June 28, 2019
2 min read

Texas man contracts rare eye infection caused by trematode

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Sarah G.H. Sapp, PhD
Sarah G.H. Sapp

A man in southern Texas became only the second person ever in the United States diagnosed with philophthalmiasis, a rare eye infection caused by a certain trematode, according to a report published in Open Forum Infectious Diseases.

The Philophthalmus species eye fluke was extracted from the bulbar conjunctiva in man’s right eye, researchers reported. Typically, the trematode infects the conjunctival and orbital tissues of birds, especially water birds and wading birds.

“Birds and other natural animal hosts are infected by eating metacercariae, an immature stage of the parasite that attaches to objects in water, such as plants or the shells of mollusks and crustaceans,” Sarah G.H. Sapp, PhD, a parasitologist in the CDC’s Parasitic Disease Branch, told Infectious Disease News. “Laboratory experiments have also shown that exposing the eye to water contaminated with cercariae, a free-swimming developmental stage of the parasite, is another way that infection can occur. It is not clear which of these routes, or if both routes, are involved in human infections.”

The authors noted that infections in nonavian species are rare, and in humans, there are only 25 cases reported in medical literature.

Photo of an adult Philophthalmus species eye fluke  
Philophthalmiasis is a rare eye infection caused by a certain trematode.
Source: cdc.gov/dpdx

Philophthalmus fluke infections in humans are extremely rare, so people should not be concerned about them, but Philophthalmus does represent an interesting example of how some veterinary parasites may infect human hosts under circumstances that we do not completely understand,” Sapp explained. “Clinicians should be aware of this possibility after other causes are ruled out.”

According to the report, the patient was a 47-year-old man who lived in southern Texas, close to the Mexico border, who complained of “recurrent subconjunctival hemorrhage on the lateral aspect of the right eye.” He reported making occasional, short-distance day trips to Mexico but did not report swimming, contact with natural bodies of water or eating unwashed vegetation. However, he said he frequently consumed raw fish and cooked crabs.

Slit lamp examination identified the trematode attached to the temporal surface of the bulbar conjunctiva under the man’s upper right eyelid, Sapp and colleagues reported. Topical tetracaine drops were administered, and the trematode was removed using forceps. The hemorrhage resolved and did not recur, the authors reported.

Sapp and colleagues said it was “difficult to determine” where the patient acquired the infection — in the U.S. or Mexico — because of his travel history, close proximity to Mexico and the occurrence of several freshwater and marine Philophthalmus species in the Americas. They were unable to identify the exact species of Philophthalmus because additional specimens were unavailable.

The primary route of transmission in humans is still unclear, Sapp and colleagues wrote. But they suspect that oral exposure is a possibility in this case due to the patient’s eating habits.

“Doctors should consider the possibility of Philophthalmus infection if live organisms of a few millimeters in size are found in the eye socket,” Sapp said. “However, diagnostic confirmation and species-level identification, particularly of rarely encountered organisms such as this, requires expertise in parasite morphology. In these cases, the services of reference laboratories with trained parasitologists, such as [the Division of Parasitic Diseases (DPDx)] at the CDC, should be consulted.” – by Marley Ghizzone

Disclosures: The authors report no relevant financial disclosures.