What defines ophthalmomyiasis interna as opposed to ophthalmomyiasis externa?

Involvement of the eyelid skin and lashes

Infestation limited to the conjunctiva and cornea

Larval infestation of the lacrimal sac

Which species is the most common cause of human ophthalmomyiasis?

Oestrus ovis, the sheep botfly Oestrus ovis, the sheep botfly, is the most common cause of human ophthalmomyiasis, although the human botfly, latrine fly, house fly and cattle botfly have all been implicated.

Dermatobia hominis, the human botfly

Which group is classically affected by ophthalmomyiasis?

Shepherds and farmers in rural areas Ophthalmomyiasis typically occurs in shepherds and farmers in rural areas, though it has also been reported in urban settings and in patients without close animal contact; other risk factors include ocular wounds, advanced age and debilitation.

Night-shift industrial workers in cold climates

Infants in neonatal intensive care

Contact lens wearers using tap water

What fundus sign does a larva migrating through the subretinal space leave behind?

White tracks in the retinal pigment epithelium A larva travelling through the subretinal space leaves white tracks in the retinal pigment epithelium, and may subsequently enter the vitreous.

A pigmented demarcation line at the ora

Radial retinal folds converging on the fovea

Sclerosed vessels in a wedge-shaped sector

What has OCT imaging through the larval tracks of ophthalmomyiasis interna revealed?

Subretinal hyporeflective areas suggesting subretinal tunnels OCT imaging through the larval tracks has revealed subretinal hyporeflective areas that may represent subretinal tunnels left by the migrating organism.

Hyperreflective nodules above the RPE

Full-thickness retinal cavitation with an operculum

Thickening of the choroid with a dilated vessel

What may slit lamp examination reveal in ophthalmomyiasis externa?

Small translucent larvae up to 5 mm on the conjunctiva Slit lamp examination may show small translucent larvae up to 5 mm long on the conjunctiva, often with mucopurulent conjunctivitis and rarely a corneal ulcer; the initial picture can mimic simple conjunctivitis.

Keratic precipitates with a hyphaema

A dense white cataract with iris atrophy

Deep stromal vascularisation with lipid

What is recommended for ophthalmomyiasis interna once the organism can be identified?

Immediate photocoagulation of the larva Immediate photocoagulation is recommended when the organism can be identified, to prevent further mechanical damage from the burrowing larva; pars plana vitrectomy is considered when photocoagulation is not possible.

Observation until the larva dies naturally

A systemic corticosteroid course alone

Intravitreal antibiotic injection

How can a larva lying in the macula be made treatable by photocoagulation?

By herding it away using an indirect ophthalmoscope beam When the larva lies in the macula it has been suggested that the bright beam of an indirect ophthalmoscope be used to herd the organism into an area that can be treated safely with photocoagulation.

By cooling the globe to immobilise the organism

By injecting air to displace it peripherally

By waiting for it to migrate under the disc

What must be anticipated once a larva dies inside the eye, whether naturally or after laser?

A significant intraocular inflammatory response Death of the larva in the subretinal space or vitreous, spontaneous or laser-induced, can provoke a significant inflammatory response, so careful monitoring and prompt topical, intraocular or systemic immunosuppression are essential.

Immediate normalisation of intraocular pressure

Spontaneous expulsion of the organism

Rapid resorption with no sequelae

Ophthalmomyiasis: DISEASE entity, diagnosis and management

DISEASE

Myiasis is the invasion of living or dead human (or animal) tissue by fly larvae. Ophthalmomyiasis refers to the larval infestation of the eye.

Ophthalmomyiasis externa refers to the infestation of the external ocular structures, including conjunctiva and cornea, while ophthalmomyiasis interna refers to intraocular penetration of larvae.

While Oestrus ovis, or the sheep botfly, is the most common cause of human ophthalmomyiasis, several other offending species have also been identified, including the human bot fly (Dermatobia Hominis) latrine fly (Fannia), house fly (Musca domestica), and cattle botfly (Hypoderma).

The female O. ovis retains her eggs within her body until they hatch, and then typically deposits her larvae into the nostrils of sheep and goats.

When they occur in the human eye, larval infections are usually restricted to the conjunctiva and cornea. O. ovis infections have also been reported in the human nose and pharynx.

Larvae do not typically survive long enough in humans to develop into their final stages. Intraocular penetration of larvae, or ophthalmomyiasis interna, is less common worldwide than ophthalmomyiasis externa.

In the United States, most reported cases of ophthalmomyiasis interna have been caused by the rodent botfly larva of the Cutebara and Hypoderma species.

Larvae have been found in the anterior chamber, the vitreous, and the subretinal space. In the latter case, it is thought that the larva travels through the subretinal space, leaving white tracks in the RPE (retinal pigment epithelium).

The larva may subsequently enter the vitreous. When the larva dies within the subretinal space or in the vitreous, it may result in intraocular inflammation.

Ophthalmomyiasis

Risk Factors

Ophthalmomyiasis typically occurs in shepherds and farmers in rural areas but has also been reported in urban areas and in patients without close animal contact.

Other reported risk factors include eye infections, ocular wounds, advanced age, debilitation, lack of nutrition and care, and poor general health.

Cases of ophthalmomyiasis have been reported in countries around the world, including Afghanistan, Pakistan, the Caribbean, Kuwait, Iraq, Libya, Tunisia, Russia, India, The United States, and Canada.

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Diagnosis

History:

History is essential in the evaluation of ophthalmomyiasis, as the patient may have a recent history of close contact with sheep or goats.

In cases of ophthalmomyiasis externa, the patient may report a history of sudden onset of pain, burning, itching, lacrimation, foreign body sensation, rhinorrhea, chemosis, and swelling.

Ophthalmomyiasis

These initial signs and symptoms of ophthalmomyiasis externa may mimic conjunctivitis. Patients with ophthalmomyiasis interna have reported symptoms including photopsias, floaters, lines through the vision, and eye pain.

Physical Examination:

In the case of ophthalmomyiasis externa, the slit lamp exam may reveal small translucent larvae up to 5mm in length on the conjunctiva.

The patient may have evidence of mucopurulent conjunctivitis, or rarely, corneal ulcer. Larval tracks are often apparent in retina in cases of ophthalmomyiasis interna.

The larva may even be visible on biomicroscopy in the anterior chamber, subretinal space, or in the vitreous.

OCT (optical coherence tomography) imaging through the tracks has revealed subretinal hyporeflective areas which may be suggestive of subretinal tunnels.

MANAGEMENT

The mainstay of treatment for ophthalmomyiasis externa is the removal of the larvae, often at the slit lamp, with topical analgesics and antibiotics.

Removal of the larvae may be difficult due to their strong attachment to tissue using their oral hooks and body spicules.

Although one successful case of treatment with ivermectin has been reported, antihelminthics have not commonly been used in the treatment of ophthalmomyiasis externa.

Interestingly, ivermectin has shown benefit as a therapy and a prophylactic treatment for bot fly infestation of livestock.

In cases of ophthalmomyiasis interna, immediate photocoagulation is recommended, if the organism can be identified, in order to prevent further mechanical damage from the burrowing larva.

If the larva is in the macula, it has been suggested to use the bright beam of an indirect ophthalmoscope to herd the organism into an area that can be safely treated by photocoagulation.

Larva within the vitreous has successfully been targeted by YAG laser. Whether the organism dies naturally or by laser treatment, the patient is at risk for a significant inflammatory response.

Therefore, careful monitoring and prompt treatment of inflammation with topical, intraocular, or systemic immunosuppression, as appropriate, is essential.

When photocoagulation is not possible, pars plana vitrectomy can be considered to remove the organism in its entirety.

While commonly used in the treatment of diffuse unilateral subacute neuroretinitis (DUSN), antihelminthic medications have not been reported in the treatment of ophthalmomyiasis interna.

REFERENCES

Saraiva V da S, Amaro MH, Belfort Jr. R, et al. A case of anterior internal ophthalmomyiasis: case report. Arq Bras Oftalmol 2006;69:741–743.
Dunbar J, Cooper B, Hodgetts T, et al. An Outbreak of Human External Ophthalmomyiasis Due to Oestrus ovis in Southern Afghanistan. Clin Infect Dis 2008;46:e124–e126.
G. S, A. M, S. H, et al. External Ophthalmomyiasis which was Caused by Sheep Botfly (Oestrus ovis) Larva: A Report of 10 Cases. J Clin Diagn Res JCDR 2013;7:539–542.
Lagace-Wiens PRS, Dookeran R, Skinner S, et al. Human Ophthalmomyiasis Interna Caused by Hypoderma tarandi, Northern Canada. Emerg Infect Dis 2008;14:64–66.
Georgalas I, Ladas I, Maselos S, et al. Intraocular safari: ophthalmomyiasis interna. Clin Experiment Ophthalmol 2011;39:84–85.

Test yourself

A few questions straight from this article.

1 / 10 0 correct

What defines ophthalmomyiasis interna as opposed to ophthalmomyiasis externa?
Ophthalmomyiasis externa denotes infestation of external ocular structures including conjunctiva and cornea, while ophthalmomyiasis interna denotes intraocular penetration of larvae and is less common worldwide.
Which species is the most common cause of human ophthalmomyiasis?
Oestrus ovis, the sheep botfly, is the most common cause of human ophthalmomyiasis, although the human botfly, latrine fly, house fly and cattle botfly have all been implicated.
Which group is classically affected by ophthalmomyiasis?
Ophthalmomyiasis typically occurs in shepherds and farmers in rural areas, though it has also been reported in urban settings and in patients without close animal contact; other risk factors include ocular wounds, advanced age and debilitation.
What fundus sign does a larva migrating through the subretinal space leave behind?
A larva travelling through the subretinal space leaves white tracks in the retinal pigment epithelium, and may subsequently enter the vitreous.
What has OCT imaging through the larval tracks of ophthalmomyiasis interna revealed?
OCT imaging through the larval tracks has revealed subretinal hyporeflective areas that may represent subretinal tunnels left by the migrating organism.
What may slit lamp examination reveal in ophthalmomyiasis externa?
Slit lamp examination may show small translucent larvae up to 5 mm long on the conjunctiva, often with mucopurulent conjunctivitis and rarely a corneal ulcer; the initial picture can mimic simple conjunctivitis.
What is the mainstay of treatment for ophthalmomyiasis externa?
Treatment centres on removing the larvae, often at the slit lamp, with topical analgesics and antibiotics; removal is hampered by oral hooks and body spicules, and anthelmintics have not commonly been used.
What is recommended for ophthalmomyiasis interna once the organism can be identified?
Immediate photocoagulation is recommended when the organism can be identified, to prevent further mechanical damage from the burrowing larva; pars plana vitrectomy is considered when photocoagulation is not possible.
How can a larva lying in the macula be made treatable by photocoagulation?
When the larva lies in the macula it has been suggested that the bright beam of an indirect ophthalmoscope be used to herd the organism into an area that can be treated safely with photocoagulation.
What must be anticipated once a larva dies inside the eye, whether naturally or after laser?
Death of the larva in the subretinal space or vitreous, spontaneous or laser-induced, can provoke a significant inflammatory response, so careful monitoring and prompt topical, intraocular or systemic immunosuppression are essential.