Human Infection by Zoonotic Eye Fluke Philophthalmus lacrymosus: What This South American Case Says (2026)

Imagine discovering a minuscule parasite, usually found lurking in the eyes of birds, has crossed over to infect a human traveler—it's a real-life scenario straight out of a medical thriller, and it's unfolding right here in South America! This eye-opening case highlights the surprising ways zoonotic diseases can bridge the gap between animals and people, sparking urgent questions about global health and wildlife encounters. But here's where it gets controversial: Could this be just the tip of the iceberg for infections we might see more of as travel and environmental changes bring us closer to wild habitats? Let's dive in and explore this fascinating—and a bit unsettling—story together, breaking it down step by step so even beginners can follow along without feeling overwhelmed.

First off, a quick disclaimer: This article is an early release version, so it's not the final word. Any updates will appear online once the official publication date arrives in the issuing month. The authors hail from esteemed institutions, including Charité–Universitätsmedizin Berlin in Germany (T. Weitzel); Clínica Alemana and Universidad del Desarrollo in Santiago, Chile (T. Weitzel and C. Aravena); Instituto de Salud Pública in Santiago (E.M. Cordero and M.I. Jercic); Universidad San Sebastián in Santiago (T. Mujica); Zoo New England in Boston, Massachusetts, USA (B.E. Phillips); University of Georgia in Athens, Georgia, USA (M.J. Yabsley); North Carolina State University in Raleigh, North Carolina, USA (G.A. Lewbart); Universidad San Francisco de Quito in Puerto Baquerizo Moreno, Ecuador (D. Páez-Rosas); Galápagos Conservancy in Puerto Ayora, Ecuador (D. Páez-Rosas); and Biologie, Épidémiologie et Analyse de Risque en Santé Animale in Nantes, France (S. Capasso). These experts bring a wealth of knowledge from parasitology, veterinary medicine, and conservation to shed light on this unusual infection.

Now, let's talk about the star of this unsettling tale: the Philophthalmus spp. parasites. These are digenean trematodes—think of them as flatworm parasites that thrive globally, primarily in the conjunctival sac of waterbirds. For those new to this, a conjunctival sac is the pocket behind your eyelids, and trematodes are a type of parasitic flatworm that can cause all sorts of trouble in hosts. Known colloquially as avian eye flukes, they lead a pretty complex life cycle. It starts with freshwater and marine snails acting as intermediate hosts—imagine these tiny creatures unwittingly carrying the parasite's eggs through water bodies. Birds, the final hosts, pick up the infection by eating metacercariae, which are encysted (capped like little cysts) on aquatic plants. Once ingested, a warm trigger in the bird's throat causes the cysts to open, allowing the fluke to migrate through the lacrimal ducts (those tear-draining tubes) into the eye's orbital cavity. Fascinating, right? But here's the twist: While birds are the usual victims, we've seen these flukes jump to mammals too, like capybaras in Brazil, Galapagos sea lions in Ecuador, and even humans on rare occasions. And this is the part most people miss—how exactly do non-bird hosts get infected? Experts aren't 100% sure, but theories include accidentally swallowing the infective stages or direct contact with them in water, opening up a whole can of worms (or should I say flukes?) for prevention strategies.

Globally, over 50 species of Philophthalmus trematodes have been named, but recent studies suggest only about 10 are truly distinct—highlighting how taxonomy can evolve with better science. In South America, these flukes have popped up in places like Brazil, Peru, Venezuela, and the stunning Galapagos Islands off Ecuador. What's more, invasive snail species could spread them even further, potentially introducing these parasites to new ecosystems. Invasive species are a hot topic in ecology; for example, think of how an introduced snail could hitchhike on boats or in aquariums, carrying pathogens across oceans. This raises eyebrows about climate change and human activity accelerating such spreads—controversial, isn't it? Some argue it's inevitable with increasing global travel, while others debate stricter biosecurity measures.

Historically, human cases of philophthalmiasis (that's the disease caused by these flukes) date back to the 19th century, with over 12 documented instances since 1939. These have occurred in Asia, Europe, and North America, often just identified by genus. But our focus is a fresh case: a 26-year-old woman from England who sought medical help in Santiago, Chile, after nine days of excruciating pain, swelling, and that eerie sensation of something moving in her right eye. Before symptoms hit, she'd spent time in Colombia (about four weeks), Ecuador including the Galapagos (around two and a half weeks), and Peru (a week). Her eye exam revealed eyelid swelling, severe chemosis (swelling of the conjunctiva, the eye's clear covering), and a follicular reaction in the inferior fornix and upper tarsal conjunctiva—nothing major on the cornea, anterior segment, or fundus. With careful inspection, doctors spotted and removed an elongated, mobile structure from the upper tarsal conjunctiva using a damp cotton swab. Poof—the foreign body feeling vanished, and follow-ups showed full recovery without issues. It's a relief to know it was treatable, but it begs the question: How many travelers brush off eye symptoms as allergies, missing something parasitic?

To confirm what was pulled out, researchers conducted in-depth morphologic studies using a temporary wet mount under an Olympus SZ61 stereo microscope, paired with an Olympus DP22 digital camera and Olympus cellSens software version 2.3. The specimen turned out to be a P. lacrymosus fluke, a single mature one with eggs. Picture an elongated oval shape, smooth exterior, no spines, and a constriction near the ventral sucker. Its widest point was behind the ventral sucker, with a subterminal oral sucker, muscular pharynx, bifurcating esophagus, and ceca reaching the back of the posterior testis. The acetabulum (a sucker for attachment) was bigger than the oral one and positioned pre-equatorially; testes were tandem, smooth, and spherical in the body's rear, intercecal area; the cirrus sac elongated just past the acetabulum's edge (cirrus hidden); genital pore median at the cecal bifurcation; ovary spherical and pretesticular; uterus long and coiled between the ventral sucker and anterior testis; vitellarium follicular with non-operculated eggs housing fully formed miracidia (early larval stages) sporting dark eyespots. They compared measurements with prior P. lacrymosus samples to nail down the identity.

Molecular confirmation followed with PCR and bidirectional Sanger sequencing targeting the nuclear ITS-2 and mitochondrial Cox1 regions. Sequences matched closely: An 861-base pair ITS-2 sequence was spot-on with P. zalophi from Galapagos sea lions, 98.6% identical to P. lacrymosus from Portuguese gulls, and 95.9% to P. lucipetus from the same birds, plus 95.6% to P. gralli from Costa Rican snails and Peruvian birds. The 396-base pair Cox1 sequence showed 99.73% identity (just one change) to P. lacrymosus from Brazilian kelp gulls, dropping to 91.90%-92.15% with Portuguese ones, and around 87% with P. lucipetus or a mysterious Japanese snail isolate. Phylogenetic trees from 17 ITS-2 and 16 Cox1 sequences further supported these connections, suggesting possible species overlaps.

Epidemiology of human philophthalmiasis remains murky. Cases span continents, with infection routes debated—some point to swimming with metacercariae directly entering the eye, others to eating contaminated food or handling it during prep. Notably, the parasite can linger in humans for months, often as a single worm causing one-sided irritation, foreign body feelings, and conjunctivitis. Vision loss is rare, except in old 18th-century reports with heavy infestations. Most worms are only ID'd to genus, leaving species uncertainty. In the Americas, P. lacrymosus has hit humans in Mexico, birds in Brazil and Venezuela, and capybaras in Brazil. This patient only touched natural waters in the Galapagos, home to P. zalophi in sea lions. Given the fluke's long human tenancy, a past Ohio case might link to Galapagos too. Marine snails like the Batillariidae family (including the West Indian false cerith) could mediate transmission there.

Her Galapagos exposure is inferred—proof needs studies on infected hosts or larvae—but molecular data hint P. zalophi might be the same as P. lacrymosus. Bird-to-sea lion spillover could explain morphological tweaks, much like adaptations in capybaras. Host plasticity or developmental stages might account for variations from classic P. lacrymosus traits, sharing features like oral sucker-to-pharynx ratios and bodies under 6 mm with P. zalophi.