Introduction

Eye infections include a broad spectrum of diseases caused by bacteria, viruses, fungi and parasites, resulting from mild conjunctivitis to severe conditions that can lead to impaired vision or blindness.1 Among parasitic eye infections, trematode eye infections represent a distinct and relatively rare subset. Trematodes, or flukes, are a diverse group of parasitic flatworms, and their infection in the eye can lead to a spectrum of pathological consequences, ranging from mild irritation to severe vision impairment or even blindness if left untreated. These parasites can infiltrate various ocular tissues, causing inflammation, tissue damage and structural alterations that compromise the visual function of the affected individual.2–4

Trematodes have complex life cycles involving intermediate hosts, snails and freshwater fish or crustaceans. For example, ocular trematodes, those of the genus Philophthalmus, are known to have complex life cycles requiring multiple hosts at different stages.5 Accordingly, snails serve as the first intermediate hosts, releasing cercariae into the water. The definitive host, typically an aquatic bird, acquires the infection through direct eye contact with cercariae or metacercariae in the water or by consuming metacercariae,6 and the definitive host can have corneal lesions with oedema and ulcers, conjunctival hyperaemia, keratitis, eye discharges and corneal abscess. Additionally, systemic signs such as dehydration and emaciation can also be seen.7 8

Trematode cercariae can penetrate the skin or mucous membranes of mammals.9 Though uncommon, humans can become an accidental host, and the role of humans as accidental hosts is highlighted in several studies from across the tropics.3 10–13 The exact path of pathogen entry into the human ocular tissue still remains unknown. Cercariae or metacercariae can enter the eye directly while bathing or swimming in freshwater14–16 or by consumption of raw or undercooked freshwater fish or crustaceans that serve as intermediate hosts for certain trematode species.12 14 Ocular trematode infections were identified among those who have engaged in freshwater-related activities. For example, several South Indian studies highlighted the possible role of trematodes in ocular granulomas in children who used to bathe in lakes or rivers.3 10 11 A year-long study conducted in Tamil Nadu, India, has found allergic conjunctival granulomas in 34 children from a single village who used the same freshwater pond for bathing. The clinical presentation was thus linked to potential trematode infections acquired through freshwater.10 Another similar study revealed evidence of trematode infection in patients who presented with clinical features of allergic conjunctival granuloma and with a history of water-related activities, especially swimming in freshwater reservoirs.11

Molecular studies conducted to identify the trematode species causing ocular infection in children and their potential vectors further indicate the high possibility of getting the infection through freshwater sources. One such study analysed the DNA isolated from surgically removed granulomas of children and from the trematode cercariae released by the snail Melanoides tuberculata found in the freshwater reservoir of those children used to bathe. Real-time and conventional PCR analysis followed by bidirectional sequencing and BLAST analysis identified maximum sequence similarity between the granuloma samples and the trematode cercariae Procerovum varium aiding the identification of the possible trematode species and vectors for ocular trematode infection in human.3 Moreover, digenic trematode DNA was detected in 6 out of 14 excised nodules of waterborne ophthalmic granulomas of the anterior chamber in a series of Egyptian patients.17 Ingestion of infected intermediate hosts of certain trematode species (eg, Procerovum spp) can introduce the infective larvae into the human digestive tract. These larvae can then penetrate the intestinal wall, enter the bloodstream and migrate to various organs, including the eyes, where they can cause localised pathology.12 However, the available information is not adequate to exclude oral inoculation in humans as not a part of the normal life cycle of those trematodes.

The clinical symptoms and signs vary, depending on the type of the infected parasite, the stage of the infection and the ocular tissues involved. Further, the host immune responses which include local defence mechanisms play a pivotal role in determining the severity of the infection.18 19 However, several typical features may aid in clinical suspicion and diagnosis such as eye redness, pain or discomfort, excessive tearing or watery eyes, itching or irritation, eye discharge, swollen eyelids, change in the appearance of the eye, photophobia, foreign body sensation and blurred or decreased vision.3 11 Granuloma formation or systemic symptoms due to the infection are some occasional clinical consequences.3 20 In the absence of timely intervention, the disease can cause serious consequences including blindness.2 Thus, delayed diagnosis can affect the quality of life of the patient and the overall disease burden. Further understanding the patient’s perspectives of the disease is pivotal to knowing its impacts and the prevention and control of these infections.

The insidious nature of these infections underscores the importance of timely detection and intervention to mitigate the risk of irreversible ocular damage. A recent study identified different trematode species causing ocular infections in Sri Lankan children denoting the need for a collective approach among clinicians and parasitologists, for improved community awareness for effective diagnosis, control and prevention of the disease.16 Thus, understanding the epidemiology, clinical presentation, risk factors and patient’s perception of ocular trematode infections is crucial in identifying effective disease control and prevention strategies. Summarising the current body of evidence not only provides insights into the aetiopathogenesis, clinico-epidemiological and diagnostic aspects, and patient perspectives related to ocular trematode infections in humans but also helps identify the areas that require further investigation. Moreover, a comprehensive synthesis of the literature enables researchers and healthcare practitioners to gain scientific information to design targeted studies and develop evidence-based guidelines for the diagnosis, management and prevention of this challenging condition.