Ophthalmomyiasis is a disease caused by infestation of the eye tissues by fly larvae (maggots). It is a type of myiasis, accounting for less than 5% of all myiasis cases 2). Fewer than 300 cases of ophthalmomyiasis have been reported in the past century 1). The first case was reported by Keyt in 1900 2).
It is classified as follows depending on the site of parasitism.
According to a systematic review of 312 cases of external ophthalmomyiasis reported from 2000 to 2022, the male-to-female ratio was 2:1, and the mean age was 32.1 years2). The most common causative species was Oestrus ovis (sheep botfly), accounting for 72.1% of all cases, followed by Dermatobia hominis (human botfly) at 5.4%2). By country, the highest numbers of reports were from India (19.9%), Jordan (16.0%), Turkey (14.4%), and Iran (8.7%)2).
The main causative species and their frequencies are shown below.
| Causative species | Frequency | Distribution |
|---|---|---|
| Oestrus ovis | 72.1% | Mediterranean coast, Asia |
| Dermatobia hominis | 5.4% | Central and South America |
| Lucilia sericata | 0.96% | Worldwide |
| Chrysomya bezziana | 0.96% | Southeast Asia, India |
Only one case has been reported in Japan, caused by Boettcherisca peregrina2). Although extremely rare, attention to future occurrences is needed due to the expansion of fly habitats associated with global warming.
Symptoms differ greatly between the external and internal types.
External Type
Foreign body sensation: The most common symptom. 43.6% report a feeling of a fly flying into the eye2).
Redness and congestion: Accompanied by conjunctival injection and eyelid redness.
Tearing and discharge: May be accompanied by mucoid to purulent discharge.
Itching: Particularly prominent with O. ovis.
Burning sensation and photophobia: Increase as inflammation progresses.
Endophthalmitis type
External ocular findings:
Findings of internal ophthalmomyiasis:
O. ovis (sheep botfly) is an obligate parasite; females deposit first-instar larvae, hatched internally, into the nasal cavities of sheep and goats. Humans are accidental hosts, and larvae cannot mature in the human body 1)5). In contrast, Calliphoridae (blowflies) are facultative parasites that typically use necrotic tissue or wounds, and do not normally prefer living tissue 1).
Risk factors for ocular myiasis include the following:
In 33% of reported cases, no risk factor was identified 2). Cases in urban areas are also increasing, and even non-farmers can become infected. Since infection can occur through accidental contact with flies, it is important to inquire about travel history to endemic areas and living environment.
Diagnosis of ocular myiasis requires a high index of suspicion. External ophthalmomyiasis symptoms resemble viral and bacterial conjunctivitis and are easily overlooked2).
For the external type, differentiation from acute conjunctivitis (viral, bacterial, allergic), corneal foreign body, and eyelid cellulitis is necessary2)5). For the intraocular type, differentiation from chorioretinitis, dry eye syndrome (reported to be misdiagnosed in early stages6)), and diffuse unilateral subacute neuroretinitis (DUSN) is required.
| Diagnostic method | External ocular type | Internal ocular type |
|---|---|---|
| Slit-lamp microscope | Direct visualization of larvae | Larvae in anterior chamber |
| Fundus examination | Not required (usually) | Migration tracks / larvae |
| Ultrasound examination | Not required (usually) | Intravitreal larvae |
The principle of treatment is prompt mechanical removal of the larvae and prevention of secondary infection.
For external ophthalmomyiasis caused by O. ovis, mechanical removal plus topical antibiotics is used in 52.8% of cases, and mechanical removal plus topical antibiotics plus topical steroids in 41.3%2).
Photocoagulation
Indications: When the larva is visible on or under the retina.
Method: Argon laser is applied to the head of the larva. Power 350–400 mW, duration 0.1–0.2 seconds, spot size 200 μm6).
Limitations: Even after the larva is killed, debris may remain in the eye, potentially causing persistent inflammation due to immunogenic substances6).
Vitrectomy
Indications: First-line treatment when the larva is present in the vitreous cavity.
Method: The larva is grasped and removed with forceps via 25-gauge pars plana vitrectomy (PPV)6).
Advantages: Immediate improvement of inflammation, restoration of transparency of the visual media, and rapid visual improvement can be expected6).
In the intraocular type, eosinophilic inflammatory reaction is suppressed preoperatively with topical dexamethasone 0.1% and systemic prednisolone (1 mg/kg/day for 7 days)6). Postoperatively, levofloxacin 0.5% eye drops are administered for 2 weeks to prevent secondary bacterial infection6).
Orazbekov et al. (2022) performed vitrectomy in 3 cases of intraocular type6). In a case where the larva had been in the eye for 1 month, postoperative visual acuity recovered to 20/32, whereas in a case of 5 months, it remained at 20/400. Early diagnosis and early surgery were shown to be directly linked to visual prognosis.
The external type is completely cured by mechanical removal of the larva. In a review of 312 cases, all reported cases were cured2). However, since recurrence can occur if the larva remains in the fornix, follow-up after 24 to 48 hours is recommended2).
Ophthalmomyiasis is classified into obligatory and facultative parasitism depending on the ecology of the causative fly1)2).
O. ovis first-instar larvae land on the conjunctival surface and attach to the conjunctival and corneal epithelium using oral hooks and body spines2). The larvae move actively within the conjunctival sac, causing mechanical tissue damage and inflammatory reactions. Calliphoridae larvae destroy tissue through both secreted proteolytic enzymes and mechanical grinding with oral hooks1).
O. ovis larvae do not mature in the human body and usually die within 10 days2). Therefore, external ophthalmomyiasis may resolve spontaneously, but rarely, depending on the host’s immune status, it can take an invasive course.
The mechanism by which larvae penetrate the sclera and enter the eye is not fully understood. The larval mouth hooks are considered the tools for penetration 6). After entry, the larvae migrate through the subretinal space, leaving characteristic white tracks on the retinal pigment epithelium (RPE). They may then enter the vitreous cavity.
When larvae die in the subretinal space or vitreous, immunogenic substances trigger an eosinophil-mediated tissue inflammatory response 6). This causes uveitis, retinal edema, and retinal detachment. The longer the larvae remain in the eye, the more irreversible the tissue damage becomes, leading to a poor visual prognosis 6).
Orazbekov et al. identified larvae removed from three cases of the intraocular type 6). Three species were identified: Stomoxys calcitrans (stable fly), Oestrus ovis, and Musca sorbens (bazaar fly), all of which have morphological features of attaching to tissue with mouth hooks and moving with spines. It is speculated that the thinner sclera in children compared to adults facilitates larval entry into the eye.
They are thought to penetrate the sclera using mouth hooks 6). In children, the sclera is thinner and the density of myofibroblasts is lower, which may make it easier for larvae to enter 6). All three intraocular cases were children aged 4 to 15 years.
Traditionally, ocular myiasis was concentrated in warm regions such as the Mediterranean coast, the Middle East, and South Asia. However, in recent years, autochthonous cases have been reported in previously non-endemic areas such as Germany, France, and China 2).
Martinez-Rojano et al. (2023) pointed out in a review of 312 cases that global warming is expanding the habitat of O. ovis 2). In Burgundy (France), a temperature rise exceeding the global average was confirmed from 1961 to 2011, and the establishment of the genus Oestrus was reported. The need for epidemiological surveillance of ocular myiasis in the context of future climate change is emphasized.
Parker et al. (2024) reported the first identification of two Calliphoridae larvae, Lucilia coeruleiviridis and Phormia regina, from a single case 3). By analyzing the complete mitochondrial genome, they succeeded in identifying closely related species that cannot be distinguished by the COI barcode region alone. Accurate identification of the causative species is useful for assessing the risk of progression to internal ophthalmomyiasis and for epidemiological studies.
