A 74-year-old California man’s insect bite led to something much more horrifying than an annoying itch: a parasitic worm infestation of his eyelid.
Doctors at the University of California Los Angeles and others detailed the upsetting turn of events in a recent case report. To make things weirder, the man contracted a species of parasite that’s never been documented in the United States before. Fortunately, upon its discovery, the worm was safely removed, and the man recovered with no issues.
“Our findings confirm the necessity of both molecular and histological studies to identify nematode infections,” the authors wrote in their report, published in the July issue of the journal Emerging Infectious Diseases.
An old-world invader
According to the report, the man was bitten by an unknown insect on his lower left eyelid.
At first, he experienced some fleeting pain, swelling, and fluid seeping from the bite. Six weeks later, however, his dermatologist noticed a firm but nontender eight millimeter-long “nodule” in the same spot. The man then visited an ophthalmologist and was given an MRI, which revealed some sort of cyst on the eyelid. Doctors initially suspected this cyst was a chalazion, an inflamed but benign lump on eyelids caused by a blocked oil gland.
Chalazions typically go away on their own after a few weeks, but five months later, the man’s lump was still there, so his doctors decided to remove and biopsy it. The surgery went off without a hitch, and the cyst was sent to a lab for further testing. It was then that doctors discovered the actual culprit: a nematode, or roundworm.
Physically, the parasite resembled a group of nematodes known as Dirofilaria; further genetic analysis confirmed that it was the species Dirofilaria repens. Like many parasites, Dirofilaria worms have a complicated life cycle, one that involves transmission between mosquitoes and their natural hosts like dogs or raccoons. Humans are accidental dead-end hosts, since the worms can’t become fully mature adults and mate inside our bodies.
In the Americas, people usually get infected by D. immitis (the cause of heartworms in dogs) or D. tenuis (native to raccoons). In Europe, Asia, and Africa, a.k.a. the Old World, the most common cause of human illness from these worms is D. repens. But this appears to be the first ever such report in the New World, according to the researchers.
How did it get here?
Some relatively recent studies have found D. repens worms in South American mammal hosts (specifically the ring-tailed coati, a type of raccoon), the researchers noted. However, a 2022 nationwide survey of dogs and cats in the U.S. failed to find any genetic traces of the worm, leaving open the question as to how and when it might have arrived here.
Given that the man reported no recent travel, the researchers do suspect he caught the worm from a domestic host in the area. And it’s possible the state’s growing presence of Aedes mosquitoes (a common vector for Dirofilaria and other diseases) may be partly to blame. “The recently increased population of Aedes mosquitoes in southern California might have contributed, but the lack of available D. repens surveys in wildlife hosts from California hampers conclusive findings,” they wrote.
Human Dirofilaria infections are rare in general, and eyelid cases of D. repens typically stay localized and can be easily removed once identified. In this case, the man was completely symptom-free six months after his surgery. Still, the appearance of a parasite in a new region is always concerning, and there should definitely be more follow-up research, the scientists say.
“The identification of D. repens nematodes in the United States warrants continued surveillance,” they wrote.
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