Article Text
PDF
Statistics from Altmetric.com
Editor,—Ocular infections with helminthic parasites are well described. The commonest organisms are filarial worms that reside in subcutaneous tissue, and are found as skin infestations or masses in the lids. Some are known to live freely in the conjunctival sac. Worms that are visible to the naked eye are often referred to as “eyeworms”, and are in the larval or adult stage of their life cycle. Thelazia callipaeda, or the oriental eye worm, is a spiruroid nematode which is the causative organism in thelaziasis, a well described condition affecting the external eye.1 It is primarily a parasite of the conjunctiva in dogs, and is also found in rabbits and humans.2 Its presence in the conjunctival sac causes lacrimation and irritation, and its frequent excursions across the cornea may cause marked discomfort and, eventually, corneal scarring. The worm also causes paralytic ectropion through its presence in the lower fornix. At least 40 cases of infection in humans have been described from China, Japan, India, Russia, Thailand, and Korea. All of these report extraocular infection only. We report the first case of Thelazia callipaeda to cause intraocular infection.
CASE REPORT
A 21 year old Chinese woman presented to the ophthalmology department at Shantou Central Hospital, Guangdong, China, complaining of decreased vision in her right eye and a floater in the right visual field. She had no photopsia or field loss, and no pain, redness, or discharge. She had no past ocular or medical history, and was otherwise well.
On examination she had a visual acuity of 6/60 in the right eye, and 6/6 in the left eye. There was no external evidence of trauma; the lids were healthy, the conjunctiva white, the cornea clear, and the anterior chamber quiet. A mild vitreous haze obscured the foveal reflex, and clearly visible within the vitreous cavity was a live, mobile, white worm. There was no retinal abnormality.
One month later she underwent a three port pars plana vitrectomy, and the worm was coaxed into a flute needle and removed intact. The patient made an uncomplicated recovery from surgery. At 6 weeks postoperatively the eye was quiet and she had a visual acuity of 6/24. On detailed examination, the worm was identified as an adult female specimen ofThelazia callipaeda. Treatment with ivermectin is recommended for cases of thelaziasis, but in the absence of preoperative and postoperative symptoms of inflammation due to infestation, pharmacological treatment was not commenced in this case.
COMMENT
Thelazia callipaeda (θηλαζω, to suck) is a member of the phylum Nematoda, order Spirusida, suborder Spirurata, superfamily Spiruroidea. Adult worms look like creamy white threads; males are 4.5–13 mm long and 0.25–0.75 mm in diameter; females are larger, ranging from 6.2 to 17 mm in length and 0.3 to 0.85 mm in diameter (Fig 1). T callipaeda is an unsegmented invertebrate with distinct oral and anal parts.3 The male is identifiable by its curved posterior (Fig 2A). In both the male and the female, the mouth is without lips (Fig 2B) and consists of two concentric rings of flattened papillae around a central opening. It should be noted that there are no hooks or sharp spines in the mouth or elsewhere on the body, and this raises the question of how this worm entered the eye.
Light micrograph of Thelazia callipaeda (magnification ×10).
(A) Electron micrograph of tail end of Thelazia callipaeda (magnification ×1000). (B) Electron micrograph of head end of Thelazia callipaeda (magnification ×1000).
Gnathostoma spinigerum, a close relative ofThelazia, is quite frequently found within the eye and causes major tissue destruction. Structurally, one of its most distinguishing features is the presence of rows of curved hooks and posteriorly angled spines in a cuff around the mouth used for boring into tissue. T callipaeda does not possess the organs to enable it to burrow through or bore its way into any body structure.
Little is known about the life cycle ofThelazia. The intermediate host isAmiota variegata, a fly of theDrosophila family.4 It is known to infect the lacrimal sac, from where it passes into the conjunctival sac. It is probably deposited in the larval form by flies settling on the eyes and this accounts for extraocular infection. The adult worm in our patient’s eye was fully developed and healthy. It had caused little inflammatory response, and appeared to thrive in the conditions provided by the vitreous cavity, suggesting the possibility of immune privilege. We cannot explain how this worm entered the eye. We put forward the suggestion that humans may be the definitive host, with dogs and rabbits being the reservoir. For intraocular colonisation, the infective stage is probably the filariform larva, the portal of entry being the skin. An alternative mode of infection may be as the larva or embryonated egg ingested with raw drinking water.