging from 11 to 20 millimeters in length were removed with the help of fine forceps.
Upon microscopic examination, the worms were identified and confirmed to be Thelazia callipaeda nematodes. Males had a ventrally curved posterior end with needle-like copulatory spicules for reproduction. Several females were found to be pregnant and had large numbers of tightly coiled first-stage larvae bursting from their uteri. Finding pregnant females confirmed that they had fully matured within the human eye.
As all adult worms were removed, no anthelmintic treatment was required. The patient was seen after two months and had fully recovered with no further symptoms, meaning he could undergo cataract surgery.1
Manual extraction is also the predominant treatment approach in reported human thelaziasis cases. The 2025 systematic review found that mechanical removal was used in 88.8% of documented cases, while antiparasitic agents were used selectively. Treatment depends on the clinical presentation and whether parasites remain after extraction.3
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The genus Thelazia (Spirurida, Thelaziidae) consists of a cosmopolitan group of spirurid nematodes that can infect domestic and wild animals, as well as humans. Adult worms are thread-like, pale white, and can reach up to 2 cm. In general literature, male adults are 4.5 to 13 mm long and 0.25 to 0.75 mm wide. Female adults are larger, ranging from 6.2 to 17 mm in length and 0.3 to 0.85 mm in diameter. However, the individual worms in the South Korean diagnosis were reported to be 11 to 20 mm.
T. callipaeda is non-segmented with a ridged cuticle. The worms have well-developed anterior and posterior ends. Both males and females lack lips, hooks, or spines. Instead, they have hexagonal mouths with two concentric rings of flat papillae surrounding a central opening. Females have a vulva located at the anterior end, close to the esophagus. Males have a posteriorly curved tail with five pairs of post-cloacal papillae. Unlike many nematode species, females are ovoviviparous, meaning embryos develop within eggs in the uterus. They then give birth to broods of first-stage larvae into tear secretions.1
How Do Tear-Feeding Flies Transmit the Parasite?
The entire life cycle of T. callipaeda depends on non-biting insect vectors, specifically, tear-feeding flies of the order Diptera, family Drosophilidae, and genus Phortica.2
Two recognized intermediate hosts of T. callipaeda are Phortica variegata, associated with transmission in Europe, and Phortica okadai, associated with transmission in China. The US Centers for Disease Control and Prevention (CDC) identifies both species as primary intermediate hosts of the parasite.7
A 2022 experimental study published in Pathogens further demonstrated that P. okadai could support the development of first-stage larvae into infective third-stage larvae. Researchers subsequently used infected flies to transmit the parasite to a rabbit under laboratory conditions.8
When the flies land on infected eyes to feed on tears, they ingest first-stage larvae released by adult females. The larvae undergo two molts within the insect over 14 to 21 days to become infectious third-stage larvae. They then migrate to the mouthparts of the insect. When the flies feed on another host, the larvae migrate to the eyeball of the new host. They then move into the conjunctival sac and develop into adult worms within 35 days. The adult worms mate on the surface of the eye.
T. callipaeda is commonly known as the oriental eye worm due to its high prevalence in the former Soviet Union, China, South Korea, Japan, Indonesia, Thailand, Taiwan, and India. Infection rates in animals vary from 5% to 42%, depending on the area. This mainly affects dogs, cats, foxes, rabbits, cows, sheep, and goats.
Since its first recorded case in Europe in 1989, it has become endemic in Italy and France. Cases have also been reported in Spain, Germany, Switzerland, the Netherlands, and Serbia. In the USA, T. callipaeda, T. californiensis, and a third unidentified species cause infections in humans.
In endemic areas, infections are most common in poorer rural communities. Children and the elderly are most commonly affected due to increased exposure to flies. Humans can act as definitive hosts, as larvae develop into reproductive adults.2
Humans are generally considered incidental or aberrant definitive hosts, while dogs and other canids are important definitive hosts in the natural transmission cycle. However, experimental evidence indicates that human-derived T. callipaeda larvae can infect intermediate hosts. In a 2022 laboratory study, researchers successfully transmitted human-derived parasites through P. okadai flies to rabbits, demonstrating the potential for onward transmission under experimental conditions.7,8
Indian cases also demonstrate that human ocular thelaziasis is not limited to East Asia. Several peer-reviewed reports have documented infections in different parts of India.
In 2018, researchers reported a 58-year-old farmer from Assam who was found to have two motile, translucent female T. callipaeda worms in his left conjunctival fornix during cataract surgery. The parasites were discovered incidentally during the procedure.5
In another report published in the Indian Journal of Ophthalmology in 2019, an 80-year-old Indian man presented with three live worms in the conjunctival sac of his right eye. Microscopic examination identified the parasites as gravid T. callipaeda. The researchers documented embryonated or unsheathed eggs and first-stage larvae in the same worm.4
These reports show that gravid eye-worm infections and cases discovered during cataract procedures have previously been documented in Indian patients. The South Korean case is therefore notable for its bilateral involvement and the removal of 11 live worms, rather than being the first documented human infection involving gravid parasites.4,5
A 2025 systematic review published in New Microbes and New Infections analyzed 134 documented human thelaziasis cases reported across 18 countries. The findings highlighted the geographical distribution and clinical presentation of this relatively uncommon parasitic infection.3
The review reported that 82.8% of documented cases originated in Asia. South Korea accounted for 33 cases, followed by China with 32, India with 18, and Japan with 12.3.
The review also identified T. callipaeda as the predominant species, accounting for 88.8% of the reported infections.
Cases of human thelaziasis are most common in the summer months (June to August). This is due to flies being active from late spring to autumn. Spring and summer are also peak times for pollen-induced allergic or bacterial conjunctivitis, which can mask the diagnosis. Additionally, the small size of the larvae and low worm numbers make them hard to see without a thorough examination.
Diagnosis can only be confirmed by removing the worms and examining their morphology.2
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Human thelaziasis can cause varying degrees of ocular inflammation, depending on the number of worms and the extent of infection. Common symptoms include foreign-body sensation, excessive tearing, eye redness, itching and conjunctivitis. Some patients may also experience eye pain and blurred vision.3,7
According to the CDC, heavier infections may cause photophobia, eyelid or surrounding tissue swelling, corneal ulceration and conjunctivitis. These complications highlight the importance of identifying and removing the parasites through appropriate ophthalmological examination.7
Reducing exposure to tear-feeding flies is an important preventive measure, particularly in endemic rural and agricultural areas. A 2025 systematic review identifies insect repellents, protective eyewear in areas where the parasite is endemic, and improved sanitation and hygiene to reduce fly populations as potential preventive measures.3
A 2023 report in Emerging Infectious Diseases also highlighted the importance of preventive measures after documenting a human reinfection with T. callipaeda in Serbia. The authors emphasised the need for prevention in areas where the parasite and its vectors are established.9
Removing 11 live, breeding Thelazia callipaeda worms from a 71 year old crop farmer in South Korea demonstrates the alarming nature of ocular zoonoses and how effective timely treatment can be, highlighting the importance of awareness when a patient presents with persistent eye discomfort.
References:
1. Song, J. E., and K. Jo. “Thelaziasis.” The New England Journal of Medicine 395, no. 17 (2026): e24.
https://www.nejm.org/doi/pdf/10.1056/NEJMicm2609683.
2. Otašević, S., M. Trenkić Božinović, A. Tasić, A. Petrović, and V. Petrović. “Thelazia callipaeda and Eye Infections.” Acta Facultatis Medicae Naissensis 31, no. 3 (2014): 171–76.
3. Bonilla-Aldana, D. K., J. L. Bonilla-Aldana, J. D. Acosta-España, R. Sah, and A. J. Rodriguez-Morales. “Thelaziasis in Humans: A Systematic Review of Reported Cases.” New Microbes and New Infections 65 (2025): 101599.
https://doi.org/10.1016/j.nmni.2025.101599.
4. Sharma, M., D. Das, H. Bhattacharjee, S. Islam, N. Deori, G. Bharali, S. Tomar, P. Bhola, and A. Deka. “Human Ocular Thelaziasis Caused by Gravid Thelazia callipaeda: A Unique and Rare Case Report.” Indian Journal of Ophthalmology 67, no. 2 (2019): 282–85.
https://doi.org/10.4103/ijo.IJO_1110_18.
5. Chowdhury, R., M. Gogoi, A. Sarma, and A. Sharma. “Ocular Thelaziasis: A Case Report from Assam, India.” Tropical Parasitology 8, no. 2 (2018): 94–97.
https://doi.org/10.4103/tp.TP_49_17.
6. Centers for Disease Control and Prevention. “Thelaziasis.” DPDx: Laboratory Identification of Parasites of Public Health Concern.
https://www.cdc.gov/dpdx/thelaziasis/.
7. Wang, L., D. Li, C. Yin, H. Tang, B. Luo, R. Yan, Y. Shen, and H. Liu. “Laboratory Culture and Life Cycle of Thelazia callipaeda in Intermediate and Definitive Hosts.” Pathogens 11, no. 9 (2022): 1066.
https://doi.org/10.3390/pathogens11091066.
8. Trenkić, M., S. Tasić-Otašević, M. Stalević, A. Petrović, M. A. Bezerra-Santos, and D. Otranto. “Prevention of Thelazia callipaeda Reinfection among Humans.” Emerging Infectious Diseases 29, no. 4 (2023): 843–45.
https://doi.org/10.3201/eid2904.221610.
(Rh/APC/MSM)
