Ocular pentastomiasis is a parasitic infection caused by the larvae of Pentastomida invading the eye. Pentastomiasis itself is a rare but increasing zoonotic disease, and ocular involvement is considered a rare manifestation.
Pentastomida, also known as tongue worms, are a group of parasitic arthropods. Based on molecular biological studies, they are now considered a type of crustacean. The species reported to infect humans belong to the following three genera.
Systemic infection is often nonspecific or asymptomatic, and the exact number of affected individuals is unknown. Autopsy studies have reported prevalence rates of 8% in Cameroon, up to 45% in Malaysia, 22% in Congo, and 33% in Nigeria [2].
It is common in endemic areas of Africa (especially Central Africa) and Southeast Asia. In these regions, bushmeat such as snakes and monkeys is an important protein source, leading to high infection risk. In non-endemic areas, immigrants and reptile keepers are at risk.
Symptoms of ocular pentastomiasis are usually unilateral. The time from onset to final diagnosis ranges from 4 days to 36 months.
Systemic pentastomiasis (visceral pentastomiasis) is generally asymptomatic. When symptomatic, it presents with various manifestations depending on the site of infection, such as acute abdomen, jaw necrosis, or gynecological complaints. Incidental findings during imaging or surgery have also been reported.
In the eye, Thelazia larvae most frequently parasitize the anterior chamber, followed by ocular adnexa and posterior chamber [1,2]. The following findings are observed depending on the location.
Anterior Segment
Ring-shaped foreign body in the anterior chamber: Peristaltic movement may be observed under slit-lamp microscopy.
Subconjunctival mass: Observed as parasitism of the ocular adnexa.
Cyclitic membrane: Formed in association with inflammatory reaction.
Annular foreign body within the lens capsule: May be accompanied by peristaltic movements.
Posterior segment
Intravitreal floating foreign body: Larvae freely floating in the vitreous cavity are observed.
Vitritis: Vitreous opacity due to inflammatory reaction to the larvae.
Subretinal annular or crescent-shaped foreign body: A characteristic finding is retinal vessels running over the parasite.
Retinal detachment: Secondary to mechanical damage or inflammation.
They are observed as ring-shaped foreign bodies in the anterior chamber during slit-lamp microscopy. Characteristic peristaltic movements may be seen. The larvae are large enough to be discernible with the naked eye.
Humans become accidental intermediate hosts by ingesting pentastomid eggs. The life cycle does not complete in the human body, making humans a parasitic dead-end.
The main risk factors are as follows.
The diagnosis of ocular pentastomiasis is primarily based on clinical evaluation, including the patient’s medical history, symptoms, and physical examination.
Suspect this disease in patients from endemic areas who consume snake or dog meat. In non-endemic areas, immigrants, veterinarians, zookeepers, and reptile keepers are at risk.
Slit-lamp examination revealing annular foreign bodies in the anterior chamber or lens capsule with visible peristaltic movement is diagnostic [3,4,5]. Fundus examination may show floating foreign bodies in the vitreous or crescent-shaped foreign bodies under the retina.
If systemic pentastomiasis is suspected, abdominal and chest X-rays may show horseshoe-shaped calcifications.
The following diseases should be differentiated.
| Differential Disease | Key Points for Differentiation |
|---|---|
| Cysticercosis | Imaging findings / endemic areas |
| Lymphoproliferative disorders | Blood tests / tissue diagnosis |
| Tuberculosis | Chest X-ray / systemic findings |
Definitive diagnosis requires surgical collection of the parasite from the eye and identification of the species using molecular biological methods. Eosinophilia on blood tests is only a supportive finding.
Most cases of pentastomiasis are asymptomatic and rarely require treatment. Since the parasite naturally dies within the body after about 2 years, treatment is not recommended for asymptomatic incidental findings.
When there is ocular involvement, surgical removal is the recommended treatment [1,2]. Early removal of the parasite minimizes exposure to inflammatory reactions and mechanical changes caused by the larvae, leading to improved prognosis.
The extraction method is selected according to the site of parasitism.
Standard drug therapy for systemic pentastomiasis has not been established. The following drugs have been reported for symptomatic systemic cases.
In both cases, clinical and radiological improvement has been observed. However, there are no reports on the efficacy of antiparasitic drugs for ocular pentastomiasis.
There are no reports demonstrating the effectiveness of antiparasitic drugs for ocular pentastomiasis. Intraocular larvae require surgical removal. Although there are reports of improvement with antiparasitic drugs in systemic pentastomiasis, standard therapy has not been established.
Pentastomids are obligate parasites, with adults parasitizing the upper respiratory tract of reptiles, birds, and mammals. They have five appendages on the head (one of which is the mouth), which is the origin of the name (Greek “penta” = five, “stoma” = mouth). Adult body length ranges from 1 to 14 cm, with females larger than males.
The life cycle of the tongue worm consists of the following stages:
Humans become accidental intermediate hosts by ingesting meat containing eggs. The larvae penetrate the intestinal wall and spread throughout the body via the bloodstream and lymphatics. Ocular involvement is thought to be hematogenous.
When larvae reach the eye, they cause tissue damage through the following mechanisms.
These changes may ultimately lead to blindness.
The definitive diagnosis of pentastomiasis has traditionally relied on morphological identification of the parasite. In recent years, molecular biological techniques (such as 18S rDNA sequencing) have advanced species identification, enabling more accurate diagnosis [2,5]. This is expected to improve the precision of epidemiological surveys and accurately assess the disease burden in endemic areas.
Recent case reports have identified a new site of parasitism: pentastomid larvae within the lens capsule. While the anterior chamber, ocular adnexa, and posterior chamber were previously considered the main sites of parasitism, the existence of intralenticular parasitism calls for an expanded search range during diagnosis.
In endemic areas, the dissemination of health education, promotion of proper cooking of bushmeat, and improvement of healthcare access are considered important for the prevention and early detection of ocular pentastomiasis. The current delay of up to 36 months from onset to diagnosis reflects a lack of medical resources and low disease awareness.
